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DYNAMICS OF MORPHOLOGICAL DIFFERENTIATION: TEMPORAL IMPACT OF GENE FLOW IN POCKET GOPHER POPULATIONS

What role does gene flow play in the evolution of natural populations? This has been an often debated question during the past two decades for at issue is a fundamental understanding of the relative roles of different evolutionary forces which may shape population and species differentiation (see Mayr, 1982). Gene flow has been viewed as a retarding element of speciation (Mayr, 1963) and the "glue" that holds populations together (Hartl, 1981). Gene flow may also provide a source of new genetic variation when migration occurs between populations with large differences in gene frequencies, such as well differentiated races. From single locus theory one can predict that ". .. if the difference between the donor populations and the population that receives immigrants is .15 or greater, gene migration is at least equally potent in evolution, and usually more so, than selection pressures of equal magnitude" (Wilson and Bossert, 1971 p. 63-64). On the other hand, Ehrlich and Raven (1969) and others have argued that gene flow may be only of minor significance in evolution because of the low dispersal propensity of many organisms (but see also Jackson and Pounds, 1979). The problem in assessing the evolutionary role of gene flow lies in the difficulty of documenting unequivocally its effects in natural populations. Indeed, few such studies are known. In the case of the water snake, Natrix sipedon, on islands in Lake Erie, selection against banded individuals is apparently balanced by migration of banded snakes from the mainland (Camin and Ehrlich, 1958). Most studies in the literature, however, involve measurement of individual movements, or dispersal, which is taken as an indirect measure of gene flow potential. Even so, these measures may vary geographically (e.g., Euphydryas editha on Jaspar Ridge [Brussard et al., 1974] as opposed to other localities [Gilbert and Singer, 1973]) so the significance of gene flow in any particular situation is difficult to ascertain from this perspective alone. Our study explores the potential impact of gene flow on morphological change in several natural populations of the pocket gopher, Thomomys bottae. Samples available from museum collections permit examination of both temporal and spatial variation over a period of less than one hundred years. Such temporal series, when examined in the context of spatial variation, are useful in distinguishing between patterns resulting from gene flow and those due to selection along environmental gradients. Pocket Gophers of the Colorado River. -Two geographic units within the Thomomys bottae complex meet along the lower Colorado River in California and Arizona (Smith and Patton, 1980). These are strongly differentiated electromorphically (mean genetic similarity is only .67), but population groupings based on either phenetic or phyletic analyses of genic data are discordant with morphological relationships suggested by previous authors. Three different systematic views of these populations are summarized in Figure 1. Thomomys b. albatus is a large, pale form inhabiting the Colorado River delta region in the Yuma Valley as well as irrigated land in the Imperial Valley of California. Thomomys b. riparius is also a large, pale form, found in the Palo Verde Valley to the north on the California side of the river. Thomomys b. chrysonotus is a small, pale gopher