Blackwell Publishing

The theory of natural selection (part 2) - What are the effects of migration?


Migration-selection balance

If selection is working against an allele within one subpopulation, but the allele is continually being introduced by migration from other populations, it can be maintained by a balance of the two processes. We can analyze the balance between the two processes by much the same arguments as we used for selection-mutation balance and heterozygous advantage. The simplest case is again for one locus with two alleles. Imagine selection in one subpopulation is working against a dominant A allele. The fitnesses of the genotypes are


The A allele has frequency p in the local population. Suppose that in other subpopulations, natural selection is more favorable to the gene A , and it has a higher frequency in them, pm on average. pm will then be the frequency of A among immigrants to our local population. In the local population, A genes are lost at a rate ps per generation. They are gained at a rate (pm -p )m per generation: m is the proportion of genes that are immigrants in a generation; immigration increases the frequency in the local population by an amount pm -p because gene frequency is increased only insofar as the immigrating population has a higher frequency of A than the local population. If the immigrating gene frequency is the same as the local gene frequency, immigration has no effect.

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