Evolutionary Relationships Within the Ensatina Eschscholtzii Complex Confirm the Ring Species Interpretation

Abstract

Sequences (644-681 bp) from the mitochondrial cytochrome b gene were obtained for 24 individuals representing the geographic range and morphological diversity of the polytypic salamander ring species Ensatina eschscholtzii. These data were used to estimate the phylogeny of components of the ring to test the biogeographic scenario underlying current interpretations of speciation in this complex. The analysis revealed high levels of nucleotide variation among subspecies. Strong subdivision was evident within the subspecies platensis and oregonensis. The phylogenetic hypothesis of minimum length that is best supported by the data contains one monophyletic group that includes populations from the southern Sierra Nevada and mountains of southern California (croceater, klauberi, and southern platensis) and another that includes populations of the southern and central coastal regions (xanthoptica and eschscholtzii) Samples of oregonensis were typically basal, but their precise branching order was unstable. Both oregonensis and platensis were paraphyletic, with several disparate lineages in oregonensis and a strong northsouth dichotomy in platensis. The data were incompatible with a biogeographic model that required all subspecies to be monophyletic but were compatible with slightly modified predictions of a model assuming stepwise colonizations from north to south down the Sierra Nevada and independently down the coastal ranges. These features provide strong support for the biogeographic scenario central to the interpretation of Ensatina eschscholtzii as a ring species. Division of this complex into separate species on the basis of the observed patterns of monophyly for mitochondrial DNA (mtDNA) is unwarranted because further sampling could reveal additional instances of paraphyly across subspecies and, more generally, because mtDNA alone should not be used to infer species boundaries