GROUP SELECTION: THE INTERACTION OF LOCAL DEME SIZE AND MIGRATION IN THE DIFFERENTIATION OF SMALL POPULATIONS.

Abstract

The effects of population subdivision on the genetic differentiation of local demes by random genetic drift have been extensively studied from a theoretical point of view (see Wright, 1931, 1943, 1969; Crow and Kimura, 1970; Bodmer and Cavalli-Sforza, 1968; Lande, 1976; Maruyama, 1977). These theoretical investigations have been focused primarily on a single component of the genetic variance, the additive genetic variance, although the potential importance of the nonadditive components of genetic variance in the local differentiation of subdivided populations is often acknowledged (e.g., Wright, 1931, 1969; Crow and Kimura, 1970 p. 242; Slatkin, 1981). Few empirical laboratory studies of the genetic effects of population subdivision have been carried out and, with one exception, the studies have been carried out with single loci or major chromosomal inversions (Wright and Kerr, 1954; Buri, 1956; Dobzhansky and Pavlovsky, 1957; McCauley and Wade, 1980; Wade and McCauley, 1980; Wade, 1982). Population structure can be characterized by two parameters: (1) Ne, the effective size of local demes; and (2) m, the amount of migration among local demes (Crow and Kimura, 1970). In our previous experimental work (Wade and McCauley, 1980; Wade, 1982), one or the other of these parameters was held constant by design. In this paper, we report the results of an experimental study in which both parameters were varied singly and in combination to measure the interaction of Ne and m in their influence on the random genetic differentiation of local demes. Specifically, we examined the effects of three different local deme sizes (N = 16,48, and 96 breeding adults) in factorial combination with three island model migration rates (m = 0%, 6.25%, and 12.5% per deme per generation). Thus, the present study significantly extends the range of population structures previously examined (Wade and McCauley, 1980; Wade, 1982) to include FST values (Wright's measure of population subdivision) in the range .01 to .10. Random genetic differentiation of local demes is necessary for interdemic selection to occur in Sewall Wright's shifting balance theory of evolution (Wright, 1931, 1978). Just as the genetic response to individual selection depends upon the extent to which the phenotypic variation among individuals is determined by genetic variation, the response to interdemic selection depends upon the amount of genetic variation among demes and its relation to the phenotypic variation among demes. For this reason, empirical studies of population subdivision and its effects on population differentiation are important in evaluating the role of interdemic selection in nature. The fraction of the observed betweendeme phenotypic variance that determines the response to interdemic selection is called the (Wade and McCauley, 1980; Slatkin, 1981; Wade, 1982). The populational heritability is the fraction of the between-deme phenotypic variance that