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GENETIC DIFFERENTIATION WITHIN SUBDIVIDED POPULATIONS OF FORMICA ANTS

A population structure in which the metapopulation is divided into small breeding units is often considered most favorable for evolution, because it facilitates genetic change and interpopulation differentiation (Wright, 1931, 1940). Such a structure may be common in nature, as electrophoretic studies have revealed considerable microgeographic differentiation in natural populations of many animals, i.e., genetic differentiation in an area which might be thought to be within the dispersal range of single individuals. This has been detected in both vertebrates (e.g., Selander, 1970; Patton and Feder, 1981) and invertebrates (e.g., Selander and Kaufman, 1975; Richmond, 1978; Jones et al., 1980; Varvio-Aho and Pamilo, 1980). One cause of such microgeographic differentiation in allele frequencies could be locally differing selection pressures which may be further enhanced by habitat choice by individuals. Alternatively, the differentiation may arise from stochastic processes. The level of genetic differentiation among partially isolated subpopulations will, in the absence of selection (and ignoring mutation), depend on effective sizes of demes and the migration rates between them. In continuous populations, local allele frequency differences may arise because of isolation by distance, and the basic conceptual unit is a neighborhood, the size of population from which the parents may be treated as if drawn at random (Wright, 1943, 1951). The effects of various population structures on genetic differentiation have been examined by using