Ht-1. The decrease in heterozygosity can also be thought of as an increase in autozygosity or the fixation index (F) through time under genetic drift. The dotted lines in each panel are levels of heterozygosity (2pq) in six replicate finite populations experiencing genetic drift. There is substantial random fluctuation around the expected value for any individual population.

progeny are sampled from a gamete pool containing only two possible alleles. Compare that with the case of outcrossing between unrelated individuals, where the alleles transmitted to progeny are sampled from a pool of four possible alleles. The probability of auto-zygosity is clearly higher with the smaller parental gamete pool associated with consanguineous mating, just as it is also higher in a smaller population where mating is random.

The faster decrease in heterozygosity in small compared to large populations can be seen in the wild. One study used the reasoning that organisms restricted to islands should have smaller census population sizes than the same species found on adjacent mainland areas (Frankham 1998). Based on the relationship between autozygosity and effective population size just derived, the expectation is that island populations should show lower levels of heterozygosity than mainland populations. Although there were some exceptions, the general pattern was that island populations had lower levels of heterozygosity than did mainland populations (Table 3.3). This is exactly the pattern expected due to the faster rate of decrease in heterozygosity in smaller populations even when mating is random. The comparison assumed that the island and mainland populations remained very similar in terms of the degree of consanguineous mating as well as genetic parameters that influence the input of genetic variation such as the migration and mutation rates. Since the comparisons were between populations of the same species that are therefore very closely related, these assumptions seem likely to be met at least approximately.

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