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 non-adaptive
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.