With population - heterozygotes mutation will introduce new...

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With population bottlenecks where the population size drops to a small number in one generation, the effective population size is not just the average of N's for each generation. To estimate the Ne, one calculates the harmonic mean population size as follows: 1/Ne = 1/t_1/Nt where t = the number of generations and Nt = the population size at each generation. Thus with population sizes of 100, 100, 20, 100 the arithmetic mean = 80 but 1/Ne = 1/4(1/100 + 1/100 + 1/20 + 1/100) = 1/4(0.08) = 0.02, so Ne = 1/0.02 = 50 . Thus the smaller population size has a disproportionate effect on the effective population size. This is a very important issue in conservation efforts concerning endangered species. DRIFT AND MUTATION Drift will tend to reduce heterozygosity (for our purposes this equals the proportion of
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Unformatted text preview: heterozygotes), mutation will introduce new alleles which will serve to increase heterozygosity. This provides yet another example of a "fight" between opposing evolutionary forces. When the mutation rate is close to the reciprocal of the population size , heterozygostity will be high (i.e., a considerable amount of variation will exist in the population). The "balance" of this equilibrium can be described by an equation for the equilibrium frequency of heterozygotes: H Å Plug in Ne's and u's to determine balance: when Ne = 1/u, H = 0.8; when u>1/Ne then heterozygosity will be higher; when u<1/Ne heterozygosity will be lower....
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