alleles that eventually fix is 4000 generations. In addition, the time to fixation is highly variable since genetic drift is a stochastic process or a random walk. The standard deviation of the time to fixation is large, at approximately 2150 generations, consistent with a broad range in times to fixation. Relatively few mutations are expected to fix, but those mutations that do go to fixation segregate for a much longer time on average than the large proportion of mutations that go to loss. While mutations are segregating before their eventual end point of fixation or loss, there is polymorphism in the population.
An additional way to understand the expected polymorphism of neutral alleles in a population is to examine the equilibrium balance between genetic drift causing alleles to go to fixation and the input of new alleles in a population by mutation. Chapter 5 showed that for the infinite alleles model of mutation, the combined processes of mutation and genetic drift produce equilibrium heterozygosity:
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