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chap 22.docx

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Maryann Vaughan

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Mechanisms of Divergence  Dispersal, vicariance, and polyploidization only create the conditions for speciation o For speciation to take place, genetic drift & natural selection have to act on mutations in a way that creates divergence in the isolated populations Genetic Drift  Effects of genetic drifts are random fixation of alleles and random loss of alleles  Drift can produce rapid genetic divergence in small, isolated populations o Its effects are most pronounced in small populations  Normally, only tiny numbers of individuals are involved in colonization events; vicariance events fragment large populations into two or more smaller ones; and polyploidization initially produces only a handful of individuals o Due to this matter, genetic drift has long been hypothesized as the key to speciation’s second stage  A general model of speciation: o Small populations that become isolated start out as a non-random sample of the ancestral population. As drift continues to occur in the small, derived population, it leads to a random loss of allele and the random fixation of existing and new alleles. As a result, the isolated population should undergo rapid genetic divergence from the ancestral population.  Bottleneck- a large-scale but short-term reduction in population size followed by an increase in population size o Has shown that when a population is reduced to a small size for a short period of time only very rare alleles tend to be lost due to drift  For drift to change allele frequencies dramatically, the founding population has to be extremely small and remain small for a significant period of time Natural Selection  Marked genetic differences have to emerge between closely related populations for speciation to occur o Drift almost always plays a role in creating these genetic differences when at least one of the populations is small o But natural selection can also lead to divergence if one of the populations occupies a novel environment or uses a novel resource  Natural selection can cause populations to diverge even when a small amount of gene flow occurs  Natural selection is a potent force for divergence Sexual Selection  Sexual selection acts on characters involved in mate choice. Changes in sexual selection can cause reproductive isolation and trigger rapid divergence  Sexual selection is a form of natural selection that results from differences among individuals in their ability to obtain mates  It has been shown that changes in the way that a population of sexual organisms chooses or acquires mates can lead to rapid differentiation from the ancestral populations  The key point is that sexual selection promotes divergence efficiently because it affects gene flow directly Secondary Contact  Suppose a particular speciation event begins with the geographic isolation of two populations and a corresponding reduction in gene flow, and then continues a
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