Adaptive radiation along a benthic/pelagic ecological axis in North America’s most diverse, endemic clade of freshwater fishes

Eastern North America is unparalleled throughout the temperate world in terms of freshwater fish biodiversity. A monophyletic group of approximately 250 cyprinid fishes, known as the open posterior myodome (OPM) clade, dominates the fish species richness in the freshwater ecosystems of this region. In this dissertation, I explore the influence of eco-evolutionary divergence along a benthic/pelagic habitat axis on the generation of this hyper-diverse group of fishes. My three chapters work synergistically to address the question: Did a historical shift from benthic to pelagic habitats by OPM cyprinids represent the invasion of an open adaptive zone and result in the simultaneous bursts of phylogenetic and ecological diversification that signify an adaptive radiation? In Chapter I, I perform the first gene tree/species tree analysis on OPM species to reconcile discordance between previous phylogenetic hypotheses as it relates to inferring the history of benthic and pelagic habitat transitions in the group. I then construct the most thoroughly sampled OPM phylogenies to date in Chapter II. Using these large-scale phylogenies and habitat-use data, I conducted ancestral state reconstructions to trace the history of benthic to pelagic habitat use during the history of the clade. I then performed lineage through time and diversification rate analyses that suggested that a period of accelerated lineage diversification followed the initial shift from benthic to pelagic habitats in the OPM radiation. In Chapter III, I recovered a significant evolutionary relationship between jaw protrusion angle (JPA) and preferred foraging height in the water column between 15 co-occurring OPM taxa. I also recovered evidence for a burst of morphological disparification in a number of individual muscoskeletal characters that are evolutionary correlated with JPA after the major benthic to pelagic shift inferred in Chapter II. Overall, the results from this dissertation suggest that an early shift from benthic to pelagic habitats in OPM cyprinids represented the invasion of an open adaptive zone and was followed by a period of rapid phylogenetic and eco-morphological evolution as species diverged to exploit vertically segregated sub-zones throughout the water column. Taken together, these results are likely the most robust evidence for an adaptive radiation to date.