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Unformatted text preview: The lack of fertility of the offspring of hybrids that are not completely sterile is usually attributed to hybrid breakdown Postzygotic reproductive isolation imposes a considerable fitness cost. Natural selection should normally favor the preemption of fitness costs associated with interspecific fertilization. 6.3 Geographical Separation of populations fosters speciation Geographically separated populations are called allopatric, evolutionarily independent. Speciation may still not occur upon barrier removal. Selection will strongly favor mating only within species upon reunification of allopatric species if there is established postzygotic reproductive isolation due to fitness costs. Natural selection actively favors the maintenance of multiple laters of mechamisms for reproductive barriers between species 6.4 The genetic mechanisms of allopatric speciation may be adaptive or nonadaptive...
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- Spring '08