GENIC DIVERGENCE AND LOCAL POPULATION DIFFERENTIATION BY RANDOM DRIFT IN THE POCKET GOPHER GENUS GEOMYS.

Abstract

With the recent development in the use of electrophoresis, there has been a surge of interest in studying genetic variation in populations and genetic differentiation of vertebrate species (Selander and Johnson, 1973). It has become increasingly obvious that one can correlate patterns of genetic variation with geographic distribution, strategies of adaptation, and the interrelationships of enzyme polymorphism and metabolism (Selander and Kaufman, 1973; Johnson, 1974; Nevo et al., 1974). Specifically, the following should be investigated to determine if correlations exist between protein variation and the biology of the organism: (1) patterns of protein variation and geographic distribution; (2) patterns of protein variation and substrate specificity of the protein; (3) patterns of protein variation and adaptive strategy; and (4) patterns of genic divergence, speciation, and evolutionary time. To test for such correlations, we chose to investigate genic variation in a genus of fossorial rodents, Geomys. At present, four species of the Geomys complex of pocket gophers inhabit a large area of the central and southeastern United States, while one species, G. tropicalis, is found only in a small area in southern Tamaulipas, Mexico (Hall and Kelson, 1959). The ranges of the five species reflect past distribution with isolation of three of the forms into disjunct populations. Geomys bursarius has the most extensive range from the western great plains, south to the Texas coast, and north into Canada. Geomys personatus is restricted to the Texas Rio Grande valley north to the Rio Grande River and is contiguous with G. bursarius from Corpus Christi to the San Antonio River. Geomys arenarius is isolated in extreme western Texas and south-central New Mexico, and G. pinetis is isolated in Alabama, Georgia, and Florida. The evolution of the genus Geomys is well-documented, based on fossil evidence. According to Russell (1968), Geomys personatus and G. arenarius probably diverged from the southern and western portions of populations of G. bursarius during Wisconsin or Recent time. Geomys pinetis and G. bursarius were separated and diverged from the ancestral stock during the midIllinoian glacial period, with post-glacial habitats of the Mississippi Valley preventing reestablishment of the forms in that region. Chromosomal evolution in the genus Geomys has been characterized by Davis et al. (1971), Kim (1972), and Baker et al. (1973). More recently, Selander et al. (1975) compared interspecific chromosomal differences and genic divergence in four species of Geomys, and although some of their data are similar to ours, they studied only four of the five extant species. In this study we have utilized data from 22 genetic loci from 25 populations of pocket gophers including all 5 species of the genus. In the process we have attempted to compare genetic variation and patterns of geographic distribution, degree of isolation, protein specificity, adaptive strategy, and speciation in the genus Geomys.