Genic and Chromosomal Differentiation in Pocket Gophers of the Geomys bursarius Group

Recently there has been a surge of interest in karyotypic variation in the vertebrates, particularly in mammals (Benirschke, 1969; Ford, 1970; White, 1973), and in the relationships of chromosomal modification to ecological adaptation and speciation. Research on karyotypic diversity in rodents, including pocket gophers (Geomyidae) (Thaeler, 1968; Patton and Dingman, 1968; Patton, 1970, 1972; Patton et al., 1972; Davis et al., 1971), Gerbillus (Zahavi and Wahrman, 1957), Spalax (Wahrman et al., 1969), Sigmodon (Zimmerman, 1970), and Peromyscus (Hsu and Arrighi, 1968; Lee et al., 1972), has demonstrated that many morphologically defined species are in fact pairs or complexes of allopatric or parapatric semispecies and species isolated to varying degree in part at least by chromosomal differences. Because extensive chromosomal reorganization can result in genetic isolation between populations as a consequence of hybrid sterility (White, 1969; Todd, 1970), it follows that speciation theoretically can occur without genic modification. Conversely, the recent discovery of homosequential species of Drosophila suggests that "it is possible for speciation and evolution to be based entirely on mutational changes occurring at the submicroscopic level" (Carson et al., 1967; Carson et al., 1970). It is therefore of interest to compare patterns of chromosomal and genic divergence in complexes of closely related species. In a group of species with a wide range of variation in chromosome morphology and number, is there a correlated pattern of genic differentiation? To answer this question, we have studied four morphologically defined species of pocket gophers of the Geomys bursarius group (Hall and Kelson, 1959; Baker and Williams, 1973), in which previous work has demonstrated an impressive range of karyotypic diversity (Davis et al., 1971; Kim, 1972; Baker et al., 1973). Of the four species of the Geomys bursarius group (Fig. 1), G. bursarius is the most widely distributed, occurring in the Plains Region, where numerous subspecies are currently recognized (Hall and Kelson, 1959). Geomys personatus and G. arenarius have moderate-sized ranges, and each is divided into several subspecies. G. personatus is parapatric with G. bursarius in southern Texas (Kennerly, 1959), and G. arenarius has a disjunct range on the sandy areas along the Rio Grande and associated tributaries in New Mexico, Texas, and Chihuahua. The fourth species, G. tropicalis, is confined to an area of old sanddune habitat of approximately 300 square kilometers near Altamira, Tamaulipas (Baker and Williams, 1973). Geomys tropicalis apparently is separated geographically from its nearest neighbor, G. personatus, which extends southward from the main part of its range in Texas on the narrow barrier islands of coastal Tamaulipas. Present information indicates a gap of 250 kilometers (Selander et al., 1962; Alvarez, 1963). On the basis of morphological evidence and geographic considerations, Russell (1968) has proposed that arenarius is cladistically allied with bursarius and that geographic isolation was imposed in a postWisconsin period of increasing aridity.