Macrogeographic Patterns of Genetic Differentiation in the Pocket Gopher Thomomys umbrinus

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Abstract
Electromorphic and chromosomal variation is analyzed in 26 populations of Thomomys umbrinus sampled from throughout the range of the species. Interpopulation levels of genie differentiation are extreme, generally exceeding values measured between conspecific populations of most animals or plants. Two principal groups of T. umbrinus are recognized based on chromosomal evidence, one with 2n = 76 chromosomes and the other with 2n = 78. Further, the 2n = 78 group (but not the 2n = 76 group) is bisected into geographic subgroups with respect to chromosome morphology and heterochromatin position. The kind and degree of chromosomal differentiation observed suggests that the three groups may be reproductively incompatible. Allozymic evidence corroborates the above groupings, and an analysis of patterns of allele sharing suggests the absence of gene flow among the groups. A cladistic analysis of electromorphic data indicates that the two 2n = 78 groups may be independently derived from the 2n = 76 lineage. The combined evidence supports the hypothesis that T. umbrinus is actually a composite of at least three biological species and confirms the observation that speciation in the genus Thomomys is unrelated to the level of genie differentiation between populations.