ICONS OF VOLUTION? WHY MUCH OF HAT ONATHAN WELLS … · The Miller-Urey experiment isn’t...

INTRODUCTION

THE PARADIGM OF EVOLUTION

Evolution is the unifying paradigm, theorganizing principle of biology.Paradigms are accepted for their overall

explanatory power, their “best fit” with all theavailable data in their fields. A paradigm func-tions as the glue that holds an entire disciplinetogether, connecting disparate subfields andrelating them to one another. A paradigm isalso important because it fosters a researchprogram, creating a series of questions thatgive researchers new directions to explore inorder to better understand the phenomenabeing studied. For example, the unifying para-digm of geology is plate tectonics; althoughnot all geologists work on it, it connects theentire field and organizes the various disci-plines of geology, providing them with theirresearch programs. A paradigm does not standor fall on a single piece of evidence; rather, itis justified by its success in overall explanato-ry power and the fostering of research ques-tions. A paradigm is important for the ques-tions it leads to, rather than the answers itgives. Therefore, the health of a scientific fieldis based on how well its central theory explainsall the available data and how many newresearch directions it is spawning. By thesecriteria, evolution is a very healthy paradigmfor the field of biology.

In his book Icons of Evolution (2000),Jonathan Wells attempts to overthrow the par-adigm of evolution by attacking how we teachit. In this book, Wells identifies ten examples

that are commonly used to help to teach evolu-tion. Wells calls these the “icons,” and brandsthem as false, out of date, and misleading.Wells then evaluates ten “widely used” highschool and college biology textbooks for sevenof these “icons” with a grading scheme that heconstructed. Based on this, he claims that theirtreatments of these icons are so rife with inac-curacies, out-of-date information, and down-right falsehoods that their discussions of theicons should be discarded, supplemented, oramended with “warning labels” (which he pro-vides).

According to Wells, the “icons” are theMiller-Urey experiment, Darwin’s tree of life,the homology of the vertebrate limbs,Haeckel’s embryos, Archaeopteryx, the pep-pered moths, and “Darwin’s” finches.(Although he discusses three other “icons” —four-winged fruit flies, horse evolution, andhuman evolution — he does not evaluate text-books’ treatments of them.) Wells is rightabout at least one thing: these seven examplesdo appear in nearly all biology textbooks. Yetno textbook presents the “icons” as a list of our“best evidence” for evolution, as Wellsimplies. The “icons” that Wells singles out arediscussed in different parts of the textbooks fordifferent pedagogical reasons. The Miller-Urey experiment isn’t considered “evidencefor evolution”; it is considered part of theexperimental research about the origin of lifeand is discussed in chapters and sections on the“history of life.” Likewise, Darwin’s finchesare used as examples of an evolutionaryprocess (natural selection), not as evidence for

ICONS OF EVOLUTION? WHY MUCH OF WHAT JONATHAN

WELLS WRITES ABOUT EVOLUTION IS WRONG

ALAN D. GISHLICKNATIONAL CENTER FOR SCIENCE EDUCATION

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evolution. Archaeopteryx is frequently pre-sented in discussions of the origin of birds, notas evidence for evolution itself. Finally, text-books do not present a single “tree of life”;rather, they present numerous topic-specificphylogenetic trees to show how relevantorganisms are related. Wells’s entire discus-sion assumes that the evidence for evolution isa list of facts stored somewhere, rather than thepredictive value of the theory in explaining thepatterns of the past and present biologicalworld.

TEXTBOOK “ICONS”:WHY DO WE HAVE THEM?

Paradigms and all their components are notnecessarily simple. To understand the depth ofany scientific field fully requires many yearsof study. It is the goal of elementary and sec-ondary education to give students a basicunderstanding of the “world as we know it,”which includes teaching students the para-digms of a number of fields of science. Inorder to do this, teaching examples must befound. It is this need to find simple, easy-to-explain, dynamic, and visual examples tointroduce a complex topic to students that hasled to the common use of a few examples —the “icons.” Yet, with our knowledge of thenatural world expanding at near-exponentialrates, the volume of new information facing atextbook writer is daunting. The aim of a text-book is not necessarily to report the “state ofthe art” as much as it is to offer an introductionto the basic principles and ideas of a certainfield. Therefore, it should not be surprisingthat introductory textbooks are frequently sim-plified and may be somewhat out-of-date. InIcons of Evolution, however, Wells makes aneven stronger accusation. Wells says:“Students and the public are being systemati-cally misinformed about the evidence for evo-lution” through biology textbooks (Wells,

2000: xii). This is a serious charge; to supportit demands the highest level of scholarship onthe part of the author.

Does Wells display this level of scholar-ship? Is Wells right? Are the “icons” out-of-date and in need of removal? And more impor-tantly, is there something wrong with the theo-ry of evolution?

In the following sections, each textbook“icon” is reexamined in light of Wells’s criti-cism. The textbooks covered by Wells areexamined as well, along with the grading cri-teria (given in the appendix of Icons [Wells,2000] and on the Discovery Institute’s web-site) that he used to assess their accuracy. Whatwas found is that although the textbooks couldalways benefit from improvement, they do notmislead, much less “systematically misin-form,” students about the theory of biologicalevolution or the evidence for it. Further, thegrading criteria Wells applied are vague and attimes appear to have been manipulated to givepoor grades. Many of the grades given are notin agreement with the stated criteria or anaccurate reading of the evaluated text. Beyondthat, Icons of Evolution offers little in the wayof suggestions for improvement of, or changesin, the standard biology curriculum. WhenWells says that textbooks are in need of cor-rection, he apparently means the removal ofthe subject of evolution entirely or the teachingof “evidence against” evolution, rather thanthe fixing of some minor errors in the presen-tation of the putative “icons.” This makesIcons of Evolution useful at most for thosewith a certain political and religious agenda,but of little value to educators.

References

Wells, J. 2000. Icons of evolution: science or myth?:why much of what we teach about evolution is wrong.Regnery, Washington DC, 338p.

Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is WrongAlan D. Gishlick, National Center for Science Education

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THE MILLER-UREYEXPERIMENT

THE EXPERIMENT ITSELF

The understanding of the origin of lifewas largely speculative until the 1920s,when Oparin and Haldane, working

independently, proposed a theoretical modelfor “chemical evolution.” The Oparin–Haldane model suggested that under thestrongly reducing conditions theorized to havebeen present in the atmosphere of the earlyearth (between 4.0 and 3.5 billion years ago),inorganic molecules would spontaneouslyform organic molecules (simple sugars andamino acids). In 1953, Stanley Miller, alongwith his graduate advisor Harold Urey, testedthis hypothesis by constructing an apparatusthat simulated the Oparin-Haldane “earlyearth.” When a gas mixture based on predic-tions of the early atmosphere was heated andgiven an electrical charge, organic compoundswere formed (Miller, 1953; Miller and Urey,1959). Thus, the Miller-Urey experimentdemonstrated how some biological molecules,such as simple amino acids, could have arisenabiotically, that is through non-biologicalprocesses, under conditions thought to be sim-ilar to those of the early earth. This experimentprovided the structure for later research intothe origin of life. Despite many revisions andadditions, the Oparin–Haldane scenarioremains part of the model in use today. TheMiller–Urey experiment is simply a part of theexperimental program produced by this para-digm.

WELLS BOILS OFF

Wells says that the Miller–Urey exper-iment should not be taught becausethe experiment used an atmospheric

composition that is now known to be incorrect.Wells contends that textbooks don’t discuss

how the early atmosphere was probably differ-ent from the atmosphere hypothesized in theoriginal experiment. Wells then claims that theactual atmosphere of the early earth makes theMiller–Urey type of chemical synthesisimpossible, and asserts that the experimentdoes not work when an updated atmosphere isused. Therefore, textbooks should either dis-cuss the experiment as an historically interest-ing yet flawed exercise, or not discuss it at all.Wells concludes by saying that textbooksshould replace their discussions of the Miller–Urey experiment with an “extensive discus-sion” of all the problems facing research intothe origin of life.

These allegations might seem serious; how-ever, Wells’s knowledge of prebiotic chemistryis seriously flawed. First, Wells’s claim thatresearchers are ignoring the new atmosphericdata, and that experiments like the Miller–Urey experiment fail when the atmosphericcomposition reflects current theories, is simplyfalse. The current literature shows that scien-tists working on the origin and early evolutionof life are well aware of the current theories ofthe earth’s early atmosphere and have foundthat the revisions have little effect on theresults of various experiments in biochemicalsynthesis. Despite Wells’s claims to the con-trary, new experiments since the Miller–Ureyones have achieved similar results using vari-ous corrected atmospheric compositions(Figure 1; Rode, 1999; Hanic et al., 2000).Further, although some authors have arguedthat electrical energy might not have efficient-ly produced organic molecules in the earth’searly atmosphere, other energy sources such ascosmic radiation (e.g., Kobayashi et al., 1998),high temperature impact events (e.g.,Miyakawa et al., 2000), and even the action ofwaves on a beach (Commeyras et al., 2002)would have been quite effective.

Even if Wells had been correct about the


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Miller–Urey experiment, he does not explainthat our theories about the origin of organic“building blocks” do not depend on that exper-iment alone (Orgel, 1998a). There are othersources for organic “building blocks,” such asmeteorites, comets, and hydrothermal vents.All of these alternate sources for organic mate-rials and their synthesis are extensively dis-cussed in the literature about the origin of life,a literature that Wells does not acknowledge.In fact, what is most striking about Wells’sextensive reference list is the literature that hehas left out. Wells does not mention extrater-restrial sources of organic molecules, whichhave been widely discussed in the literature

since 1961 (see Oró, 1961; Whittet, 1997;Irvine, 1998). Wells apparently missed the vastbody of literature on organic compounds incomets (e.g. Oró, 1961; Anders, 1989; Irvine,1998), carbonaceous meteorites (e.g., Kaplanet al., 1963; Hayes, 1967; Chang, 1994;Maurette, 1998; Cooper et al., 2001), and con-ditions conducive to the formation of organiccompounds that exist in interstellar dust clouds(Whittet, 1997).

Wells also fails to cite the scientific litera-ture on other terrestrial conditions under whichorganic compounds could have formed. Thesenon-atmospheric sources include the synthesisof organic compounds in a reducing ocean


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Researcher(s) Year Reactants Energy source Results Probability Miller 1953 CH4, NH3, H2O, H2 Electric discharge Simple amino acids, unlikely

organic compoundsAbelson 1956 CO, CO2, N2, NH3, H2, Electric discharge Simple amino acids, unlikely

H2O HCNGroth and Weyssenhoff 1957 CH4, NH3, H2O Ultraviolet light Simple amino acids (low under special conditions

(1470–1294 ?) yields)Bahadur, et al. 1958 Formaldehyde, Sunlight Simple amino acids possible

molybdenum oxide (photosynthesis)Pavolvskaya and 1959 Formaldehyde, nitrates High pressure Hg lamp Simple amino acids possiblePasynskii (photolysis)Palm and Calvin 1962 CH4, NH3, H2O Electron irradiation Glycine, alanine, aspartic under special conditions

acidHarada and Fox 1964 CH4, NH3, H2O Thermal energy 14 of the “essential” under special conditions

(900–1200º C) amino acids of proteinsOró 1968 CH4, NH3, H2O Plasma jet Simple amino acids unlikely

Bar-Nun et al. 1970 CH4, NH3, H2O Shock wave Simple amino acids under special conditions

Sagan and Khare 1971 CH4, C2H6, NH3, H2O, Ultraviolet light (>2000 Simple amino acids (low under special conditions

H2S ?) yields)Yoshino et al. 1971 H2, CO, NH3, Temperature of 700°C Glycine, alanine, unlikely

montmorillonite glutamic acid, serine, aspartic acid, leucine, lysine, arginine

Lawless and Boynton 1973 CH4, NH3, H2O Thermal energy Glycine, alanine, aspartic under special conditions

acid, ?-alanine, N-methyl-?-alanine, ?-amino-n-butyric acid.

Yanagawa et al. 1980 Various sugars, Temperature of 105°C Glycine, alanine, serine, under special conditionshydroxylamine, aspartic acid, glutamic inorganic salts, acid

Kobayashi et al. 1992 CO, N2, H2O Proton irradiation Glycine, alanine, aspartic possible

acid, ?-alanine, glutamic acid, threonine, ?-aminobutyric acid, serine

Hanic, et al. 1998 CO2, N2, H2O Electric discharge Several amino acids possible

Figure 1. A table of some amino acid synthesis experiments since Miller–Urey. The “probabili-ty” column reflects the likelihood of the environmental conditions used in the experiment.Modified from Rode, 1999.

(e.g., Chang, 1994), at hydrothermal vents(e.g., Andersson, 1999; Ogata et al., 2000), andin volcanic aquifers (Washington, 2000). Acursory review of the literature finds more than40 papers on terrestrial prebiotic chemical syn-thesis published since 1997 in the journalOrigins of life and the evolution of the bios-phere alone. Contrary to Wells’s presentation,there appears to be no shortage of potentialsources for organic “building blocks” on theearly earth.

Instead of discussing this literature, Wellsraises a false “controversy” about the lowamount of free oxygen in the early atmos-phere. Claiming that this precludes the sponta-neous origin of life, he concludes that“[d]ogma had taken the place of empirical sci-ence” (Wells, 2000:18). In truth, nearly allresearchers who work on the early atmospherehold that oxygen was essentially absent duringthe period in which life originated (Copley,2001) and therefore oxygen could not haveplayed a role in preventing chemical synthesis.This conclusion is based on many sources ofdata, not “dogma.” Sources of data includefluvial uraninite sand deposits (Rasmussen andBuick, 1999) and banded iron formations(Nunn, 1998; Copley, 2001), which could nothave been deposited under oxidizing condi-tions. Wells also neglects the data from pale-osols (ancient soils) which, because they format the atmosphere–ground interface, are anexcellent source to determine atmosphericcomposition (Holland, 1994). Reduced pale-osols suggest that oxygen levels were very lowbefore 2.1 billion years ago (Rye and Holland,1998). There are also data from mantle chem-istry that suggest that oxygen was essentiallyabsent from the earliest atmosphere (Kump etal., 2001). Wells misrepresents the debate asover whether oxygen levels were 5/100 of 1%,which Wells calls “low,” or 45/100 of 1%,which Wells calls “significant.” But the con-

troversy is really over why it took so long foroxygen levels to start to rise. Current datashow that oxygen levels did not start to risesignificantly until nearly 1.5 billion years afterlife originated (Rye and Holland, 1998;Copley, 2001). Wells strategically fails to clar-ify what he means by “early” when he discuss-es the amount of oxygen in the “early” atmos-phere. In his discussion, he cites researchabout the chemistry of the atmosphere withoutdistinguishing whether the authors are refer-ring to times before, during, or after the periodwhen life is thought to have originated. Nearlyall of the papers he cites deal with oxygen lev-els after 3.0 billion years ago. They are irrele-vant, as chemical data suggest that life arose3.8 billion years ago (Chang, 1994; Orgel,1998b), well before there was enough freeoxygen in the earth’s atmosphere to preventMiller–Urey-type chemical synthesis.

Finally, the Miller–Urey experiment tells usnothing about the other stages in the origin oflife, including the formation of a simple genet-ic code (PNA or “peptide”-based codes andRNA-based codes) or the origin of cellularmembranes (liposomes), some of which arediscussed in all the textbooks that Wellsreviewed. The Miller–Urey experiment onlyshowed one possible route by which the basiccomponents necessary for the origin of lifecould have been created, not how life came tobe. Other theories have been proposed tobridge the gap between the organic “buildingblocks” and life. The “liposome” theory dealswith the origin of cellular membranes, theRNA-world hypothesis deals with the origin ofa simple genetic code, and the PNA (peptide-based genetics) theory proposes an even sim-pler potential genetic code (Rode, 1999). Wellsdoesn’t really mention any of this except tosuggest that the “RNA world” hypothesis wasproposed to “rescue” the Miller–Urey experi-ment. No one familiar with the field or the evi-


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dence could make such a fatuous and inaccu-rate statement. The Miller–Urey experiment isnot relevant to the RNA world, because RNAwas constructed from organic “buildingblocks” irrespective of how those compoundscame into existence (Zubay and Mui, 2001).The evolution of RNA is a wholly differentchapter in the story of the origin of life, one towhich the validity of the Miller–Urey experi-ment is irrelevant.

WHAT THE TEXTBOOKS SAY

All of the textbooks reviewed contain asection on the Miller–Urey experi-ment. This is not surprising given the

experiment’s historic role in the understandingof the origin of life. The experiment is usuallydiscussed over a couple of paragraphs (seeFigure 2), a small proportion (roughly 20%) ofthe total discussion of the origin and early evo-lution of life. Commonly, the first paragraphdiscusses the Oparin-Haldane scenario, andthen a second outlines the Miller–Urey test ofthat scenario. All textbooks contain either adrawing or a picture of the experimental appa-ratus and state that it was used to demonstratethat some complex organic molecules (e.g.,simple sugars and amino acids, frequentlycalled “building blocks”) could have formedspontaneously in the atmosphere of the earlyearth. Textbooks vary in their descriptions ofthe atmospheric composition of the early earth.Five books present the strongly reducingatmosphere of the Miller–Urey experiment,whereas the other five mention that the currentgeochemical evidence points to a slightlyreducing atmosphere. All textbooks state thatoxygen was essentially absent during the peri-od in which life arose. Four textbooks mentionthat the experiment has been repeated success-fully under updated conditions. Three text-books also mention the possibility of organicmolecules arriving from space or forming at

deep-sea hydrothermal vents (Figure 2). Notextbook claims that these experiments conclu-sively show how life originated; and all text-books state that the results of these experi-ments are tentative.

It is true that some textbooks do not mentionthat our knowledge of the composition of theatmosphere has changed. However, this doesnot mean that textbooks are “misleading” stu-dents, because there is more to the origin oflife than just the Miller–Urey experiment.Most textbooks already discuss this fact. Thetextbooks reviewed treat the origin of life withvarying levels of detail and length in “Originof life” or “History of life” chapters. Thesechapters are from 6 to 24 pages in length. Inthis relatively short space, it is hard for a text-book, particularly for an introductory class likehigh school biology, to address all of thedetails and intricacies of origin-of-life researchthat Wells seems to demand. Nearly all textsbegin their origin of life sections with a briefdescription of the origin of the universe andthe solar system; a couple of books use a dis-cussion of Pasteur and spontaneous generationinstead (and one discusses both). Two text-books discuss how life might be defined.Nearly all textbooks open their discussion ofthe origin of life with qualifications about howthe study of the origin of life is largely hypo-thetical and that there is much about it that wedo not know.

WELLS’S EVALUATION

As we will see in his treatment of theother “icons,” Wells’s criteria for judg-ing textbooks stack the deck against

them, ensuring failure. No textbook receivesbetter than a D for this “icon” in Wells’s eval-uation, and 6 of the 10 receive an F. This islargely a result of the construction of the grad-ing criteria. Under Wells’s criteria (Wells,2000:251–252), any textbook containing a pic-


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ture of the Miller–Urey apparatus couldreceive no better than a C, unless the caption ofthe picture explicitly says that the experimentis irrelevant, in which case the book wouldreceive a B. Therefore, the use of a picture isthe major deciding factor on which Wells eval-uated the books, for it decides the grade irre-spective of the information contained in thetext! A grade of D is given even if the textexplicitly points out that the experiment usedan incorrect atmosphere, as long as it shows apicture. Wells pillories Miller and Levine forexactly that, complaining that they bury thecorrection in the text. This is absurd: almost alltextbooks contain pictures of experimentalapparatus for any experiment they discuss. It isthe text that is important pedagogically, not the

pictures. Wells’s criteria would require thateven the intelligent design “textbook” OfPandas and People would receive a C for itstreatment of the Miller–Urey experiment.

In order to receive an A, a textbook mustfirst omit the picture of the Miller–Urey appa-ratus (or state explicitly in the caption that itwas a failure), discuss the experiment, but thenstate that it is irrelevant to the origin of life.This type of textbook would be not only scien-tifically inaccurate but pedagogically defi-cient.

WHY WE SHOULD STILL TEACHMILLER–UREY


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Figure 2. Textbook treatments of the Miller–Urey experiment. Textbooks are listed in order ofincreasing detail (AP/College textbooks highlighted; note that Futuyma is an upper-level col-lege/graduate textbook).

The Miller–Urey experiment representsone of the research programs spawnedby the Oparin-Haldane hypothesis.

Even though details of the model for the originof life have changed, this has not affected thebasic scenario of Oparin–Haldane. The firststage in the origin of life was chemical evolu-tion. This involves the formation of organiccompounds from inorganic molecules alreadypresent in the atmosphere and in the water ofthe early earth. This spontaneous organizationof chemicals was spawned by some externalenergy source. Lightning (as Oparin andHaldane thought), proton radiation, ultravioletradiation, and geothermal or impact-generatedheat are all possibilities.

The Miller–Urey experiment represents amajor advance in the study of the origin of life.In fact, it marks the beginning of experimentalresearch into the origin of life. Before Miller–Urey, the study of the origin of life was mere-ly theoretical. With the advent of “spark exper-iments” such as Miller conducted, our under-standing of the origin of life gained its firstexperimental program. Therefore, the Miller–Urey experiment is important from an histori-cal perspective alone. Presenting history isgood pedagogy because students understandscientific theories better through narratives.The importance of the experiment is more thanjust historical, however. The apparatus Millerand Urey designed became the basis for manysubsequent “spark experiments” and laid agroundwork that is still in use today. Thus it isalso a good teaching example because it showshow experimental science works. It teachesstudents how scientists use experiments to testideas about prehistoric, unobserved eventssuch as the origin of life. It is also an interest-ing experiment that is simple enough for moststudents to grasp. It tested a hypothesis, wasreproduced by other researchers, and providednew information that led to the advancement

of scientific understanding of the origin of life.This is the kind of “good science” that we wantto teach students.

Finally, the Miller–Urey experiment shouldstill be taught because the basic results are stillvalid. The experiments show that organic mol-ecules can form under abiotic conditions. Laterexperiments have used more accurate atmos-pheric compositions and achieved similarresults. Even though origin-of-life research hasmoved beyond Miller and Urey, their experi-ments should be taught. We still teach Newtoneven though we have moved beyond his workin our knowledge of planetary mechanics.Regardless of whether any of our current theo-ries about the origin of life turn out to be com-pletely accurate, we currently have models forthe processes and a research program thatworks at testing the models.

HOW TEXTBOOKS COULD IMPROVETHEIR PRESENTATIONS OF

THE ORIGIN OF LIFE

Textbooks can always improve discus-sions of their topics with more up-to-date information. Textbooks that have

not already done so should explicitly correctthe estimate of atmospheric composition, andaccompany the Miller–Urey experiment with aclarification of the fact that the correctedatmospheres yield similar results. Further, thewealth of new data on extraterrestrial andhydrothermal sources of biological materialshould be discussed. Finally, textbooks ideallyshould expand their discussions of other stagesin the origin of life to include PNA and someof the newer research on self-replicating pro-teins. Wells, however, does not suggest thattextbooks should correct the presentation ofthe origin of life. Rather, he wants textbooks topresent this “icon” and then denigrate it, inorder to reduce the confidence of students inthe possibility that scientific research can ever


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establish a plausible explanation for the originof life or anything else for that matter. IfWells’s recommendations are followed, stu-dents will be taught that because one experi-ment is not completely accurate (albeit in hind-sight), everything else is wrong as well. This isnot good science or science teaching.

References

Anders, E. 1989. Pre-biotic organic matter from cometsand asteroids. Nature 342:255–257.

Andersson, E. and N. G. Holm. 2000. The stability ofsome selected amino acids under attempted redox con-strained hydrothermal conditions. Origins of Life andthe Evolution of the Biosphere 30: 9–23.

Chang, S. 1994. The planetary setting of prebiotic evo-lution. In S. Bengston, ed. Early Life on Earth. NobelSymposium no. 84. Columbia University Press, NewYork. p.10–23.

Commeyras, A., H. Collet, L. Bioteau, J. Taillades, O.Vandenabeele-Trambouze, H. Cottet, J-P. Biron, R.Plasson, L. Mion, O. Lagrille, H. Martin, F. Selsis, andM. Dobrijevic. 2002. Prebiotic synthesis of sequentialpeptides on the Hadean beach by a molecular engineworking with nitrogen oxides as energy sources.Polymer International 51:661–665.

Cooper, G., N. Kimmich, W. Belisle, J. Sarinana, K.Brabham, and L. Garrel. 2001. Carbonaceous meteoritesas a source of sugar-related organic compounds for theearly Earth. Nature 414:879–882.

Copley, J. 2001. The story of O. Nature 410:862-864.

Hanic, F., M. Morvová and I. Morva. 2000.Thermochemical aspects of the conversion of thegaseous system CO2—N2—H2O into a solid mixture ofamino acids. Journal of Thermal Analysis andCalorimetry 60: 1111–1121.

Hayes, J. M. 1967. Organic constituents of meteorites, areview. Geochimica et Cosmochimica Acta 31:1395–1440.

Holland, H. D. 1994. Early Proterozoic atmospherechange. In S. Bengston, ed. Early Life on Earth. NobelSymposium no. 84. Columbia University Press, NewYork. p. 237–244.

Irvine, W. M., 1998. Extraterrestrial organic matter: areview. Origins of Life and the Evolution of theBiosphere 28:365–383.

Kaplan, I. R., E. T. Degens, and J. H. Reuter. 1963.

Organic compounds in stony meteorites. Geochimica etCosmochimica Acta. 27:805–834.

Kobayashi, K., T. Kaneko, T. Saito, and T. Oshima.1998. Amino acid formation in gas mixtures by highenergy particle irradiation. Origins of Life and theEvolution of the Biosphere 28:155–165.

Kump, L. R., J. F. Kasting, M. E. Barley. 2001. Rise ofatmospheric oxygen and the “upside-down” Archeanmantle. Geochemistry, Geophysics, Geosystems –G3, 2,paper number 2000GC000114.

Maurette, M. 1998. Carbonaceous micrometeorites andthe origin of life. Origins of Life and the Evolution of theBiosphere 28: 385–412.

Miller, S. 1953. A production of amino acids under pos-sible primitive earth conditions. Science 117:528–529.

Miller, S. and H. Urey. 1959. Organic compound syn-thesis on the primitive earth. Science 130:245–251.

Miyakawa, S., K-I. Murasawa, K. Kobayashi, and A. B.Sawaoka. 2000. Abiotic synthesis of guanine with high-temperature plasma. Origins of Life and Evolution of theBiosphere 30: 557–566.

Nunn, J. F. 1998. Evolution of the atmosphere.Proceedings of the Geologists’ Association 109:1–13.

Ogata, Y., E-I. Imai, H. Honda, K. Hatori, and K.Matsuno. 2000. Hydrothermal circulation of seawaterthrough hot vents and contribution of interface chem-istry to prebiotic synthesis. Origins of Life and theEvolution of the Biosphere 30: 527-–537.

Orgel, L. E. 1998a. The origin of life – a review of factsand speculations. Trends in Biochemical Sciences23:491–495.

Orgel, L. E., 1998b. The origin of life — how long didit take? Origins of Life and the Evolution of theBiosphere 28: 91–96.

Oró, J. 1961. Comets and the formation of biochemicalcompounds on the primitive Earth. Nature 190:389-390.

Rasmussen, B., and R. Buick. 1999. Redox state of theArchean atmosphere; evidence from detrital heavy min-erals in ca. 3250-2750 Ma sandstones from the PilbaraCraton, Australia. Geology 27: 115–118.

Rode, B. M., 1999. Peptides and the origin of life.Peptides 20: 773–786.

Rye, R., and H. D. Holland. 1998. Paleosols and theevolution of atmospheric oxygen: a critical review.American Journal of Science 298:621–672.

Washington, J. 2000. The possible role of volcanicaquifers in prebiologic genesis of organic compounds


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and RNA. Origins of Life and the Evolution of theBiosphere 30: 53–79.


Whittet, D. C. B. 1997. Is extraterrestrial organic matterrelevant to the origin of life on earth? Origins of Lifeand the Evolution of the Biosphere 27: 249–262.

Zubay, G. and T. Mui. 2001. Prebiotic synthesis ofnucleotides. Origins of Life and Evolution of theBiosphere 31:87–102.


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DARWIN’S “TREE OF LIFE”

PHYLOGENETIC TREES

In biology, a phylogenetic tree, or phyloge-ny, is used to show the genealogic relation-ships of living things. A phylogeny is not

so much evidence for evolution as much as itis a codification of data about evolutionary his-tory. According to biological evolution, organ-isms share common ancestors; a phylogenyshows how organisms are related. The tree oflife shows the path evolution took to get to thecurrent diversity of life. It also shows that wecan ascertain the genealogy of disparate livingorganisms. This is evidence for evolution onlyin that we can construct such trees at all. Ifevolution had not happened or common ances-try were false, we would not be able to discov-er hierarchical branching genealogies fororganisms (although textbooks do not general-ly explain this well). Referring to any phylo-genetic tree as “Darwin’s tree of life” is some-what of a misnomer. Darwin graphically pre-sented no phylogenies in the Origin of Species;the only figure there depicts differential ratesof speciation. If anyone deserves credit forgiving us “trees of life,” it is Ernst Haeckel,who drew phylogenies for many of the livinggroups of animals literally as trees, as well ascoining the term itself.

WELLS’S SHELL GAME

Wells uses phylogenetic trees to attackthe very core of evolution — com-mon descent. Wells claims that text-

books mislead students about common descentin three ways. First, Wells claims that text-books do not cover the “Cambrian Explosion”and fail to point out how this “top-down” pat-tern poses a serious challenge to commondescent and evolution. Second, he asserts thatthe occasional disparity between morphologi-cal and molecular phylogenies disproves com-

mon descent. Finally, he demands that text-books treat universal common ancestry asunproven and refrain from illustrating that“theory” with misleading phylogenies.Therefore, according to Wells, textbooksshould state that there is no evidence for com-mon descent and that the most recent researchrefutes the concept entirely. Wells is complete-ly wrong on all counts, and his argument isentirely based on misdirection and confusion.He mixes up these various topics in order toconfuse the reader into thinking that whencombined, they show an endemic failure ofevolutionary theory. In effect, Wells plays theequivalent of an intellectual shell game, put-ting so many topics into play that the “ball” ofevolution gets lost.

THE CAMBRIAN EXPLOSION

Wells claims that the CambrianExplosion “presents a serious chal-lenge to Darwinian evolution”

(Wells, 2000:41) and the validity of phyloge-netic trees. The gist of Wells’s argument is thatthe Cambrian Explosion happened too fast toallow large-scale morphological evolution tooccur by natural selection (“Darwinism”), andthat the Cambrian Explosion shows “top-down” origination of taxa (“major” “phyla”level differences appear early in the fossilrecord rather than develop gradually), whichhe claims is the opposite of what evolutionpredicts. He asserts that phylogenetic treespredict a different pattern for evolution thanwhat we see in the Cambrian Explosion. Thesearguments are spurious and show his lack ofunderstanding of basic aspects of both paleon-tology and evolution.

Wells mistakenly presents the CambrianExplosion as if it were a single event. TheCambrian Explosion is, rather, the preserva-tion of a series of faunas that occur over a 15–20 million year period starting around 535 mil-


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lion years ago (MA). A fauna is a group oforganisms that live together and interact as anecosystem; in paleontology, “fauna” refers to agroup of organisms that are fossilized togetherbecause they lived together. The first faunathat shows extensive body plan diversity is theSirius Passet fauna of Greenland, which isdated at around 535 MA (Conway Morris,2000). The organisms preserved become morediverse by around 530 MA, as the Chenjiangfauna of China illustrates (Conway Morris,2000). Wells erroneously claims that theChenjiang fauna predates the Sirius Passet(Wells, 2000:39). The diversification contin-ues through the Burgess shale fauna of Canadaat around 520 MA, when the Cambrian faunasare at their peak (Conway Morris, 2000). Wellsmakes an even more important paleontologicalerror when he does not explain that the “explo-sion” of the late Early and Middle Cambrian ispreceded by the less diverse “small shelly”metazoan faunas, which appear at the begin-ning of the Cambrian (545 MA). These faunasare dated to the early Cambrian, not thePrecambrian as stated by Wells (Wells,2000:38). This enables Wells to omit thesteady rise in fossil diversity over the ten mil-lion years between the beginning of theCambrian and the Cambrian Explosion (Knolland Carroll, 1999).

In his attempt to make the CambrianExplosion seem instantaneous, Wells alsogrossly mischaracterizes the Precambrian fos-sil record. In order to argue that there was notenough time for the necessary evolution tooccur, Wells implies that there are no fossils inthe Precambrian record that suggest the com-ing diversity or provide evidence of moreprimitive multicellular animals than those seenin the Cambrian Explosion (Wells, 2000:42–45). He does this not by producing originalresearch, but by selectively quoting paleonto-logical literature on the fossil record and

claiming that this proves that the fossil recordis complete enough to show that there were noprecursors for the Cambrian Explosion ani-mals. This claim is false. His evidence for this“well documented” Precambrian fossil recordis a selective quote from the final sentence inan article by Benton et al. (2000). While thepaper’s final sentence does literally say thatthe “early” parts of the fossil record are ade-quate for studying the patterns of life, Wellsleaves out a critical detail: the sentence refersnot to the Precambrian, but to the Cambrianand later times. Even more ironic is the factthat the conclusion of the paper directly refutesWells’s claim that the fossil record does notsupport the “tree of life.” Benton et al. (2000)assessed the completeness of the fossil recordusing both molecular and morphologicalanalyses of phylogeny. They showed that thesequence of appearance of major taxa in thefossil record is consistent with the pattern ofphylogenetic relationships of the same taxa.Thus they concluded that the fossil record isconsistent with the tree of life, entirely oppo-site to how Wells uses their paper.

Wells further asserts that there is no evi-dence for metazoan life until “just before” theCambrian explosion, thereby denying the nec-essary time for evolution to occur. Yet Wells isevasive about what counts as “just before” theCambrian. Cnidarian and possible arthropodembryos are present 30 million years “justbefore” the Cambrian (Xiao et al., 1998).There is also a mollusc, Kimberella, from theWhite Sea of Russia (Fedonkin and Waggoner,1997) dated approximately 555 million yearsago, or 10 million years “just before” theCambrian (Martin et al., 2000). This primitiveanimal has an uncalcified “shell,” a muscularfoot (Fedonkin and Waggoner, 1997), and aradula inferred from “mat-scratching” feedingpatterns surrounding fossilized individuals(personal observation; Seilacher, pers.


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comm.). These features enable us to recognizeit as a primitive relative of molluscs, eventhough it lacks a calcified shell. There are alsoPrecambrian sponges (Gehling and Rigby,1996) as well as numerous trace fossils indi-cating burrowing by wormlike metazoansbeneath the surface of the ocean’s floor(Seilacher, 1994; Fedonkin, 1994). Trace fos-sils demonstrate the presence of at least oneancestral lineage of bilateral animals nearly 60million years “just” before the Cambrian(Valentine et al., 1999). Sixty million years isapproximately the same amount of time thathas elapsed since the extinction of non-aviandinosaurs, providing plenty of time for evolu-tion. In treating the Cambrian Explosion as asingle event preceded by nothing, Wells mis-represents fact — the Cambrian explosion isnot a single event, nor is it instantaneous andlacking in any precursors.

Continuing to move the shells, Wellsinvokes a semantic sleight of hand in resur-recting a “top-down” explanation for the diver-sity of the Cambrian faunas, implying thatphyla appear first in the fossil record, beforelower categories. However, his argument is anartifact of taxonomic practice, not real mor-phology. In traditional taxonomy, the recogni-tion of a species implies a phylum. This is dueto the rules of the taxonomy, which state that ifyou find a new organism, you have to assign itto all the necessary taxonomic ranks. Thuswhen a new organism is found, either it has tobe placed into an existing phylum or a new onehas to be erected for it. Cambrian organismsare either assigned to existing “phyla” or newones are erected for them, thereby creating theeffect of a “top-down” emergence of taxa.

Another reason why the “higher” taxonom-ic groups appear at the Cambrian Explosion isbecause the Cambrian Explosion organismsare often the first to show features that allowus to relate them to living groups. The

Cambrian Explosion, for example, is the firsttime we are able to distinguish a chordate froman arthropod. This does not mean that the chor-date or arthropod lineages evolved then, onlythat they then became recognizable as such.For a simple example, consider the turtle. Howdo you know a turtle is a turtle? By the shell.How would you recognize the ancestors of theliving turtle, before they evolved the shell?That is more complicated. Because its ances-tors would have lacked the diagnostic featureof a shell, ancestral turtles may be hard to rec-ognize (Lee, 1993). In order to locate theremote ancestors of turtles, other, more subtle,features must be found.

Similarly, before the Cambrian Explosion,there were lots of “worms,” now preserved astrace fossils (i.e., there is evidence of burrow-ing in the sediments). However, we cannot dis-tinguish the chordate “worms” from the mol-lusc “worms” from the arthropod “worms”from the worm “worms.” Evolution predictsthat the ancestor of all these groups was worm-like, but which worm evolved the notochord,and which the jointed appendages? In his argu-ment, Wells confuses the identity of the indi-vidual with how we diagnose that identity, afailure of logic that dogs his discussion ofhomology in the following chapter. If the ani-mal does not have the typical diagnostic fea-tures of a known phylum, then we would beunable to place it and (by the rules of taxono-my) we would probably have to erect a newphylum for it. When paleontologists talk aboutthe “sudden” origin of major animal “bodyplans,” what is “sudden” is not the appearanceof animals with a particular body plan, but theappearance of animals that we can recognize ashaving a particular body plan. Overall, howev-er, the fossil record fits the pattern of evolu-tion: we see evidence for worm-like bodiesfirst, followed by variations on the wormtheme. Wells seems to ignore a growing body


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of literature showing that there are indeedorganisms of intermediate morphology presentin the Cambrian record and that the classic“phyla” distinctions are becoming blurred byfossil evidence (Budd, 1998, 1999; Budd andJensen, 2000).

Finally, the “top-down” appearance ofbody-plans is, contrary to Wells, compatiblewith the predictions of evolution. The issue tobe considered is the practical one that “large-scale” body-plan change would of courseevolve before minor ones. (How can you varythe lengths of the beaks before you have ahead?) The difference is that, many of the“major changes” in the Cambrian were initial-ly minor ones. Through time they becamehighly significant and the basis for “body-plans.” For example, the most primitive livingchordate Amphioxus is very similar to theCambrian fossil chordate Pikia. Both are basi-

cally worms with a stiff rod (the notochord) inthem. The amount of change between a wormand a worm with a stiff rod is relatively small,but the presence of a notochord is a major“body-plan” distinction of a chordate. Further,it is just another small step from a worm witha stiff rod to a worm with a stiff rod and a head(e.g., Haikouella; Chen et al., 1999) or a wormwith a segmented stiff rod (vertebrae), a head,and fin folds (e.g., Haikouichthyes; Shu et al.,1999). Finally add a fusiform body, fin differ-entiation, and scales: the result is somethingresembling a “fish” (Figure 3). But, as soon asthe stiff rod evolved, the animal was suddenlyno longer just a worm but a chordate — repre-sentative of a whole new phylum! Thus these“major” changes are really minor in the begin-ning, which is the Precambrian–Cambrianperiod with which we are concerned.


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Figure 3. Stepwise evolution of vertebrate features as illustrated by living and fossil animals.

CONGRUENCE OF PHYLOGENIES

BASED ON DIFFERENT

SOURCES OF DATA

Wells also points to the occasionallack of congruence between molec-ular- and morphology-based phylo-

genies as evidence against common descent.(Molecular phylogenies are based on compar-isons of the genes of organisms.) Wells omitsthe fact that the discrepancies are frequentlysmall, and their causes are largely understood(Patterson et al., 1993; Novacek, 1994).Although not all of these discrepancies can yetbe corrected for, most genetic and morpholog-ical phylogenies are congruent for 90% of thetaxa included. For example, all phylogenies,whether morphological or molecular, considerall animals bearing amniotic eggs to be moreclosely related to one another than to amphib-ians. Within this group, all reptiles and birdsare more closely related to each other than theyare to mammals. Finally, birds and crocodilesare more closely related to each other than tolizards, snakes, and the tuatara (Gauthier et al.,1988; Gauthier, 1994). The only group whoseplacement varies for both molecular and mor-phology data sets is turtles. This is due to aphenomenon called “long branch attraction” orthe “Felsenstein Zone” (Huelsenbeck andHillis, 1993). Long branch attraction is causedwhen a organism has had so much evolution-ary change that it cannot be easily compared toother organisms, and due to the nature of themethodology used to evaluate phylogeny, itcan appear to be related to many possibleorganisms (Felsenstein, 1978; Huelsenbeckand Hillis, 1993). This is the case for turtles.Turtles are so morphologically and geneticallydifferent from the rest of the reptiles that theyare hard to place phylogenetically (Zardoyaand Meyer, 2001). Still, researchers have nar-rowed down the possible turtle relationships to

a few possibilities (Rieppel and deBraga,1996; Lee, 1997; deBraga and Rieppel, 1997;Zardoya and Meyer, 1998; Rieppel and Reiz,1999; Rieppel, 2000; Figure 4), and none ofthese claim turtles are mammals. The uncer-tainty over the precise placement of turtleswith respect to other groups, however, does notmean that they did not evolve. Unfortunately,genes can never be totally compared to mor-phology since genetic trees cannot take fossiltaxa into account: genes don’t fossilize. Nodiagnostic tool of science is perfect. Theimperfections in phylogenetic reconstructiondo not make common ancestry false. Besides,are these extremely technical topics reallyappropriate for introductory textbooks?

Instead of clearly discussing these actualphylogenetic issues, Wells invents one thatisn’t even real. He cites a 1998 paper thatplaced cows phylogenetically closer to whalesthan to horses, calling that finding “bizarre”(Wells, 2000:51). Yet this is not “bizarre” atall; it was expected. All the paleontologicaland molecular evidence points to a whale


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Figure 4. Amniote relationships based ondifferent sources of data. Note that the onlygroup whose position varies is turtles.

orgin within artiodactyls, and further to thefact that artiodactyls (cows, deer, antelopes,pigs, etc.) are not more closely related to peris-sodactyls (horses, rhinos, and the tapir) thanthey are to whales (Novacek, 1992, 2001).Wells makes this statement smugly, as if tosuggest that everyone should think that thissounds silly. Unfortunately, it is Wells’s criti-cism that is silly.

THE UNIVERSAL COMMONANCESTOR

Finally, Wells cites the “failure” of molec-ular phylogeny to clarify the position ofthe Universal Common Ancestor as

proof that there is no common ancestry for anyof life. Here, Wells mixes up the differentscales of descent in order to tangle the readerin a thicket of phylogenetic branches. He is


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Figure 5. The traditional view of phylogenetic relationships for the three “domains” of life com-pared to Woese’s view. Note that the only difference lies in whether there is a single “root” at thebase of the tree. Regardless, eukaryotes, archaeans, and bacteria all share a common ancestor onboth, although Woese does posit a greater degree of lateral trasfer for single-celled organisms.

attacking the notion that life originated withone population, and that all life can trace itsancestry back to that population, the UniversalCommon Ancestor (UCA). The problem hasbeen that it is hard to determine relationshipswhen there is nothing to compare to. How doyou compare “not life” to “life”? We have nofossils of the earliest forms of life, and the highdegree of genetic change that has occurred inthe 3.8 billion years since the early stages oflife make it nearly impossible to reconstructthe “original” genetic code. This does notinvalidate the concept of common ancestry; itjust makes it difficult and potentially impossi-ble to untangle the lineages. And this does notmean that there is not one real lineage: theinability to determine the actual arrangementof “domains” at the base of the tree or to char-acterize the UCA does not make the UCA anyless real than the inability to characterize lightunambiguously as either a wave or a particlemakes light unreal.

Some authors (e.g., Woese, 1998) go furtherand suggest that there is no “UCA”; rather,they suggest, life arose in a soup of competinggenomes. These genomes were constantlyexchanging and mixing, and thus cellular lifemay have arisen multiple times. Wells misrep-resents the statements of those scientists tomake it look as if they are questioning theentirety of common ancestry, when what theyquestion is just the idea of a single commonancestor at the base of life. Further, when somesuggest that we should abandon the search forthe UCA, they do not mean that they don’tthink it existed. They mean only that it may bea waste of time to try to find it given the cur-rent technology and methods at our disposal.Regardless of the status of a UCA, which is atthe base of the tree of life, the entire debate hasnothing to do with the branches of the tree —the shared descent of eukaryotes, of animals,or common descent among vertebrates, arthro-

pods, or angiosperms (Figure 5). That is still alot of evolution that Wells’s inaccurate attackon the idea of a UCA does nothing to dispel.


The concept of common ancestry is at thecore of evolution. The very idea thatdifferent species arise from previous

forms via descent implies that all living thingsshare a common ancestral population at somepoint in their history. This concept is support-ed by the fossil record, which shows a historyof lineages changing through time. Becauseevolution is the basis for biology, it would besurprising if any textbook teaching contempo-rary biology would portray common descentother than matter-of-factly.

Textbooks treat the concept of commondescent in basically the same way as do scien-tists; they accept common ancestry of livingthings as a starting point, and proceed fromthere. Phylogenies thus appear in many placesin a text, which makes it very hard to evaluateexactly how textbooks “misrepresent” biologi-cal evolution using trees. Most texts show aphylogeny in chapters discussing systematicsand taxonomy. In this section there is usually atree of “kingdom” or “domain” relationships,which may be what Wells considers a treeshowing “universal” common ancestry; unfor-tunately, his discussion is too vague for a read-er to be sure whether that is what he is refer-ring to. Many textbooks show additional, moredetailed trees in their discussions of differenttaxonomic groups. In terms of textbook pre-sentations, then, there is no single “Darwin’stree of life” presented in some iconic state, butmany various phylogenies shown in the appro-priate sections of most books. Textbooks alsopresent trees in the chapters on processes andmechanisms of evolution, in the “Origin oflife” or “History of life” chapters, and in chap-ters dealing with individual taxonomic groups.


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Figure 6. Evaluation of Wells’s grading of Textbook Icon #2, “Darwin’s Tree of Life.”Parenthetical notations indicate the number of phylogenetic trees shown in the book.

This is because phylogenetic trees are not partof the “evidence for evolution,” but rathergraphical representations of the history,genealogy, and taxonomy of life. No textbookmisrepresents the methods that are used toconstruct trees or the trees themselves,although some trees contain out-of-date rela-tionships and occasionally incorrect identifica-tions of organisms pictured in them. Whentextbooks cover the Cambrian period, the rapidappearance of many body plans is discussednot as a “paradox” for evolutionary theory, butas an interesting event in the history of life —which is how paleontologists and evolutionarybiologists consider it.


Overall, Wells’s grading system for this“icon” is so nebulous that it is hard tofigure out exactly how he evaluated

the textbooks at all. The “Universal CommonAncestor” is far different from the “CambrianExplosion.” These deal with very differentplaces in the “tree of life” as well as very dif-ferent issues in evolution. Wells’s grades seemlargely based on presentations of “commonancestry.” For example, according to Wells, ifthe textbook treats common ancestry as “fact,”then it can do no better than a D. In order to geta C or better, a book must also discuss the“top-down” nature of the Cambrian explosionas a “problem” for evolution; if a book onlymentions the Cambrian Explosion, it gets a D.Here Wells does not even apply his gradingscheme consistently (Figure 6). For example,Wells chastises textbooks (Miller and Levine’sin particular) for not discussing the CambrianExplosion, yet most of the textbooks hereviews actually mention it (Figure 6) andMiller and Levine devote an entire page (p.601) to it. Many of the reviewed textbooks dis-cuss the Cambrian period in the history of lifesections, but do not specifically call it an

“explosion.” These discussions usually men-tion that it was a “rapid” origin of animalgroups. Does Wells actually require that thebook explicitly mention the “CambrianExplosion” by name? If so, it should have beenspecified in the criteria. Or is it that it he onlylooked for “Cambrian Explosion” in the noto-riously spotty indexes of the textbooks? Areevaluation suggests that five of the books towhich he gives an F should receive, even byhis criteria, a D. Finally, one text (Miller andLevine’s) even mentions the confusing natureof the basal divergence of life caused by later-al transfer, yet this discussion can receive nocredit in the grading. This is because althoughWells considers the “phylogenetic thicket” tobe extremely important to reject universalcommon ancestry, he apparently does not con-sider it important enough to account for it inhis grading scheme. All of this calls into ques-tion how well Wells actually reviewed the textshe graded as well as whether his grades haveany utility at all.

WHY WE SHOULD CONTINUE TOTEACH COMMON DESCENT

There is no reason for textbooks to sig-nificantly alter their presentations ofcommon descent or phylogenetic trees.

As long as biological evolution is the paradigmof biology, common descent should be taught.All living organisms that reproduce have off-spring that appear similar to, but not exactlylike, their parents. We can observe descentwith modification every day, and like Darwin,we can confidently extrapolate that it has goneon throughout the history of life. Through thisprocess, small differences would accumulateto larger differences and result in the evolutionof diversity that we see today and throughoutthe history of life.

The concept of descent allows us to maketestable predictions about the fossil record and


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the genetics of organisms. For example, wepredict that all animals sharing a commonancestor would have a similar genetic code,use the same cellular processes, and so on;these predictions are confirmed by biochem-istry and genetics. In terms of fossils, wewould expect to see animals with transitionalmorphologies in the past, as well as animalsthat appear similar, but not identical, to thoseliving today. We also predict that these organ-isms, both past and present, can be arrangedinto a branching hierarchy of forms, whichappears much like a genealogy. This is whatthe biological community considers science;this is what we should teach.

HOW TEXTBOOKS COULD IMPROVETHEIR PRESENTATIONS

OF PHYLOGENY

There is always room for improvement inthe presentations of the concept of com-mon descent. Textbooks could improve

by updating the phylogenies, many of whichare now out-of-date. They should also removediscussions of “phenetics” (an outdated formof phylogenetic reconstruction and classifica-tion) from the phylogenetic reconstruction sec-tions, and expand discussions of cladistics andmore modern descent-based taxonomies.Finally, textbooks should make a clear distinc-tion between molecular clocks and geneticphylogenies, something many fail to do clear-ly. However, to make textbooks conform toWells’s criteria would be to misrepresent theentire life sciences and to deprive students ofpedagogically useful visual representations ofthe unity of life.

References

Benton, M. J., M. A. Wills, and R. Hitchin. 2000.Quality of the fossil record through time. Nature403:534–537.

Budd, G. E. 1998. Arthropod body-plan evolution in theCambrian with an example from anomalocarid muscle.Lethaia 31:197–210.

Budd, G. E., 1999. Does evolution in body patterninggenes drive morphological change –– or vice versa?Bioessays 21:326–332.

Budd, G. E. and S. Jensen. 2000. A critical reappraisalof the fossil record of the bilaterian phyla. BiologicalReviews 75:253–295.

Chen, J.-Y., D.-Y. Huang, and C.-W. Li. 1999. An earlyCambrian craniate-like chordate. Nature 402:518–522.

Conway Morris, S. 2000. The Cambrian “explosion”:slow fuse or megatonnage? Proceedings of the NationalAcademy of Science 97:4429–4439.

DeBraga, M., and O. Rieppel. 1997. Reptile phylogenyand the interrelationships of turtles. Zoological Journalof the Linnean Society 120:281–354.

Eernisse, and A. G. Kluge. 1993. Taxonomic congru-ence versus total evidence, and amniote phylogenyinferred from fossils, molecules, and morphology.Molecular Biology and Evolution 10:1170–1195.

Fedonkin, M. A., 1994. Vendian body fossils and tracefossils. In S. Bengston, ed. Early Life on Earth. NobelSymposium no. 84. Columbia University Press, NewYork. p. 370–388.

Fedonkin, M. A., and B. M. Waggoner. 1997. The LatePrecambrian fossil Kimberella is a mollusc-like bilater-ian organism. Nature 388:868–871.

Felsenstein, J. 1978. Cases in which parsimony or com-patibility methods will be positively misleading.Systematic Zoology 27:401–410.

Gehling, J. G., and J. K. Rigby. 1996. Long expectedsponges from the Neoproterozoic Ediacara fauna ofSouth Australia. Journal of Paleontology 70:185–195.

Gauthier, J. A., 1994. The diversification of theamniotes. In D. R. Prothero and R. M. Schoch eds.Major Features of Vertebrate Evolution. PaleontologicalSociety Short Courses in Paleontology 7:129–159.

Gauthier, J. A., A. G. Kluge, and T. Rowe. 1988.Amniote phylogeny and the importance of fossils.Cladistics 4:105–209.

Huelsenbeck, J. P., and D. M. Hillis, 1993. Success ofphylogenetic methods in the four-taxon case. SystematicBiology 42:247–264.

Knoll, A. H., and S. B. Carroll. 1999. Early animal evo-lution: emerging views from comparative biology andgeology. Science 284:2129–2137.


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Lee, M. S. Y., 1993. The origin of the turtle bodyplan:bridging a famous morphological gap. Science261:1716–1720.

Lee, M. S. Y. 1997. Pareiasaur phylogeny and the originof turtles. Zoological Journal of the Linnean Society120:197–280.

Martin, M. W., D. V. Grazhdankin, S. A. Bowring, D. A.D. Evans, M. A. Fedonkin, and J. L. Kirschvink. 2000.Age of Neoproterozoic Bilatarian body and trace fossils,White Sea, Russia: implications for metazoan evolution.Science 288:841–845.

Novacek, M. J. 1992. Mammalian phylogeny: shakingthe tree. Nature 356:121–125.

Novacek, M. J. 1994. Morphological and molecularinroads to phylogeny. In L. Grande and O. Rieppel, eds.Interpreting the Hierarchy of Nature: from SystematicPatterns to Evolutionary Process Theories. AcademicPress, New York, p. 85–132.

Novacek, M. J. 2001. Mammalian phylogeny: Genesand supertrees. Current Biology 11:R573–575.

Patterson, C., D. M. Williams, and C. J. Humphries.1993. Congruence between molecular and morphologi-cal phylogenies. Annual Review of Ecology andSystematics 24:153–188.

Rieppel, O. 2000. Turtles as diapsid reptiles. ZoologicaScripta 29:199–212.

Rieppel, O., and M. deBraga. 1996. Turtles as diapsidreptiles. Nature 384:453–455.

Rieppel, O., and R. R. Reiz. 1999. The origin and earlyevolution of turtles. Annual Review of Ecology andSystematics 30:1–22.

Seilacher, A. 1994. Early multicellular life: LateProterozoic fossils and the Cambrian explosion. In S.Bengston, ed. Early Life on Earth. Nobel Symposiumno. 84. Columbia University Press, New York. p.389–400.

Shu, D.-G., H.-L. Luo, S. Conway Morris, X.-L. Zhang,S.-X. Hu, L. Chen, J. Han, M. Zhu, Y. Li, and L.-Z.Chen. 1999. Lower Cambrian vertebrates from southChina. Nature 402:42–46.

Valentine, J. W., D. Jablonski, and D. H. Erwin. 1999.Fossils, molecules and embryos: new perspectives onthe Cambrian explosion. Development 126:851–859.


Woese, C. R. 1998. The universal common ancestor.Proceedings of the National Academy of Science95:6854–6859.

Xiao, S., Y. Zhang, and A. H. Knoll. 1998. Three-dimen-sional preservation of algae and animal embryos in aNeoproterozoic phosphorite. Nature 391:553–558.

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HOMOLOGY

HOMOLOGY

Homology is a specific explanation ofsimilarity of form seen in the biologi-cal world. Similarities can often be

explained by common descent; features areconsidered homologous if they are shown to beinherited from a common ancestor. For exam-ple, although the arms of four-limbed verte-brates externally appear quite different, allhave the same basic underlying skeletal andmuscular pattern. Such shared patterns are bestexplained by the inference that they wereinherited from a common ancestor that alsohad this pattern. Proposed homologies areevaluated using comparative anatomy, genet-ics, development, and behavior.

WELLS RIDES THE HOMOLOGYMERRY-GO-ROUND...

Wells claims that homology is used ina circular fashion by biologistsbecause textbooks define homology

as similarity inherited from a common ances-tor, and then state that homology is evidencefor common ancestry. Wells is correct: thissimplified reading of homology is indeed cir-cular. But Wells oversimplifies a complex sys-tem into absurdity instead of trying to explainit properly. Wells, like a few biologists and manytextbooks, makes the classic error of confusingthe definition of homology with the diagnosisof a homologous structure, the biological basisof homology with a procedure for discoveringhomology. In his discussion, he confuses notonly the nature of the concept but also its his-tory; the result is a discussion that would con-fuse anyone. What is truly important here isnot whether textbooks describe homology cir-cularly, but whether homology is used circu-larly in biology. When homology is properlyunderstood and applied, it is not circular at all.

...BACK TO THE FUTURE

Before 1859, the year Darwin publishedthe Origin of Species, homology wasdefined as similarity of structure and

position, and distinguished (although inconsis-tently) from “analogy,” which was defined assimilarity of function but not necessarily ofstructure and position. An example of homolo-gy and analogy are the wings of birds and bats.The arms of birds and bats would be consid-ered homologs because they have the samestructure and position in both animals. Theirwings, however, are analogs. Both wings havethe same function (flight), yet the bird’s wingis made of feathers, and the bat’s is made ofskin. They are different structures.

The pre-Darwinian basis for similarity wasthe idea of an “archetype.” The archetype,however, was never clearly defined. The ideabelongs to a morphological theory that camefrom the German transcendentalist philoso-phers of the late 1700s and early 1800s. It waslargely out of fashion by the 1840s, butRichard Owen, who codified this distinction,was dedicated to a philosophy of transcenden-tal causes, as many historians of science havenoted (e.g. Russell, 1916; Desmond, 1982;

Rupke, 1993; Padian, 1995a, 1997). Yet the pattern of the biological world more

resembles a genealogy than a gallery of cook-ie-cutter “archetypes.” Darwin accounted forthe similarities in structure and position amongvery different animals as being the result ofnatural selection working on shared ancestralpatterns. The concept of homology shiftedfrom reflecting a vague “archetype” to reflect-ing descent with modification.

Today, biologists still diagnose homologousstructures by first searching for structures ofsimilar form and position, just as pre-Darwinian biologists did. (They also search forgenetic, histological, developmental, and


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behavioral similarities.) However, in our post-Darwin period, biologists define a homologousstructure as an anatomical, developmental,behavioral, or genetic feature shared betweentwo different organisms because they inheritedit from a common ancestor. Because not allfeatures that are similar in two organisms arenecessarily inherited from a common ancestor,and not all features inherited from a commonancestor are similar, it is necessary to teststructures before they can be declared homol-ogous. To answer the question, “could this fea-ture in these groups be inherited from a com-mon ancestor?” scientists compare the featureacross many groups, looking for patterns ofform, function, development, biochemistry,and presence and absence. Many features aretested simultaneously against genealogythrough a process that Kluge (1997; see alsoKluge, 1998, 1999 for discussions of inde-pendent homology tests) termed testing “mul-tiple ad hoc hypotheses of homology.”

If, considering all the available evidence,the distribution of characteristics across manydifferent groups resembles a genealogical pat-tern, it is very likely that the feature reflectscommon ancestry. Future tests based on morefeatures and more groups could change thoseassessments, however — which is normal inthe building of scientific understanding.Nevertheless, when a very large amount ofinformation from several different areas(anatomy, biochemistry, genetics, etc.) indi-cates that a set of organisms is genealogicallyrelated, then scientists feel confident in declar-ing the features that they share are homolo-gous. Finally, while judgments of homologyare in principle revisable, there are many casesin which there is no realistic expectation thatthey will be overturned.

So Wells is wrong when he says that homol-ogy assumes common ancestry. Whether a fea-ture reflects common ancestry of two or more

animal groups is tested against the pattern itmakes with these as well as other groups.Sometimes, though not always, the patternreflects a genealogical relationship among theorganisms — at which point the inference ofcommon ancestry is made. Today, the testingprocess is carried out by a method called “phy-logenetic systematics” or “cladistics,” whichcan be done without assuming an evolutionaryrelationship among the groups — but descentwith modification is the best explanation forthe patterns the comparisons of features itreveals.

Evolution and homology are closely relatedconcepts but they are not circular: homologyof a structure is diagnosed and tested by out-side elements: structure, position, etc., andwhether or not the pattern of distribution of thetrait is genealogical. If the pattern of relation-ships looks like a genealogy, it would be per-verse to deny that the trait reflects commonancestry or that an evolutionary relationshipexists between the groups. Similarly, the close-ness of the relationship between two groups oforganisms is determined by the extent ofhomologous features; the more homologousfeatures two organisms share, the more recenttheir common ancestor. Contrary to Wells’scontention, neither the definition nor the appli-cation of homology to biology is circular.

As mentioned, new evidence from variousfields of biology has expanded our understand-ing of homology beyond just anatomical struc-tures. Anatomical homologies, behavioralhomologies, developmental homologies, andgenetic homologies can be independentlydiagnosed and tested.

Behavioral homology recognizes features ofanimal behavior that can be traced to commonancestry. For example, consider the nestingpractices of birds and crocodilians. Both ofthese groups share the behaviors of nest-build-ing, parental care of young, and “singing” to


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defend territory and attract mates. Most peopleknow birds do these things, but fewer knowthat their cousins the alligators and crocodilesdo these things as well. They inherited thesebehaviors from a shared ancestor. Because ofhomology, we infer these behaviors for theirextinct ancestors as well; thus it came as nosurprise when fossils of many non-aviandinosaurs were found nesting with their young(Horner and Makela, 1979; Horner, 1982;Clark et al., 1999).

Developmental homologies are features inthe developmental programs of organisms. Anexample of this is the “pharyngeal pouches”that nearly all vertebrates acquire to somedegree during their development, but whichbecome very different structures in the adults.For example, the embryological pharyngealpouches of jawless chordates (e.g.,Amphioxus, hagfishes, and lampreys) developinto pharyngeal arches and slits, which supportthe gill structure and allow water to exit thepharynx after passing over the gills. In jawedvertebrates, such as sharks and fish, the pha-ryngeal pouches develop into gill supports andportions of the jaw skeleton. In land verte-brates (tetrapods), these arches and pouchesdevelop into jaw skeleton and musculature, butother pouches/arches, which in gill bearingvertebrates developed into gill structure, nowdevelop into ear bones and cavities, and thy-roid and tracheal cartilages (Gilbert, 2000).The evolution of the different adult pharyngealmorphologies of vertebrates are the results ofalterations of these embryonic structures andtheir components through the developmentalprogram (Graham, 2001).

Today we also recognize genetic homolo-gies. There are similar genes that control thedevelopment of non-homologous features. Forexample, there is a gene, named “Pax6,” pos-sessed by fruit flies, mice, and many otherorganisms, which influences the development

of the eye. Biologists hypothesize that the geneis inherited from a common ancestor not onlybecause of its biochemical similarity but alsobecause of its distribution in numerous taxa.However, the actual eyes that the gene formsare not a result of common ancestry — theirshared ancestor most likely lacked eyes,although it may have had light-sensing ability.The eyes of flies, mice, and many other crea-tures are of different structure and position andare not historically continuous, yet the Pax6gene is historically continuous and responsiblefor them all. This homologous gene functionsas a “switch” that triggers the development oflight-sensing organs (Gilbert, 2000), but the“downstream” genes that they trigger are nolonger the same: they govern different devel-opmental programs and thus build structurallydifferent eyes in flies, mice, and other organ-isms. The relatively new field of evolutionarydevelopmental biology (evo-devo for short)deals with these processes. The discoveriesmade in just the last 10 years in this field havegreatly increased our understanding of homol-ogy, and have made the picture more complex.Wells nearly ignores this important new fieldin his discussion, a surprising omission for onewhose background includes a degree in biolo-gy.

HOMOLOGY, EVOLUTION, ANDTHE NATURE OF SCIENCE

Some formulations of the concept ofhomology appear to be circular, but asdiscussed above, because there is an

external referent (the pattern that characteris-tics take across groups) that serves as an inde-pendent test, the concept, properly defined andunderstood, is not. Wells’s claim that homolo-gy is circular reveals a mistaken idea of howscience works. In science, ideas frequently areformulated by moving back and forth betweendata and theory, and scientists regularly distin-


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guish between the definition of a concept andthe evidence used to diagnose and test it.

Homology is in fact no more circular thanthe methods used in geology to determinepaleogeography and plate tectonics. For exam-ple, in the 1920s, Alfred Wegener used theshape of the continents, the correlation of rockstrata, the correlation of fossil organisms, andthe position of glacial striations as evidence forhis proposal that the continents were oncejoined in one supercontinent and have subse-quently “drifted” to their current locations.Today, geologists can estimate where a certainsection of a continent used to be by looking atpolar wander, paleomagnetism, glacial stria-tions, correlation of strata and fossils, andshape. Is this any more circular than the rea-soning for homology? No. Evidence was usedto infer that continents had moved, and thenthe concept of plate tectonics was applied todifferent data to determine the positions ofcontinents at different times. The analogy toplate tectonics is also relevant to Wells’s impli-cation (Wells, 2000:77) that we don’t fullyunderstand the mechanisms of homology: themechanism of sea floor spreading may not yetbe fully understood, but the continents stillmove.


The presentations of homology in thetextbooks reviewed by Wells differ onlyin the lengths of their discussions.

Overall, textbooks give homology (usuallyincluding discussions of analogy and vestigialfeatures) 2–10 paragraphs (Figure 7). Becausethe shorter introductory textbooks have littlespace to devote to the complexities of howhomology is defined, diagnosed, and applied,their explanations verge on the circular. Thelonger upper-level textbooks make a clearerdistinction between the explanation for homol-ogy (common ancestry) and using sets of

homologies to reconstruct relationships(Figure 7). All textbooks include diagrams ofthe forelimbs of various vertebrates, and allbut one color-code homologous elements foreasy identification. Guttman includes a secondfigure showing homologous bones in a numberof tetrapods and one fish skull, clearly illus-trating how skulls have been reshaped.Futuyma, Guttman, and Campbell, et al.include the best discussions and illustrations ofhomology, but nevertheless earn a D fromWells.

Most textbooks include discussions of anal-ogy and vestigial structures along with discus-sions of homology. Analogous features are fea-tures with similar functions (but not necessari-ly similar structures) that are not inheritedfrom a common ancestor but evolved conver-gently, whereas vestigial features are remnantstructures that have been retained from previ-ous forms. Wells notably leaves out any men-tion of analogous features or vestigial struc-tures from his evaluation (such as the limb gir-dles of snakes or the limb girdles of whalescited by most textbooks).

WELLS’S TEXTBOOK EVALUATION

According to Wells, textbooks shouldexplain that homologies are similari-ties of structure and function due not

to common ancestry but to a common “arche-type” or basic plan on which all forms werebased (Wells is remarkably cagey as to what hemeans by “archetype”). When Wells proposesthat textbooks revert to a pre-Darwinian viewof homology, he doesn’t explain what thatwould mean for biology or biology teaching.He doesn’t explain that it would replace atestable model (descent) with a non-existent,untestable, transcendentalist construct. Wellsis vague because he merely wants to advancehis position and the archetype is consistentwith some notion of special creation, as


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favored by proponents of “intelligent design”creationism and their allies.

Wells’s grades (he gives only Ds and Fs)appear to correlate with the length of the text-book’s coverage (Figure 7). For example, allbooks given Ds devote well over 200 words tothe discussion of homology, whereas the threebooks given an F devote fewer than 200words. This is because the difference betweena D and an F for Wells is whether the bookdefines homology “circularly.” Therefore, theability to treat homology “well” (meaning a D)depends largely on how much space is devotedto the discussion of it. Wells does, however,allow the book to have a picture. In order toreceive a B or better, textbooks must definehomology as similarity of structure and posi-tion and state that homology is based on the

concept of an “archetype.” Further, theyshould state that an “archetype” could meanmany things, not just common ancestry. Healso wants textbooks to state — inaccurately— that mechanisms such as genetics anddevelopmental programs do not account forhomology. Finally, he wants textbooks to statethat the concept of homology is “controver-sial.” This scheme is rigged for failure becausecontemporary biology does not considerhomology to be either controversial or basedon archetypes. There is certainly no reason toaccept these grading criteria.


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Figure 7. Examination of grades applied by Wells for Icon #3. Parentheticals refer to addition-al coverage. Plus (+) numbers reflect secondary treatments of homology and convergence inphylogenetic recontstruction sections of the text.

WHY WE SHOULD STILL TEACH THATHOMOLOGY IS A RESULTOF COMMON ANCESTRY

As our current knowledge of biologysuggests, there is no reason to doubtthe fact that the patterns of structures,

behavior, genes, and developmental programsfit best with the hypothesis that all organismsshare common ancestors. Many of the similar-ities among these widely divergent groups area result of that ancestry. The questions current-ly being debated in biology are not whetherhomology is real, but rather what structures arehomologous and how we may best determinehomology (because our diagnostic approachesare fallible). This type of discussion of relia-bility of methodology is typical for science inall fields, not just biology. Descent is the basisfor homology; similar genes, acting throughdevelopment, convey homology between gen-erations. Genes build structures through theirinteractions in the developmental program.Therefore genes, development, and similarityof structure and position are discovery proce-dures for homology; they help biologists todetermine evolutionary relationships. This fitsthe patterns and processes we observe in thenatural world; this is what we should teach.

HOW TEXTBOOKS COULD IMPROVETHEIR DISCUSSIONS OF HOMOLOGY

The biggest flaw in textbook descriptionsof homology is that they, like Wells,tend to confuse the definition of homol-

ogy with the diagnosis of homologous fea-tures. Textbooks need to state explicitly thathomologies are similarities seen in the biolog-ical world that are best explained as being dueto common descent. They should then explainhow homologous structures are diagnosed bytheir similar structure and position, biochemi-cal basis, developmental path, and so on. Amore detailed and lengthened discussion

would help to remove the appearance of circu-larity caused by oversimplified descriptions.Describing how homology is used as a tool todiscover evolutionary relationships wouldmake it a much more pedagogically usefulconcept for students because it would showthem how evolutionary biologists use anatom-ical observations to discover evolutionary rela-tionships. Finally, adding the notions of multi-ple layers of homology from genetics anddevelopmental biology would better show stu-dents just how different lines of evidence con-verge to support homologies and phylogenies.Textbooks should not follow Wells’s sugges-tion to say that homology is merely similarityin structure and position, nor should they statethat there are “other” reasons for homologiesbeyond inheritance from a common ancestor.To revert to Wells’s 19th-century notion ofhomology would leave students unprepared toparticipate in 21st-century science.

References

Clark, J. M., M. A. Norell, and L. M. Chiappe. 1999. Anoviraptorid skeleton from the Late Cretaceous of UkhaaTolgod, Mongolia, preserved in an avianlike broodingposition over an oviraptorid nest. American MuseumNovitates 3265:1–36.

Desmond, A. 1982. Archetypes and Ancestors:Palaeontology in Victorian London 1850-1875, Blond& Briggs, London 287p.

Gilbert, S. F. 2000. Developmental Biology, SixthEdition. Sinauer Associates, Sunderland 749p.

Graham, A. 2001. The development and evolution ofpharyngeal arches. Journal of Anatomy 199:133–141.

Horner, J. R. 1982. Evidence of colonial nesting and‘site fidelity’ among ornithischian dinosaurs. Nature297: 675–676.

Horner, J. R. and Makela, R. 1979. Nest of juvenilesprovides evidence of family structure among dinosaurs.Nature 282: 296–298.

Kluge, A. G. 1997. Testability and the refutation and cor-roboration of cladistic hypotheses. Cladistics 13:81–96.


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Kluge, A. G. 1998. Total evidence or taxonomic con-gruence: cladistics or consensus classification.Cladistics 14:151–158.

Kluge, A. G. 1999. The science of phylogenetic system-atics: explanation, prediction, and test. Cladistics15:429–436.

Padian, K. 1995a. Pterosaurs and typology: archetypalphysiology in the Owen-Seeley dispute of 1870. InW.A.S. Sarjeant, ed. Vertebrate Fossils and theEvolution of Scientific Concepts. Gordon and Breach,Yverdon, Switzerland p. 285–298.

Padian, K. 1997. The rehabilitation of Sir RichardOwen. BioScience 47: 446–453.

Rupke, N. A. 1993. Richard Owen’s vertebrate arche-type. Isis 84:231–254.

Russell, E. S. 1916. Form and function: a contributionto the history of animal morphology. John Murray,London. Reprint of 1982, University of Chicago Press,Chicago 383p.



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HAECKEL’S EMBRYOS

ERNST HAECKEL ANDCOMPARATIVE EMBRYOLOGY

Ernst Haeckel (1834–1919) is both a heroand a villain in the biological communi-ty. He was a prominent figure in the late

nineteenth-century comparative anatomy com-munity and is famous for his phylogenetictrees, anatomical illustrations, support for evo-lution, and strong personality. He is perhaps aswell known, and considerably misunderstood,for his studies in embryology and his dictumthat “ontogeny recapitulates phylogeny,”called the Biogenetic Law. Haeckel espousedthe view that evolution generally proceeds byplacing each innovation on top of a previousone, like adding layers on a cake. Therefore,the embryo of an “advanced” organism shouldpass through (“recapitulate”) the adult stagesof more “primitive” forms as it develops.However, repeated observations of develop-ment by other workers (e.g., Wilhelm His,Walter Garstang, Wilhelm Roux, AdamSedgwick, Gavin de Beer, and others; seeGilbert, 1991, or Gould, 1977 for a detailedhistory) clearly showed that embryos do not gothrough adult stages of lower forms; rather,they share many common features in develop-ment. No biologist has accepted the biogeneticlaw for many decades and it may have been acaricature of Haeckel’s actual views anyway.Much of Haeckel’s developmental work isnow considered invalid, and some historians ofscience have provided reasonable evidence tosuggest that he manipulated his drawings to fithis preconceived views about developmentand evolution. Haeckel’s views about the pro-gressive nature of evolution are no longeraccepted.

Regardless of Haeckel’s accuracy or precon-ceptions, comparative embryology continues

to be central to our understanding of evolution.Comparative embryology shows how differentadult structures of many animals have thesame embryonic precursors. These shareddevelopmental features suggest that many ani-mals have ancestors in common. Further com-parative embryology shows that closely relat-ed animals show a unity of developmental pat-tern, particularly in earlier stages, and havemore developmental features in common thando more distantly related organisms. The factthat certain incipient structures such as pha-ryngeal pouches or arches exist in all verte-brate embryos yet develop into very differentadult structures suggests that they all share acommon ancestor whose embryo had pharyn-geal pouches (at least at some stage in devel-opment). In this way, developmental similari-ties that are inherited from a common ancestorare homologous, just like the patterns of bonesin adult limbs.

DEVELOPING AN ARGUMENT

Wells’s entire chapter on embryologyamounts to little more than a mis-reading of Darwin, Haeckel, and

others, combined with a general failure toacknowledge recent work on Haeckel and hisembryos by Gould, Richardson, and others. Init, he conflates ideas in history of developmen-tal biology with ideas of contemporary devel-opmental biology. He also fails to recognizeclose to 60 years of work in developmentalbiology and thus completely omits any discus-sion of the real developmental evidence forevolution. It almost seems that Wells’s goal isto discredit the entire field of comparativeembryology by proxy, employing a bait-and-switch between Haeckel and Darwin. Wells’sploy is reminiscent of a child’s false logicproof. It goes like this: Darwin relied onHaeckel, Haeckel was a fraud, thereforeDarwin is a fraud.


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The charge that Ernst Haeckel intentionally“faked” his drawings is irrelevant. Regardlessof his intent, the drawings that Haeckel madeare incorrect, especially in what he labeled asthe “first stage.” But it really does not matterwhat Haeckel thought or whether his drawingsare accurate: modern comparative embryologydoes not stand or fall on the accuracy ofHaeckel any more than modern physics standsor falls on the accuracy of Kepler or Newton.Historically, Wells actively ignores the accu-rate work of many of Haeckel’s predecessorsand contemporaries (such as William andJeffrey Parker, Hans Gadow, Hans Selenka,Heinrich Rathke, Virgil Leighton, HugoSchauinsland, and Alfred Voeltzkow, to namea few). Haeckel and von Baer were not theonly embryologists in nineteenth-century sci-ence, but you wouldn’t know that from readingWells. Worse, Wells speciously extends his cri-tique of Haeckel to the present day. Wellsimplies that textbooks misrepresent the studyof developmental programs as evidence forevolution by accusing them of using Haeckel’sinaccurate drawings, in effect accusing text-books that show any embryos of “mindlesslyrepeating” Haeckel. The important question iswhether textbooks, and more importantlydevelopmental biologists, still rely onHaeckel’s work. The answer is no, but thatdoesn’t stop Wells from acting as if they do.

Wells sets up a straw man in his bait-and-switch, starting with Darwin’s famous asser-tion that embryology represented the “singlestrongest class of facts” in favor of his theory.Here Wells misrepresents both early embryol-ogy and Darwin’s own words. When quotingboth Darwin and other historical figures, hequotes them out of context, leaves out impor-tant parts of quotes, and even changes theorder of their appearance, all to misrepresenttheir real meaning and intent. Wells also con-flates “recapitulation” — that is, that embryos

go through the adult stages of their ancestors— with the idea that shared features ofembryos give insight into their phylogeneticrelationships. Failing to distinguish theseallows Wells to avoid dealing with the actualevidence for shared developmental features invarious embryos and to dismiss the entire fieldas based on an outdated and outright refutedclaim, one that embryologists know to be falsebut cling to anyway because of an ideologicalcommitment to evolution. Wells should knowbetter, as the holder of a Ph.D. in cell anddevelopmental biology.

REWRITING HISTORY FORTHE GREATER GLORY OF

THE REV. MOON

In the introduction to Icons, Wells statesthat he first became aware of the problemsin evolutionary theory when he was “fin-

ishing his Ph.D. in cell and developmentalbiology” (Wells, 2000:xi). He claims that heknew that the drawings of embryos presentedin textbooks were false because he was adevelopmental biologist. Shortly thereafter, heclaims, his observation was confirmed byother scientists. Before that seminal event, hesays, “I believed almost everything I read inmy textbooks” (Wells, 2000:xi). This state-ment is inconsistent with other claims ofWells’s. According to statements made byWells in a sermon on a Unification Churchwebsite (http://www.tparents.org/library/unifi-cation/talks/wells/DARWIN.htm), he went tograduate school with the specific intent ofattacking evolution: “Father’s words, my stud-ies, and my prayers convinced me that I shoulddevote my life to destroying Darwinism” andhe believed that its weakest point was devel-opmental biology. “I was convinced thatembryology is the Achilles’ heel ofDarwinism; one cannot understand how organ-isms evolve unless one understands how they


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develop. In 1989, I entered a second Ph.D. pro-gram, this time in biology, at the University ofCalifornia at Berkeley. While there, I studiedembryology and evolution.” So it was not somuch a “revelation” as it was a plan. If Wellsis so revisionist about his own history, how canwe trust him with the history of science?

DEVELOPMENTAL ANATOMY, DARWIN, AND EVOLUTION

Wells opens the chapter by telling uswhat Darwin thought about devel-opment and evolution. Wells uses

about 5 different quotes from the Origin in anattempt to show that Darwin was advocatingrecapitulation in spite of what the data showed.To do this, he distorts the history. Wells tries toconnect Darwin to Haeckel so that he can usethat to dismiss Darwin. Wells says that Darwinwas not an embyrologist and thus he relied onHaeckel (Wells, 2000:81). Anyone familiarwith the history of biology knows that this isimpossible. Haeckel did not publish hisAnthropogenie until 1874 (where the much-maligned embryo drawings first appear), 15years after the publication of the Origin. (Itshould also be noted that the drawings referredto by Wells [2000] are not from Haeckel butredrawn from the first edition ofAnthropogenie in a textbook by Romanes[1892; see figure 10a]. In later editions ofAnthropogenie, Haeckel corrected some of theerrors of the first edition drawings [Richardsonand Keuck, 2002; personal observation].)Wells quotes Darwin’s praise of Haeckel in hissixth and final edition of the Origin in such away as to obscure the fact that Darwin laudsHaeckel for his phylogenies, not his embryol-ogy. The quote is not even from the embryolo-gy section of the book; rather it comes fromthe classification section, in the final sentenceof which Darwin praises Haeckel for usinghomologous features (including but not limit-

ed to developmental ones) to generate classifi-cations for organisms. Darwin is praising theapplication of his theory by Haeckel.

Although Darwin did not use Haeckel onembryology, he did use von Baer. RecognizingDarwin’s use of von Baer, Wells then accusesDarwin of “misusing” von Baer’s work, twist-ing the data to fit his views. But Darwin doesnot. Wells claims that von Baer’s embryologi-cal laws are incompatible with Darwin’s con-clusions, but they are not. Von Baer may havedisagreed with Darwin about his conclusions,but his laws do not prohibit development elu-cidating common ancestry. Darwin came to adifferent conclusion from the same body ofevidence — this is not “distorting” the evi-dence. Darwin was making a general inductiveargument and searched for data that could testthe general proposition of common descent; heargued that von Baer’s data could be reinter-preted in terms of common ancestry. This wasno more a “misuse” of von Baer than wasAlfred Wegener’s reinterpretations of the dataof geology in light of mobile continents. Newscientific theories always use previous data. IsWells implying that evolutionary biology can-not cite any research that predates 1859? IsWells implying that developmental sequencessuch as those illustrated by von Baer and oth-ers are not data?

That Darwin and all modern evolutionistsadvocate some form of the “Biogenetic Law”is the central falsehood of this chapter; in factthe entire “resurrecting recapitulation” sectiondoes nothing but assert this. But Wells fails toexplain fully what recapitulation means. Thereare a number of meanings for “recapitulation”that Wells conflates in order to tar the entirefield of embryology with a biogenetic brush.As he says in a footnote, a “plain reading” ofDarwin shows that Darwin was advocatingrecapitulation — but just what kind? (1) Anembryo of an “advanced” form goes through


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all the adult stages of all its ancestors. This is acaricature of Haeckelian recapitulation, whichis false, and few scientists ever believed it any-way. (2) Evolution proceeds as an “add-on”process so that there is a general progression ofembryological stages from “primitive” to“advanced” forms. This more traditional read-ing of recapitulation is also false and has notbeen accepted for nearly a century. (3) Allclosely related organisms go through all thesame stages of development and always looksimilar. This is vague: how closely related isclosely related? How are the stages individuat-ed? But however these questions are answered,this reading of recapitulation would generallybe agreed to make too sweeping a claim. (4)Some parts of developmental sequences (andsome specific characters of them) in closelyrelated animals share more specific similarities(in pattern, sequence, position, etc. of develop-mental features) with each other than withthose of more distantly related animals. That’sbasically true. All modern biologists recognizethat all stages of development are open tomodification. This is generally the type of“recapitulation” accepted by the post-Haeckelian embryologists (such as FrankLillie) cited by Wells, as well as by currentembryologists, but Wells treats it derisively asif it were exactly what Haeckel thought.Finally, a “plain reading” of Darwin showsthat he was suggesting something between (2)and (3); even though he was not an embryolo-gist, he had a more sophisticated notion ofembryology and development than does Wells.

Wells chides Darwin and nineteenth-centuryembryologists for saying that the “earliest”stages of development are similar when in factthey are not. However, “earliest” is Wells’sword, not Darwin’s. It does not appear in anyof the quotes that Wells uses. Indeed, in theentire section on embryology in the Origin, theword “earliest” only appears once, in a quota-

tion from von Baer. Does “earliest” reflectDarwin’s belief, or is he merely reporting vonBaer’s? This is important because numerousscholars have made the mistake of confusingDarwin’s reporting of what others thought withhis expression of his own views (Padian,1999). So apparently has Wells. But it reallydoes not matter what Darwin thought: just asmodern embryology does not rely on Haeckel,neither does modern evolutionary biologyslavishly follow Darwin’s beliefs.

It is also important to understand what nine-teenth-century scientific workers may havemeant by the use of “embryo” and “earlystage.” For many workers in the nineteenthcentury, developing organisms weren’t calledembryos until they reached the tailbud (phylo-typic) stage. During earlier stages, they werecalled “developing ovum” or “developing egg”(see Barry, 1839, or just about any embryolo-gy work from 1820 to 1900). What this meansis that Haeckel, von Baer, and others, have adifferent meaning for “early embryo.” YetWells interprets them using modern defini-tions.

Wells also criticizes the field of comparativeembryology for the way it chooses its data andfor its names for embryonic structures. First,Wells emphasizes the disparity of “earliest”developmental stages, accusing biologists of“choosing” taxa (animals) that look most sim-ilar for illustrations in textbooks and else-where. He criticizes Haeckel for not using ani-mals such as monotremes in his work. Butdevelopmental sequences for monotremeswere not available in 1874. Monotreme devel-opmental sequences were not known ordescribed until 1884 (Caldwell, 1887; Hughesand Hall, 1998), and it was the developmentalfeatures monotremes shared with marsupialsthat led Caldwell to conclude that monotremeswere indeed mammals (Caldwell, 1887). Wasthe sample of organisms available to Haeckel


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biased? Yes, but only in the sense that earlyembryologists worked with the animals thatwere available to them. Most specimens of“exotic” animals were shipped to researchersby explorers and received in varying states ofdecay (Caldwell, 1887). Most nineteenth-cen-tury embryologists loved to describe the devel-opment of any animal they could. And Haeckelwas continually updating and adding neworganisms to his embryonic series as theycame available. Contrary to what Wellsimplies, there was no attempt to limit the data,and the sample was not “chosen” for any par-ticular reason.

Today, embryologists work mainly with“model organisms,” which were largely cho-sen for practical reasons such as ready avail-ability, small body size, large litter size, rapid

sexual maturity, rapid reproduction, ability fordevelopment to occur in the laboratory, andability to live indoors for several generations(Bolker, 1995). They were not chosen to “sup-port evolution” as Wells implies. In fact, themodel organism that is the subject of Wells’sown dissertation, Xenopus, was not the origi-nal “model” amphibian. The discovery thatXenopus does not need a breeding season wasa boon to embryologists and led to itsserendipitous adoption as a model organism(Gurdon and Hopwood, 2000). How Wellsknows that “model organisms” were chosen tomislead is unclear, especially given his ownuse of model organisms later in his chapter.Wells doesn’t show developmental sequencesfor any of the organisms he complains othersdon’t show. Why not? Because there is no evi-


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Figure 8. Developmental sequences of various vertebrates shown in phylogenetic context. Notethe shared similarities of some closely related taxa, particularly the amniotes (modified fromRichardson et al. 1998).

dence for his insinuation that developmentalbiologists treat their data selectively in order tohide something. The fact that embryologiststend to present, at least in textbooks, develop-mental sequences for which there is good datadoes not refute the idea that closely relatedtaxa, should, and do, have more shared simi-larities in developmental programs than moredistantly related taxa (Figures 8, 9). Wells triesto support his claim by using a quote byDarwin in which he states that embryos of thesame “class” are most similar in their earlieststages. Wells then says that the quote is false,and cites how the different “classes” of verte-brates are very different in their “earliest”stages. This is merely a semantic sleight-of-hand, a bait-and-switch. Darwin is not talkingabout different “classes.” Wells leaves outimportant information, as usual.

In the figures of embryos (Wells, 2000:95,especially stage 4, “gastrulation”), Wells’sillustrator resorts to a number of graphic tricksin order to make the embryos appear more dif-ferent than they are. First, the embryos are notshown from the same rotational angles. Thechicken is shown in a different position thanthe other “Haeckel’s first stage” embryos.

Second, they are not all scaled the same. In thefigure showing the neural crest infolding, theturtle and chicken are shown at a large scale,neglecting the large yolk they sit on, while thehuman is shown as part of the whole develop-ing ovum, so that the germinal disc and primi-tive streak formation are shown differently,even though it is shared by all amniotes(Schaunislaund, 1903; Nelson, 1953; Cruz,1997; Schoenwolf, 1997; Figure 9). Also pic-tured is a frog embryo, despite its indirectdevelopment, which is very different from thatof the other vertebrates pictured. Many of thegeneral “differences” in early embryo develop-ment that Wells mentions are a result of organ-ization due to the yolk size rather than beingspecific differences in the basic body-plan ofthe embryo (Arendt and Nübler-Jung, 1999).

Embryos do reveal phylogenetic informa-tion in terms of specific shared features, sharedearly developmental features such as the for-mation of a germinal disc and primitive streakin all amniotes or the neural crest cells of allvertebrates. The presence, and sequence ofdevelopment, of eyes, ears, somites, limbs,guts, nerve cords, tails, organs, etc. are indi-vidual features that no one would deny are


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Figure 9. Embryos of various amniotes shown during somite stage. All amniotes go through thesame sequence of development: primitive streak–neural tube–somite formation.

present in vertebrates and not present in thesame way in other animals. These are individ-ual characters whose developmental featuresare treated as shared features in reconstructingvertebrate evolution; these features do notalways have to be in agreement, and some ani-mals can show unusual derived features earlyin development, such as the snake’s tail(Figure 8).

Wells’s treatment of comparative embryolo-gy is remarkably limited; for example, henever discusses invertebrate development. Yetthere are plenty of shared developmental pat-terns there as well. Despite the very differentappearance of echinoderm, hemichordate, andchordate embryos, they all share the deuteros-tome condition, in which the first cell openingbecomes the anus, before they diverge to theiradult body plans. Or what about the tro-chophore larvae of most protostomes and spi-ral cleavage shared by annelids, arthropods,mollusks (Nielsen, 1995; Fell, 1997)? Thenauplius larvae of crustaceans (Gilbert, 1997)or the verliger larvae and development of gas-tropods, which go through flexure, torsion, anddegeneration of muscles on one side of thebody, suggestive of their evolutionary history(Nielsen, 1995; Collier, 1997)? These are justa few of the specific similarities of the kindthat Wells implies do not exist. Similarities inembryonic sequences are data — characters bywhich we can discover shared similaritiesamong organisms that can be used to recon-struct their relationships. Using such data inphylogeny is not the same as using those char-acters in any “recapitulationist” way.

Finally, Wells concludes by attackingprominent biologists such as Gould andFutuyma for supposedly not knowing the truthabout Haeckel, saying that this is “a confessionof ignorance not likely to inspire confidence inthe quality of our biology textbooks” (Wells,2000:107). Wells’s own misrepresentations of

the letter and spirit of the concepts and authorshe presents do little to inspire confidence inwhat he says about Haeckel or embryology ingeneral. Even if Wells were right aboutHaeckel’s work and Darwin’s use of it, whatHaeckel and Darwin thought doesn’t matter;embryology has moved beyond them. Wellsneeds to show a lack of specific similarities tosupport his case. Is Wells actually claimingthat there are no shared features in develop-ment at all? That a chicken gets a planula whilethe duck gets a naupius? If so, he needs toshow it, but Wells never gets to specifics —apparently because the specifics aren’t there.Innuendo and accusations of fraud do not cut itin science.

WHAT TEXTBOOKS SAY

For any textbook to show Haeckel’sdrawings themselves as unqualifiedstatements of developmental anatomy or

to advocate “recapitulation” in a Haeckeliansense would be inexcusable, but none of thetextbooks reviewed by Wells appear to do so.Wells gleefully excoriates Futuyma for usingHaeckel’s drawings (Figure 10a), but appar-ently in his fit of righteous indignation, he for-


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Figure 10a. Romanes (1892) embryo draw-ings reproduced in Futuyma (1998:653).

got to read the text, in which the drawings arediscussed in a historical context — statingwhy Haeckel is wrong — and Futuyma has anentire chapter devoted to development andevolution. Guttman (Figure 10b) uses them inan explicitly historical context as well. Wellsstates that books use “Haeckel’s drawings, orredrawn versions of them” (Wells, 2000:255),but this is not true. Figures 10a–j showHaeckel’s drawings compared to the drawingsin the textbooks reviewed by Wells. It can beclearly seen that a majority of the drawings arenot “redrawn.” Some textbooks show more

accurate drawings (Miller and Levine,Johnson, Biggs, Kapicka and Lundgren;Figures 10f,g,h); some use photos (Campbell,Reese and Mitchell, Mader; Figures 10i,j);only Starr and Taggart (Figure 10c), Raven andJohnson in their development chapter alongwith accurate drawings and photos; (Figure10d), and Schraer and Stolze (but redrawn andcorrected; Figure 10e) use what could be con-sidered embryos “redrawn” from Haeckel. Notextbook discusses embryology in any way


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Figure 10b. Romanes (1892) embryosreproduced and placed in historical con-text in Guttman (1999:718)

Figure 10c. Romanes (1892) embryosredrawn in Starr and Taggart (1998:317).

Figure 10d. Embryos redrawn and some-what corrected in Raven and Johnson(1999:288).

Figure 10e. Embryos redrawn and correctedin Schraer and Stolze (1999:582)

that could be considered strongly “recapitula-tionist.” In most textbooks, embryology is pre-sented in just one or two paragraphs, making ithard to discuss all the complexities of devel-opment. At a high school level, the aim of thebook is to convey some basic concepts of biol-ogy, not to confuse students with the complex-ity of a subject.


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Figure 10f. Original embryo drawings inMiller and Levine (2000:283).

Figure 10g. Original embryo drawings inJohnson (1998:179).

Figure 10h. Embryo drawings in Biggs et al.(1998:433). Identical drawings appear in theevolution chapter (p.416) of Raven andJohnson (1999).

Figure 10i. Embryo photos in Campbell,Reese, and Mitchell (1999:424).

Figure 10j. Embryo photos in Mader(1998:298).

WELLS’S “WELL-DEVELOPED”GRADING SCHEME

The grading scheme employed by Wellsis designed for failure. This is becauseWells assumes all drawings to be

“redrawn” from Haeckel and gives any bookwith a drawing an F (Figure 11). Wells doesnot explain how one would determine whetherthey are simply redrawn from Haeckel; in anycase none of the books appear to contain mind-lessly redrawn figures (Figure 10a–j). Using

more accurate pictures only earns a book a D.In order to earn a C or higher, a book must notuse “misleading drawings or photos.” Thisamounts to complaining that textbooks should-n’t allow students to be misled by reality!Wells does not specify what kind of drawingsor photos would not be misleading. Thus Wellsapparently thinks that all visual presentationsof embryos are misleading, whether they areaccurate or not. Wasn’t Wells the one com-plaining about selective use of data? He actu-


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Figure 11. Wells’s grades for the embryology sections of textbooks.

ally attacks Mader and Campbell, Reese, andMitchell, for using “misleading photos”because they show embryos of a chick and ahuman, which he says “just happen” to have astronger resemblance than would embryosfrom any other “classes” of vertebrate. Wells iswrong: a chick embryo at that stage looksmuch more like an alligator embryo than amammal embryo (comparisons made fromNelson, 1953, Schaunisland, 1903, and Reese,1915). This is in accordance with the predic-tions of evolutionary theory, because an alliga-tor and a chicken share a more recent ancestorwith each other than they do with a mammal,and thus should have more similar a develop-mental program. Wells also chides Mader forsaying that embryos “have many features incommon” (Wells, 2000:103–104). Does Wellsassert that they have no features in common? Ifso, he should document it. Having failed to dothis, Wells merely labels anything he does notlike “misleading.” Wells also takes exceptionto the colloquial term “gill slits,” which is acommonly used non-technical term for pha-ryngeal pouches. Wells implies that by usingthis term, biologists and textbooks are sayingthat all animals’ embryos have gills. This ispatently false. No textbook reviewed evenimplies the presence of gills in embryos. Thequestion is what these structures are and whatthey become, not what they are called. Usingthe terms “gill slits” automatically results in aC even if the textbook contains no images, andregardless of its content. Campbell, Reese, andMitchell, and Guttman both contain entirechapters devoted to developmental biology inwhich they do discuss some of the “early stagedifferences” that Wells suggests they do not.They receive no credit for these extensivetreatments (Figure 11).

WHY WE SHOULD STILL TEACHCOMPARATIVE EMBRYOLOGY

Despite changes in how we view the roleof developmental programs as reflec-tions of evolutionary history, we can

still see how the same embryonic structuresdevelop into different adult structures. Weobserve the unity of developmental plan in allvertebrates. This is what we see, and noamount of wishful thinking on the part of evo-lution detractors can change that. There is noreason to let their baseless complaints andcharacter assassination dissuade biologyteachers from presenting the evidence to stu-dents.


COMPARATIVE EMBRYOLOGY

Textbooks could largely improve the pre-sentations of embryology by lengthen-ing their discussions of it, and by using

photos rather than cartoonish drawings. Theycould also be more explicit about how embry-onic precursors develop into different adultstructures. Finally, adding discussions of Hoxgene complexes (master developmental con-trol genes) and evolutionary developmentalbiology would help bring the books up-to-datein their treatment of developmental biology.We are learning more about the evolutionaryhistory and underpinnings of developmentalprograms every day. We are learning howdevelopmental programs are the source ofmuch of the evolutionary novelty that naturalselection shaped. Wells ignores all this. To fol-low Wells’s advice would arrest the develop-ment of students’ knowledge.


39

References

Arendt, D. and K. Nübler-Jung. 1999. Rearranging gas-trulation in the name of yolk: evolution of gastrulationin yolk-rich amniote eggs. Mechanisms of Development81:3–22.

Barry, M. 1839. Researches in Embryology. — SecondSeries. Philosophical Transactions of the Royal Societyof London 129:307–380

Bolker, J. A. 1995. Model systems in developmentalbiology. BioEssays 17:451–455.

Caldwell, M. A. 1887. The embryology of Monotremataand Marsupialia — Part I. Philosophical Transactionsof the Royal Society of London, B 178:463–486.

Collier, J. R. 1997. Gastropods, the snails. In S. F.Gilbert and A. M. Raunio, eds. Embryology: construct-ing the organism. Sinauer and Associates, Sunderland p.189–217.

Cruz, Y. P. 1997. Mammals. In S. F. Gilbert and A. M.Raunio, eds. Embryology: constructing the organism.Sinauer and Associates, Sunderland p. 459–489.

Fell, P. E. 1997. The concept of larvae. In S. F. Gilbertand A. M. Raunio, eds. Embryology: constructing theorganism. Sinauer and Associates, Sunderland p. 21–28.

Gilbert, S. J., ed. 1991. A conceptual history of modernembryology. Johns Hopkins Press, Baltimore, 266p.

Gilbert, S. F. 1997. Arthropods: the crustaceans, spidersand myriapods. In S. F. Gilbert and A. M. Raunio, eds.Embryology: constructing the organism. Sinauer andAssociates, Sunderland p. 237–257.

Gould, S. J. 1977. Ontogeny and Phylogeny. TheBelknap Press, Cambridge, 501p.

Gurdon, J. D., and N. Hopwood. 2000. The introductionof Xenopus laevis into developmental biology: ofempire, pregnancy testing and ribosomal genes.International Journal of Developmental Biology44:43–50.

Hughes, R. L., and L. S. Hall. 1998. Early developmentand embryology of the platypus. PhilosophicalTransactions of the Royal Society of London, B353:1101–1114.

Nelson, O. E. 1953. Comparative embryology of thevertebrates. Blackiston, New York, 982p.

Nielsen, C. 1995. Animal evolution: interrelationshipsof the living phyla. Oxford University Press, New York,467p.

Padian, K. 1999. Charles Darwin’s views of classifica-

tion in theory and in practice. Systematic Biology48:352–364

Reese, A. M. 1915. The Alligator and its Allies. G. P.Putnam and Sons, New York 358p.

Richardson, M. K., J. Hanken, M. L. Gooneratne, C.Pieau, A. Raymond, L. Selwood, and G.M. Wright.1997. There is no highly conserved embryonic stage inthe vertebrates: implications for current theories of evo-lution and development. Anatomy and Embryology196:91–106.

Richardson, M. K., J. Hanken, L. Selwood, G. M.Wright, R. J. Richards, and C. Pieau. 1998. Haeckel,embryos, and evolution. Science 280:983–984.

Richardson, M. K. and G. Keuck. 2002. Haeckel’s ABCof evolution and development. Biological Reviews,77:495–28.

Romanes, G. J. 1892. Darwin and After Darwin.Volume1: The Darwinian Theory. Open Court, Chicago.

Schauinsland, H. 1903. Beitrage zur entwicklungs-geschichte und anatomie der wirbeltiere. Zoologica her-ausgeg, C. Chun, Stuttgart.

Schoenwolf, G. C. 1997. Reptiles and birds. In S. F.Gilbert and A. M. Raunio, eds. Embryology: construct-ing the organism. Sinauer and Associates, Sunderland p.437–458.



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ARCHAEOPTERYX

ARCHAEOPTERYX: THE FOSSIL

Contrary to Wells’s subtitle,Archaeopteryx is not a “missing link.”The term “missing link” is an outdated

term that does not accurately reflect the waybiologists and paleontologists think about fos-sils. We prefer not to talk about “missinglinks” or “intermediate forms,” but ratherintermediate features. Archaeopteryx has fea-tures intermediate between those of livingbirds and ancient reptiles; along with manyother fossils, it preserves ancestral featureswhile it shows descendant novelties.Archaeopteryx retains the ancestral “reptilian”features of a long bony tail, clawed hands,teeth, and many others. It also has the derived“avian” features of feathers and poweredflight. Archaeopteryx, along with otherdinosaur fossils, shows the evolution of avianfeatures and flight. These fossils show thatmany features thought of as unique to a certaingroup of animals were also shared by some oftheir ancestors; this helps paleontologists toreconstruct the evolutionary history of livinganimals. When many fossils are looked at intheir genealogical context, they blur the linesbetween the normally recognized taxonomicgroups (most of which were based originallyonly on living forms). Archaeopteryx is fre-quently used for pedagogical purposes becauseit is easy to recognize its mixture of “bird” and“reptile” features and because it played an his-torical role in helping to cement Darwin’s the-ory (it was discovered 2 years after publicationof the Origin). Textbook authors likeArchaeopteryx for these reasons and oftenillustrate their discussions with pictures of theBerlin specimen, one of the most beautiful fos-sils ever discovered, and remarkably complete.Textbooks also use Archaeopteryx as an exam-

ple of how fossils are important for showingtransitional features of evolution, and how thefossil record is good evidence that evolutionhas occurred.

WELLS MISSES MORETHAN THE LINKS

Wells objects to textbook treatmentsof Archaeopteryx as a transitionalform or as an “ancestor” of birds.

Wells wants textbooks to say thatArchaeopteryx was not an “ancestor” becausemodern birds are not descended from it andthat its transitional status is “controversial.”Wells claims that Archaeopteryx has been“quietly shelved” by paleontologists and thatthe search for a “missing link” betweendinosaurs and birds goes hopelessly on “asthough Archaeopteryx had never been found”(Wells, 2000:138). Paleontologists would findthis surprising. By making such claims, Wellsexposes the depths of his ignorance of phylo-genetic methodology, paleontology, and avianevolution.

Wells is clearly confused by Archaeopteryx,“transitional forms,” and ancestors. First of all,Wells asserts that Archaeopteryx is no longerconsidered a transitional form or an “ances-tor.” Wells is correct, but only in a specializedsense, not appropriate in the context of hisgeneralized discussion. We cannot — and donot — say for certain that the animal that wecall Archaeopteryx was actually geneticallytransitional to living birds, or that it was adirect genetic ancestor of living birds.However, in a less strict sense (that appropriateto Wells’s discussion), Archaeopteryx has agreat many transitional features between livingbirds and Mesozoic dinosaurs: if it was not adirect ancestor, it was surely a close collateralancestor (see below).

Second, there is no such thing as a “missinglink,” and paleontologists are not looking for


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them. Paleontologists collect, survey, andreconstruct past forms of life. Some of thesefossil organisms have features that illustratethe path evolution took to reach the forms wesee today. We can think of these organisms asshowing transitional or ancestral features.Paleontologists are also not looking for ances-tors, but rather features of ancestors.Paleontologists distinguish between lineal andcollateral ancestors. Lineal ancestors are thosethat are directly ancestral to living organisms:your lineal ancestors are your father and moth-er, grandfathers and grandmothers, and so on.Collateral ancestors are those organisms thatshare an ancestor with living organisms: yourcollateral ancestors are your uncles, great-uncles, cousins, second cousins, and so on.Paleontologists do not claim to be able to iden-tify lineal ancestors. Without observational orgenetic evidence, how could you ever knowthat a fossil organism left any offspring? It isnot the ancestry that is important to paleontol-ogists, but rather the ability to reconstruct thefeatures of those ancestors. This is a powerfuland important concept, one completely lost onWells.

To illustrate this powerful approach, let’ssay you wanted to know something about yourown ancestors. If you knew your ancestorscame from a certain small village in France inthe 1600s, you could return to that village and,even if you can’t locate their graves, you mightfind those of many of their contemporaries inthe churchyard. A collection of artifacts fromany of those people would give you a perfect-ly adequate idea of the characteristics, culture,possessions, and daily life of your directancestors (Padian and Angielczyk, 1999).Using similar methods for similar reasons,paleontologists try to uncover features ofancestors, not the ancestors themselves.

Even Wells’s claim that paleontologists donot think Archaeopteryx is “ancestral” is incor-

rect. Archaeopteryx has no features that wouldactually disbar it from being a direct ancestorof living birds. Whether it was a direct ances-tor of today’s birds or not is irrelevant:Archaeopteryx exhibits unique features of thelast ancestor it shared with birds, so, regardlesswhether it is a lineal ancestor, it still preservesfeatures that indicate what the last ancestor ofArchaeopteryx and birds may have been like.In other words, Archaeopteryx has many fea-tures intermediate between those of itsdinosaurian ancestors and its avian descen-dants, which is exactly what would be predict-ed by evolution. No amount of stridency onWells’s part can change that.

When paleontologists reconstruct relation-ships of living and fossil organisms, they usethe features of both living and fossil organ-isms. This allows them to reconstruct the fea-tures of the ancestors and get a pretty good pic-ture of what the ancestors were like.Phylogenetic systematics, commonly called“cladistics,” is the method that nearly all biol-ogists use to determine relationships, whetherthey work on dinosaurs or dinoflagellates, andwhether they use molecules or morphology. Itssimplicity, objectivity, testability, repeatability,utility, and firm rooting in the principle ofdescent has led to its near-universal applica-tion. Contrary to Wells’s characterization,cladistics is not a search for “missing links” ordirect ancestors, but for shared evolutionaryfeatures. The basic idea behind cladistics isthat when novel features arise, they are passedon to descendents. Therefore, these “derivedfeatures” should be more informative in recon-structing relationships than those that are pres-ent across a larger group. For example, if apopulation of animals evolve stripes on theirbacks and all their descendants continue tosport stripes, then all the members of thatspecies that have stripes are probably moreclosely related to each other than they are to


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those without stripes. It is that simple, yetWells’s discussion of cladistics reveals that heeither does not grasp the method or has nointerest in explaining it properly.

In the nearly two pages devoted by Wells toa discussion of cladistics (Wells, 2000:118–119), he states that cladistics is based on over-all similarity. Yet as stated above, cladistics isnot based on mere similarity, but insteadfocuses on a special kind of similarity — fea-tures that are derived, or evolutionary novel-ties. Evolutionary novelties help to show rela-tionships and thus are “phylogeneticallyinformative.” In contrast, similarities that arenot evolutionary novelties are “ancestral” fea-tures and are not phylogenetically informative.For example, a derived feature of primates isan opposable thumb; this feature is phyloge-netically informative because it allows us togroup all primates together to the exclusion ofother mammals. On the other hand, a five-fin-gered hand is an ancestral feature and not phy-logenetically informative because we wouldnot group all animals possessing a five-fin-gered hand together to the exclusion of thosethat do not. For example, we do not proposethat all five-fingered mammals are more close-ly related to each other than they are to three-fingered, two-fingered, or one-fingered mam-mals. So cladistics is not based on mere simi-larity. Further, paleontologists who applycladistic methods to the problem of avian evo-lution do not think that how flight evolved is“irrelevant”; in fact we specifically use clado-grams to inform our models of how flight (andother things) evolved. By rooting our explana-tions in phylogenies, we can move beyondsubjective models, and constrain our hypothe-ses (e.g., Witmer, 1995; Padian, 1995). Theseexplanations can also serve as yet anotherindependent test of our phylogenies (Padian,2001a, 2001b).

Wells then accuses cladistics of “rearrang-

ing” the evidence, stating that the supposed“ancestors” of Archaeopteryx are millions ofyears younger. First of all, none of these more“recent” avian-like dinosaurs thought to beclosely related to Archaeopteryx (e.g.,troodontids and dromaeosaurs) are considered“ancestors”; rather, they retain ancestral fea-tures that show us what the ancestors ofArchaeopteryx were like. Here again Wellsmistakes lineal for collateral ancestry. Second,the statement that there are no fossils of theseclose cousins of Archaeopteryx until “millionsof years” later is false. Fossils of non-avianmaniraptor dinosaurs, which are closely relat-ed to the ancestors of Archaeopteryx, havebeen found in rocks dating to the same age asthose in which Archaeopteryx has been found(Jensen and Padian, 1989); this discovery wasreported over 10 years ago. Wells apparentlyhas not done his homework very well.

Despite Wells’s claims to the contrary,Archaeopteryx is still an important contributorto our knowledge of transitional features, andit clearly shows the dinosaurian ancestry ofbirds (Figure 12). To confirm this, all one hasto do is peruse any piece of literature on theorigin of birds. Papers on Archaeopteryx andbird evolution appear in many journals eachyear, and there is even an entire journal (calledArchaeopteryx) devoted to the study ofArchaeopteryx and its environment. Ratherthan consult the vast body of literature on theorigin of birds, Wells appears to base much ofhis discussion on two popular works, one tech-nical — The Mistaken Extinction by LowellDingus and Tim Rowe (1998) — and the sec-ond non-technical — Taking Wing, by PatShipman (1998). Both are excellent books.However, during the same period when Wellsapparently wrote Icons (1998–1999), well over50 papers were published that in some waydealt with Archaeopteryx and the dinosaurianorigin of birds. A number of these were very


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Figure 12. Archaeopteryx (bold) shown in evolutionary context with respect to crocodilians, non-avian dinosaurs, and birds. Some relevant features plotted.

important (e.g., Britt et al., 1998; Padian andChiappe, 1998; Forster et al., 1998; Ji et al.,1998; Burgers and Chiappe 1999; Chiappe etal. 1999; Clark et al., 1999; Garner et al., 1999;Norell and Makovicky, 1999; Ostrom et al.,1999; Wagner and Gauthier, 1999; Xu et al.,1999), yet Wells cites none of them.

Wells also ignores the many fossil discover-ies of feathered non-avian dinosaurs fromLiaoning, China (see Figure 13), which shouldplay an important role in any discussion ofavian origins, save for one notable exception.In an attempt to discredit the entire field, Wellsbrings up “Archaeoraptor,” which he regardsas a “hoax” and indicative of the sloppy sci-ence that paleontologists do. In fact Wellsspends the remaining third of the chapter try-ing to use “Archaeoraptor” in an attempt toslander the field of paleontology. Here too, hegets most of the facts wrong.

“Archaeoraptor” was a fossil bought at theTucson Gem and Mineral Show for SteveCzerkas, a knowledgeable dinosaur enthusiastand skilled sculptor and artist. Its remainscame from the Liaoning area of China, whichhas produced numerous beautifully preservedfossils of fish, mammals, lizards, and bothavian and non-avian dinosaurs. Many of thesewere preserved with their body coverings,such as fur or feathers, intact. So it was notunexpected to see an allegedly new find fromthere that combined features of fossil birds andclosely related dromaeosaurid dinosaurs, espe-cially given the large body of evidence sug-gesting that birds evolved from thesedinosaurs. The fossils of Liaoning are collect-ed by local villagers and farmers who knowthat “complete” specimens, particularly thosewith feathers, are preferred by scientists andcollectors. Therefore, a cottage industry hassprung up around using parts to enhance ormake “whole” specimens (Chiappe et al.,1999). These constructed specimens are very

well done and can fool an untrained eye, whichis more or less what happened with“Archaeoraptor.” The first paleontologists tosee the specimen were immediately suspiciousbecause the prevalence of composite speci-mens was already known, and its distributionof features were not what would be expected inan avian-like dinosaur. We would not expect itto have the arms of a primitive bird and thelegs of a non-avian theropod. Even though anumber of paleontologists were skeptical,National Geographic went ahead with an arti-cle that featured this specimen along with twoothers. This became an embarrassment forNational Geographic when, at nearly the sametime it ran its article, computerized axialtomography (CAT) scanning of the specimenshowed it to be a composite. As it turns out, thelegs of the specimen belong to the counterslabof a tiny non-avian theropod calledMicroraptor (Xu et al., 2001); a full descrip-tion of the composite was published by Roweet al. (2001). To view the scans of the compos-ite, visit the UT Austin CT lab website(www.ctlab.geo.utexas.edu/pubs/nature2000).

Wells concludes that this sorry episodeoccurred because of “the cladists’ desire toprove their theory. Just as the need for a miss-ing link between apes and humans led toPiltdown man, so the need for a missing linkbetween dinosaurs and birds paved the way forthe ‘Piltdown bird.’” (Wells, 2000:125). Notso. The people who bought and promoted thespecimen weren’t cladists, and they never per-formed a cladistic analysis or attempted toplace the specimen in a phylogeny. Piltdownman was an intentional hoax played on scien-tists, and the hoax was revealed by scientistswhen the specimen was studied. The forgery of“Archaeoraptor” was discovered by scientificinvestigation as well, and it was cladists TimRowe, Xu Xing, and Phil Currie who uncov-ered it. The name “Archaeoraptor” was never


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formally published as a scientific name, andhas no scientific standing — the animal neverexisted. This doesn’t prevent Wells from itali-cizing the name as if it were a real species.Further, the specimen was never consideredimportant to our understanding of avian evolu-tion. This doesn’t stop Wells from pretendingotherwise, as if it were somehow important,even crucial, to the idea that birds are descend-ed from dinosaurs.

Returning to Archaeopteryx, Wells thenresorts to a classic creationist taxonomy game.In this game, the creationist says that scientistshave to choose whether a fossil belongs to onetaxonomic group or another. So, in the case ofArchaeopteryx, it has to be a bird or a reptile.Then the creationist says that because it hasfeathers it is a bird, and therefore because it isa “bird” it cannot be a transitional form. Ineffect the transitional features of the fossil aredefined out of existence. This is a classic cre-ationist ploy, and nothing new; it is what wehave seen for decades from Duane Gish andHenry Morris. Wells uses a slightly differentapproach, claiming that if Archaeopteryx andbirds are just dinosaurs, then humans are justfish, which — he implies — is absurd. But thisis another case of Wells trying to use semanticsto negate the evidence of evolution, just as hedid with the Cambrian Explosion.

Here Wells exploits the systematic practiceby which all groups of organisms must be“monophyletic,” that is, consist of an ancestorand all of its descendants. In Wells’s rathernaïve example, “fish” must be taken to includehagfishes, lampreys, sharks, goldfish and otherrayfins, coelocanths, and lungfishes. If “fish”were defined that way, then tetrapods (all ani-mals that have four limbs) would indeed be“fish” and “fish” would become another namefor “vertebrate.” But “fish” is not a taxonomicname; it is a colloquial term, and as a Ph.D.biologist, Wells should know that. Real sys-

tematists don’t use the term “fish” except in arestricted sense referring either to a subgroupthat is monophyletic such as Actinopterygia orto “rayfins” (things like goldfish, trout, sword-fish, etc.) — the vast majority of living “fish-es.” Humans are vertebrates; so are fishes.Birds, by phylogenetic relationship, aredinosaurs. Just as dogs are canids, and alsomammals, and also tetrapods and vertebrates.Consider a mailing address: just because youlive on 1010 Main Street does not mean thatyou don’t live in Peoria or in Illinois, or thatsomeone living on 411 South Street doesn’tlive in the same town or state.

Wells’s most ridiculous treatment of “sci-ence” in this chapter is when he takes childishshots at paleontologists. This is another popu-lar creationist tactic: attacking the character ofa prominent scientist or scientific field. In fact,he devotes six pages to making fun of paleon-tologists at a Florida symposium withoutappearing to understand what they were say-ing. Worse yet, Wells completely misrepre-sents the proceedings. For example, he claimsthat a “cladistic analysis” showed a specimenpresented there, called “Bambiraptor,” to bean ancestor of Archaeopteryx, yet no “cladisticanalysis” was mentioned in either the descrip-tion (Burnham et al., 2000) or the conferenceproceedings. To my knowledge, no cladisticanalysis has ever been performed on that spec-imen. Wells then claims to be appalled that inthe reconstruction, “Bambiraptor” was showncovered in feathers even though none werefound fossilized with it. But other fossilizeddromaeosaurid dinosaurs are found covered infeathers (e.g. Xu et al., 1999; Ji et al., 2001;Norell et al., 2002) and so are the more basalOviraptorids (Ji et al., 1998). The even morebasal Compsognathids are found with down-like feathers as well (Chen et al., 1998; seeFigures 12, and 13). So it is conservative toreconstruct “Bambiraptor” with a covering of


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Figure 13. Some examples of feathered dinosaurs discovered in Laioning, China.

feathers. Besides, the reconstruction is a pic-ture, not scientific evidence — a confusion ofWells’s revealed further in the peppered mothschapter. By Wells’s logic, we shouldn’t acceptthe likelihood of fur on a fossil sabertooth cat.Is this the kind of “critical thinking” Wellswants us to teach our students?

Finally, Wells charicatures the conferencepresentation of Kevin Padian, who not only isa respected paleontologist but also happens tobe the president of NCSE. Padian’s talk was acritique of the hypothesis that birds evolvedfrom something other than dinosaurs. Wellslikens Padian’s talk to an “old lawyers’ joke”about a “cracked kettle.” Wells even says thatPadian was not trying to be funny, and that itwould be unkind to compare his talk to thejoke, yet he continues the ad hominem attacksummarizing Padian’s talk as a joke. Wells’ssummary, however, looks nothing like eitherthe abstract that Padian submitted, whichWells (as a conference attendee) received, orthe text of the talk he gave. In particular,Padian never called the critics of the dinosauri-an origin of birds “unscientific,” just their crit-icisms. He never accused them of “selectiveinterpretation” of the evidence; he just saidthat they did not use accepted methodologiesto evaluate the evidence. He never said thatscientists reject their methodology regardlessof the evidence; he said that we cannot evalu-ate their methodology because they do not pro-vide one. Finally, Padian’s conclusion was notthat there was no controversy, but that the con-troversy over bird origins was journalistic, notscientific (Padian, pers. comm.). If Wells wastaking notes at the conference, he didn’t do avery good job.

Although Wells smugly chides paleontolo-gists for their supposed views about bird evo-lution, he has not attended any meetings of theSociety of Vertebrate Paleontology or theOstrom Symposium on the origin of birds. He

has no training or expertise in the field.Instead, he relies on caricatures of paleontol-ogy and paleontologists, and lampoons theentire field, treating scientists as if they were abunch of dinosaur-loving buffoons who areeasily fooled and misled. This is not science orscholarship; this is tabloid journalism.


Textbooks cover Archaeopteryx withvarying degrees of brevity, frequentlygiving only a paragraph to

Archaeopteryx, usually in the section on rep-tiles or birds or in the history of life section.The lengths of the paragraphs vary from 54words to well over 500 (Figure 14), and theaverage length falls around 200 words.Archaeopteryx is frequently used as an exam-ple of a transitional form between reptiles(dinosaurs) and birds. Eight of the books treatit as showing a dinosaurian ancestry for birds,while two state that the ancestry is simply rep-tilian (Figure 14). Few of these books treatArchaeopteryx well and most of these discus-sions are garbled and contain factual errorsabout Archaeopteryx. For example, Guttmancontains numerous errors, even suggesting thatit could not fly. Wells apparently does not evenknow enough about the topic to point this out.Wells only singles out the two books that usethe word “link” in their descriptions, Maderand Schraer and Stolze. The most accurate dis-cussions can be found in Raven and Johnson,Campbell et al., and Johnson. Archaeopteryx issometimes used as an example of how fossilscan elucidate evolutionary relationships. Fewbooks use Archaeopteryx as direct evidencefor evolution; some books (e.g., Johnson)instead use the origin of whales as the princi-pal example of a transitional sequence.


48


In grading textbooks on Archaeopteryx, thegrading scheme, as usual, seems skewed tofail the books. Any book that does not

describe the transitional status ofArchaeopteryx between reptiles and birds as“controversial” gets a D. As mentioned above,there is no controversy about whether it istransitional, i.e, possesses structural featuresboth of its reptilian ancestors and of birds. Toget better than a D, a book would have to pres-

ent scientifically incorrect data. What is mostpuzzling is that some books are given ratherhigh grades compared to those given for other“icons.” Close examination of these bookssuggests that Wells misgraded them (Figure14). For example, Wells gives Campbell,Reese, and Mitchell a B, yet they clearly statethat Archaeopteryx is a transitional formbetween dinosaurs and birds, for which a C orD would have been a more accurate gradegiven Wells’s criteria. This negligent applica-tion of his own criteria calls into question the


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Figure 14. Textbooks’ treatment of and Wells’s grades for Archaeopteryx.

rigor of Wells’s evaluation and the value of hisgrades whether or not one accepts his idiosyn-cratic criteria.

WHY ARCHAEOPTERYX STILL FLIESIN TEXTBOOKS

If anything, the value of Archaeopteryx as apedagogical tool is increasing with all thenew discoveries of feathered dinosaurs

from China. Literally every new fossil discov-ery has added to the utility of Archaeopteryx.Archaeopteryx is still one of our best examplesof a fossil that preserves ancestral featureswhile showing descendant novelties.Archaeopteryx is but one of many fossilsshowing a clear genealogical connectionbetween dinosaurs and birds (Figure 12).Much like Mark Twain’s, the reports of itsdeath are greatly exaggerated.

HOW TEXTBOOKS COULD IMPROVETHE USE OF ARCHAEOPTERYXAND TRANSITIONAL FORMS

Textbooks could improve their explana-tions of transitions in evolution byfocusing on transitional features (not

forms or individual animals) that are borne bya series of closely related organisms. Further,textbooks should be clear in presenting theidea that in general fossils are not consideredto be direct ancestors, but as records of ances-tral features. Finally, in discussions ofArchaeopteryx, textbooks need to tighten uptheir descriptions and check their facts aboutthe history of both Archaeopteryx and thedinosaur–bird relationship. Textbooks shouldbe clear that birds are descendants of dinosaursand that there are no other credible potentialancestral groups; they should also augmenttheir rather short discussions of avian evolu-tion with some of the new fossil evidence fromChina where non-avian dinosaurs have beenfound with feathers (Figure 13). Wells’s claims

about Archaeopteryx are simply inaccurate. Tofollow his lead would mislead students intothinking that fossils tell us nothing about evo-lutionary relationships. Considering the factthat Wells doesn’t understand ancestry or phy-logenetic reconstruction, and he isn’t evenaware of Archaeopteryx’s status in paleontol-ogy, should we really be inclined to trust any-thing he says on these topics?

References

Britt, B. B., P. J. Makovickey, J. A. Gauthier, and N.Bonde. 1998. Postcranial pneumaticity inArchaeopteryx. Nature 395:374–376.

Burnham D. A., K. L. Derstler, P. J. Currie, R. T. Bakker,Z. Zhou and J. H. Ostrom. 2000. Remarkable new bird-like dinosaur (Theropoda: Maniraptora) from the UpperCretaceous of Montana. University of KansasPaleontological Contributions 13:1–14.

Burgers, P., and L. M. Chiappe 1999. The wing ofArchaeopteryx as a primary thrust generator. Nature399:60–62.

Chen, P.-J., Z.-M. Dong, and S.-M. Zhen. 1998. Anexceptionally preserved theropod dinosaur from theYixian formation of China. Nature 391:147–152.

Chiappe, L. M., S.-A. Ji, Q. Ji, and M. A. Norell. 1999.Anatomy and systematics of the Confuciusornithidae(Theropoda: Aves) from the late Mesozoic of northeast-ern China. Bulletin of the American Museum of NaturalHistory. 242:1–86.

Clark, J. M., M. A. Norell, and L. M. Chiappe. 1999. Anoviraptorid skeleton from the Late Cretaceous of UkhaaTolgod, Mongolia, preserved in an avianlike broodingposition over an oviraptorid nest. American MuseumNovitates 3265:1–36.

Dingus, L. and T. Rowe. 1998. The mistaken extinction:dinosaur evolution and the origin of birds. W. H.Freeman and Company, New York 332p.

Forster, C. A., S. D. Sampson, L. M. Chiappe, and D. W.Krause. 1998. The theropod ancestry of birds: new evi-dence from the Late Cretaceous of Madagascar. Science279:1915–1922.

Garner, J. P., G. K. Taylor, A. L. R. Thomas. 1999. Onthe origins of birds: The sequence of character acquisi-tion in the evolution of avian flight. Proceedings of the


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Royal Society of London, Biological Sciences Series B266:1259–1266.

Jensen, J. A., and Padian, K. 1989. Small pterosaurs anddinosaurs from the Uncompahgre fauna (Brushy BasinMember, Morrison Formation: ?Tithonian), LateJurassic, western Colorado. Journal of Paleontology.63:364–373.

Ji, Q., P. J. Currie, M. A. Norell, and S.-A. Ji. 1998. Twofeathered dinosaurs from northeastern China. Nature393:753–761.

Ji, Q., M. A. Norell, K.-Q. Gao, S.-A. Ji, and D. Ren.2001. The distribution of integumentary structures in afeathered dinosaur. Nature 410:1084–1088.

Norell, M., Q. Ji, K. Gao, C. Yuan, Y. Zhao, L. Wang.2002. ‘Modern’ feathers on a non-avian dinosaur.Nature 416:36–37.

Norell, M. A., and P. J. Makovicky. 1999. Important fea-tures of the dromaeosaurid skeleton II: information fromnewly collected specimens of Velociraptor mongolien-sis. American Museum Novitates 3282:1–45.

Ostrom, J. H., S. O. Poore, and G.E. Goslow. 1999.Humeral rotation and wrist supination: Important func-tional complex for the evolution of powered flight inbirds? Smithsonian Contributions in Paleobiology89:301–309.

Padian, K. 1995b. Form and Function: The evolution ofa dialectic. In J.J. Thomason ed. FunctionalMorphology and Vertebrate Paleontology CambridgeUniversity Press p. 264–277.

Padian, K. 2001a. Cross-testing adaptive hypotheses:phylogenetic analysis and the origin of bird flight.American Zoologist 41:598–607.

Padian, K. 2001b. Stages in the origin of bird flight:beyond the arboreal–cursorial dichotomy. In J.A.Gauthier and L. M. Gall eds. New Perspectives on theOrigin and Early Evolution of Birds. Yale UniversityPress, New Haven p.255–273.

Padian, K., and K. D. Angielczyk. 1999. Are there tran-sitional forms in the fossil record? In P.H. Kelley, J.R.Bryan, and T.A. Hansen eds. The Evolution–CreationControversy II: Perspectives on science, religion, andgeological education. Paleontological Society Papers5:47–82.

Padian, K. and L. M. Chiappe. 1998. The origin andearly evolution of birds. Biological Reviews of theCambridge Philosophical Society 73:1–42.

Rowe, T., R. A. Ketcham, C. Denison, M. Colbert, X.Xu, and P. J. Currie. 2001. Forensic paleontology: theArchaeoraptor forgery. Narure 410:539–540.

Shipman, P. 1998. Taking wing: Archaeopteryx and theevolution of bird flight. Simon and Schuster, New York336p.

Wagner, G. P., and J. A. Gauthier. 1999. 1,2,3 = 2,3,4: asolution to the problem of the homology of the digits inthe avian hand. Proceedings of the National Academy ofScience 96:5111–5116.


Witmer, L. M. 1995. The extant phylogenetic bracketand the importance of reconstructing soft issues in fos-sils In J.J. Thomason ed. Functional Morphology andVertebrate Paleontology Cambridge University Press p.19–33.

Xu, X., X.-L. Wang, and X.-C. Wu. 1999. A dro-maeosaurid dinosaur with a filamentous integumentfrom the Yixian Formation of China. Nature 401: 262–266.

Xu, X., Z. Zhou, and X. Wang. 2001. The smallestknown non-avian theropod dinosaur. Nature 408:705–708.


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PEPPERED MOTHS

THE STORY OF THE PEPPERED MOTH

Industrial melanism in peppered moths isone of the most frequently used examplesof natural selection in action. This is large-

ly because of its pedagogical simplicity — it isa straightforward example that is visual anddynamic — and its copious documentation.Industrial melanism refers to the darkening ofcolor that occurred in a number of species ofinsects following the Industrial Revolution.This change appears to be related to theincrease in pollutants in the environment.Before the Industrial Revolution, individualsof the moth species Biston betularia (com-monly called the “peppered moth”) were pre-dominantly white with black speckles. By theend of the 1800s, they were predominantlycharcoal grey. This change was well docu-mented and led Tutt (1896) to hypothesize thatthis change was a result of pollution-stainedtrees’ affecting the camouflage potential of themoths. This change was termed “industrialmelanism.” In the 1950s, Bernard Kettlewelldecided to test the hypothesis that naturalselection was working on the differential cam-ouflage of the moths. In order to do this, hereleased marked light and dark moths into pol-luted and non-polluted forests. He found thatbirds appear to prey selectively on light mothsin polluted forests and on dark moths in non-polluted forests and so documented the idea ofnatural selection of these color patterns inmoths by birds. After anti-pollution laws tookeffect and the bark lightened, the moth popula-tions in formerly polluted areas returned toprevious color distributions.

HOW MANY MOTHS CAN DANCEON THE TRUNK OF A TREE?

DISTRACTION BY IRRELEVANT DATA

Wells disagrees with the results of theresearch on industrial melanism inthe peppered moth, and manipulates

the literature and the data to fit his views. Hepoints out that the “problem” of the pepperedmoths is far from simple. His discussion cen-ters on three points where he believes text-books are in error, alleging that (1) the daytimeresting places of peppered moths invalidatesKettlewell’s experimental results; (2) the pho-tos of the moths are “staged”; and (3) therecovery patterns of populations dominated bylight moths after the levels of pollution werereduced do not fit the “model,” although he isunclear as to what the “model” is. All three ofthese objections are spurious. They are distrac-tions from the general accuracy of the storyand its value in showing the effects of naturalselection on genetic variability in natural pop-ulations.

First, Wells argues that the story is seriouslyflawed because “peppered moths in the wilddon’t even rest on tree trunks” (Wells,2000:138). He repeats this point throughoutthe chapter. However, it is both false and irrel-evant, and only serves as a distraction to leadthe reader away from the actual story of themoths. Contrary to Wells’s assertions, datagiven by Majerus (1998:123) indicate that themoths do indeed rest on the trunks of trees25% of the time. The rest of the time mothsrest in branches (25%) or at branch-trunk junc-tions (50%). The facts have been pointed outrepeatedly to Wells; his response has beenmostly to claim that moths don’t rest on“exposed” tree trunks (Wells, 2002 www.dis-covery.org/viewDB/index.php3?program=CRSC&command=view&id=1144). But this isnot what he said in the text of Icons, which


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remains flatly wrong. Moths are found all overtrees, which is not a surprise (Clarke et al.,1994) and it is mentioned in the references thatWells cites.

To clear up any confusion, no researcherdoubts that the peppered moth rests in trees(Clarke et al. 1994; Majerus 1998), whichmeans that the resting substrate is bark. Entiretrees are stained by pollution — the leaves,twigs, branches, trunks, and the surroundingground (Kettlewell, 1973) — and so the colorsof the moths are relevant no matter where onthe tree they rest — trunks, trunk-branch junc-tions, branches, twigs, and even the leaves.Wells’s argument implies that predatory birdscan only see moths that are on exposed trunks.By making this argument, however, Wellsshows an apparent ignorance of the ecology ofbirds and woodland ecosystems. If you walkinto any forest, you can see that the birds flyfrom tree to tree, branch to branch, and hunt atall levels of the forest. Woodland species ofbirds that prey on moths and other insects liveand hunt in the canopy (the leafy part of thetrees). These birds are not hunting from out-side, soaring above the trees like hawks, asWells’s argument would require.

In the scientific literature, there is extensivediscussion of the hunting behavior of birds,including those that hunt peppered moths.Ornithologists have shown the woodlandecosystem to be vertically stratified by compe-tition between different bird species. Thiszonation means that there are skilled predatorspatrolling all levels of the forest: the trunks,trunk-branch joints, branches, and highercanopy (Colquhoun and Morley, 1943;Hartley, 1953). Further, birds learn to distin-guish their prey against various backgroundsand preferentially hunt prey in locations wherethey have found it in the past and that birdsselectively prey on the more visible moths(Pietrewitz and Kamil, 1977, 1981). In other

words, birds hunt the prey they can see andhunt it where it is, not where it isn’t. Therefore,no matter where the moth rests in the tree, it isvisible to predatory birds, and thus its differen-tial camouflage is important.

The purpose of Wells’s distraction is to putthe actual experiments into question and makeit sound as if the textbook authors are eithermistaken, or intentionally trying to fool stu-dents. The insinuation is that becauseKettlewell released the moths during the day,they did not find “normal” resting places.Whether or not this is so, the release and cap-ture experiments took place over a number ofdays, so the moths were able to take up posi-tions of their choosing, even if the first daywas not perfectly “natural” (Kettlewell, 1955,1956, 1973). Kettlewell’s experiments werenot perfect — few field experiments are — andthey may have magnified the degree of selec-tion, but all serious researchers in the fieldagree that they were certainly not so flawed asto invalidate his conclusion.

In his second objection, Wells ties theKettlewell experiments to textbooks by con-stantly repeating the statement that the illustra-tive photos were “staged” (Wells, 2000:150);the important issue here is not how the photoswere made, but rather their intent. Wellsimplies that the photos purport to show a “life-like” condition to prove that moths rest ontrunks. This is not the case. The photos aremeant to demonstrate the visibility of the dif-ferent forms of the moth on polluted andunpolluted trees. It is absurd to expect a pho-tographer to just sit around and wait until twodifferently colored moths happen to alight sideby side. Further, how the photos were pro-duced does not change the actual data. Birdseat moths and they eat the ones that they seemore easily first. The textbook photos neverclaim to depict a real-life situation, and it isimproper to imply otherwise.


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The third criticism, and the only scientificone that Wells levels, deals with the recoveryof the light form of the moth following theinstitution of pollution control laws. The mainthrust of his argument is that because therecovery of light-colored lichens does not cor-relate with the recovery of the light form of themoths, the entire story is incorrect. Wellsexploits the fact that the original researchersthought that the camouflage of the light mothsdepended on the presence of lichen. However,the light forms recovered before the lichensdid; therefore, Wells concludes, natural selec-tion has nothing to do with the story. Althoughit is true that the moths are well-camouflagedagainst lichens, and lichens are destroyed bypollution, nevertheless the camouflage of themoths ultimately depends upon the color of thetrees, which reflect the amount of soot stainingthe trees. Although lichens play a role in cam-ouflage, they are not necessary. This is whathappened: pollution was reduced, the trees gotlighter, then the moths got lighter. Further, inall areas, the light moths have recovered, aspredicted by the hypothesis. This is clearlystated in the literature (e.g., Grant et al., 1998),but it does not fit Wells’s story, and he justignores it.

TEXTBOOK TREATMENT OFTHE PEPPERED MOTHS

All but one of the textbooks (Campbell,Reese, and Mitchell) reviewed inIcons cover the peppered moths and

present the basic story correctly. Again, how-ever, the coverage is limited to only a coupleof paragraphs (Figure 15), varying from 117 toover 500 words. Miller and Levine devotemore than a page to the story, and even discusssome of its complexity, suggesting that thestory is not as simple as it seems.

GREY AREA GRADES

Like the grading schemes for the other“icons,” this one is stacked against thetextbooks as well. Even books that have

more extensive discussions of the problemsand details (such as Miller and Levine) can atbest earn a D. Like the grading schemes forMiller-Urey and Haeckel’s embryos, it is basedlargely on the presence or absence of pictures(Figure 15). Explaining the peppered mothstory without photos (as in Biggs et al.), gar-ners a peculiar X grade. In order to get an A orB, a book must contain pictures of moths in“natural” resting places. Given Wells’s expla-nation that these are unknown, presentingthose would be impossible. How can textbooksbe expected to do that? A C would be awardedto a textbook that (1) used “misleading” pic-tures, but (2) referred to them as “staged,” and(3) stated that the results of the experiment arein doubt. Any standard textbook discussion ofthe issue, even if it mentions that the story ismore complicated, is given a D. So as usual,this is a “no win” situation. This falls intoWells’s pattern of requiring the books to “crit-icize” their examples, although the criticismshe insists on are largely fallacious.

WHY WE CAN STILL TEACHPEPPERED MOTHS AS AN EXAMPLE

OF NATURAL SELECTION

Although there will always be details ofthe peppered moth story that we do notfully understand, its status as an exam-

ple of natural selection is not even remotely indoubt. There is a clear correlation between pol-lution levels and moth color. Even if bird pre-dation may not be the only factor involved inthe selection of one color over another, obser-vations show that bird predation and substratecolor play the major roles in natural selectionof the color of peppered moths. There aremany areas in science where our knowledge is


54

incomplete, but that does not mean we shouldnot teach about them. There are things we stilldo not know about gravity, but no one isdemanding that examples of gravity in actionbe removed from textbooks.


PEPPERED MOTHS

For the most part, textbook coverage ofthe peppered moth story is adequate. Asalways, expanding the discussion would

improve the coverage in the textbooks thatcover it briefly. Textbooks could qualify thecaptions with a statement that the picturesillustrate differential camouflage in order toclear up any misunderstanding (howeverunlikely) as to the meaning of the photos. Abetter way for books to improve the topic is byadding other examples of natural selection act-

ing on genetic variation. Some books alreadycover sickle-cell anemia in humans (Figure15). Other possible examples include antibiot-ic resistance in bacteria and myxomatosis virusin rabbits in Australia. The key here is toexpand the exposure of students to the manyexamples of natural selection-driven evolu-tionary change (e.g., Endler, 1986). What iscurious about Wells’s criticism of the pepperedmoth is that he says in Icons that he accepts“microevolution.” The peppered moths are anexample of “microevolution,” so why does hehave a problem with teaching it?


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Figure 15. Wells’s grades for textbooks’ treatments of peppered moths.

References

Clarke, C. A., B. S. Grant, F. M. M. Clarke, and T.Asami. 1994. A long term assessment of Biston betular-ia (L.) in one UK locality (Caldy Common near WestKirby, Wirral), 1959-1993, and glimpses elsewhere.Linnean 10:18–26.

Colquohoun, M. K, and A. Morley. 1943. Vertical zona-tion in woodland bird communities. Journal of AnimalEcology 12:75–81.

Endler, J. AL. 1986. Natural Selection in the Wild.Monographs in Population Biology 21. PrincetonUniversity Press, Princeton, 336p.

Grant, B.S., A.D. Cook, C.A. Clarke, and D.F. Owen.1998. Geographic and temporal variation in the inci-dence of melanism in peppered moth populations inAmerica and Britain. Journal of Heredity 89:465–471.

Hartley, P. H. T. 1953. An ecological study of the feed-ing habits of the English titmice. Journal of AnimalEcology 22:261–288.

Kettlewell, H. B. D. 1955. Selection experiments onindustrial melanism in the Lepidoptera. Heredity9:323–342.

Kettlewell, H. B. D. 1956. Further selection experimentson industrial melanism in the Lepidoptera. Heredity10:287–301.

Kettlewell, H. B. D. 1973. The evolution of melanism:the study of a recurring necessity; with special referenceto industrial melanism in the Lepidoptera. ClarendonPress, Oxford. 423p.

Majerus, M. E. N. 1998. Melanism: evolution in action.Oxford University Press, New York. 338p.

Pietrewicz, A. T. and A. C. Kamil. 1977. Visual detec-tion of cryptic prey by blue jays Cyanocitta cristata.Science 195:580–582.

Pietrewitz, A. T., and A. C. Kamil. 1981. Search imageand the detecting of cryptic prey: an operant approach.In A. C. Kamil and T. D. Sargent eds. Foraging behav-ior. Ecological, ethological and psychologicalapproaches. Garland Press, New York, p. 311–331.

Tutt, J. W. 1896. British Moths. George Routledge,London 368p.



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“DARWIN’S FINCHES”

THE STORY OF“DARWIN’S” FINCHES

Darwin’s finches,” along with Hawaiianhoneycreepers and African cichlids,are frequently used as examples of

adaptive radiation. In an adaptive radiation, a“founder” species enters a new environmentwith many unoccupied niches. This speciesexpands (radiates) and evolves adaptations tofit these niches better. The process of becom-ing adapted to these different niches may leadto, and in these cases has led to, the formationof new species. All the species of finches onthe Galápagos Islands appear morphologicallyvery similar, varying mostly in terms of beaksize and behavior; they all look very much likea species of finch from the mainland of SouthAmerica. This suggests that all the finches onthe Galápagos are descended from one originalcolonist species that went through an adaptiveradiation. Because of the small, isolated envi-ronment of the Galápagos, the finches havebecome the topic of extensive study into natu-ral selection. The studies that have been con-ducted on the finches show strong selection forlarger beaks during droughts. These data showthat climatic changes can have profoundeffects on the morphology of a species andpotentially lead to the formation of newspecies. When Darwin visited the Galápagos,he observed and collected some of the finchspecies, believing that they represented a verydiverse set of birds that were not closely relat-ed. Their significance was not recognized untillater, when ornithologist John Gould pointedout that the birds were all closely related finch-es (Desmond and Moore, 1991). But becauseDarwin originally collected some of the speci-mens and because the finches showed so much

evidence for evolution and natural selection,they have been dubbed “Darwin’s finches.”This has led many people to conclude (mistak-enly) that Darwin’s theory of evolution wasspecifically inspired by the finches.

A LEGEND IN HIS OWN MIND

Wells apparently feels the need toattack the finches largely becausethey are an “icon” in need of

destruction; the chapter on the finches is per-haps the most poorly conceived section in thebook. Wells initially focuses on the “biologicalurban legend” that the finches inspired Darwinto compose his theory of evolution. Of coursethis has nothing to do with whether or not thefinches are a good example of an adaptiveradiation. Therefore, his “requirement” thattextbooks specifically mention that the finches“played no role” in Darwin’s formulation ofnatural selection is irrelevant, only servingWells’s efforts to portray evolutionary biolo-gists as people who just “make things up.”This is like saying that because Betsy Ross didnot really sew the U.S. flag, the flag does notactually exist. Wells even goes so far as tobrand the finches a “legend” — what is he try-ing to imply? Finally, Wells’s assertion thatDarwin was not inspired by the finches is notexactly correct. Although Darwin did not real-ize the significance of the finches until afterGould pointed it out to him in 1837, he thennoted that the different species of finches wereisland-specific like the other Galápagos ani-mals and suggested that they too were descen-dants of a mainland ancestor. Darwin madeextensive notes about the finches in his diaries(Desmond and Moore, 1991). The finches,then, did play a role in the formulation ofDarwin’s theory and they became an importantpart of his evidence for the role of naturalselection in evolution; they were not a “specu-lative afterthought” as Wells claims.


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After branding the finches a “legend,” Wellsswitches gears and discusses the finches them-selves, acknowledging the strength of the evi-dence for an adaptive radiation, given the sim-ilarities of the different species. Wells almostseems to accept that the finches are descendedfrom a common ancestor; at least, he does notargue explicitly against it. But he demands thatthere be direct evidence for speciation by nat-ural selection; in his attempt to explain howthis demand could be met, the remainder of thechapter degenerates into a series of nonsequiturs. This is particularly apparent inWells’s discussion of what would constitute“direct” evidence.

Suggesting that the work of Grant and Grantclaimed to be that direct evidence, he discuss-es their experimental work on finch beak vari-ation. The most detailed selection work on thefinches was done by the husband and wifeteam of Peter and Rosemary Grant. For overtwo decades, the Grants and their studentshave monitored the sizes of the beaks of someof the finches on one small island (Grant,1999). They have documented that the size ofthe finch beaks is correlated to the relativerainfall on the island, and thus to the abun-dance and hardness of the food. During dryyears larger beak size is selected for, whileduring wet years the beak size is more varied.Wells acknowledges that the beaks vary andthat this shows natural selection. He seems toaccept that the changes in beak shape arecaused by natural selection in reaction todrought-caused changes in the food supply.These data are some of the most compellingfor natural selection in the wild — somethingthat even Wells has a hard time denying.However, he then contends that because thebeak shape returns to a pre-drought size distri-bution, that no “net” evolution has occurred.But this is a mysterious contention. Naturalselection occurred. If the droughts had contin-

ued, larger beak sizes would continue to beselected for, but the droughts did not.Evolutionary theory would predict that if cli-mate oscillates, morphology would oscillate aswell. The finches fit the predicted pattern.Speciation would require selection to be moreconstant than a couple of years here or there. Itis not unreasonable to extrapolate that if just acouple of years of drought can have that sig-nificant an effect on beak size, then extendeddroughts could cause such variations tobecome fixed in a population, and lead to spe-ciation. This is no different than extrapolationsof unknown orbits. When a new comet is dis-covered, its orbit is calculated based on a fewshort-term observations. We assume that theforces acting on the comet are constant andthus we can predict its position in 10, 20, 100,etc. years. If gravity varied, then these extrap-olations would be in doubt. In the case of thefinches, climate varied and the extrapolationschanged. Does Wells not allow scientists tomake reasonable extrapolations based on dataand observations? If so, physicists must be upnext for Wells’s scorn. Perhaps what is mostinteresting about Wells’s discussion of this“icon,” however, is that in chapter 7 on thepeppered moths, he denies natural selectionentirely, when he could have made the sameargument — that “no net evolution occurred”because the distribution of dark and lightforms of the moths returned to pre-industriallevels just as the finch beaks return to pre-drought levels. For finches he accepts naturalselection, but for the peppered moths he doesnot.

Wells goes on to complain about the extrap-olations of speciation rates based on theGrants’ data, complaining that the finchesaren’t an example of natural selection-drivenspeciation because no new species of finchesarose during the duration of the Grants’ study.However, no one would expect speciation to


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occur on that scale, and the Grants neverclaimed to expect it either. And how wouldyou recognize a new species had formed?More importantly, one wonders how Wellswould recognize new species based on his gar-bled discussion of species concepts (Wells,2000:172-173), where he claims that oneshould “expect” “true” species to be separatedby more than “just” beak shape and song pat-tern. This is important because in order to doc-ument speciation, you need a model by whichto recognize species. Wells provides none, andcannot even manage to explain the currentlyaccepted models properly.

Wells makes much of how the species offinches are freely hybridizing and may in factbe merging. He claims that in order to be“true” species, they should be separated by“more than beak shape and song pattern”(Wells, 2000:172). However, such a separationis a perfectly acceptable definition of speciesbased on Recognition Concept (Paterson,1985), according to which species are separat-ed by behaviors that lead animals to recognizepotential mates. This species definition iswidely accepted amongst animal workers,which Wells should know, having a Ph.D. inbiology. If Wells does not, one would expecthim to learn it as minimum required researchbefore critiquing others’ diagnosis of species.Whether the species are merging or divergingis unimportant because both divergence andmerging are forms of long-term evolutionarychange. If indeed selection favors hybrids, asWells appears to think, then the separatespecies will merge. That’s still evolution andspeciation by natural selection because thenew hybridized form will be a new speciesfavored by natural selection.

TEXTBOOK COVERAGE OFTHE FINCHES

Textbooks use the finches to illustrate awide variety of concepts, from the his-tory of evolutionary theory to adaptive

radiation, natural selection, taxonomy, phy-logeny, and niche partitioning. Textbooks thatdiscuss the finches in an historical context gen-erally devote a paragraph or two to the finch-es, sometimes in the discussion of how Darwinconstructed his theory. Finches also frequentlyappear in sections dealing with patterns of evo-lution as an example of natural selectionand/or adaptive radiation. Only the upper-levelbooks discuss the Grants’ work specifically.Space allotted to the finches vary from a fewwords to a few pages (Figure 16). In terms ofthe historical discussion, most books discussthe finches in connection with Darwin’s visitto the Galápagos Islands. Few books explicitlycredit the finches as Darwin’s inspiration,however. Most do discuss the fact that theywere part of his overall evidence that he col-lected on his voyage. Many books treat thefinches as an example of an adaptive radiation.Some books discuss the finches as examples ofnatural selection and niche splitting instead;these discussions occur in the chapters on evo-lutionary processes or patterns. In Raven andJohnson, the finches are treated in detail; thediscussion includes an accurate summary ofthe historical story and the work of the Grants.This book mentions the finches as an exampleof adaptive radiation along with the Africancichlids.

BIRD-BRAINED GRADING

Due to the diversity of treatment of thefinches in textbooks, it is hard to eval-uate the textbook coverage under

Wells’s grading scheme. The grading schemeemployed for the finch icon is perhaps thestrangest of all Wells’s schemes. Like others,


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this grading scheme appears constructedspecifically for failure. First, Wells objects totextbooks using the finches as an example ofadaptive radiation, and he incorrectly equatesan “adaptive radiation” with the “origin ofspecies by natural selection” in his grading cri-

teria. Adaptive radiation is a description of apattern and makes no statement as to theprocess — which the “origin of speciation bynatural selection” does. This is importantbecause one can document an adaptive radia-tion without knowing the process by which it


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Figure 16. Wells’s grades of textbook treatments of Darwin’s finches. Plus numbers refer to addi-tional treatments of finches.

occurred. He also wants textbooks explicitly tostate that the beak shape oscillates. Further, inorder for a book to get an A, a B, or even a C,the book must explicitly point out that thefinches had nothing to do with Darwin’s for-mulation of the theory of evolution. Whilebooks should not suggest that the finches weremore important in the formulation than theywere, it is interesting that in order to get a goodgrade, Wells insists that the books assert thenegative. In his grading scheme, this meansthat any treatment of the finches that does notexplicitly say that the finches did not inspireDarwin automatically gets a D, even if it men-tions the beak size oscillation or evidence ofmerging. Thus the only criterion for the books’grade is the statement of an unnecessary pieceof information — that Darwin was not inspiredby finches. This has no pedagogical value andisn’t even wholly true; even if it were whollytrue, it has no bearing on the theory of evolu-tion one way or the other. This brings up thequestion of Wells’s real intent. His true goalsare made apparent by the grades themselves.Wells grades many of the books needlesslylow. When reevaluated on Wells’s own criteria,many of the books given a D or F should havbeen given a C (Figure 16). Is Wells simplylooking for any excuse to damage textbooks’reputations?

WHY WE CAN STILL USETHE GALÁPAGOS FINCHESAS A TEACHING EXAMPLE

The finches clearly show adaptive radia-tion and were important to Darwin’sresearch. Their inclusion in texbooks is

perfectly legitimate and should not change.The best way textbooks could improve theirpresentations of adaptive radiation is toinclude other examples such as Hawaiian hon-eycreepers or African cichlids as well. Thereare numerous examples of adaptive radiation;

the more of those that we teach to students, thebetter they will understand evolution.Comically, Wells never really objects to thefinches as an example of natural selection,even concluding that “In this limited sense, thefinches provide evidence for Darwin’s theory”(Wells, 2000:173). If that is the case, what’sthe big deal?

References

Desmond, A., and J. Moore. 1991. Darwin: the life of atormented evolutionist. Warner Books, New York, 808p.

Grant, P. R. 1999. The ecology and evolution ofDarwin’s finches. Princeton University Press, Princeton,492p.

Paterson, H. E. H. 1985. The recognition concept ofspecies. Transvaal Museum Monograph 4:21–29.



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CONCLUSION

ICONS — SHOULD WE KEEP THEM?

The role of primary and secondary edu-cation is to pass on a certain body ofaccepted knowledge and basic concepts

to students in order to prepare them to learnmore. The question is whether the criticismsleveled by the author of Icons would aid us inthat goal; the resounding answer is no. Theeducational program that would result fromimplementing the suggestions contained inIcons would have just the opposite result: Itwould seriously hamper science education andleave students unprepared for the future inwhich science, and biology in particular, willplay an increasing role.

Figure 17 shows the grades that Wells giveseach textbook, the number of pages each text-book specifically devotes to evolutionary the-ory compared to the total number of pages thatcontain evolutionary content (such as phyloge-netic relationships), and when evolution is firstmentioned in the text. Although the amount oftext devoted to evolution varies widely in text-books, the coverage as a percentage of the totaltext that evolution is given is very small —usually less than 10% of the total book. Wellsevaluates only four high school textbooks inthe review, and those have a far smaller treat-ment of evolution than do the college texts.

It is clear from Wells’s treatment of the“icons” and his grading scheme that his inter-est is not to improve the teaching of evolution,but rather to teach anti-evolutionism. UnderWells’s scheme, teachers would be hostile toevolution as part of biology instruction. Wellsand his allies hope that this would open thedoor to alternatives to evolution (such as“intelligent design”) without actually having

to support them with science. In order to get a “good” grade from Wells,

that is to portray a piece of evidence for evolu-tion “accurately” (in Wells’s opinion), onemust mention it and then proceed to criticize it.This is not standard pedagogical practice; if anexample is that bad, it should be removed fromthe biology curriculum, rather than introducedand then criticized. What we see is a pattern ofgrading to create bias rather than accuracy.Rewriting textbooks to criticize evolutionserves no teaching purpose (teaching is a pos-itive endeavor, not negative), yet it is clearfrom the grading that this is the goal of theauthor. What’s worse is that the grading crite-ria are not even consistently applied. There isno pedagogical or factual basis for thesegrades, and they should not be taken seriously.To follow Wells’s advice would not only resultin mis-education about evolution, but about allof biology and other sciences as well. Goodteaching may value critical thinking, but itdoes not value wanton criticism for the sake ofcriticism.

Finally, in his zeal to attack the textbooks’treatments of evolution, Wells misses a chanceto provide a good listing of actual errors intextbooks. A study of the textbooks that Wellsevaluated uncovered factual errors, inexactwordings, and garbled explanations of biolog-ical phenomena that Wells either did not noticeor considers unimportant. This lack of docu-mentation of real textbook errors is yet anoth-er failure of Wells’s effort. Far from beingtracts of “evolution propaganda,” as Wellsimplies, many biology textbooks devote toolittle space to evolution, especially in earlychapters. Most of evolution is reserved for themiddle of textbooks; it is frequently given lesscoverage than ATP cycles or photosynthesis.The topics of which Wells is so critical amountto only a small fraction of any given textbook.In fact, evolutionary biologists consider the


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lack of coverage of evolution, and the failureto interweave it throughout the entire book, tobe the greatest deficit of textbooks.

In conclusion, the scholarship of Icons issubstandard and the conclusions of the bookare unsupported. In fact, despite his touted sci-entific credentials, Wells doesn’t produce asingle piece of original research to support hisposition. Instead, Wells parasitizes on otherscientists’ legitimate work. He could not havewritten the “Haeckel’s embryos” chapter with-

out the work of Richardson et al. (1997, 1998),or the “peppered moths” chapter withoutCoyne (1998) and Majerus (1998), or the“Archaeopteryx” chapter without Shipman(1998). Even then, Wells’s discussions are rifewith inaccuracies and out-of-date information.Wells seems to think that scientific theories aresupported by certain “keystone” pieces of evi-dence, removal of which causes the theory tocollapse. Paradigms in science work whenthey provide solutions and further research;


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Figure 17. Grades given to textbooks in comparison to the coverage of evolution given in the text.AP and College level texts shaded. Parenthetical numbers under the heading “number of pagesdevoted to evolution” refer to the number of pages in the “evolution” chapter.

their health is not tied to single examples. Theparadigm of evolution is not tied to a singlepiece of evidence.

If that is the case, why “defend” the “icons”at all? If evolution doesn’t need them, why notjust replace them? The answer is simple: Thereis no reason to throw out good teaching exam-ples unless the criticisms leveled against themare valid. We should not just acquiesce toWells’s arguments unless they have merit. Justas no piece of evidence becomes a teachingexample without extensive testing, no exampleshould be removed on the basis of one poorlyargued, inaccurate, and tendentious book. Ineach case, it is Wells’s arguments that arewanting, not the “icon.”

When Alfred Wegener first proposed histheory of continental drift, he was laughed atand ridiculed. What did he do? Did he form anon-profit advocacy group and lobby stateschool boards and lawmakers to force teachingof “evidence against” geosynclinal theory?Write a book called Icons ofUniformitarianism? Evaluate and grade earthscience textbooks and demand that they berewritten to remove examples of “border-lands”? No. He went back and did moreresearch. He found like-minded colleaguesand they produced research. He fought in thepeer-reviewed literature. He produced originalresearch, not polemical popular tracts or poli-tics. Eventually his ideas were adopted by thewhole of geology — not through politics butbecause of their overall explanatory power. IfWells and his colleagues want “intelligentdesign” to succeed, they need to produce thatresearch. Until they do, evolution remains thereigning paradigm and the “icons” are perfect-ly acceptable teaching aids.

References

Coyne, J.A. 1998. Not black and white. Nature 396:35–36.

Majerus, M. E. N. 1998. Melanism: evolution in action.Oxford University Press, New York, 338p.

Richardson, M. K., J. Hanken, M. L. Gooneratne, C.Pieau, A. Raymond, L. Selwood, and G.M. Wright.1997. There is no highly conserved embryonic stage inthe vertebrates: implications for current theories of evo-lution and development. Anatomy and Embryology196:91–106.

Richardson, M. K., J. Hanken, L. Selwood, G. M.Wright, R. J. Richards, and C. Pieau. 1998. Haeckel,embryos, and evolution. Science 280:983–984.

Shipman, P. 1998. Taking wing: Archaeopteryx and theevolution of bird flight. Simon and Schuster, New York,336p.



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