Agr. Orgen Maiz (Hart 1999)

Journal of Archaeological Method and Theory, Vol. 6, No. 2, 1999

Maize Agriculture Evolution in the EasternWoodlands of North America: A DarwinianPerspectiveJohn P. Hart1

David Rindos' coevolution theory remains the most comprehensive applicationof Darwinian theory to issues of prehistoric agriculture evolution. While his the-ory has drawn attention, there has been a lack of subsequent development of theapplication of Darwinian theory to prehistoric agricultural evolution. Combin-ing Sewall Wright's shifting balance theory of evolution with aspects of Rindos'coevolution theory provides important new insights into the processes of croptransmission between regions. Using these theories, a model is developed for theadoption and subsequent evolution of maize agriculture in the Eastern Woodlandsof North America.

INTRODUCTION

Fifteen years ago, David Rindos (1984; see also Rindos, 1980) published whatremains the most comprehensive application of Darwinian theory to issues of pre-historic agricultural evolution. Rindos was interested primarily in coevolutionaryrelationships between plant and human populations, which in some cases resultedin agriculture: "an integrated set of animal behaviors that affect the environmentinhabited by domesticated plants throughout the whole life cycle of those plants"(Rindos, 1984, p. 256). He was less interested in the transmission of crops be-tween human populations in different regions. While Rindos addressed this topicin several later publications (Rindos, 1989; Rindos and Johannessen, 1991), he did

1 Anthropological Survey, New York State Museum, 3122 Cultural Education Center, Albany, New York12230. E-mail: [email protected].

KEY WORDS: maize agriculture; evolution; shifting balance theory; Eastern Woodlands.

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l072-5369/99/0600-0137$16.00 1999 Plenum Publishing Corporation

not provide the level of theoretical development in those articles that he did in hisearlier, general work. In this article, I extend the application of Darwinian theoryto the issue of crop transmission between regions by integrating aspects of Rindos'coevolutionary theory with Sewall Wright's (1932, 1978a) shifting balance theoryof evolution and its fitness landscape metaphor. I do this by developing a modelof the adoption and subsequent evolution of maize (Zea mays) agriculture in theEastern Woodlands of North America.

As demonstrated in this article and by Rindos, evolutionary biology can serveas a source of robust theory for the investigation of prehistoric agricultural evo-lution. Not to take advantage of Darwinian theory's powerful explanatory frame-work for organic evolution can be done only to the detriment of archaeologicalexplanations.

THE NATURAL STATE MODEL

Despite major changes in archeological method and theory during the sec-ond half of the twentieth century, archeological metaphysics have not changed(Meltzer, 1979); they remain essentialistic (Dunnell, 1986; Lyman et al., 1997;O'Brien and Holland, 1995). Consistent with other assessments of essentialism inWestern thought (e.g., Hull, 1965; Mayr, 1982), the essentialist metaphysic pro-vides continuity to twentieth-century archaeology. One manner that essentialismis expressed in archaeology is the natural state model as defined by Sober (1994).Under the natural state model, all members of a type or kind should reflect thenatural state, or essence, of the kind regardless of spatial and temporal location.Variation between members of a kind results from interfering forces "frustratingtheir natural tendency" (Sober, 1994, p. 168). Explanations for the departure ofeach individual from the natural state are sought in established lists of interferingforces or through the identification of new interfering forces.

Change under the natural state model occurs when one natural state is replacedby another and, as a result, can only be saltational (cf. Mayr, 1988, p. 172; O'Brienand Holland, 1990, p. 37). Change can occur in members of a kind as they striveagainst interfering forces to reach their natural state; the natural state of a kindis unchanging, but members of the kind change to realize their natural state (seePopper, 1957, p. 33). Variation within a kind does not play a role in change becauseit is simply the result of interfering forces that do not affect the natural state.

The natural state model is particularly evident in archaeological interpreta-tions of the evolution of prehistoric agriculture. Interpretations of the adoption andevolution of maize agriculture in the Eastern Woodlands of North America pro-vide a good illustration of the effects of the natural state model on archaeologicalinterpretation.

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INTERPRETATIONS OF EASTERN WOODLANDS MAIZEAGRICULTURE EVOLUTION

A great deal of attention has been given to the adoption and evolution of maizeagriculture in the Eastern Woodlands during the twentieth century because of itscommonsense association with "cultural complexity." Maize agriculture has beenconsidered an important factor in the evolution of complex culture-historic taxasuch as Hopewell and Mississippian2 (Hall, 1980). While no one has explicitlydefined a natural state for maize agriculture in the Eastern Woodlands, it is clear thatthe natural state model has influenced and continues to influence interpretationsof maize agriculture's adoption and evolution. Based on described assumptionsabout the effects of maize agriculture's adoption, its natural state can be definedas effective and highly productive. In other words, it constitutes a focal economysensu Cleland (1966) rather than a set of human behaviors sensu Rindos (1984).3In some interpretations, it was the adoption of maize agriculture in its natural statethat allowed complex culture-historic taxa to arise or, alternatively, resulted in theirdemise. In other interpretations, it was interfering forces pushing maize agriculturefrom its natural state that led to the decline of complex taxa and the ameliorationof those forces that allowed new complex taxa to arise.

For example, early on, the adoption of maize agriculture in its natural statewas viewed as a necessary condition for the origin of Hopewell (e.g., Ford andWilley, 1941; Griffin, 1943, 1946, 1952; Hyde, 1962; Morgan, 1952, Prufer, 1965;Spaulding, 1952; Willey, 1966; Willey and Phillips, 1958; Wray, 1952; but seeCaldwell, 1958). Maize agriculture was thought to have been immediately effec-tive and highly productive, which allowed the emergence of Hopewell, with itsinferred sociopolitical complexity. The end of the Hopewell taxon, the so-called"Hopewellian decline," was thought by some to have resulted from a climaticdeterioration. This was an interfering force that pushed maize agriculture fromits natural state, decreasing or preventing maize production (e.g., Griffin, 1960;Struever and Vickery, 1973; Vickery, 1970). Also reflecting the natural state model,others saw the adoption of maize in its natural state as the cause of the "Hopewelliandecline" by resulting in a focal economy that destabilized regional trade networksand other intergroup contacts (e.g., Brain, 1976; Cleland, 1966; Dragoo, 1976;Ford, 1974). To some, maize agriculture was able to reach its natural state at thistime by the removal of interfering forces with the amelioration of climate and theadoption of better-adapted maize (e.g., Stothers and Yarnell, 1977).

During the 1980s, accelerator mass spectrometry (AMS) dating and stable car-bon isotope analysis (SCIA) of human bone collagen redefined how the relationship2Presently Hopewell is dated to ca. 2000-1500 B.P. (Yerkes, 1988) and Mississippian to ca. 950-350 B.P. (Rogers, 1995).

3I thank Robert Dunnell for pointing out this distinction.

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between Hopewell and maize agriculture was envisioned. AMS dating originallycast doubt on the presence of maize agriculture during Hopewellian times (Conardet al., 1984), but later results confirmed its presence at some sites (Chapmanand Crites, 1987; Ford, 1987; Riley et al., 1994). SCIA studies, on the otherhand, suggested a lack of maize in Hopewellian diet (Bender et al., 1981; van derMerwe and Vogel, 1978). These results changed the questions asked about therelationship between maize agriculture and Hopewell to, "Why, if maize agricul-ture was known, did it not play an important role in Hopewellian subsistence?"Reflecting the influence of the natural state model, answers to this question con-sisted of the identification of interfering forces such as human ignorance of maizeagriculture's potential (Ford, 1985; King, 1987) and environmental constraints(Watson and Cowan, 1992). Increasingly, elite control of production and ceremo-nial use of maize have been cited as interfering forces (e.g., Fritz, 1993; Hastorf andJohannessen, 1994; Scarry, 1993; Wymer, 1994).

The next complex culture-historic taxon, Mississippian, has also been asso-ciated with maize agriculture in its natural state (e.g., Deuel, 1935; Griffin, 1946;Krieger, 1948; McKern, 1939; cf. Smith, 1994). Like Hopewell, maize agriculturewas thought necessary to support the inferred Mississippian sociopolitical com-plexity. The removal of interfering forces through the amelioration of climate (e.g.,Griffin, 1960), diffusion of a better-adapted variety of maize (e.g., Galinat, 1965,1968; Galinat and Campbell, 1967; Galinat and Gunnerson, 1963), or develop-ment of new agricultural management techniques (e.g., Fowler, 1969; Willey andPhillips, 1958) allowed maize agriculture to reach its natural state.

During the last three decades, the introduction or evolution of better-adaptedvarieties of maize has been perhaps the most frequently cited mechanism forovercoming interfering forces, primarily unfavorable climatic conditions, therebyallowing Mississippian maize agriculture to reach its natural state (e.g., Dragoo,1976; Fiedel, 1987; Galinat, 1985, 1988; Gibbon, 1972; Gonzalez, 1994; Griffin,1990; Keegan and Butler, 1987; Lathrap, 1987; Muller, 1978;Rindos, 1989; Rindosand Johannessen, 1991; Schroedl and Boyd, 1987; Schroedl et al., 1990; Smith,1989; Stoltman, 1978; Watson and Cowan, 1992; Yarnell, 1993; but see Ford,1981; Fritz, 1990, 1992, 1993). The introduction of beans (Phaseolus vulgaris)has been viewed as a means by which maize agriculture overcame nutritionaldisadvantages and therefore reached its natural state (e.g., Hammett, 1997; Muller,1978; Stoltman, 1978; Stoltman and Baerreis, 1983). Others continued to arguethat the development of specialized agricultural management techniques allowedmaize agriculture to reach its natural state (e.g., Gibbon, 1972; Riley, 1987) or thatit was simply the adoption of maize agriculture in its natural state that permittedthe Mississippian emergence (e.g., Munson, 1988).

The natural state model, then, has provided an underlying metaphysical conti-nuity to archaeological interpretations of maize agriculture adoption and evolutionduring the twentieth century. While theory has changed, interpretations have been

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based on the implicit assumption that maize agriculture has a natural state andthat interfering forces have prevented it from reaching that natural state at varioustimes.

POPULATION THINKING

One problem with the influence of the natural state model on interpretationsof maize agriculture adoption and evolution in the Eastern Woodlands is that pop-ulation genetics has discredited the idea that species have natural states (Sober,1984, 1994). According to Sober (1994, p. 181), ''The science which describesthe laws governing the historical origins of variation within a speciespopulationgeneticsmakes no appeal to such 'natural tendencies.' Rather, this frame in-variant 'natural tendency'this property that an organism is supposed to haveregardless of what environment it might be inhas been replaced by a frame rel-ative propertynamely, the phenotype that a genotype will produce in a givenenvironment." Maize, like all species, is composed of populations of individualswith unique genotypes and phenotypes. Genotypes determine phenotypes withinthe contexts of the environments in which individuals exist (Briggs and Walters,1997, pp. 114-123). What connect individuals of a species together is not a naturalstate but descent from common ancestors and the ability of the genetic code tocombine from two unique individuals in sexually reproducing species and produceviable offspring.

By the same token, the human species consists of populations of unique indi-viduals, each of whom has a unique genotype and phenotype. As with all species,the genotype determines the phenotype within the context of a given environment.However, the phenotype also consists of an extremely rich array of non-geneticallydetermined learned behaviors that vary considerably from individual to individualboth within and between populations. Commonalties of this learned behavior arewhat define ethnographic study units generally referred to as cultures. As in allother species, human populations do not have natural states.

Because maize agriculture is formed on the basis of relationships betweenmaize and human populations, it also does not have a natural state. FollowingSober (1994), variation in maize agriculture is a frame-relative property determinedby the traits that are produced as a result of human and plant interactions ina given setting, as well as by the traits of each of those populations. It is thepopulation thinking metaphysic that allows this conceptualization. According toMayr (1982, 1988), who coined the term, population thinking is diametricallyopposed to essentialism. Like the natural state model, population thinking viewsvariation as "real." However, rather than looking at variation between individualsas effects of interfering forces that require explanation, individual variation is astate of higher level organization, the population. Variation at any given time is


contingent on previous states of variation within the population and the forces atwork on that variation. Under the natural state model, the essence is natural andvariation is unnatural. Population thinking, however, "accepts variation as a factin its own right, capable of playing a causal role as well as being something thatcan itself be explained" (Sober, 1984, p. 156; also see Schiffer, 1996).

DARWINIAN EVOLUTION

The introduction of population thinking was a critical component of Darwin'stheories of evolution (Mayr, 1988). The basic principles of Darwinian evolutionthrough natural selection are very simple. As related by Lewontin (1970, p. 1),(1) individuals within a population have different phenotypes as expressed bymorphology, physiology, and/or behaviors; (2) different phenotypes have differ-ent survival and reproductive rates or different fitnesses in different environments;and (3) this fitness is heritable. As related by Lloyd (1988, p. 11), "It specifiesno mechanism for inheritance, no explanation for the differential contribution tofuture generations of different phenotypes, and no particular level of entity thatevolves." This is important because we are interested in the evolution of bothgenetically determined (maize) and learned and genetically determined (human)systems of inheritance. Darwinian theory is a powerful explanatory system thatmay be applied either to system working independently or to both systems work-ing dependently (Rindos, 1980, 1984). Because behavior is part of the humanphenotype, separate explanatory systems are not needed for these different sys-tems. The fact that most human behavior is learned rather than genetic, is largelyirrelevant. Since Dunnell's (e.g., 1978, 1980, 1982) seminal publications, a largebody of literature has attempted to apply Darwinian theory to archaeology and, byextension, prehistoric human behavioral evolution (e.g., Barton and Clark, 1997;Lyman and O'Brien 1998; Maschner, 1996; O'Brien and Holland, 1990, 1992;Teltser, 1995). This literature presents cogent arguments for this application thatneed not be repeated here in detail. Rather, definitions and explanations of termsare offered when necessary.

Proximate and Ultimate Causation

There are two broad categories of causation, proximate and ultimate, rec-ognized under Darwinian theory (Mayr, 1982; Rindos, 1984). Proximate causesare functional and are the subject of "how" questions, while ultimate causes areevolutionary and are the subject of "why" questions (Dunnell, 1982, pp. 9-12;Mayr, 1982, pp. 67-69). Proximate causation is concerned with immediate or on-tological responses of organisms to their environment, while ultimate causationis concerned with phylogenetic history (Sober, 1993, p. 6). Mayr (1982, p. 68)

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provides the following example in biology: "The proximate causation of sexualdimorphism might be hormonal or some genetic growth factors, while sexual se-lection or a selective advantage of differential utilization of the food niche mightbe the ultimate causation."

Proximate causation studies are represented in archaeology in part by op-timization models based on evolutionary ecology or microeconomic theory. AsBettinger and Richerson (1996, pp. 223-225) remind us, these studies are part ofa successful Darwinian research program (but cf. Boone and Smith 1998; Lymanand O'Brien, 1998; cf. O'Brien and Wilson, 1988; Pryor 1986, 1988). There havebeen numerous optimization studies that deal in part with the adoption and/or in-tensification of maize agriculture in the Eastern Woodlands (e.g., Gardner, 1992;Gremillion, 1996; Hart, 1990; Keegan and Butler, 1987; Winterhalder and Goland,1997). The question asked is, How do human populations optimize their diets givenavailable resources?

In a microeconomic-based model, maize will enter the diet of a populationwhen its marginal cost, the input necessary to increase output by one unit, fallswithin an acceptable range (e.g., Earle, 1980). Maize production may be intensifiedas its marginal cost is lowered relative to that of other resources as the result,for example, of innovative technology, changes in maize productivity, or somecombination thereof. Alternatively, maize production may also be intensified if itsmarginal cost of production does not decrease but the acceptable marginal cost levelincreases. These models can be classified as populationist because they generallyrely on the decisions and activities of individuals or small groups (but see Lymanand O'Brien, 1998). However, they do not address the issues of the processes ofmaize agricultural evolution because they rely on proximate causes. They can veryeasily fall into the trap of the natural state model by calling, for example, on thenatural expandability of maize agriculture, the beneficial physiology of the maizecob, or the appearance of a more productive variety of maize (e.g., Keegan andButler, 1987). It is only by reference to ultimate causation that the natural statemodel can be avoided.

Selection and Domesticated Taxa

Charles Darwin directly and purposefully modeled his theory of evolutionthrough natural selection on the work of animal and plant breeders (Browne,1995; Dennett, 1995; Desmond and Moore, 1992). He spent considerable timeinterviewing and sending questionnaires to animal breeders and compiling theiranswers for data on the effects of selection on domestic animals. He became anexpert domestic pigeon breeder to achieve better understanding of the methods ofanimal breeders and of selection. ''The notion of selection in 'natural selection'is, in fact, modeled on the plant and animal breeder's 'pick of the litter,' workwith which Darwin was both practically and theoretically acquainted" (Depew


and Weber, 1995, p. 5). Darwin (1859) ultimately devoted the first chapter of Onthe Origin of Species to "Variation Under Domestication." His continued interestin the evolution of domesticated taxa as illustrative of the mechanism of selectionis evident in his devotion of an entire two volume set (Darwin, 1875) and portionsof other volumes (e.g., Darwin, 1876) to developing this same topic. Sober (1984,p. 19) suggests that Darwin believed that "artificial" selection was an inadvertentexperiment by humans on the theory of natural selection. "Artificial selection isnot selection that takes place outside nature, but selection that occurs within aparticular niche found in nature" (Sober, 1984, p. 19; also see Schiffer, 1996).

Following the pattern started by Darwin, many of the evolutionary theoriesand models developed by early twentieth-century theoretical population geneticistsincluding Fisher, Haldane, and Wright were based on knowledge of and practicalexperience with domesticated plant and animal breeding (Provine, 1986). In fact,Sewall Wright, now recognized as one of the most influential twentieth-centuryevolutionary theorists (Provine, 1986, p. 233), also achieved high status as ananimal breeding theorist (Provine, 1986; Wright, 1978b). As related by Provine(1986, p. 142), "From very early in his career, Wright saw evolution in natureas deeply related to what he knew of evolution in domestic populations." One ofWright's most important contributions to evolutionary theory was his shifting bal-ance theory of evolution and its associated metaphor of fitness landscapes (Provine,1986).

Fitness Landscapes

Wright (1932) introduced the fitness landscape metaphor to provide a qual-itative means of presenting his complex mathematical theories of evolution (e.g.,Wright, 1931). It is based on the fundamental Darwinian principle that certaingenotypes of a species are more fit than others in a given environment. In pop-ulations of sexually reproducing species, there is an almost-infinite number ofpotential genotypes. Most of these never arise, but each can be assigned a fitnessvalue under a given set of environmental conditions. Those genotypes that aremore fit will have higher values than those that are less fit. In the very simplestrepresentation of a fitness landscape, each gene has just two alleles, and there aretwo dimensions. The intersection points of the x and y axes represent possiblegene combinations, each with a fitness value. Contour lines are drawn around thefitness values of the genotypes, creating the topographic map of a rugged fitnesslandscape with numerous peaks and valleys (Fig. 1). The location of a populationon the landscape at any time is determined by the genetic history of that populationand the prevailing conditions. Selection will always push a population up the near-est peak, whether or not that peak is the highest one on the landscape: a portrayalof selection as an algorithm (Dennett, 1995, pp. 50-51). In current terminology,lower peaks are local optima, while the highest peak is the global optimum. The

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Fig. 1. Simple fitness landscape (from Wright, 1932, p. 3S8).

most fit genotypes will come to occupy the peak, while less fit genotypes willoccupy the fitness space around that peak. The spread of the population aroundthe peak is dependent, at least in pan, on the strength of stabilizing selection. ToWright (1932, p. 163), the key problem in evolutionary biology was how a popu-lation occupying a fitness peak can move across valleys to ascend a higher peak,given that selection will always act against downward movement into the valleys.

In an oft-reprinted figure, Wright (1932, p. 361) illustrated various kinds ofevolutionary change on a simple fitness landscape (Fig. 2). In each of the illustra-tions, the heavy dashed line represents the original position of a large panmictic(freely interbreeding) species that has lived in constant conditions long enoughfor each gene to have reached equilibrium. According to Wright (1932, p. 360),under these conditions further evolution will occur only when there is a favorablegenetic mutation that changes the landscape, increasing the elevation of the peakon which the species resides. Selection will then push the population to the top ofthe higher elevation. This is likely to happen only rarely.

In Fig. 2A, in contrast, stabilizing selection has moderated, allowing thespecies to spread out more widely on the slopes of the peak. This can be facilitatedby an increase in the general mutation rate. The species' spread may eventuallybe broad enough so that it comes under the influence of another higher peak. InFig. 2B, stabilizing selection has become more stringent and/or there has beena decrease in the general mutation rate. Because the amount of variation in thespecies decreases through these processes, it now occupies a much smaller space

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on the fitness peak. This process diminishes the possibility that the species will beable to capture another higher fitness peak (Wright, 1932, pp. 361-362). Figure 2Cillustrates the effects of changes in environmental (biotic and/or abiotic) conditions.Wright (1932, p. 362) suggests that a species under severe selection, as in Fig. 2B,is likely to get trapped in a valley or pit and become extinct because it lacksthe genetic variability necessary to climb out of the low-fitness area, explore thefitness landscape, and ascend a new peak. Alternatively, a species that has beenunder moderate selection, as in Fig. 2A, will "merely be kept continually in motion"through genetic drift (Wright, 1932, p. 362; cf. Van Valen, 1973). Such a speciesis likely to come under the influence of a nearby peak, and selection will push itup that peak if conditions remain constant.

Figure 2D illustrates a situation in which the species' size has decreasedsubstantially. Specific alleles become fixed at almost every locus, irrespective ofselection, through random genetic drift. The species moves down the slope of thepeak to a valley and extinction as a result of the deleterious genetic effects ofextreme interbreeding. In Fig. 2E, by contrast, with an intermediate species size,slight inbreeding, and random genetic drift, the species moves away from the peakand wanders across the landscape in the vicinity of the peak. Over time, the speciesmay come under the influence of another peak. This is most likely when thepeak that the species originally occupies is low. Over a very long period of time,the species may ascend the highest peak in the vicinity (Wright, 1932, p. 362).Figure 2F illustrates Wright's (1932, p. 363) shifting balance theory of evolution,

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Fig. 2. Examples of evolutionary change on a simple fitness landscape (fromWright, 1932, p. 361) in which N is the population size; n, the deme size; U, themutation rate per generation; 5, the selection coefficient; and m, the populationexchange between demes.

which I discuss in detail below. Although Wright originally conceived of fitnesslandscapes to illustrate genotypic evolution, the applicability of the concept to bothgenotypes and phenotypes is widely recognized in current evolutionary biology(e.g., Kauffman, 1993; Niklas, 1997; Whitlock, 1997; see also Simpson, 1944).

As discussed by Wright (1932), the fitness landscapes illustrated in Figs. 1and 2 are overly simplistic; in actuality, fitness landscapes are n-dimensional (seeKauffman, 1993; Niklas, 1997). The number of dimensions is equal to the numberof genes or phenotypic traits being modeled, plus an added dimension for fitness(Wright, 1932, pp. 356-357). The ruggedness of the fitness landscape (the numberof peaks and their height) is determined by epistasis (Kauffman, 1993; Whitlock,1997; Whitlock et al., 1995), which occurs when an allele at one locus affectsthe expression of an allele at another locus. There can also be epistasis of pheno-typic traits, thereby determining the ruggedness of phenotypic adaptive landscapes(Kauffman, 1993; Niklas, 1997; Whitlock et al., 1995). The fitness landscape con-cept, then, is deceptively simple. While the two-dimensional representation is eas-ily visualized, the underlying concepts of the representation are highly complex.

The Shifting Balance Theory of Evolution

Wright's shifting balance theory (SBT) consists of three phases in populationsthat are divided into many partially isolated demes. In Phase I, demes move acrossfitness landscapes through random genetic drift, exhibiting temporary decreases infitness as they cross valleys (Fig. 2F). This is the stochastic phase of the process. InPhase II, one or more demes come under the influence of a new adaptive peak(s).The selection algorithm pushes those demes to the top of the new peak(s). InPhase III of SBT, demes that ascend higher fitness peaks exhibit an increase in pop-ulation size and send out more migrants than less fit demes. These migrants matewith members of less fit demes and pass on the new favorable gene combinations,eventually resulting in the less fit deme ascending the higher peak. This processincreases the fitness of the entire population. Individuals from the more fit demesmay also migrate to and inhabit localities where less fit demes have become extinct.

SBT is based on an assumption of demes occupying different positions onrugged fitness landscapes. Recent mathematical modeling (e.g., Kauffman, 1993;Whitlock et al., 1995) suggests that rugged landscapes are common. It is notat all clear, however, whether the population structure envisioned by Wright iscommon (Coyne et al., 1997). But, as discussed below, this population structurewas probably common for prehistoric maize agriculture in the Eastern Woodlands(also see Hall, 1980, p. 427). The movement of demes from one fitness peak to theslopes of another through random genetic drift, although theoretically possible, ismore problematic (Coyne et al., 1997; Whitlock, 1995). Movement across valleysrequires a temporary decrease in demic fitness. The probability of this happening isexceedingly small in natural populations (Whitlock, 1995). As discussed by Lewin


(1986), descent from a fitness peak is most likely to occur if it is rapid becauseslow drift off of the peak will be counteracted by selection.

There are, however, mechanisms such as randomly generated increases inphenotypic variance (Whitlock, 1995) that result in the temporary flattening offitness landscapes making such transitions more probable. Other mechanisms, suchas environmental change (Whitlock, 1997; Wright, 1932), random fluctuationsin selection, and mutation (Coyne et al., 1997, p. 650), can also result in peakshifts. Additionally, it is likely that ridges connect many fitness peaks (Whitlock,1997). This means that deterministic processes such as mass selection can result inpeak shifts, eliminating the need for stochastic processes such as drift. As arguedby Coyne et al. (1997), the multifaceted nature of evolution almost ensures thatphases I and II of SBT as presented by Wright occur in nature, but it is not clearthat they are common phenomena. However, as I argue below, stochastic processesprobably played important roles during SBT phases I and II in the initial diffusionand subsequent evolution of maize to the Eastern Woodlands.

Likewise, Phase III is a much more complicated process than originally pro-posed by Wright. Recent modeling has called into question several of Wright'sprinciple assumptions about consistency of population structure and the effectsof recombination, as well as migration rates and directions (Coyne et al., 1997;Gavrilets, 1996). Gavrilets (1996, p. 1041) concludes that Phase III is most readilyaccomplished when it is initiated by peripheral demes with few neighboring demes(see also Crow et al., 1990; Garcia-Ramos and Kirkpatrick, 1997; cf. Mayr, 1963).

Consequences of Being a Plant

A primary consequence of being a plant is a sessile existence (Bradshaw,1972; Levin, 1988; Linhart and Grant, 1996). This makes plants highly sensitive totheir immediate physical environments, so much so that there can be considerablegenetic variation in sexually reproducing species over as little as a few meters.In many natural populations, pollen and seeds travel only short distances. "As aresult, species are broken up into vast arrays of small populations which are so smallin diameter tha[t] they can match the size of extremely localized variations of theenvironment, variation occurring even over a distance of a few metres" (Bradshaw,1972, p. 28). Therefore, even a continuously distributed species cannot be thoughtof as panmictic (e.g., Billington et al., 1988). This population structure matches atleast some of the expectations for SBT (Bradshaw, 1972; Levin, 1988).

According to Linhart and Grant (1996, pp. 241-242), there is little questionthat genetic heterogeneity within plant species is correlated directly with environ-mental heterogeneity, the direct result of selection operating on very fine scales.Variation in environmental features such as elevation, exposure, and moisture act asbarriers to gene flow in plant populations and enhance genetic isolation betweensemi-isolated and isolated populations. "These two forces work synergistically,

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producing genetic heterogeneity in natural populations; this is one of the strongestgeneralizations currently known about differentiation at either large or small tem-poral and spatial scales in plants" (Linhart and Grant, 1996, p. 242).

Even in cases where there is a strong homogenizing force from gene flow,small-scale genetic differentiation persists for some traits. Random forces likegenetic drift can result in genetic differences between demes, providing the op-portunity for demes to come under the influence of new fitness peaks. Linhartand Grant (1996, p. 242) do not believe that these factors account for correlationsbetween environmental and genetic heterogeneity; they suggest that selection isthe primary force. However, as argued by Wright (1932, 1986), random processesbring demes within range of new fitness peaks.

Mating experiments with plants from different environments result in out-breeding depression, where offspring are not as well adapted as the parent gen-erations to a specific environment (Linhart and Grant, 1996). As a result manyplant populations are likely to have population structures necessary for phases Iand II of SBT. Important for the introduction and subsequent evolution of maize inthe Eastern Woodlands is the sensitivity of plants to their immediate environment,resulting in this population structure.

MAIZE EVOLUTION IN THE EASTERN WOODLANDS

A number of authors have postulated that the entire suite of maize vari-eties encountered in the Eastern Woodlands before major changes wrought byEuroamerican farmers could be derived from a single introduction from the South-west (e.g., Ford, 1985; Jones, 1968; King, 1987). While this is possibly true, itseems very unlikely that a single introduction can account for the perpetuation ofmaize as a crop in the Eastern Woodlands. At the time of its introduction(s) into theEastern Woodlands, maize had been an agricultural domesticate for millennia; thatis, it was dependent on humans throughout its life cycle for successful reproduc-tion and perpetuation of local populations (Rindos, 1984). Like all plants, maize issensitive to its immediate environment. Maize populations from the source areaswere adapted to their locally evolved agroecologies. Maize populations occupiedfitness peaks of varying heights, with each peak representing a local optimum as-sociated with a specific agroecology. In some cases, the evolution of agroecologiesin the source areas diminished the microenvironmental variation encountered bylocal maize populations and resulted in fairly consistent stabilizing selection oncethe populations ascended fitness peaks.

The movement of a relatively small amount of maize from an establishedagroecology over long distances into a new environment is equivalent to an evo-lutionary bottleneck or a founder event (King, 1987; Mayr, 1963). Because only asmall portion of the population is represented after one of these events, sampling er-ror will result in, among other things, (1) changed gene frequencies, (2) breakdown


of coadapted gene complexes, and sometimes (3) increased additive genetic vari-ability (Cheverud and Routman, 1996).

It can be assumed that only a small quantity of maize was involved in anyintroductory event into the Eastern Woodlands. This maize was sown in a newmacroenvironment, which may or may not have been significantly different fromthat of its parent population (cf. Fritz, 1992; Gremillion, 1996). It was also sownin a different microenvironment in the form of a previously evolved agroecologyif it was introduced to agriculturists, or by the lack of an established agroecologyif introduced to nonagriculturists. This is true whether or not the traits of theagroecology from its place of origin were transmitted and carefully recreated bythe adopting population. Soil conditions, moisture regimes, competing vegetation,predation, and other factors affecting germination, vegetative development andmaturation, reproductive organ development, and kernel maturation were differentfrom those experienced by the parent population.

The results of each founder event varied based on the genetic composition ofthe maize, environmental conditions at various scales at the point of introduction,and population size. Each event resulted in a new fitness landscape. The placeon that landscape the founder population occupied was largely a result of chancein the sample of genetic variation and of environmental conditions at the placeof introduction (cf. Mayr, 1963); survival of the new maize population dependedon genetic, environmental, and human behavioral variables and population struc-ture. A number of factors acted against the establishment of a perpetuating maizepopulation from any single introduction to the Eastern Woodlands.

Initial Introductions

Several sampling events occurred with each introduction of maize to theEastern Woodlands. First, the maize kernels represented a sample of the geneticvariation of a parent population that may have been either genetically homogeneousor heterogeneous depending on the genetic and selective history of that population.If from a population under severe stabilizing selection, and therefore geneticallyhomogeneous, then genetic variation in the founder population was low. If from apopulation under weak to moderate stabilizing selection, and therefore geneticallyheterogeneous, then genetic variation in the founder population was less than in theparent population. Second, selection of a sample from the introduced kernels forsowing may have further diminished genetic variation. The percentage of kernelssown from an introductory event depended on the number of kernels availableand the desired number of plants. If only a fraction of the available kernels wasplanted, then additional genetic variation was lost in that initial and any subsequentsowings from the original seed stock. Third, the selection of kernels from the firstgeneration of plants for sowing in the subsequent year may have further diminished

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the genetic variation of the founder population. The number of plants producinggrain determined the population from which the seed for the next year's crop wasselected. It is likely that not all of the kernels planted in the first year germinated.Of those that did, not all would have reached vegetative and sexual maturity,and not all kernels produced would have been viable, further diminishing geneticvariation. If the available kernels were not selected randomly but rather fromone or a few cobs, then the genetic variation of the subsequent generation wasdiminished. Each of these samples determined the amount of genetic variation inthe founder population, resulting in a more genetically homogeneous populationthan the parent population. The sampling events determined, in part, and modifiedthe fitness landscape of the founder population and determined where on a newlandscape it was located.

It is unlikely that all of the management techniques used in creating andmaintaining the agroecology of the donating human population were or could betransferred to the adopting human population (see Linton, 1924; Witthoft, 1949).Some adopting human populations probably sowed the maize in an existing agroe-cology that evolved with indigenous domesticates. Other adopting human popu-lations, whether agriculturists or not, probably attempted to recreate managementtechniques described by the donating human population. Various environmentalconditions prevented that recreation regardless of the efforts of the adopting humanpopulation. As a result, founder populations were not sown in the agroecologicalconditions in which the parent population evolved.

The founder population was subject to selective pressures not encounteredby its parent population, including macroclimatic variables, microenvironmentalvariables of the new agroecology, and in some cases interspecific competition forhuman attention from established crops. Macroclimatic variables and the bioticand abiotic variables of the new agroecology determined successful germination,vegetative growth and maturation, flowering, pollination, and kernel maturationand viability. Human behavior determined the size of the population and the sam-ple of kernels sown. The adopting human population must have been willing andknowledgeably able to provide the care necessary for the new maize population tobecome established (Rindos and Johannessen, 1991). Indifferent behavior towardthe founder maize population decreased its chances of even short-term perpetua-tion. Such indifference resulted from the marginal cost of managing the new croprelative to older, established crops and/or wild foods.

Factors that overrode marginal cost considerations, for example, adoption forceremonial and/or religious reasons, may have encouraged some adopting humanpopulations to effectively manage the new maize population. Alternatively, insome instances the marginal cost of maize production was probably low enoughrelative to the accepted level of marginal cost for the adopting human populationto expend the necessary time and energy to ensure the successful perpetuation of asmall maize population. Despite efforts made by the adopting human population,


stochastic events including late or early frosts, floods, drought, and predation bypests resulted in destruction of the founder maize population or prevented itskernels from maturing. Without a source of additional seed, establishment of thefounder population was precluded.

If the founder population became established without gene flow from an-other population, and the population size was kept small by the adopting humanpopulation because of marginal cost or other factors, then it was subject to in-breeding depression. Maize is particularly susceptible to decreases in yield as aresult of inbreeding depression over relatively short periods of time (Hallauer andMiranda, 1988; Jugenheimer, 1976; Mangelsdorf, 1974). In the absence of geneflow, it is likely that newly established small maize populations as envisionedin the garden crop scenario of Ford (1981, 1985) and others (e.g., Fowler 1969;Fritz, 1993; Gardner, 1992; Gremillion, 1996; Smith, 1992a) became extinct as aresult of inbreeding depression. If, as suggested by Hall (1980) and others (e.g.,Criddlebaugh, 1985), most maize was consumed in its green state and only a fewplants were left to mature as the season allowed, genetic variation would declinerapidly and extinction would occur because of inbreeding depression. As the yielddecreased and the marginal cost of maize production increased, the adopting hu-man population would have abandoned it as a crop. Even if maize was adoptedfor religious or ceremonial reasons, in the absence of gene flow from other pop-ulations, extended inbreeding resulted in extinction of small populations after arelatively short period of time. Small founder populations may also rapidly ac-cumulate slightly deleterious mutations that can also result in extinction (Niklas,1997, p. 95).

Given the obstacles to the establishment of founder populations, it is veryunlikely that a single founder event was responsible for the adoption of maizein the Eastern Woodlands. There were probably many founder events in manylocations in the East, the majority of which likely ended over varying lengths oftime in extinction.

Establishment

For a maize founder event to have been successful, it was necessary forthe founder population to be divided into many, partially isolated demes (as inSBT) over a short period of time by (1) dispersion within an already dispersed(Fuller, 1981; Smith, 1992b, p. 240) adopting human population; (2) dispersionamong interacting human populations; or (3) multiple introductions by a donatingpopulation or populations, into interacting adopting populations. These events werenot mutually exclusive and overcame the size limitations imposed on an undividedand isolated founder population. The first two events occurred any time after theinitial adoption but probably before severe inbreeding depression occurred. For

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the third, introductory events and subsequent gene flow had to take place overrelatively short periods of time to prevent inbreeding depression and extinction ofthe original founder populations, assuming that they were not subsequently splitinto partially isolated demes.

In the first two events, each deme represented a sample of the genetic vari-ation of the original founder population. In the third event, there was potentiallygreater initial genetic variation because each introduction represented a differentsample of the parent population or because different introductions originated fromdistinct parent populations. Its genetic and physical environments determined eachdeme's position on the fitness landscape. Through selection, drift, mutation, and/orvariation in level and source of gene flow, the demes diverged from one another.Limited gene flow among demes, from exchange of seed stock and rare interdemicpollination, prevented severe inbreeding depression. This resulted in the mainte-nance of greater genetic variation than occurred within an undivided and isolatedfounder population.

Splitting the population into many demes located in varied settings also dimin-ished the risk of the population becoming extinct from stochastic events (Dimmick,1994, pp. 237-238; Johannessen et al., 1970, p. 396). Some demes became extinctbecause of localized stochastic environmental events, but because other demeswere not affected, seed stock was available to replace the extinct demes. This con-stituted an unintentional regional divided risk strategy (Hart, 1993). Some demesbecame extinct if deleterious alleles were fixed by drift and/or inbreeding in theabsence of sufficient gene flow. Other demes became extinct through indifferenthuman behavior. However, it is less likely that a founder population split into par-tially isolated demes became rapidly extinct than an undivided population isolatedfrom gene flow.

Each deme was under severe selective pressure from microenvironmental con-ditions and human management behavior of the local agroecology. Established ornewly created agroecologies into which maize was adopted probably varied con-siderably (Morrison, 1996, p. 585), resulting in different selective contexts. Demesascended the slopes of the nearest fitness peak if genetic variation permitted. Giventhat agroecologies of the adopting populations had not evolved with maize, anddeme size was kept small by human behavior, the peaks probably were mostly low.

Demes probably ascended fitness peaks fairly rapidly, resulting in at leastshort-term perpetuation. Given the lack of coevolved agroecologies, however,whenever a deme was relocated to a new field its position on the fitness land-scape probably changed (Fig. 2C). Relocated demes were likely to be positionedon the slopes of a peak rather than on a peak, which resulted in lowered fitness.If there was sufficient genetic variation, then selection pushed the deme up slope.If there was insufficient genetic variation, then the deme became extinct. Demesunder these conditions approximated the conditions of Van Valen's (1973) RedQueen theory, where the environment changes rapidly enough that a population


never gains on it. Selection pushing demes up slopes under these conditions sim-ply prevents them from becoming extinct (Sober, 1984, p. 174). Field movementswere probably frequent among some adopting human populations (Smith, 1992b,p. 243), taking place often enough to prevent some demes from completely as-cending fitness peaks.

The establishment of maize founder populations set the stage for diffusionof maize within the Eastern Woodlands. A founder population, or on a finer scale,its demes, was successful if its marginal costs of production were at an accept-able level, and as a result, the human population continued to sow its seed. Theprocesses described above undoubtedly occurred hundreds if not thousands oftimes as maize was introduced into and subsequently diffused across the East-ern Woodlands. The diffusion of maize from its points of introduction resultedin greater genetic variation through random genetic drift, mutation, different se-lective contexts, and recombination as populations from different parental stockscrossed. Populations were variously isolated and partially isolated from other pop-ulations depending on distance, interaction among adopting human populations,and physiological traits such as flowering time. Multiple points of introductionand subsequent diffusion decreased the chances of extinction of maize in the East-ern Woodlands (Johannessen et al., 1970), although the extinction of demes andpopulations was an ongoing process.

During the early phases of evolution in the Eastern Woodlands the relationshipbetween maize and adopting human populations was one of predator and prey.Maize had little or no effect on the reproductive success of the adopting humanpopulation. Human populations fed on maize because it was available as a result ofits low marginal cost of production relative to other resources. Selection forces weremostly of nonhuman origin (Johannessen et al., 1970, pp. 412-413; King, 1987,p. 17; Mangelsdorf, 1974, pp. 145, 208). Humans selected seed for sowing, butgiven a small deme size and limited gene flow from other demes and populations,the demes were probably fairly genetically homogeneous. As a result, exceptfor partially determining the immediate microenvironmental conditions of demes,intentional human activities probably had little effect on maize evolution (seeRindos, 1984, pp. 90-91).

Demes adopted into existing agroecologies competed for human attentionwith other incidental, specialized, and/or agricultural domesticates (Rindos, 1984)located in and outside of the agroecology. Competition among domesticates wasan important component in the early evolution of maize agriculture in the East-ern Woodlands (Rindos, 1980). The fitness landscapes of domesticates within anevolving agroecology were linked (Kauffman, 1993, p. 243): the movement of onedomesticate on its fitness landscape changed the landscape of the other domes-ticates in the agroecology if that move resulted in changed human managementbehavior. Maize ascending a higher fitness peak may ultimately have resulted inlowered fitness of other domesticates by deforming their fitness landscapes, al-though the reverse was also true.

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A maize population could increase in size and therefore fitness through theestablishment of new demes. The only way the size of a maize deme could increase,however, was for its interacting human population to sow additional seed. Thisoccurred if its marginal costs decreased relative to other resources if, for example,(1) more grain was produced per productive maize plant, and/or (2) more plantsreached sexual maturity, reproduced, and produced grain, and/or (3) management(including technological) innovations made maize production less costly. The firsttwo were the result of selection pushing a maize deme up a higher fitness peak aftersome stochastic event put it in reach of that peak or as it gradually ascended theoriginal peak to which it was attracted. The third probably happened only after oneof the others increased yield or by changes in management behaviors that affectedthe entire agroecology.

The Establishment and Perpetuation of Coevolutionary Relationships

As a maize deme became more productive it changed from a minor resourcewith little investment in production to a valued resource engendering greater inputsof labor if marginal costs were lowered enough relative to other resources or ifthe accepted level of marginal cost increased sufficiently. The deme entered intoa coevolutionary relationship with the human population as it became more suc-cessful in the agroecology. Increased human attention and any subsequent increasein management effort changed the fitness landscape of the deme. As a result, itmay have come under the influence of a new, perhaps higher, fitness peak andwas pushed up that peak by selection. After it ascended the new peak, stabilizingselection acted to maintain the deme on that peak. As in earlier phases of the rela-tionship, however, movement of fields resulted in more or less significant changesin a maize deme's fitness landscape. Increased human labor input shifted the fitnesslandscape and brought the maize deme under the influence of a higher fitness peak.If the maize deme ascended this peak, then yields increased, which resulted in thehuman population increasing the amount of grain sown and, thus, the demic fitnessbecause the marginal cost per unit of human input was lowered. These processesoccurred on a continuing basis throughout the Eastern Woodlands as maize popu-lations became established in specific locales, representing a continuation of SBTphases I and II.

The crossing of two previously isolated maize demes was an important mech-anism for generating new genetic variation on which selection could act (King,1987, p. 13). Many of the resulting progeny of such crosses were less well adaptedto prevailing conditions than either parental generation because of outbreedingdepression. Crossing of two previously isolated demes may have resulted in thecreation of new fitness landscapes and the ascension of new and potentially higherfitness peaks if new gene combinations were favorable within the context of thelocal agroecologies.


In some instances the crossing of previously isolated demes resulted in hetero-sis. This is a phenomenon where the hybrids have higher yields than either parentalpopulation (Hallauer and Miranda, 1988, p. 337; Jugenheimer, 1976, p. 55), result-ing in an immediate boon to the human population. In order for this higher yieldto continue, new crossed seed must have been produced annually because secondand subsequent generations of such crosses likely had marked decreases in yield(Jugenheimer, 1976, p. 87). While the phenomenon of heterosis by annually cross-ing inbred lines is an important aspect of modern commercial maize production(Hallauer and Miranda, 1988), it is unlikely to have been a component of prehistoricmaize evolution in the Eastern Woodlands.

Phase III of SBT may have resulted in other demesand perhaps the entirelocal populationascending the higher fitness peak. This happened if both traitsof the better-adapted maize deme and specifically associated human behaviors andartifacts were successfully transmitted. Continuation of the higher levels of pro-duction within the human and maize populations depended on the successful intra-and intergenerational transmission of the management techniques associated withhigher maize yields. It is likely that a human population or segment of a dispersedpopulation grew and consumed large quantities of maize when other populationsor segments of a dispersed population did not. However, because of a lack of bi-ological reproductive success and/or successful transmission of learned behavior,neither the maize traits nor the management practices spread to other maize demesand human populations, nor were they maintained within the deme or populationin which they evolved. An increased yield in one deme may have failed to spread toother demes for a variety of reasons, including failure of new behavioral traits to beadopted by other segments of dispersed populations. Selection can work only ontrait variants that are successfully transmitted. Some higher-yielding maize demeswere adapted to specific conditions of local agroecologies that were not presentor could not be replicated in other agroecologies. This would be especially true ofactivities specific to a particular microenvironmental setting. Failure of Phase IIIof SBT to occur eventually resulted in the loss both of new maize traits and of newhuman behavioral traits.

The initial establishment of coevolutionary relationships between maize demesand human populations added further complexity to the evolutionary mosaic withinthe Eastern Woodlands. Previously described processes continued to occur asmaize diffused to and became established in new areas. Coevolutionary relation-ships did not develop in all cases, and maize demes and populations became extinctas a result of the mechanisms discussed earlier. The development of coevolution-ary relationships between some human populations and maize demes resulted inincreased yield within some of those demes. However, these relationships werenot always perpetuated if the human population was not successful in intra- and in-tergenerationally transmitting the relationship. Alternatively, the establishment ofcoevolutionary relationships in some instances set in motion processes that resultedin substantial intensification of maize production.

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Intensification of Maize Agricultural Production

As human populations and maize populations/demes developed coevolution-ary relationships resulting in changes in the traits of both populations, they mayboth have ascended higher fitness peaks. As the marginal cost of maize productiondecreased relative to other resources or as the acceptable level of marginal costincreased, more human attention was focused on maize production. Managementbehaviors, their associated artifacts, and agroecologies evolved and become moreeffective as selection favored particular behavioral traits and artifacts. Maize fit-ness increased as the interacting human population sowed more kernels, increasingthe number of plants in the agroecology. Because of increased food availability inthe form of higher maize yields and corresponding changes in human consump-tion and reproductive behaviors, human fitness may also have increased (Buikstraet al., 1987).

Traits of the coevolved relationship, including the maize itself, human behav-ior, and associated artifacts, spread to other human populations and were perpet-uated intergenerationally. Maize traits spread as a result of seed stock exchangefrom higher-yielding demes being sown in other agroecologies. Other demes andpopulations may have ascended the higher fitness peak, although, as discussed pre-viously, this was not inevitable. New human behavioral traits and their associatedartifacts must also have spread to recreate the coevolved relationship of the higher-yielding deme and its interacting human population. Exchange of seed stock fromthe higher-yielding demes resulted in a faster spread of favorable maize traits, as-suming an adequate sample of the original deme's genetic variation and coevolvedhuman behavioral traits and associated artifacts. This is especially likely if onlyseed from higher-yielding demes was planted and human behavior and associatedartifacts were closely replicated. Because it is unlikely that microenvironmentalconditions were replicated exactly, a period of lower fitness probably occurred. Hu-man populations with higher fitness as the result of coevolutionary relationshipswith high-yield maize established more daughter settlements than less reproduc-tively fit populations. New demes were thus created with those traits, althoughsampling error and microenvironmental factors may have resulted in lower fitnessin newly established maize demes.

These processes resulted in bringing other populations and demes to thehigher fitness peak under Phase III of SBT. In some instances, Phase III of SBTresulted in the spread of higher yield traits over a wide area. As maize became morecommon in the environment, increasing agricultural yields may have also resultedin higher human fitness (Rindos, 1984). Increased maize production resulted ininnovative management behaviors as more effort was expended in its production.These behaviors in turn modified the fitness landscape of the maize deme, bringingit within the attraction sphere of sometimes higher fitness peaks. Selection favoredthe spread of those trait variants that had the greatest positive effects on yield,modifying the fitness landscapes of other demes and potentially bringing them


within the attraction sphere of higher peaks. Selection pushed these populationsup the higher peaks. This further increased yields per unit of human labor input. Thehigher-yield maize and its corresponding human behavioral traits and associatedartifacts spread through and between populations. As yields increased per unitof human labor input, additional maize plants were sown, increasing the maizepopulation's fitness.

These coevolutionary processes occurred over varying temporal scales, de-pending on local conditions, and were added to processes already described,resulting in greater complexity in the maize agricultural mosaic in the EasternWoodlands. Regional maize agricultural traditions evolved that included distinctraces and varieties of maize adapted to local agroecologies. These traditions mayhave shared traits analogous to those in other areas of the Eastern Woodlands, de-spite lack of interaction between human populations. The agroecologies of theseregional traditions occupied high fitness plateaus on which there were numerouslow peaks that in turn were occupied by local variants.

Maize demes on the plateaus experienced annual fluctuations in yield butmaintained high yields over time. Regional divided risk strategies generally en-sured adequate maize production for human populations in most years (Hart, 1993).Stabilizing selection was very stringent, maintaining populations on the peaks andlowering genetic variation within demes. Movement of agricultural fields generallyhad little effect on deme fitness because human input into maize-centered agroe-cologies was great enough to minimize microenvironmental variation (Hart, 1990);fitness landscapes did not change significantly as a result of field movements. Fit-ness landscape changes did occur as a result of macroenvironmental events whoseeffects on agroecologies could not be controlled though human labor. Deleteriousmacroenvironmental events resulted in maize populations occupying valleys onchanged fitness landscapes. These populations may have lacked genetic variationnecessary to ascend ascending valley and peak slopes, resulting in marginal costshigh enough for the human population to abandon production, at least temporarily.Favorable macroenvironmental changes altered fitness landscapes, creating higherfitness plateaus that regional agricultural traditions ascended by the evolution ofindividual demes and through Phase III of SBT.

Discussion

The importance of the fitness landscape concept and the application of SBTto the evolution of maize agriculture in the Eastern Woodlands is that it forces afocus on local evolutionary trajectories and change within local populations. Inpresenting the model, I have been careful to avoid references to time frames forthe various processes. There has been continual debate since Darwin's publicationof On the Origin of Species about the ability of small, incremental, opportunis-tic changes under the forces of selection to produce new species. However, thereis little doubt that selection is the primary force behind adaptive evolution in a

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species' populations (Briggs and Walters, 1997; Gould, 1997). The evolutionaryprocesses described above took place over varying lengths of time depending onlocal conditions. While Darwinism is often associated with gradualism, the rapid-ity of evolution varies (Lyman and O'Brien, 1998), and evidence for its speed isoften dependent on the available scale of resolution. Rindos (1984) demonstratedhow Darwinism can readily accommodate rapid intensification of agricultural pro-duction. There is no reason to expect a priori either rapid or slow evolution of maizeagriculture in a given region. The speed of evolution must be clearly demonstratedwith evidence from the archaeological record.

I have also avoided ascribing any of the processes discussed in the modelto a particular period of time or culture-historic taxon, such as Hopewell orMississippian. Such ascriptions exclude variation within the time period or taxon.Variation is seen only as difference between time periods and/or taxa. Variationwithin those units is interpreted as noise or, in the case of the natural state model, asthe result of interfering forces. Smith's (1992b, p. 111) complex mosaic of maizeagriculture beginning around 800 B.P. is an essentialist conceptualization of vari-ation (see Dunnell, 1994). The mosaic to which Smith refers consists of variationbetween culture-historic taxa. With Darwinian evolution, a complex mosaic canoccur at any time and on any spatial scale.

Each of the processes described occurred at any given time and place de-pending on the evolutionary history of the maize population and its demes, theinteracting human populations, and the agroecologies in a region once maize wasadopted. Some of the processes obviously could not have occurred until othershad taken place and they were repeatable. Maize agricultural evolution was anopportunistic and continuous process, and variation was continuous throughoutthe Eastern Woodlands from the time of its original successful adoptions.

ARCHAEOLOGICAL EVIDENCE

A number of lines of evidence from the archaeological record have been usedrecently to address questions of the timing of maize agriculture's introductionand its subsequent evolution in the Eastern Woodlands and regions thereof. Thisevidence generally consists of a few scattered occurrences of charred maize cobfragments and kernels from archaeological sites and a few pollen grains, oftenrecovered from nonarchaeological contexts. SCIA of human bone collagen hasalso become a standard source of evidence. There remains, however, a paucity ofevidence for pre-1150 B.P. maize agriculture in the Eastern Woodlands. As I discussbelow, one reason for this may be the manners in which such evidence is gathered.

AMS Dating of Macrobotanical Remains

The macrobotanical evidence for early maize in the Eastern Woodlands isnot controversial where AMS dating of maize remains from good archaeological


Table I. Pre-1150 B.P. Maize AMS Dates and Pollen in the Eastern Woodlands

Site

Grand Banks

CraneEdwin Harness

Icehouse BottomHolding

Tuskegee PondDismal SwampB. L. BigbeeFort CenterLake Shelby

Location

Ontario

IllinoisOhio

TennesseeIllinois

TennesseeVirginiaMississippiFloridaAlabama

Date (B.P.)

AMS dates1250 801500 1501570 901450 3501720 1051730 851775 1002017 502077 70

PollenCa. 1500Ca. 2200Ca. 2400Ca. 2500Ca. 3500

Reference

Crawford et al. (1997)

Conard et al. (1984)Ford (1987)

Chapman and Crites (1986)Riley et al. (1994)

Delcourt et al. (1986)Whitehead (1965)Whitehead and Sheehan (1985)Sears (1982)Fearn and Liu (1995)

contexts yield an early date consistent with those contexts. Such is the case with atleast five Eastern Woodlands sites (Table I). AMS dating has been used to disproveapparently early maize at a number of sites in the riverine interior that had previ-ously been used as evidence of Hopewellian maize agriculture (cf. Conard et al.,1984; Smith, 1992b; Struever and Vickery, 1973). There remain a large number ofsites with maize in apparently pre-1150 B.P. contexts based on radiocarbon datesof associated materials that have not been subjected to AMS dating (e.g., Crawfordet al., 1997, Table 1; Riley et al., 1994, Table 2). Some of these occurrences aremore widely accepted (e.g., Scarry, 1990; Wymer, 1992, 1994) than others (e.g.,Adovasio and Johnson, 1981). However, caution is generally urged in the accep-tance of these cases until AMS dating is secured (cf. Crawford et al., 1997, p. 118;Riley et al., 1994, p. 495).

The presence of macrobotanical maize remains representing food prepara-tion at an open-air site is a function of the length and intensity of maize use,length of site occupation, accidental loss and charring of maize, and depositionof charred maize in an environment favorable for preservation for many hun-dreds of years (see, e.g., Miksicek, 1987). As discussed by Lopinot (1992, p. 56),"Whichever preparation method was utilized [for early maize consumption], thechances of loss, eventual carbonization, archaeological recovery, and identificationwould be relatively rare" compared to taxa of the indigenous seed crop complex.Detection of macrobotanical maize remains at a site is dependent on use of flota-tion recovery, sampling intensity in the field and laboratory, and identificationefforts.

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When maize becomes ubiquitous at archaeological sites in a region, thereis often evidence for increased reliance on storage, suggesting the use of maizeover extended periods of time each year (e.g., Hart and Sidell, 1996; Smith 1992b).Despite this, when not found in contexts reflecting in situ burning of stored produceor use of cobs as fuel, maize generally is recovered in relatively small quantitiescompared to the number of fragments a single cob can produce (Chilton, 1999;Fritz, 1992; see, e.g., Wymer 1992, 1994). Prior to apparently intensive use, maizeprobably entered the archaeological record only rarely, and that which did probablyrepresents more than casual use. The recovery of small amounts of maize at anearly site probably reflects a level of use that is rarely ascribed to pre-1150 B.P.contexts.

Early maize confirmed through AMS dating has been recovered only in well-preserved contexts such as beneath burial mounds (Ford, 1987) and/or throughintensive sampling efforts at open-air habitation sites (e.g., Chapman and Crites,1987; Crawford et al., 1997; Riley et al., 1994). Sample size has an importanteffect on the number of taxa represented in a sample (richness); the discovery ofrare classes requires large samples (Kintigh, 1989). As a result, large samples arerequired from early sites to determine whether or not maize is present. For example,at the Holding site, flotation of 5340 liters of soil from features and midden depositsyielded only 11 kernels, 4 cupules, 3 cob fragments, and 1 embryo (Riley et al.,1994, p. 492). Few sites receive this level of sampling, and it is probable that maizehas not been recovered and/or identified from early sites at which it was preservedin small amounts. Maize often seems to appear suddenly and ubiquitously in thearchaeological record of a region. Without intensive efforts to find maize in earliercontexts, such occurrences cannot be taken as evidence of a sudden adoptionof maize agriculture, nor can they be taken as evidence of the immigration ofagriculturists from other regions (e.g., Snow, 1995).

In early uses, when maize was probably a minor crop at best, it is very unlikelythat it is represented in the archaeological record because of its low level of use, es-pecially if sites were occupied for only short periods of time. Early macrobotanicalremains are more likely to be represented in the archaeological record only aftermaize had become a common and frequently, even if only seasonally, consumedcrop. This occurred after a maize deme had evolved in and became adapted to anagroecology, ascended a higher fitness peak, and most likely, began to establisha coevolutionary relationship with the human population. The early, direct-datedmaize at Holding, Icehouse Bottom, Harness Mound, Crane, and Grand Banks doesnot represent the first introduction of maize in their respective regions. Rather, theyprobably reflect the evolution of maize demes and concomitant increased use ofthe crop by the local human populations over an unknown period of time. Despiteemphatic statements to the contrary (e.g., Smith, 1992b, p. 203), we do not knowthe timing of maize's introduction into the Eastern Woodlands or regions thereofbased on macrobotanical remains.


Pollen

Maize pollen from pre-1150 B.P. contexts has been reported from at least fivelocations, four of which predate the earliest AMS-dated macrobotanical remains(Table I). Unlike AMS-dated macrobotanical remains, maize pollen from earlycontexts is generally controversial, especially in contexts predating early mac-robotanical remains. Smith (e.g., 1992a, p. 110, 1992b, p. 272), for example, allbut dismisses purported pre-2000 B.P. pollen in his discussions of early maize inthe Eastern Woodlands, relying instead on directly dated macrobotanical remainsfor convincing evidence of early maize. Purportedly early maize pollen has beenchallenged on the basis of (1) potential misidentification [e.g., Lake Shelby (cf.Eubanks, 1996; Fearn and Liu, 1995, 1997)], (2) the recovery of only a single grainand lack of corroborating macrobotanical remains [e.g., Lake Shelby (Crawfordet al., 1997)], (3) extrapolation of dates [Dismal Swamp (cf. Crawford et al., 1997;Whitehead, 1965)], (4) possible root disturbance [B. L. Bigbee (cf. Crawford et al.,1997; Whitehead and Sheehan, 1985)], and (5) context and possible contamina-tion [Fort Center (cf. Crawford et al., 1997; Milanich, 1994; Sears, 1982; Smith,1986)], among others. Only Tuskegee Pond, where maize pollen represented 1 to2% of the total upland pollen sum from 1500 B.P. to the present (Delcourt et al.,1986) and is corroborated by macrobotanical remains at a nearby archaeologicalsite (Icehouse Bottom) (Chapman and Crites, 1987), is uncontroversial.

Fearn and Liu (1995) agree that direct dated macrofossils in good archaeo-logical contexts are the best evidence for the presence of early maize but argue that"in the humid eastern United States, pollen is more likely to be preserved than aremacrofossils on land, and in well chosen sites, more likely to be stratigraphicallyin place" (p. 111). As a result, pollen may be the only trace of early maize in theEastern Woodlands. Citing discussions of maize pollen preservation by Bryantand Hall (1993), among others, they argue that because maize pollen is poorly dis-persed, its presence at a location is "direct evidence of maize cultivation" (Fearnand Liu, 1995, p. 115).

Maize produces vast quantities of pollen (Faegri and Iverson, 1975, p. 53;Mangelsdorf, 1974, p. 58; Traverse, 1988, p. 376; Weatherwax, 1955, p. 103), verylittle of which escapes the maize field (Raynor et al., 1972; Traverse, 1988, p. 381).In fact, only 180 m is needed to isolate two maize fields reproductively (Briggsand Walters, 1997, p. 214). The chance of preservation of escaped pollen grains isvery low. Deposition and rapid burial in lake and pond sediments present the bestchance for preservation. However, Whitehead and Sheehan (1985, p. 135) reportthat "mud-water interface samples from lakes in areas of intensive cultivationin northern Indiana contain less than 0.1% maize pollen." The preservation ofrelatively large amounts of pollen in Tuskegee Pond is fortuitous and may simplybe the result of its location near maize fields. Maize pollen did not occur in BlackPond, 4 km north of Icehouse Bottom, until 450 to 300 B.P. (Delcourt et al., 1986).

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The preservation of maize pollen from early contexts is a function of theamount of pollen produced over time (size of plot, length of presence), the amountthat escaped the plot, and the amount that landed in contexts favorable for preser-vation over tens of hundreds of years. Its detection depends on palynologicalsampling, including placement, number and size of cores, number of samples pre-pared from cores, number of pollen grains counted, the amount of maize pollenrelative to that of other taxa, and efforts to identify maize pollen after standardcounts are completed. As maize is a relatively minor plant within a region, andone that produces large, poorly dispersed pollen in much smaller amounts than domany arboreal and herbaceous species with more widely dispersed pollen (Faegriand Iversen, 1975, p. 53), the chance for preservation and subsequent detectionof early maize pollen is very low. In fact, special sampling techniques have beendevised to identify rare pollen such as maize in samples after standard counts arecompleted (Bryant and Hall, 1993).

While skepticism about contexts and dates of purportedly early maize pollenis warranted, early instances should not be dismissed out of hand. Given that largeamounts of early maize pollen are rare in the Eastern Woodlands except in unusualcircumstances like Tuskegee Pond, the presence of a maize pollen grain in earlycontexts should trigger intense efforts to identify additional grains from the sample.A single grain of maize pollen in the absence of evidence for dislocation fromlater sediments may very well be evidence for early maize in an area, especially ifconfirmed by additional finds at that location or elsewhere in the region.

Pollen recovered from early contexts cannot be considered the result of its firstintroduction into a particular region based on all of the factors operating againstits preservation and subsequent discovery. Rather, such occurrences represent acertain threshold of use that was unlikely to have occurred immediately followingintroduction. Cases such as Tuskegee Pond and Fort Center probably representlocal maize populations that had ascended a high fitness peak relative to thoseinhabited by newly introduced populations, as well as the formation and perpetu-ation of coevolutionary relationships with human populations. The single grain atLake Shelby and the few grains at B. L. Bigbee are more difficult to assess but are,nonetheless, intriguing potential evidence of early maize agriculture. If corrobo-rated by additional finds in the region from good contexts, they may also representa degree of use beyond that probably achieved after the initial adoption of maize.

Stable Carbon Isotope Analysis

Since the initial studies in the late 1970s and early 1980s (Bender et al.,1981; van der Merwe and Vogel, 1978; Vogel and van der Merwe, 1977), SCIAof human bone collagen has become a standard means of obtaining evidenceof the levels of prehistoric maize consumption in the Eastern Woodlands (e.g.,


Ambrose, 1987; Boutton et al., 1991; Boyd, 1996; Broida, 1983, 1984; Buikstra,1992; Buikstra and Milner, 1991; Buikstra et al., 1987, 1988, 1994; Conard, 1988;Farrow, 1986;Greenlee, 1990, 1998;Katzenberg, 1993; Katzenberg and Schwarcz,1986; Katzenberg et al., 1993, 1995; Larsen et al., 1992; Lynott et al., 1986;Medaglia et al., 1990; Rose et al., 1991; Schurr, 1992; Schurr and Redmond,1991; Schwarcz et al., 1985; Scuilli, 1995; Stothers and Bechtel, 1987; Wagner,1987). The attractiveness of SCIA is that, unlike botanical evidence of maize, it islinked to the consumption behavior of specific humans. The methods and theoryof SCIA have been discussed often and have been reviewed in detail recently byAmbrose (1993), Pate (1994), and Schoeninger and Moore (1992).

The original SCIA studies (Bender et al, 1981; van der Merwe and Vogel,1978) were used to address issues of the initiation and intensification of maizeconsumption in several areas of the Eastern Woodlands. Many subsequent stud-ies have been used at least in part for the same purpose. The results of SCIAstudies concerned primarily with chronological issues are summarized in Table II.These studies have become part of the accepted literature on maize agriculturalevolution in the Eastern Woodlands and are used as evidence in support of variousinterpretations (e.g., Smith, 1992b, p. 275).

The studies listed in Table II, and others that address one or a few sites, havebeen used to infer different trajectories for intensification of maize consumptionbetween regions and to demonstrate broader pan-regional trends (e.g., Buikstra,1992). In the former case, line plots using mean

Maize Agriculture Evolution

Fig. 3. Line plot comparing the Central Mississippi Valley (Lynott et al., 1986) and western LakeErie (Stothers and Bechtel, 1987) 13C values.

used to demonstrate interregional differences, as in Fig. 3. In the latter case mean

166

are interpreted as being representative of broader trends in the levels of maizeconsumption within regions. Differences in values for roughly contemporaneoussamples from different regions are interpreted to reflect different trajectories inlevels of maize consumption.

The basic premise of these arguments, then, is that when maize was adoptedby a population, it was consumed at a relatively constant level by all membersof that population. Any variation in

from numerous contemporary sites to allow an examination of changing variantfrequencies. Only then can explanations for the change be constructed. The presentcollection of SCIA study results cannot be used with the model developed in thisarticle to address issues of the adoption and early evolution of maize agriculturein any region (but see Greenelee, 1998).

Several authors have rationalized the use of small samples in SCIA studiesto represent maize consumption levels at specific sites and, by extension, regionalpopulations. Boutton et al. (1991, p. 380) ask, "How well does that single individualrepresent the isotopic composition of the larger population from which he or shewas sampled?" For an answer they did SCIA of 51 individuals dating to ca. 550 B.P.at the Crow Creek site in South Dakota. All of these individuals were recoveredfrom a mass grave of 486 massacre victims. Archaeological evidence indicated asubsistence system that included maize agriculture. The S13C values had a meanof -11.3 0.8%o and a range of -13.5 to -9.6%o. Boutton et al. (1991, p. 381)conclude that the "small standard deviation (0.8%o) associated with the entiresample and the relatively small range (3.9%o) of values suggest the results froma single individual should be reasonably representative of the larger populationfrom which he or she was sampled." By implication, then, the almost 4.0%o rangeof values in this large sample is unimportant and these results and Boutton andco-workers' interpretation of them are applicable to the sites in their study area ofsoutheastern Missouri and northeastern Arkansas: those individuals analyzed arereasonably reflective of the population to which they belonged. By extension, thatpopulation is representative of all populations encompassed by a particular periodof time and/or culture-historic taxon.

Katzenberg et al. (1993, p. 268) suggest that, given the expense and destructivenature of SCIA, use of the smallest sample required to provide a "representativepicture of the diet" is justified. They argue that the "assumption that a few indi-viduals represent the diet of the community is logical based on what is knownabout the monotony of diets, over the long term, from the archaeological recordand from the ethnohistoric literature" (Katzenberg et al., 1993, p. 268; see alsoSchwarcz, 1991). They list as support for this supposition the homogeneity of

history of the population? (2) Was the population under selective pressure? If so,what was the mode of selectionstabilizing, directional, or diversifyingandhow strong was the selection? (3) Was the population experiencing drift? (4) Howdoes this population compare to others belonging to contemporary regional popu-lations? (5) How does the population at this time compare to the same populationat earlier and later times? These questions can be answered only with additionallarge samples from contemporary sites and with similar samples from sites pre-and postdating Crow Creek. In any case, using a site or sites from one region toestablish expectations for variability in another region is untenable.

A number of SCI A studies have identified variation in samples from sites and,as reviewed by Schurr (1992, pp. 301-304), often have been used in attempts todetermine whether there were gender-specific differences in maize consumption.Buikstra et al. (1987, p. 72; also see Buikstra, 1992) take a different approach tointrasite variation in their west-central Illinois study. "Intracommunity variationin maize dependence is indicated by the broad range of values from the Helton andLedders sites. . . . This suggests that during the time of transition there was wideinter-individual variability in dietary maize. The development of agriculture wasapparently not a uniform process at the community level, for there is no evidencefor a regular sequence of modest increments of maize in the diet, equally shared byall members of the prehistoric community. Instead it appears that this was a timeof experimentation, with a great deal of local variability." Schurr (1992, p. 301)attributes this variation to the multicomponent nature of these sites and suggeststhat the samples "may span significant periods of time or reflect rapid changes insubsistence. . . ." In fact, dividing the Helton Mound sample into three temporalunits, Buikstra et al. (1987, p. 72) note that there is greater intraunit than interunitvariation. These are important observations that are generally consistent with whatwould be expected under a populationist model. This kind of discussion is rare inthe SCIA literature (see also Greenlee, 1996, 1998) and Buikstra and co-workers'observations have not been developed into a populationist explanation on maizeevolution in west-central Illinois (see Buikstra, 1992).

Because of its relative stability, bone collagen was established as the prefer-able material on which to perform SCIA (van der Merwe and Vogel, 1978; Vogeland van der Merwe, 1977; see also Ambrose, 1993; Schoeninger and Moore,1992). Questions were later raised (e.g., Krueger and Sullivan, 1984) about whichpart of the diet determined the carbon content of bone collagen versus bone ap-atite. Several studies were published in 1993 indicating that the carbon content ofcollagen and apatite are determined by different dietary factors (Ambrose, 1993;Ambrose and Norr, 1993; Tieszen and Fagre, 1993; see also Pate, 1994; Rileyet al., 1994). Using controlled dietary experiments with laboratory rats, Ambroseand Norr (1993) determined that the carbon content of apatite is reflective of thewhole diet, while the carbon content of collagen is reflective of dietary protein.This is significant, because carbon from maize will be underrepresented in

1993, p. 31). "At low levels of maize consumption it is likely that carbon frommaize would be underrepresented in collagen, especially when humans had highprotein diets. Apatite carbonate would more closely reflect the true percentage ofmaize in the diet" (Ambrose and Norr, 1993, pp. 31-32). Tieszen and Fagre (1993,p. 153) reached similar conclusions based on independent controlled dietary ex-periments with laboratory rats. They concluded that "bioapatite is a better generalpredictor of total food intake than is collagen. . . ." The implication of these studiesis that SCIA studies based on collagen are incapable of detecting consumption ofmaize until it is a major source of protein in the diet.

Based on these studies and on other questions raised about SCIA duringthe late 1980s and early 1990s (see Ambrose, 1993; Buikstra and Milner, 1991;Parkington, 1991; Tieszen, 1991), Tykot et al. (1996) argue that both bone ap-atite and collagen must be analyzed and reported in SCIA studies of prehistoricdiet to enable a fuller understanding of maize consumption levels. According toPate (1994, p. 193), with the use of appropriate laboratory controls, postmortemdiagenesis is not a limiting factor for SCIA of apatite, especially of tooth enamelapatite. As demonstrated by Conard (1988) and Tykot et al. (1996), SCIA of bothcollagen and apatite is possible from the same skeletal populations. However, aspointed out by Greenlee (1998, p. 301), in temperate areas su

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Agr. Orgen Maiz (Hart 1999)

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