Speciation is especially likely in small isolated populations.

1. Allopatric speciation 3 steps: (1) isolation (geographic) (2) divergence (mainly by drift) (3) reproductive isolation (in secondary contact) • Isolation • without migration, populations can diverge • allopatry – geographic separation prevents gene flow • Isolation can occur through: • colonization of a new habitat • - cuts off gene flow • - possible change in natural selection • vicariance – species range is divided • e.g., sinking/rising of land bridge, separation of continents Speciation is especially likely in small isolated populations.

3. Allopatric speciation 3 steps: (1) isolation (geographic) (2) divergence (mainly by drift) (3) reproductive isolation (in secondary contact) • Divergence • after isolation, no gene flow, so populations can diverge • originally thought divergence was primarily by genetic drift • Divergence by genetic drift • most pronounced in small populations • small groups of colonizers • small isolated peripheral populations divergence by genetic drift is likely Speciation is especially likely in small isolated populations.

4. Allopatric speciation 3 steps: (1) isolation (geographic) (2) divergence (mainly by drift) (3) reproductive isolation (in secondary contact) • Reproductive isolation • after divergence in isolation, populations may no longer be able • to produce viable offspring • postzygotic reproductive isolation – hybrid offspring are unfit • - predicted to evolve in isolation • Secondary contact • after divergence in isolation, populations come back together • hybrid matings take place, but offspring are unfit  • reinforcement – natural selection for avoiding hybrid matings • reinforcement completes reproductive isolation • prezygotic reproductive isolation – hybrid matings don’t occur

5. Other ways for speciation to work 3 steps: (1) isolation (not necessarily geographic) (2) divergence (not just by drift) (3) reproductive isolation (secondary contact not necessary) • Isolation • without gene flow, populations can diverge • Other methods of isolation: • chromosomal changes including polyploidy • - matings become incompatible

7. Other ways for speciation to work 3 steps: (1) isolation (not necessarily geographic) (2) divergence (not just by drift) (3) reproductive isolation (secondary contact not necessary) • Isolation • without gene flow, populations can diverge • Other methods of isolation: • chromosomal changes including polyploidy • - matings become incompatible • sympatric speciation – two groups in the same place • - isolation by biology rather than geography especially habitat or food preferences  promote mating within groups  expose groups to diff. selective factors

8. Sympatric speciation 3 steps: (1) isolation (different host plants) (2) divergence (natural selection) (3) reproductive isolation • Example:apple maggot flies • mate and lay eggs on apples or hawthorns • each host plant selects for different traits • because mating occurs on host fruit, • assortative mating occurs • and gene flow is prevented

9. Other ways for speciation to work 3 steps: (1) isolation (not necessarily geographic) (2) divergence (not just by drift) (3) reproductive isolation (secondary contact not necessary) • Other mechanisms of divergence • natural selection – isolated populations experience different env’s • - group A fitness depends on different traits than group B fitness • sexual selection – isolated populations differ in sexual selection • affects gene flow directly • may diverge until they fail to recognize each other as mates  • prezygotic reproductive isolation (can evolve first) • divergence in sympatry – strong selection can allow divergence • - even with low amounts of gene flow • alleles introduced through gene flow will be unfit • - assortative mating must prevent most gene flow between groups • (mating between individuals experiencing same environment)

10. Other ways for speciation to work 3 steps: (1) isolation (not necessarily geographic) (2) divergence (not just by drift) (3) reproductive isolation (secondary contact not necessary) • Reproductive isolation • prezygotic reproductive isolation – hybrid matings don’t occur • can evolve through sexual selection, drift, or natural selection • or by reinforcement (natural selection to avoid hybrid matings) • postzygotic reproductive isolation – hybrid offspring are unfit • - can evolve by drift or natural selection • Secondary contact • not required if prezygotic reproductive isolation evolves independently • not required if the new species never come in contact