Selection in Natural Populations

It is clear to even the most casual observer that the environment is in a constant state of flux. It must be the case that fitness differences between genotypes are also in a constant state of flux.

J. H. Gillespie, The Causes of Molecular Evolution ( 1991)

The discovery of abundant protein genetic variation stimulated evolutionary biologists to take a more critical look at their models and to build more biologically realistic models. This reevaluation has included several largely independent goals. Two of the responses were at least partially prompted by the genetic load that would be generated if the variation were maintained by balancing selection. First, a quick response by three papers in the journal Genetics ( King 1967; Milkman 1967; Sved, Reed, and Bodmer 1967) questioned the traditional calculation of genetic load and devised models of fitness determination that generated little segregational load. Second, concern about an intolerable segregational load spurred the development and elaboration of a theory concerning the evolutionary dynamics of functionally equivalent, or neutral, allelic variation ( Crow and Kimura 1970; Kimura 1983; Kimura and Ohta 1971; Nei 1975). Third, emphasis shifted from single-locus models to multilocus models, and biological reality was increased to examine the effect of environmental heterogeneity on the distribution of genotypes.

This chapter begins by reviewing the models of environmental heterogeneity and concludes by examining the empirical studies of the influence of environmental heterogeneity on genetic variability.

Levene ( 1953) first illustrated the importance of environmental heterogeneity for the maintenance of genetic variation. He modeled a randomly mating population distributed over several environmental patches that imposed different selection regimes on

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