Great Basin Naturalist


Genetic variation across the known geographic range of Thomomys townsendii was examined by starch gel electrophoresis and nondifferentially stained karyotypes. All specimens examined had a diploid number of76, but some populations possessed one pair of acrocentric chromosomes in an otherwise biarmed karyotype. Electrophoretic analyses of 16 populations revealed Thomomys townsendii to be among the least variable of pocket gopher species studied. Of 27 loci examined, 17 were monomorphic and heterozygosity values were low (H = .000–.028, = .012). Genic differentiation between populations was also low (S = .896–.998, = .956) and revealed little concordance with current subspecific units. The most marked differentiation is between the Honey Lake Valley/Humboldt River region and the Snake River drainage. Within each of these regions population variation is, in some cases, greater within than between currently defined subspecies. F-statistics show that the greatest genic differentiation is seen between populations, and a comparison of subspecies within a region shows the greatest homogeneity. The evolutionary history of Thomomys townsendii is discussed in the context of both the physiographic history of the region and the affinities of this species to Thomomys bottae. Results of this study and the patterns of differentiation found in the morphological analyses of the accompanying paper (Rogers 1991) indicate that the only consistent pattern to be discerned from the overall morphological and genetic homogeneity of Thomomys townsendii is that between populations of the Humboldt and Snake River drainage systems, which are here assigned to Thomomys townsendii nevadensis and Thomomys townsendii townsendii, respectively.