Modularity underlies the morphological evolution of Appalachian salamanders

Modularity is a pattern whereby traits evolve as interrelated systems, resulting in lower covariation between modules than within. Modularity may facilitate diversification by permitting the phenotype to independently respond to selection. The Appalachian Mountains are a biodiversity hotspot for lungless salamanders (Plethodontidae). Despite their ecological diversity, putative catalysts of their radiation in the Appalachians are poorly understood. We test whether (i) Appalachian salamanders exhibit modularity in functional, morphological characters and (ii) if the rate of evolution-a fundamental macroevolutionary signal of ecological opportunity-varies among modules and if their rates are correlated. We find that Appalachian salamanders exhibit modularity under two different models in which their head, trunk, limbs, and tail or the head, body, and limbs act as separate modules. The three major clades, Plethodon, Eurycea, and Desmognathus, vary in the presence and extent of modularity. Across Appalachian salamanders, modules exhibit non-significant differences in rates of evolution (~1.5-fold), with the trunk and tail modules exhibiting the slowest and fastest rates, respectively. We also find that the rate of modular evolution can be highly correlated, suggesting that modules may respond similarly to ecological opportunity. These results suggest that modularity underlies the morphological diversity of Appalachian salamanders by permitting localized regions of the phenotype to respond independently to selection, but not evolve at different rates.