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allele frequencies since it is easier to summarize a diallelic locus in a population as two allele frequencies rather than three genotype frequencies. But remember that genotype frequencies are affected by genetic drift too, and genotype frequencies can always be obtained by multiplication given allele frequencies under the assumption of random mating.

Up to this point, the beakers of micro-centrifuge tubes and computer simulations only considered cases of alleles at equal initial frequencies (p = q = 0.5). Figure 3.4 shows results of computer simulations for genetic drift where initial allele frequencies are p = 0.2 and p = 0.8 with identical population sizes of N = 25. Identical simulations under the assumptions of the Wright-Fisher model were used to produce both Figs 3.3 and 3.4, so the results in the two cases can be compared directly. Initial allele frequencies do influence the outcome of genetic drift in the simulations shown in Fig. 3.4. The lower initial allele frequency is associated with more frequent loss of the allele (five of six replicates) while the higher initial allele frequency fixed in five of six replicates. These results are consistent with what would occur in a much larger number of replicate simulations with the same initial allele frequencies and population size. A larger sample would show that the probability that an allele reaches fixation under genetic drift is the same as the initial allele frequency. This makes intuitive sense, since with a lower initial frequency a population is closer in frequency to loss than to fixation. If the direction and magnitude of genetic drift in allele frequencies is random, there is a better chance of reaching zero on average than reaching one. This same pattern would be true for any initial values of the allele frequency closer to zero or to one, except in the special case of equal allele frequencies where the chances of fixation or loss for an allele would be equal.

Under the Wright-Fisher model the following are general conclusions about the action of genetic drift in finite populations:

• the direction of changes in allele frequency is random;

• the magnitude of random fluctuations in allele frequencies from generation to generation increases as the population size decreases;

• fixation or loss is the equilibrium state if there are no other processes acting to counteract genetic drift or reintroduce genetic variation;

• genetic drift changes allele frequencies and thereby genotype frequencies; and

• the probability of eventual fixation of an allele is equal to its initial frequency (or the probability of ultimate loss of an allele is equal to one minus its initial frequency).

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