How do new species originate How do we

How do new species originate? How do we account for the great diversity of life in the biosphere? www. cdnn. info/news http: //bio 1903. nicerweb. com www. condorjourneys-adventures. com www. botany. wisc. edu

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. Sturnella neglecta Sturnella magna www. sacsplash. org www. stephenbodio. com 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.

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. mongabay. com www. cirrusimage. com www. richard-seaman. com

How do gene pools of similar species remain separate from each other? • A catastrophe (earthquake, fire, new highway, and so on) may North rim separate one population in two Kaibab smaller populations. squirrel* • 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). South rim • We do not know if these two populations have been isolated for Abert enough generations that mutations squirrel ** would have accumulated that have made the two populations reproductively isolated from each * Sciurus aberti kaibabensis and other. **Sciurus aberti Geographical Isolation

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? 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. 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. 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. Note the long ovipositor on the fig wasp. Why is this adaptive? Fig wasp laying her eggs amongst the ovules of a developing fig fruit (www 3. imperial. ac. uk/) Emerging new generation of fig wasps (image from www. morning-earth. org)

Temporal isolation is another prezygotic reproductive barrier Ø Two species may breed at different times of the day, or in different seasons, or in different years. Spilogale gracilis Image from http: //fwp. mt. gov/ Ø 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 putorius Image from www. cedarcreek. umn. edu

Different courtship behaviors may isolate two species Below, four similar looking beetles with very different courtship behaviors– Posterior courtship behavior in Linsleya beetles Mounting behavior in Meloe beetles Linear courtship behavior in subfamily Meloinae beetles Anterior courtship behavior in Tegrodera beetles Images from www. ipmworld. umn. edu

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. Both shells must coil in the same direction A boa’s hemipene www. proexotics. com And be sure to check out the amazing video of the four-headed penis of the spiny anteater at http: //scienceblogs. com/neurophilosophy/ 2007/10/experimenting_with_a_fourheaded_penis. php (scroll halfway down) Four-headed penis of the spiny anteater

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. 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. 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. 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. This is true of some of the salamander subspecies of the genus Ensatina in California. We describe this situation as hybrid inviability or reduced hybrid vigor. 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. www. utc. edu • 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)*: (a) (b) (c) www. talkreason. org • 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 worldfamous chestnut plantation is at the east end of the Sleeping Giant. * But see the following: www. ncbi. nlm. nih. gov/pmc/articles/PMC 1289946/pdf/j rsocmed 00204 -0031. pdf !

Hybrids might have low fertility due to chromosomal incompatibilities v 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. v Only some of the resulting gametes will have full sets of genetic information. https: //qspace. library. queensu. ca

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.

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 v By this means, small subsets of a common ancestral population* leave at different times, each traveling in its own direction. * Often on the mainland v Upon arrival in a suitable habitat**, each such “founder” group will become established, responding to the unique environmental conditions of its new location. ** often an island

The Hawaiian islands provide many such examples of rapidly evolving populations • 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 • • 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. Campbell’s Biology, 7 th ed.

Dams built by beavers or humans create ponds and lakes and separate populations http: //oakridgevisitor. com www. mvhs 65. com Ø Studies of striped bass (Morone saxatilis) were carried out by Irwin Beitch 10 years after the completion of a large dam in southeastern Virginia. Ø Kidney morphologies of populations above (freshwater) and below (near the Chesapeake Bay) the dam had become significantly different from each other. www. highlandlakesflyfishing. com

Land masses that were connected during the Triassic Period later separated, taking life forms with them. “Gondwana” http: //earthscience. files. wordpress. com www. palaeos. com Check out the very cool animation at http: //www. divediscover. whoi. edu/images/gondwana 2. gif!

Ratite birds– an example of early evolution after Gondwana broke apart Cassowary (www. unfamiliar-image. co. uk ) Rhea www. animals. nationalgeographic. com www. uwm. edu Emu 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) Kiwi

Alternative models of speciation * In such a situation, however, there might exist subtle microhabitat differences within the same space. *

Ecological isolation within the same space ( sympatric speciation) A hypothetical example of how microhabitat differences might provide environmental pressures for genetically different members of a species to speciate in adjacent habitats: palm trees growing on volcanic soil (high in ash) or calcareous soil (of coral reef origin) NOTE: IF THIS HAPPENS AT ALL, IT’S VERY RARE

Another example of microhabitat differences possibly leading to sympatric speciation v The maggot fly Rhagoletis pomonella has a stable parasitic relationship with the American hawthorn tree, in which the fly lays its eggs. v In the seventeenth century apple trees were introduced to North America by the early colonists. v Over the years scientists began to observe the fly infecting apple trees, as well as hawthorns. v 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). v 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) ♀

Polyploidy can occur during mitotic cell division

Sudden speciation by polyploidy (happens mostly in higher plants) • 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 (2 n) reproductive cells could result. • Self fertilization of such diploid gametes would then produce tetraploid (4 n) 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

Speciation by polyploidy http: //trc. ucdavis. edu/biosci 10 v/bis 10 v/media/ch 13/wheat_speciation. html

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

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