BIOL 4P08 Lecture Notes - Allopatric Speciation, Disruptive Selection, Evolutionary Radiation

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Prezygotic barriers operate before fertilization: mechanisms that operate before fertilization prezygotic reproductive barriers may prevent individuals of different species or populations from interbreeding, habitat isolation, temporal isolation, mechanical isolation, gametic isolation, behavioural isolation. If individuals of two different populations lack complete prezygotic reproductive barriers, postzygotic reproductive barriers may still prevent gene exchange: low hybrid zygote viability, low hybrid adult viability, hybrid infertility. Individuals that mate with individuals of the related species should evolve prezygotic reproductive barriers more rapidly that allopatric pairs of species. Hybrid zones may form if reproductive isolation is incomplete. If contact is re-established between formerly isolated populations before complete reproductive isolation has developed, members of two populations may interbreed. Three outcomes of such interbreeding are possible: in hybrid offspring are as fit as those resulting from mating within each population; hybrids may spread through both populations and reproduce with other individuals.

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