Cutter_Lecture_06 - GeneticDrift(2) 1...

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Genetic Drift (2) 1
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1. Allele frequencies fluctuate at random gen to gen Eventually, 1 allele becomes fixed 2. Genetic variation declines and ultimately is lost 3. At a given time, a neutral allele’s probability of fixation equals its frequency in the population Previous history or frequency change don’t matter The initial frequency of a new mutation is 1/2N The probability of fixation of a new mutation is thus 1/2N Summary of Evolution by Genetic Drift 2
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4. Different populations with the same initial allele frequency (p) will diverge A fraction p of the populations fix for one allele, a fraction 1 p lose the allele 5. Among initially identical subpopulations, the average allele frequency does not change But the frequency of heterozygotes decreases, due to fixation by drift within each subpopulation The variance among demes in allele frequency increases with time 6. Evolution by drift proceeds faster in small populations The average time to fixation for a new mutation is 4N e generations Summary of Evolution by Genetic Drift 3
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Mutation ADDS genetic variation into a population Genetic drift ELIMINATES genetic variation from a population Both mutation and drift are random processes Contrasting stochastic evolutionary forces 4
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Elephant seals Currently ~30,000 individuals Exceptionally low molecular variation NO allozyme polymorphism at 24 loci Consistent with bottleneck in past Historical records indicate hunting bottlenecked population to ~20 animals in 1890’s High variance in male repro success probably caused N e < 20 Lab experiments confirm that predictive power of genetic drift theory operate appropriately In natural populations, make inferences about observed patterns given the verified theory Sometimes other data can further confirm inferences from observed patterns Inferences about natural populations 5
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What effects does gene flow have on genetic variation and evolution? Transfer of breeding individuals between subpopulations What fraction of all genetic variation in the species is due to differences between populations? Compared to within population variation? Migration (gene flow) and genetic drift 6
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Migration mixes alleles between populations Acts to homogenize populations Counters the tendency for genetic drift to cause divergence between subpopulations Homogenization due to gene flow 7 5 different subpopulations with different initial p
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Wright’s F statistics = fixation indices Hierarchical version of the inbreeding coefficient F F I S = fixation index for I ndividuals within S ubpopulations Equivalent to typical “F” from only 1 population F S T = fixation index for S ubpopulations relative to T otal Measure of differentiation between subpopulations Fraction of genetic variation partitioned between subpopulations Also, F I T How to quantify population structure? 8
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This note was uploaded on 04/22/2009 for the course EEB 318 taught by Professor Cutter during the Spring '09 term at University of Toronto- Toronto.

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Cutter_Lecture_06 - GeneticDrift(2) 1...

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