Taxonomy is the science of describing and categorizing organisms. Mountain yellow-legged frogs are a member of the Rana boylii group (R. boylii, R. muscosa, R. sierrae, Rana cascadae, Rana luteiventris, Rana pretiosa, Rana aurora, Rana draytonii), and were originally classified as two subspecies of R. boylii. This included R. boylii muscosa in the Transverse Ranges and R. boylii sierrae in the Sierra Nevada (Camp 1917). Based on additional analysis, Zweifel (1955) separated the two subspecies from R. boylii and classified them as a single species, Rana muscosa. This classification remained unchanged for more than 50 years, despite lingering questions arising from the considerable variation in coloration and morphology of R. muscosa across its range. A recent study analyzed variation in DNA, morphology, and mating calls, and revealed that R. muscosa is in fact two species (Vredenburg et al. 2007). Surprisingly, the geographic split between the species is not between the Transverse Ranges and the Sierra Nevada, but is instead within the southern Sierra Nevada.
Under the new classification (Vredenburg et al. 2007), Rana muscosa ("southern mountain yellow-legged frog") is found in the Transverse Ranges and in the Sierra Nevada south of Mather Pass. Rana sierrae ("Sierra Nevada yellow-legged frog") is found in the Sierra Nevada north of Mather Pass. The two species are referred to collectively as "mountain yellow-legged frogs". R. muscosa and R. sierrae differ in numerous respects, but the most noticeable difference is the longer legs of R. muscosa compared to R. sierrae. The mating call is also quite different between the two species.
The most immediate consequence of this new classification is that the mountain yellow-legged frog is even more endangered than previously believed. This is a consequence of the remaining populations being divided between two species instead of one. For additional details, see the Current Status section.
The genus, Rana, has also ungone recent taxonomic revision (Frost et al. 2006), with many subgenera being elevated to genera. As a consequence, the names Rana muscosa and Rana sierrae would be changed to Lithobates muscosa and Lithobates sierrae, respectively. However, the new taxonomy is not yet universally recognized. As stated on the AmphibiaWeb site, "The majority of the new names proposed by Frost et al. (2006) are disconnected from the vast amount of previous amphibian research that has taken place, and thus adopting this new taxonomy hinders effective dissemination of information. At some point, when the hypotheses of relationships in Frost et al. have been tested with additional data, better taxon sampling, and more realistic analytical methods, changes may be in order. At least for the time being however, we have elected to retain the traditional names for amphibian taxa pending publication of a phylogenetic taxonomy for additional branches of the living amphibian portion of the Tree of Life." Therefore, I retain use of the genus name, Rana, throughout this website.