MICROSPATIAL GENETIC HETEROGENEITY IN POCKET GOPHERS: NON‐RANDOM BREEDING AND DRIFT

In the absence of selection, the genetic structure of populations will reflect the interaction of population size, amount of gene exchange, and breeding patterns. Populations characterized by high gene flow and random mating will be much less structured than those in which the reverse conditions hold. Thus, it will be in those species that are subdivided into small, discrete breeding units, or demes, that genetic drift and demic extinction can serve as strong evolutionary forces. If demes are sufficiently small and temporally closed, sampling error can override all but the strongest selection regimes (see, for example, Wright, 1931, 1969; Kimura, 1955). Moreover, local extinctions, while they may enhance the probability of drift by increasing the likelihood of inbreeding within surviving demes, may also act to maintain genetic heterogeneity among subdivisions of a population (Wright, 1940). Examples of species with highly structured populations in which the structure results, at least in part, from small population size and constraints to movement due to social factors and/or inherently low vagility include house mice (Petras, 1967; Selander, 1970; but see Baker, 1981) and land snails (Selander and Kaufman, 1975; Jones et al., 1977). Pocket gophers of the North American rodent family Geomyidae provide an example of how population size, local differential selection pressures, and genetic drift can interact to form populations highly structured on both a local and regional level. The Thomomys bottae complex is comprised of more than 200 described races (Hall and Kelson, 1959), based primarily on adaptive color variation and body size, and exhibits a concomitantly greater degree of chromosomal variability than any other known mammal (Patton, 1972). While selection regimes may be partly responsible for this variation (i.e., local cryptic matching of pelage color and soil color), the general biological attributes of gophers which stem from their fossorial habitus (such as low vagility, male and female exclusive territories, relatively low population densities, and patchy distribution) provide substrates for the action of stochastic factors in population divergence (Patton, 1972; Penney and Zimmerman, 1976). Indeed, in recent summaries of macrogeographic patterns in genic (=electromorphic) variability in this complex, the major determinants of within and between population variability were hypothesized to be gene flow and the temporal stability of populations (Patton and Yang, 1977; Patton and Feder, 1978). In order to further understand both the extent and controlling factors of among population heterogeneity, the genetic structure of pocket gopher (Thomomys bottae) populations has been examined in detail at Hastings Natural History Reservation, Carmel Valley, California, using electrophoretic techniques. At this site, gophers are generally confined to grassland fields of small size, varyingly isolated from one another by road cuts, stream courses, or patches of vegetation largely uninhabited by gophers (Fig. 1). Densities of animals in this area are moderate to high and no human-induced perturbations