Body Shape Evolution in Sunfishes: Divergent Paths to Accelerated Rates of Speciation in the Centrarchidae
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Vertebrate evolutionary radiations are the result of divergence along a variety of ecological and behavioural axes. In addition, the potential for clades to evolve along any one axis can be strongly influenced by the types and levels of phenotypic variation that are exposed to natural selection. Understanding the factors that promote and constrain morphological diversification is a central goal of evolutionary biology. Here we use the sunfishes (Centrarchidae), a perciform family containing three main clades (Lepomis, Micropterus, and a basal clade), to explore this question with respect to variation in body shape. We gathered morphological data from 26 of the 38 centrarchid species using geometric morphometrics and analyzed the resultant shape data over a time-calibrated phylogenetic tree. We find that centrarchids partitioned body shape early in their evolutionary history, a pattern that is largely associated with expansion into divergent foraging niches and elaboration of sexual ornamentation. The morphological disparity of each clade was tightly linked to integration: those clades with high disparity (Lepomis, basal clade) were the least integrated, while the opposite trend was observed in Micropterus. We also find evidence for an increase in speciation rate at the node leading to Lepomis and Micropterus, and a decline in speciation for the basal clade. Our data lead us to suggest different hypotheses for explaining accelerated speciation in Micropterus and Lepomis: invasion of a novel pursuit-predator niche that reduced resource competition (Micropterus), and the elaboration of opercular morphology (Lepomis), a trait that is linked to reproductive behaviour and facilitates mate recognition in communities with many sunfish species.
KeywordsFreshwater fish Integration Disparity Geometric morphometrics Phylogenetic methods
The authors wish to thank the Katherine Doyle (UMass, Amherst) and Caleb McMahan (Chicago Field Museum) for access to specimens. Thanks to Israel Del Toro and Benjamin Allen Concannon Smith for providing live centrarchid images. We also wish to acknowledge Elizabeth Dumont and an anonymous reviewer for comments and discussion on early versions of the manuscript. This work was funded by an OEB research grant awarded to A.J.S, the Department of Biology at UMass, Amherst, start-up funding from WSU to W.J.C, and from Glasgow University to K.J.P.
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