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that heterozygosity declines by a factor of 1 - —~

e every generation caused simply by sampling from a finite population that results in some autozygous genotypes every generation. Recall that this is exactly the same result that was obtained for the rate of loss of genetic variation with the Markov chain model. The degree of sampling varies directly with the effective population size, so that the rate of increase in auto-zygous genotypes also depends directly on the effective population size. The expected heterozygosity from equation 3.52 is shown in Fig. 3.19 for four different effective population sizes over 50 generations. For comparison, heterozygosity in six independent replicate populations experiencing genetic drift are also plotted. The random trajectories of heterozyg-osity in these individual populations make clear that equation 3.52 provides an expectation for average heterozygosity taken across a large number of replicate populations or numerous independent neutral loci if applied to a single population.

There are two conclusions that can be drawn from the interrelationship between autozygosity and the effective population size. First, genetic drift causes populations to become more inbred in the sense that autozygosity and homozygosity increase even though mating is random. An important distinction is that genetic drift causes heterozygosity to decrease due to the fixation and loss of alleles. In contrast, consanguineous mating decreases heterozygosity by changing genotype frequencies but does not impact allele frequency. Genetic drift produces homozyg-osity since ultimately one allele reaches fixation while consanguineous mating produces homozygous genotypes for all alleles in the population. Second, mating systems where there is consanguineous mating cause genetic variation in populations to behave, from the perspective of heterozygosity, as if the effective population size were smaller than it would be under complete random mating. For example, when a parent self-fertilizes, the alleles transmitted to its

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