How do new species originate?
www.cdnn.info/news
www.condorjourneys-adventures.com
How do we account forthe great diversity of life
in the biosphere?
www.botany.wisc.edu http://bio1903.nicerweb.com
What is a species?
We say that all dogs are in the same species, but that Norway maples and sugar maples are in different species. How do we know that? What do we mean?
What we mean by species is a group of individuals with the potential to interbreed under natural conditions and produce fertile offspring. By this definition each species has its own gene pool that does not mix with the gene pools of other species.
www.thestonemill.com
http://oregonstate.edu
Shortcomings of this definition
As discussed earlier, this species definition does not work for species that reproduce only asexually.
Moreover, it is not always feasible to determine whether mating is occurring between two groups in the same habitat.
And sometimes members of two species look so much alike that we have a hard time telling them apart.
We sometimes make mistakes in concluding that two groups are in different species, when in fact they do share genes through mating.
The eastern and western meadowlarks (above, S. magna and S. neglecta, respectively) look similar, but their songs and other behaviors are sufficiently different that they would not interbreed, if they met in nature.
www.sacsplash.org
Sturnellaneglecta
www.stephenbodio.com
Sturnellamagna
And mimicry in nature can confuse biologists even more
• As an adult, the butterfly on the left, Monarch danaus, is similar to the viceroy butterfly on the right, Limenitis archippus.
• But compare their appearance as larvae! www.richard-seaman.com
www.mongabay.com
www.cirrusimage.com/
How do gene pools of similar species remain separate from each other?
• A catastrophe (earthquake, fire, new highway, and so on) may separate one population in two smaller populations.
• A famous example is the two populations of Kaibab squirrels, isolated geographically from each other by the Grand Canyon (perhaps 6 to 7 million years ago).
• We do not know if these two populations have been isolated for enough generations that mutations would have accumulated that have made the two populations reproductively isolated from each other.
North rim
Kaibab
squirrel*
South rim
Abert
squirrel **
Geographical Isolation* Sciurus aberti kaibabensis and
**Sciurus aberti
An extreme example of geographical isolation
--New Haven Register, January 31, 2010
Habitat Isolation
Two other ways to ask this question are as follows: What keeps different species from breeding with each other? How to they maintain their reproductive isolation?
Such barriers may occur before fertilization has a chance to occur, or after fertilization.
A first step toward reproductive isolation is often habitat isolation within the same area.
An example is two garter snake species in the genus Thamnophis. One species spends most of its time in water, while the other is primarily terrestrial.
http://sparkleberrysprings.com www.wildherps.com
An example of ecological isolation by different habitats
• There exist 750 wasp species (!) each of whose members lay their eggs in the flower of a different fig species. These fig species share the same habitat but bloom at different times. This keeps the different wasp species reproductively separate from each other.
Emerging new generation offig wasps (image from
www.morning-earth.org)
Fig wasp laying her eggsamongst the ovules of a
developing fig fruit (www3.imperial.ac.uk/)
Note the longovipositor on thefig wasp. Why is
this adaptive?
Temporal isolation is another pre-zygotic reproductive barrier
Two species may breed at different times of the day, or in different seasons, or in different years.
For example, even though the territories of the western and eastern spotted skunks, Spilogale gracilis, and S. putorius, overlap, S. gracilis mates in the late summer while S. putorius reproduces in the late winter.
Spirogale putoriusImage from www.cedarcreek.umn.edu
Spilogale gracilisImage from http://fwp.mt.gov/
Different courtship behaviors may isolate two species
Below, four similar looking beetles with very different courtship behaviors–
Mounting behavior inMeloe beetles
Posterior courtshipbehavior in Linsleya
beetles
Anterior courtshipbehavior in
Tegrodera beetles
Linear courtshipbehavior in subfamilyMeloinae beetles
Images from www.ipmworld.umn.edu
Sexual selection by coloration can act as a reproductive barrier
Biologists at the University of Leiden in the Netherlands raised two cichlid species (Pundamilia pundamilia and P. nyererei) together in two different aquaria. One included natural light and the other monochromatic orange light.
Under natural light the two species looked very different from each other, but under orange light they were indistinguishable from each other.
Campbell’s Biology, 7th ed.
Cichlid experiment, concludedRESULTS:
With natural light, in which the two species looked different from each other, the females mated only with males of their species.
However, under orange lighting the females mated with males of either species indiscriminately and produced fertile offspring.
INTERPRETATION AND CONCLUSION:The species are very similar to each other, differing only in their coloration.
This color difference acts as a prezygotic reproductive barrier.
The two species very likely separated from each other only recently in their evolutionary past.
P. pundamilia
P. nyerereiCichlid images from www.fishecology.ch
Mechanical isolation may keep two species reproductively separate
• In such a situation, even if mating is attempted, structural differences in their reproductive organs prevent copulation.
And be sure to check out the amazing video ofthe four-headed penis of the spiny anteater at http://scienceblogs.com/neurophilosophy/
2007/10/experimenting_with_a_fourheaded_penis.php(scroll halfway down)
www.proexotics.com
A boa’shemipene
Both shells mustcoil in the same
direction
Four-headedpenis of thespiny ant-
eater
Coiling direction matters to a snail!Shells of three closely related snail species (image a)
Two snails whose coiling directions are the same are able to mate (image b)
The genitalia of two snails with opposite coiling direction do not match up (see image c).
Gametic isolation
Sperm of one species might not be able to survive in the reproductive tract of another species.
Or biochemical differences might prevent the sperm of one species from getting through the protective barrier of another species’ ova (in both plants and animals).
Thus, even though members of two such species copulated, fertilization would not occur.
For example, sea urchins of more than one species might reproduce in the same place at the same time, but the sperm of species A might be incompatible with the ova of species B and vice versa.
www.dailygalaxy.com
Postzygotic barriers may exist between two “subspecies”
For example, if cross fertilization does occur, the resulting hybrid offspring may not complete embryonic development or may be frail.
We describe this situation as hybrid inviability or reduced hybrid vigor.
This is true of some of the salamander subspecies of the genus Ensatina in California.
http://evolution.berkeley.edu/
Reduced hybrid fertility• Even if hybrids between two
species or subspecies (two emerging species) are survive and are healthy, they might not be fertile.
• Chromosome incompatibilities might prevent pairing at Prophase I of meiosis and thus limit gamete production.
• This keeps the two gene pools effectively separate from each other.
• The well known example of reduced hybrid fertility is the mostly sterile mule (see image b, below), produced by a mating between a male donkey (a) and a female horse (c)*:
• Another example comes from the work of a friend of mine, Sandra Anagnostakis. She crosses different chestnut species in an attempt to produce a tall, straight, blight-resistant tree with chestnut fruits. Her world-famous chestnut plantation is at the east end of the Sleeping Giant.
* But see the following: www.ncbi.nlm.nih.gov/pmc/articles/PMC1289946/pdf/jrsocmed00204-0031.pdf !
www.talkreason.org/
(a) (b) (c)
www.utc.edu
Photographs taken by scientistSandra Anagnostakis, at theworld-famous Chestnut Plantation (located at the eastend of Sleeping Giant StatePark)
Hybrids might have low fertility due to
chromosomal incompatibilities For example, in a hybrid
between a parental stock and a stock with translocations, the chromosomes will not match up well at Prophase I of meiosis.
Only some of the resulting gametes will have full sets of genetic information.
https://qspace.library.queensu.ca
And then there the weird triploid all-female fish species!
• Iberian minnows of the all-female species known as Tropidophoxinellus alburnoides “borrow” chromosomes from a male of a closely related species, but the resulting generation throws away those male chromosomes during the process of meiosis and production of ova.
• Other all-female fish species don’t even accept the chromosomes from the sperm that contact their ova; the sperm serve only to activate embryonic development of the ova.
• There exist such situations in some reptiles, amphibians, and insects, as well!
http://www.genetics.org/cgi/reprint/146/3/983Poeciliopsis: http://jhered.oxfordjournals.org/cgi/content/full/96/1/32
Hybrid breakdown: A final reproductive isolating mechanism
Some hybrids are robust and fertile and mate either with each other or with a parent (!), but the offspring of such matings may be either weak or infertile.
That minimizes the gene flow between the two species.
For example, some rice species follow this pattern. Healthy hybrids have been produced experimentally, in attempts by farmers to increase crop yield by combining genetic traits of both parents.
However, attempts to cross two different such hybrids often fail; the two rice strains that produced these hybrids are well on their way to becoming different species.
Explanation for hybrid breakdown in the case of rice crosses
Various strains of cultivated rice have been produced by artificial selection.
Over time they have diverged genetically from their common ancestor and have each accumulated different recessive mutations.
Crosses between such strains often bring together harmful mutant alleles, often resulting in weak or sterile hybrid offspring. * “N” means normal, and “S” means sterile.
** The Rf allele restores fertility, but the rf allele does not.
* *
Some images of rice plants*
http://bicol.da.gov.ph/tips/hybrid-rice.jpg
www.msstate.edu
www.fao.org/ag/agl/agll
* What part of the rice plant do we actually eat?
And how about seedless fruits? What’s that all about?
www.heinens.com
www.plantsciences.ucdavis.edu
Summary of the different mechanisms for reproductive isolation of species
(or have different niches)
*
* The terms “prezygotic” and “postzygotic” are sometimes used instead of “premating” and “postmating”, respectively.
*
How do new species form?• In addition to distilling Darwin’s theory into a few succinct
observations and inferences, Ernst Mayr has given us the two major conditions under which speciation can occur:
(1) Interbreeding between the populations of two emerging species must be blocked, by their becoming isolated from each other.
(2) Enough time must pass during this time of isolation for mutations to accumulate and for the two populations to become genetically distinct from each other. They must become sufficiently different from each other that if the two populations were brought together again, they would not be able to share their gene pools.
• These differences might be caused by genetic drift or accumulation of different mutations (as we have discussed), or through different environmental pressures that result in natural selection within each of these now separated populations.
Speciation can occur with or without geographic isolation
*
* In such a situation, however, there might exist subtle micro- habitat differences within the same space.
Biologists believe that most new species arise from allopatric* speciation
We will consider the following examples of allopatric speciation:
• Adaptive radiation, resulting from small groups leaving a larger population and colonizing new environments
• Geological and climactic changes separating members of a population
• Human activities that split a population into separate subgroups
* literally, “other country”
“Adaptive radiation” is well known to occur in island populations
By this means, small subsets of a common ancestral population* leave at different times, each traveling in its own direction.
Upon arrival in a suitable habitat**, each such “founder” group will become established, responding to the unique environmental conditions of its new location.
* Often on the mainland
** often an island
The Hawaiian islands provide many such examples of rapidly evolving populations
• The Hawaiian islands are located ~3,500 km (about 2200 miles) from the nearest continent.
• Very young, these islands arose volcanically; the youngest (“Big Island”) appeared only about a million years ago and still demonstrates active eruptions.
• The five silversword species below are all very closely related, based on molecular studies, yet look quite different from each other.
• Most Hawaiian species are endemic; they can be found nowhere else.
http://ops.fhwa.dot.gov/freight/freight_analysis/state_info/hawaii
Campbell’s Biology, 7th ed.
A population may be divided by a number of different geological and climactic events
Global warming causes sea levels to rise, separatng contiguous land masses into several separate large masses and smaller islands.
For example, North America looked very different during the Cretaceous period (~90 MYA)! (See the map on the left.)
www.rankinstudio.com
Dams built by beavers or humans create ponds and lakes and separate populations
Studies of striped bass (Morone saxatilis) were carried out by Irwin Beitch 10 years after the completion of a large dam in southeastern Virginia.
Kidney morphologies of populations above (freshwater) and below (near the Chesapeake Bay) the dam had become significantly different from each other.
www.mvhs65.com
http://oakridgevisitor.com
www.highlandlakesflyfishing.com
Land masses that were connected during the Triassic Period later
separated, taking life forms with them.
http://earthscience.files.wordpress.com
www.palaeos.com/
Check out the very cool animation athttp://www.divediscover.whoi.edu/images/gondwana2.gif!
“Gondwana”
Ratite birds– an example of early evolution after Gondwana broke apart
The above map shows the distribution of living and recently extinct* ratite birds. Key: C = Cassowary; E = Emu; e = Elephant Bird*; K = Kiwi; m = Moa*; O = Ostrich; R = Rhea (see www.grisda.org)
Cassowary(www.unfamiliar-image.co.uk )
Emu www.uwm.edu/ Rhea www.animals.nationalgeographic.com
Kiwi
Highways and housing developments split populations and cause habitat destruction
• In what way can such fragmentation lead to the evolution of new species?
• What happens to species members on the edge of the habitat?• Could fragmentation cause species extinction?• Example: Howler and spider monkeys in fragmented landscapes in
southern Mexico (www.primatesmx.com).
Contiguous, undisturbed forest Fragmented forest
Hybridization between closely related species can lead to extinction
EXAMPLE: Unhybridized (“pure”) Hawaiian ducks (Anas wyvilliana) are rare today and exist only on the island of Kauai. On the other Hawaiian islands, where mallards have been introduced by hunters and have interbred with the Hawaiian duck, the hybrids are sterile and are replacing pure-bred Hawaiian ducks. Hawaiian duck
(Anas wyvilliana )www.en.wikipedia.org
Female mallard duck(Anas platyrhynchos)
www.upload.wikimedia.org
And their facilitators (www.guidedventures.com )
Ecological isolation within the same space ( sympatric speciation)
One of many examples of how microhabitat differences might provide environmental pressures for genetically different members of a species to specialize in different ways: palm trees in a habitat that contains both volcanic soil (high in ash) and calcareous soil (of coral reef origin)
Another example of microhabitat differences leading to sympatric speciation
The maggot fly Rhagoletis pomonella has a stable parasitic relationship with the American hawthorn tree, in which the fly lays its eggs.
In the seventeenth century apple trees were introduced to North America by the early colonists.
Over the years scientists began to observe the fly infecting apple trees, as well as hawthorns.
The two fruit fly populations tend not to intermingle, even if in the same habitat, and thus are unlikely to interbreed. When offspring become adults, they tend to lay their eggs in the same kind of fruit from which they had hatched (perhaps based on a type of olfactory imprinting).
Over time these two populations have become genetically different from each other and may be on their way to becoming separate species.
Apple maggot flies(www.sciencecases.org/maggot_fly)
♂ ♀
Occasional meiotic errors may lead to polyploidy– a common source of sympatric speciation
• Sometimes DNA replication occurs normally (during the S) phase in (especially in plants), but problems arise during meiosis.
• If chromosomes failed to separate from their homologs, then diploid (2n) reproductive cells could result.
• Self fertilization of such diploid gametes would then produce tetraploid (4n) offspring.
• This occurs surprisingly often in plants; it is estimated that 25 to 50% of all plant species evolved through polyploidy.
www.departments.oxy.edu
Sympatric speciation by the appearance of polyploidy
http://trc.ucdavis.edu/biosci10v/bis10v/media/ch13/wheat_speciation.html
Polyploidy can occur during mitotic cell division
Create a polyploid plant in your dorm room!
The colchicine that was applied to a bud in the above example (http://waynesword.palomar.edu) interferes with mitotic spindle formation. Why (how) would that cause tetraploid cells to form from mitotic cell division of diploid cells?
Other common examples of polyploid plant species
* Triploid crops: banana, apple, ginger* Tetraploid crops: durum wheat, macaroni wheat,
maize, cotton, potato, cabbage, leek, tobacco, peanut
* Hexaploid crops: chrysanthemum, bread wheat, oat
* Octaploid crops: strawberry, dahlia, pansies, sugar cane
CHALLENGE: Why are polyploids with odd numbers of sets of chromosomes (e.g., triploid species) sterile? And bananas are sterile, so how do we get new banana plants?
http://ask.metafilter.com
Why triploid species are sterile
New species evolve, but other species become extinct– Why?
Study the vertical axes to see why these two graphs are so different from each other:
www.dailygalaxy.com www.austinchronicle.com
The future is yours; you have the ability (and the challenge) to bring about change.
www.greenfacts.org
Note logarithmicscale
Can you list and explain at least six human activities that accelerate
species extinction rates?
If humans can cause species extinctions, do we also have the ability to take actions to prevent such permanent losses of biodiversity?Think about what steps you can take to help preserve Gaia.
And finally, you could name a new species after yourself, a friend, or . . .
And an ethical question: Should people be allowed to pay money for the right to name a new species?
Agathidium bushi, on the left,from www.forskning.no
Agathidium cheneyi, on the left,from www.masdebate.com
And a lichen has been named And a lichen has been named after President Obamaafter President Obama
• "I discovered the new species in 2007 while doing a survey for lichen diversity on Santa Rosa Island in California," said Kerry Knudsen, the lichen curator in the UCR Herbarium. “I named it Caloplaca obamae to show my appreciation for the president's support of science and science education.“
ScienceDaily (Apr. 16, 2009)
