We investigated genetic variation within the Great Plains narrow-mouthed toad, Gastrophryne olivacea, across its geographic range in the United States and Mexico. An analysis of mitochondrial DNA (mtDNA) from 105 frogs revealed remarkably low levels of genetic diversity in individuals inhabiting the central United States and northern Mexico. We found that this widespread matrilineal lineage is divergent (ca. 2% in mtDNA) from haplotypes that originate from the western United States and western coast of Mexico. Using a dataset that included all five species of Gastrophryne and both species of the closely related genus Hypopachus, we investigated the phylogenetic placement of G. olivacea. This analysis recovered strong support that G. olivacea, the tropically distributed G. elegans, and the temperately distributed G. carolinensisconstitute a monophyletic assemblage. However, the placement of G. pictiventris and G. usta render Gastrophryne paraphyletic with respect to Hypopachus. To complement our mitochondrial analysis, we examined a small fragment of nuclear DNA and recovered consistent patterns. In light of our findings we recommend (1) the resurrection of the nomen G. mazatlanensis Taylor (1943) for the disjunct western clade of G. olivaceaand (2) the tentative placement of G. pictiventris and G. usta in Hypopachus. To explore possible scenarios leading to low levels of genetic diversity in G. olivacea, we used mismatch distributions and Bayesian Skyline plots to examine historic population expansion and demography. Collectively these analyses suggest that G. olivacea rapidly expanded in effective population size and geographic range during the late Pleistocene or early Holocene. This hypothesis is consistent with fossil data from northern localities and contemporary observations that suggest ongoing northern expansion. Given our findings, we suspect that the rapid range expansion of G. olivacea may have been facilitated by ecological associations with open habitats and seasonal water bodies.

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