Phylogeographic incongruence of codistributed amphibian species based on small differences in geographic distribution

Codistributed species may display either congruent phylogeographic patterns, indicating similar responses to a series of shared climatic and geologic events, or discordant patterns, indicating independent responses. This study compares the phylogeographic patterns of two similarly distributed salamander species within the Pacific Northwest of the United States: Cope's giant salamander (Dicamptodon copei) and Van Dyke's salamander (Plethodon vandykei). Previous studies of P. vandykei support two reciprocally monophyletic lineages corresponding to coastal populations, located from the Olympic Mountains to the mouth of the Columbia River, and inland populations within the Cascade Mountains. We hypothesized that D. copei would have a congruent phylogeographic pattern to P. vandykei due to similarity in distribution and dependence upon similar stream and stream-side habitats. We test this hypothesis by estimating the phylogeny of D. copei using approximately 1800bp of mitochondrial DNA and comparing it to that of P. vandykei. Sympatric populations of D. copei and of P. vandykei display an identical phylogeographic pattern, suggesting similar responses within their shared distribution. Populations of D. copei occurring outside the range of P. vandykei displayed high levels of genetic divergence from those sympatric to P. vandykei. Overall, phylogeographic patterns between the two species were ultimately incongruent due to the high divergence of these allopatric populations. These results provide an example of codistributed species displaying overall incongruent phylogeographic patterns while simultaneously displaying congruent patterns within portions of their shared geographic distribution. This pattern demonstrates that a simple dichotomy of congruent and incongruent phylogeographic patterns of codistributed species may be too simplistic and that more complex intermediate patterns can result even from minor differences in species' ranges.