Morphological convergence and character displacement in two species of polymorphic salamanders (genus Plethodon) in eastern Tennessee

In situations involving the co-occurrence ecologically similar species, one of two different evolutionary responses is often expected. If sympatry results in competition over a shared resource, character displacement would be a favored outcome, while morphological convergence is an alternative outcome if the species have similar responses to the shared environment. In this study, I examine cranial morphology and dorsal coloration of two ecologically similar salamander species (Plethodon serratus, the Southern Red-backed Salamander, and P. ventralis, the Southern Zig-zag Salamander) to evaluate the alternative hypotheses of character displacement or convergence. Results from linear morphometrics analyses showed no significant shifts in morphology of either species suggestive of either character displacement or convergence in any of the measured traits. However, geometric morphometric analyses showed significant morphological convergence of the two species in sympatry. In contrast, the presence or absence of a dorsal stripe showed evidence of character displacement, corroborating an earlier claim made by Highton. These results are unexpected in that features associated with feeding (cranial morphology) are expected to often exhibit character displacement if dietary resource partitioning is an important mechanism of coexistence, whereas coloration might be expected to show local convergence if its primary function is camouflage or aposematism. Convergence might reflect the overwhelming influence of developmental responses to shared environments or convergent adaptation to local prey communities. Displacement with respect to color polymorphism might be consistent with frequency-dependent selection maintaining alternative ways of being cryptic