07_Genetic_Drift__Founder_Effect__and_Ge (3)

07_Genetic_Drift__Founder_Effect__and_Ge (3) - BI SC 002...

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BI SC 002 LECTURE 7—GENETIC DRIFT, FOUNDER EFFECT, AND GENE FLOW Draft: November 10, 2010 Being an isolated population shifts how a group evolves. Being isolated limits the gene pool—if there is no immigration or emigration, no new alleles (and thus no new traits) can enter a population. Ultimately, isolation can lead to speciation, but that’s the next lecture. Today we talk about how isolated populations function on the microevolutionary (small) scale. Populations can fluctuate in size from time to time. During a period when a population is small, an uncommonly high frequency of a recessive allele may occur. This is often due not to a selective pressure but to random chance. If there are a lot of heterozygotes present in the parents, many more recessive offspring may occur. This idea is called genetic drift. Genetic drift is the random fluctuation of an allele due to chance alone. Say, for instance, that there is a population of frogs—some are striped and some are not. If a cold winter kills many frogs in the population, it may be just at random that more striped ones survive than non-striped. The population will now shift towards striped. Having a stripe doesn’t give a reproductive advantage over not having a stripe; it was strictly random chance that the ones who survived happened to have a stripe. If a few individuals migrate away from the main population, only a limited portion of that original population’s gene pool is carried with the immigrants. If they form a new population, that population will only have that limited gene pool. This is called founder effect . That is, the new population’s alleles are representative only of those individuals who founded the population. Several examples of founder effect can be found in Native American groups.
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07_Genetic_Drift__Founder_Effect__and_Ge (3) - BI SC 002...

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