Biogeography in the Death Valley region: evidence from springsnails (Hydrobiidae: Tryonia)

Allozyme and mitochondrial DNA variation were analysed to examine evolution of the nine species of springsnails (genus Tryonia) living in the Death Valley system (Owens and Amargosa basins) of southeastern California and southwestern Nevada. Both allozyme and mtDNA evidence indicate that this highly endemic fauna is non-monophyletic. Species from the upper Amargosa basin comprise a clade most closely related to snails living in the Colorado basin. Snails from the lower Amargosa basin (Death Valley trough) reflect a complex evolutionary history and two of these species are more closely related to an estuarine species from western California than to other snails of the region. These results indicate a commonality of pattern with the well-studied Death Valley pupfishes (Cyprinodon), which also are non-monophyletic and include species that are most closely related to Colorado basin congeners. These biogeographic patterns are interpreted within the context of a recently proposed model for the early history of the lower Colorado River.