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ch6outline - ALLOPATRIC SPECIATION 6.1 The Biological...

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ALLOPATRIC SPECIATION 6.1- The Biological species concept is based on the reproductive isolation of organisms that are given the opportunity to mate B.S.C = way of defining when speciation occurred, makes reproductive isolation key for separation pops. into multiple species. Reproduction Isolation = failure of organisms to reproduce successfully when placed together in normal mating circumstances Prezygotic Isolating Mechanisms o Different habitats or mating sessions o Lack of sexual attraction o Coital Problems o Fertilzation Problems 6.2 – Species may be isolated by a failure to reproduce that occurs after fertilization, which will select for isolation before fertilization Postzygotic Isolating Mechanisms act after fertilization occurs. In most animal groups, hybrids are mostly inviable, sterile, or have greatly reduced fertility Hybrid Breakdown: Occurs due to a failure of the hybrid’s genetic system to produce gametes that can successfully combine with the gametes of the parental species, or with the gametes of other hybrids. 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.
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