Dietary specialization drives adaptation, convergence, and integration across the cranial and appendicular skeleton in Waterfowl (Anseriformes)

Convergence provides strong evidence for adaptation, reflecting similar responses to recurring selection pressures. Waterfowl (order Anseriformes) have repeatedly evolved morphotypes putatively correlated with foraging ecology (e.g., dabbler, grazer, diver), making them an ideal model for testing hypotheses of adaptive evolution. Here, we estimated the most robust time-calibrated phylogenetic tree for the waterfowl to date and quantified the shape of the skull, femur, tibiotarsus, and tarsometatarsus of 118 extant waterfowl species using geometric morphometrics. Multivariate generalized evolutionary models provide strong support for convergent phenotypic evolution across lineages that share dietary characteristics. We recovered a significant relationship between the evolution of diet and foraging behavior and the evolution of skull shape. We found a stronger correlation between the evolution of foraging behavior and the evolution of hindlimb shape, but diet evolution was also significantly related to hindlimb shape. Derived terrestrial and diving lineages consistently showed greater rates of phenotypic evolution than surface swimming and wading lineages. We also find evidence for integration between the hindlimb elements and the skull. These results suggest that the morphological diversity of waterfowl is largely explained by adaptive evolution in response to different foraging ecologies.