The Role of Migration in the Genetic Structure of Populations in Temporally and Spatially Varying Environments. III. Migration Modification

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Abstract
Evolution may act on migration rates in such a way as to maximize the geometric mean fitness of the population. This maximization involves a decrease in migration when there is, in some sense, more spatial than temporal variation in the environment and an increase when there is more temporal than spatial variation. There may or may not be an intermediate level of migration which maximizes the geometric mean fitness depending on the values of the various parameters. As with other modifier loci the rate of change in frequency of the migration alleles is very small, of the order of the product of the linkage disequilibrium and the selection coefficient of the primary locus. This analysis is only concerned with a single selected locus. The general impression from much of the work on modifier theory is that selection will usually either maximize or minimize the parameters under consideration (Karlin and McGregor, 1974). In fluctuating environments intermediate levels of the parameters may be achieved by modification. This is suggested to be the case for the modification of recombination in the simulations of Charlesworth (1976). If so, a good deal of the explanation for the values of genetic parameters observed in natural populations may reside with the effects of fluctuating environments.