GROUP SELECTION: MIGRATION AND THE DIFFERENTIATION OF SMALL POPULATIONS.

It is well known that, in the absence of selection, the subdivision of a large population into isolated demes will result in the genetic differentiation of the population subunits owing to random genetic drift (Wright, 1931, 1969; Crow and Kimura, 1970). Wright (1931, 1978) considered selection among randomly differentiated local demes to be one of the "greatest creative forces for evolutionary change" and this interdeme selection was the basis for his "shifting balance theory" of evolution. Selection at any level of biological organization, however, depends upon the interaction of phenotypes and environments and the response to selection is determined by the amount of genetic variation underlying the phenotypic variability. For this reason the study of the relationship between genetic drift and the differentiation of populational phenotypes is important to evaluating Wright's shifting balance theory and the role of interdeme selection in evolution. In evolutionary discussion, selection on a trait generally implies a relationship of covariance between values of the trait and fitness. Strictly speaking, the opportunity for selection depends upon the interaction of the phenotype with the environment; the response to selection depends, in addition, upon the nature of the relationships between the phenotypic variation and the genetic variation. Using laboratory populations of the flour beetle, Tribolium castaneum, Wade and McCauley (1980) and McCauley and Wade (1980) showed that population subdivision in the absence of migration can result in the rapid phenotypic and genotypic differentiation of local demes for traits affecting the rate of population increase. We introduced the term "population heritability" (Wade and McCauley, 1980) in order to relate the observed phenotypic differentiation of the isolated demes to the process of intergroup selection. The "population heritability" is that fraction of the observed between-deme phenotypic variance which is heritable in that demes descended by colonization from the same "parental" population resemble one another and that parent in their populational characteristics. Motivated in part by these experimental studies with Tribolium, Slatkin (1981) conducted an analytic study of the effect of population subdivision on the between-deme variance in the average value of a quantitative character. These empirical and theoretical studies are relevant to both aspects of interdemic selection, the opportunity for interdemic selection and the response to interdemic selection. First, both studies found that the rate of phenotypic differentiation of an array of demes is curvilinear over time and inversely proportional to effective deme size. Thus, the opportunity for interdemic selection, which depends upon the amount of phenotypic variation among demes, decreases as effective deme size increases. That is, the rate of differentiation of populational phenotypes parallels the theoretically expected rate of genetic differentiation among the demes. In the empirical studies, however, the among-deme phenotypic variance achieved values in the treatments with large effective deme sizes (Ne = 21.0 and Ne = 40.1) equal to those in the treatments with smaller effective deme sizes (Ne = 5.8 and Ne = 11.0) within 11 generations (cf. Wade and McCauley, 1980 p. 803, Table 1 and p. 804, Fig. 2).