also the expected allele frequency in both subpopulations at equilibrium.

The main conclusion of the two-island model is that the equilibrium allele frequencies in the two subpopulations are the average allele frequency of the total population when the two migration rates are equal. This conclusion holds when there are a larger number of subpopulations, a result that will be useful to remember when considering the process of gene flow in an island model in combination with another process such as genetic drift.

Infinite island model

One of the most widely used models of the process of gene flow among a set of subpopulations is Wright's

(1931, 1951) infinite island model. Gene flow takes the form of all subpopulations being equally likely of exchanging migrants with any other subpopulation, equivalent to a complete absence of isolation by distance. In addition, the size and migration rate of each subpopulation is most commonly assumed to be equal. The total population is made up of an infinite set of subpopulations each of size Ne with m percent of each subpopulation's gene copies exchanged at random with the rest of the population every generation (see Fig. 4.12b). Using this model it is possible to approximately relate the degree of differentiation among subpopulations to a function of the effective population size and the amount of migration.

Infinite island model An idealized model of population subdivision and gene flow that assumes an infinite number of identical subpopulations (demes) and that each subpopulation experiences an equal probability of gene flow from all other subpopulations.

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