Speciation How species evolve

SPECIES – BIOLOGICAL CONCEPT Population or groups of populations whose members have the potential to interbreed and produce fertile offspring with each other, but not with members of other species Emphasizes reproductive isolation

Speciation Process by which one species splits into two or more species, accounts for both the unity and diversity of life.

Reproductive Barriers Depending on whether they function before or after zygotes form, reproductive barriers are categorized as prezygotic or postzygotic.

Habitat isolation (lack of opportunities to encounter each other) Figure 14.3_1 Reproductive barriers keep species separate (part 1) The garter snake Thamnophis atratus lives mainly in water. The garter snake Thamnophis sirtalis lives on land.

Temporal isolation (breeding at different times or seasons) The eastern spotted skunk (Spilogale putorius) breeds in late winter. The western spotted skunk (Spilogale gracilis) breeds in the fall. Figure 14.3_2 Reproductive barriers keep species separate (part 2)

Behavioral isolation (different courtship rituals ) Figure 14.3_3 Reproductive barriers keep species separate (part 3) The blue-footed booby (Sula nebouxii) performs an elaborate courtship dance. The masked booby (Sula dactylatra) performs a different courtship ritual.

Video: Blue-Footed Boobies Courtship Ritual

Video: Albatross Courtship Ritual

Video: Giraffe Courtship Ritual

Mechanical isolation (physical incompatibility of reproductive parts) Figure 14.3_4 Reproductive barriers keep species separate (part 4) Heliconia pogonantha is pollinated by hummingbirds with long, curved bills. Heliconia latispatha is pollinated by hummingbirds with short, straight bills.

Gametic isolation (molecular incompatibility of eggs and sperm or pollen and stigma) Figure 14.3_5 Reproductive barriers keep species separate (part 5) Purple sea urchin (Strongylocentrotus purpuratus) Red sea urchin (Strongylocentrotus franciscanus)

Reduced hybrid viability (hybrid development or survival impaired by interaction of parental genes) Some species of salamander can hybridize, but their offspring do not develop fully or, like this one, are frail and will not survive long enough to reproduce. Figure 14.3_6 Reproductive barriers keep species separate (part 6)

Reduced hybrid fertility (vigorous hybrids that cannot produce viable offspring) The hybrid offspring of a horse and a donkey is a mule, which is robust but sterile. Figure 14.3_7 Reproductive barriers keep species separate (part 7)

Reduced hybrid fertility (vigorous hybrids that cannot produce viable offspring) The rice hybrids on the left and right are fertile, but plants of the next generation (middle) are sterile. Figure 14.3_8 Reproductive barriers keep species separate (part 8)

Modes of speciation Geographically separated from other populations, a small population may become genetically unique as its gene pool is changed by natural selection, mutation, or genetic drift.

ALLOPATRIC Populations segregated by geographic barrier Geographic barrier = ocean, mtn. Range, etc. Conditions favoring o Small population at fringe o Better chance gene pool already different o Different selection factors Adaptive radiation – emergence of numerous species from common ancestor introduced into environment

SYMPATRIC New species arise within range of parent species Reproductive isolation without geographic barrier Plants – polyploidy (multiple sets of chromosomes) Animals – group fixed on resources not used by parent population

GENETIC CHANGE Adaptive divergence 2 parts of population adapt to different environments Each accumulates different gene pools Reproductive barriers develop Usually secondary to change in gene pool May be side effect of sexual selection

Remember: Species do not evolve because of need. Biological diversity exists and the environment selects. Evolution is not deliberate; it is reactive. Species do not deliberately change. There is no plan.

Models for speciation 1 species gradually evolves into new species • GRADUALISM   PUNCTUATED EQUILIBRIUM 1 species gradually evolves into new species Represents microevolution Big changes occur through the accumulation of small changes • Long periods of stasis (equilibrium) punctuated by episodes of speciation Species undergo most changes when first branch from parent; then change little Species develop in spurts of rapid change Not slow and gradual

Tempo of speciation The time interval between speciation events varies from a few thousand to tens of millions of years. Punctuated pattern Gradual pattern Time

Animation: Macroevolution

You should now be able to Distinguish between microevolution and speciation. Compare the definitions, advantages, and disadvantages of the different species concepts. Describe five types of prezygotic barriers and three types of postzygotic barriers that prevent populations of closely related species from interbreeding. Explain how geologic processes can fragment populations and lead to speciation.

You should now be able to Explain how reproductive barriers might evolve in isolated populations of organisms. Explain how sympatric speciation can occur, noting examples in plants and animals. Explain how sexual selection can lead to speciation. Explain why isolated islands are often sites of speciation.

You should now be able to Describe the circumstances that led to the adaptive radiation of the Galápagos finches. Explain how hybrid zones are useful in the study of reproductive isolation. Compare the gradual model and the punctuated equilibrium model of evolution.