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Evolution in Finite Populations (Coalescent theory and Genealogy of Genes,…
- Evolution in Finite Populations
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Demography, Biogeography & Drift
even a brief reduction in population size can cause effective population size to be drastically reduced, thus causing drift to operate very strongly (below are 2 processes of how population can reduce in size drastically)
1. Population Bottlenecks
-allele frequencies change drastically because (a) sampling effects that occurs during reduction of popn size, (b) accelerated pace of drift after popn reduction
-biggest changes in allele frequency occur during the bottleneck
-bottleneck reduces genetic variation within a popn
-when popn are restored to original size, rate of allele fluctuations slows
-bottleneck causes divergence BTWN populations within a species
2. Founder Effect
-changes in allele frequency that results from sampling effects that occur when small no. of individuals from a large popn colonize a new area and found a new popn
-allele frequencies in founders deviate by chance from those in large popn
-if island popn is smaller than mainland popn, the effect of genetic drift will be greater as well. The probability of an allele going to fixation is equal to its initial frequency on the island
-fixation of particular allele WITHIN popn & high between-popn variation = hallmark of genetic drift
Interplay of Drift, Mutation & NS
if drift were the only evolutionary process occurring, any finite popn will eventually become entirely homozygous. However, in practice, popn do not become homozygous as mutation provides continuous supply of genetic variation.
Selection and drift operate simultaneously on a popn to determine allele frequency change over generations
factors that reduce effective popn size (popn bottlenecks, set ratio biases) reduce the chance that a beneficial mutation will be fixed
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