DISSERTATION RESEARCH: Integrating Fossils and Molecules to Trace Ecological Divergence and Convergence in Marine Bivalves

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Closely related biological species are thought to exclude one another from a particular habitat because they compete for similar food and habitat resources. Yet, many closely related species inhabit the same geographic space. This research dissects the tension between 1) the ecological pressure on closely related, co-occurring species to use different resources, and 2) the evolutionary tendency for closely related species to play similar ecological roles. By studying ecologically important traits in pairs of closely related species in the fossil record, the researchers will study the ecology of each species when it is, first, isolated from, and then co-occurs with its near relative, giving a direct window into the nature of evolutionary divergence or convergence of those species as they interact. Thus, the balance of pressures pushing species closer in form (as they enter the same environment) and pushing them apart (as they come into ecological competition) can be assessed directly. This research will collaborate with members of the ongoing Bivalve Tree of Life effort and give one of the first examples of ecological evolution over time among closely related bivalve species. Finally, this project will train one graduate student and contribute course material to the researchers? interactive Science Café on the role of ecology in evolution for middle school students at the Chicago Museum of Science and Industry.The researchers will investigate two prominent ecological hypotheses that suggest co-occurring, closely related species should differ in their ecological traits to ease direct competition for resources. ?Character displacement? predicts that competition between species after they co-occur drives their ecological divergence whereas ?species sorting? involves the co-occurrence of species that already differed ecologically before coming into sympatry. Character displacement and species sorting will be tracked in pairs of related species by using the fossil record to trace shifts from allopatry to sympatry and any corresponding change in ecological traits through time. This project will construct a molecular phylogeny for a genus of marine bivalves along the American coasts, Chione (9 extant species plus 3 extinct species fitted into the molecular tree via analysis of character matrices), which coupled with spatially explicit fossil data will provide unique access to the relation between character displacement, species sorting, and phylogenetic relatedness. Given the tendency for ecological similarity to correlate with relatedness, the researchers expect character displacement and species sorting to operate along a continuum, where character displacement predominates between more closely related species and species sorting predominates between more distantly related species. However, ecological selection imposed by the environment may trump selection mediated by competition. In that case, species will express similar ecological traits where they co-occur.