RESUMEN
The widespread, lowland toad Bufo valliceps has an unusual distribution in North and Middle America that straddles two major biogeographic areas; previous morphological studies of this species suggested the existence of two species. We used mitochondrial DNA sequences to examine the phylogeography of this species and discovered the existence of two distinct clades. We recognize these as two species: B. valliceps and B. nebulifer. These molecular data support morphological data from previous studies. Our results show low levels of molecular variation in a morphologically uniform temperate species (B. nebulifer) and high levels of molecular variation in a morphologically variable tropical species (B. valliceps), providing an example of molecules matching morphology. Two biogeographic hypotheses are tested to explain the current distribution of these species, based on a calibrated rate of evolution and the percent sequence divergence between the two species. A more recent Pleistocene dispersal event, followed by vicariance associated with rising sea level, is rejected in favor of an earlier Miocene-Pliocene vicariant hypothesis associated with the formation of the Trans-Mexican Neovolcanic Belt.