Ecological opportunity spurred adaptive radiation of Central Mexican Silversides: evidence from functional and morphometric traits.

Dispersal to a geographic area or colonizing a new habitat can lead to ecological opportunity, which predicts that in absent or reduced competition, lineages can diversify to exploit available resources, ultimately leading to adaptive radiation. We investigated the role of colonizing new environments with novel community contexts in Central Mexican Silversides, a putative adaptive radiation. We explored macroevolutionary dynamics of speciation and phenotypic evolution across New World Silversides in different community settings. We analyzed geometric morphometric, linear, and functional traits, three classes of morphological data rarely evaluated jointly in a single group. We found that Central Mexican Silversides, which occupy a region characterized by low species richness, and isolated freshwater habitats, experienced elevated rates of phenotypic evolution and speciation. In the absence of ecological opportunity, other lineages of Menidiinae experienced constant evolutionary rates through time and patterns consistent with nonadaptive diversification dynamics. We found that traits related to piscivory and burst-swimming have the highest rates of evolution in Central Mexican Silversides. We also show that functional traits and linear morphometrics better capture among lineage variation than body shape data. Our study shows that unique paleogeographic and ecological settings can promote adaptive radiation in clades that otherwise experience steady-state diversification dynamics.