Character Displacement via Aggressive Interference in Appalachian Salamanders

Ecological character displacement occurs when sympatric species compete with one another, resulting in morphological divergence. Theoretically, character displacement can evolve from a number of ecological interactions, such as exploitation, interference, or predation, but most examples describe species competing exploitatively for limiting resources (typically food). Here I report a case of character displacement evolving from aggressive behavioral interference, found in a well-studied system of terrestrial salamanders of the genus Plethodon. Using geometric morphometrics, I found parallel shifts in head shape from allopatry to sympatry in both P. jordani and P. teyahalee, and found significantly greater morphological divergence in sympatry relative to allopatry. Both findings are consistent with ecological character displacement. I also show a significant association between morphology and aggressive behavior, demonstrating a direct link between interference competition and morphology. I present a modified set of the criteria for character displacement that are appropriate for discriminating character displacement via aggressive interference from other possible evolutionary mechanisms. For this example, empirical support satisfying five of the six criteria for character displacement is found. These results provide evidence that morphological variation can be generated by mechanisms other than resource exploitation, which has profound implications for interpreting patterns of biological diversity.