Phylogeographic Patterns in Mitochondrial Dna of the Desert Tortoise (xerobates Agassizi), and Evolutionary Relationships among the North American Gopher Tortoises.

Restriction-fragment polymorphisms in mitochondrial DNA (mtDNA) were used to evaluate population-genetic structure in the desert tortoise Xerobates agassizi and to clarify evolutionary affinities among species of the gopher tortoise complex. Fourteen informative endonucleases were employed to assay mtDNAs from 56 X. agassizi representing 22 locations throughout the species' range. The mtDNA genotypes observed were readily partitioned into three major phylogenetic assemblages, each with striking geographic orientation. Overall, the X. agassizi mtDNA genotypes typify a common phylogeographic pattern, in which broad genetic uniformity of populations is interrupted by geographic features that presumably have functioned as dispersal barriers. The geologic history of the Colorado River area, which includes extensive marine incursions, may account for the marked mtDNA divergence between eastern and western X. agassizi assemblages. In mtDNA comparisons among the four species of the gopher tortoise complex, both UPGMA and Wagner parsimony analysis strongly support the recognition of two distinct species groups previously suggested by traditional systematic approaches. Furthermore, the mtDNA data identify the eastern X. agassizi assemblage as the probable inceptive lineage of X. berlandieri. Results from both intra- and interspecific comparisons illustrate how clues to historical events may be present in the geographic structure of mtDNA phylogenies.