Coincident transitions across elevation and origins of functional innovations drove the phenotypic and ecological diversity of lungless salamanders

Ecological opportunity (EO) is an important catalyst for evolution. Whereas theory often centers around a lineage encountering a source of EO in isolation, in practice they experience numerous sources of opportunity, either concurrently or sequentially. Such multiplicity can obscure the macroevolutionary signature of EO. Here, we test the effects of elevation (a proxy of the "mountain effect") and an array of functional innovations on the evolutionary history of plethodontid salamanders, a diverse and charismatic radiation of lungless amphibians. Functional innovations unlock access to novel microhabitats, ultimately enabling sub-lineages to occupy a diverse range of ecological niches, particularly in lowland areas where those niches are more abundant. Consistent with expanded ecological opportunity, such transitions to lower elevation result in rapid phenotypic evolution. At high elevation, by contrast, rates of phenotypic evolution and phenotypic disparity decline, reflecting a loss of phenotypically extreme ecological specialists. Transitions in elevation and the origin of innovations appear largely coincident among lungless salamanders, suggesting myriad sources of EO. The magnitude of the "mountain effect" on evolutionary rates (∼10-fold) is on par or greatly exceeds that of islands, lakes, and coral reefs on other iconic vertebrate radiations. Therefore, we find that elevation acts as a major ecological moderator and, in concert with functional innovations, shapes the ecological and phenotypic diversity of lungless salamanders.