Ecological Filtering Shapes Morphological Diversity Across Flow Regimes in Neotropical Characiforms

Colonizing novel habitats can increase ecological opportunity, promoting rapid phenotypic evolution as species adapt to new niches. However, colonization of a novel habitat can limit phenotypic diversity without changing the rate of evolution if the transition is recent and ecological filtering favors only a few highly adaptive phenotypes. We examine body shape diversity in Neotropical characiforms across flow regimes using linear morphometrics integrated with phylogenetic comparative methods, including disparity metrics, ancestral state reconstructions, and evolutionary model fitting, to explore the role of time, evolutionary rate, and functional constraints in explaining patterns of morphological diversity. Among sampled extant lineages, reconstructed high-flow habitat colonization was rare until the last 60 million years, after which it steadily increased. Low-flow fishes exhibit greater body shape disparity than high-flow species, though evolutionary rates are similar. Low body shape diversity in high-flow habitats results from a combination of two factors. First, characiforms have occupied high-flow habitats for less time, having recently transitioned from low-flow environments. Second, the body shapes of high-flow lineages are not a random subset of low-flow lineages. Instead, they are restricted to intermediate body shapes and some unusually dorsally flattened and elongated forms, likely reflecting the mechanical demands imposed by fast-flowing water. Consistent with this interpretation, phylogenetic comparative analyses supported a multiple-optima Ornstein-Uhlenbeck model (OUM), indicating distinct adaptive body shape optima for high- versus low-flow regimes. Higher body shape diversity in low-flow habitats appears to reflect longer occupation, fewer flow-related constraints, and environmental stability over 100 million years. Our study highlights how patterns of phenotypic diversity emerge on broad landscapes through the interplay between habitat-specific environmental factors and evolutionary time.