MARKO, P.B.; Univ. of North Carolina, Chapel Hill: Divergence of Geminate Marine Species and the Timing of Biological Separation Between the Tropical Western Atlantic and Eastern Pacific Oceans

To develop hypotheses regarding the processes that generate biological diversity, we need to understand the timing of both environmental change and species diversification. Geminate species pairs, morphologically similar species in which one inhabits the tropical eastern Pacific (EP) and the other the western Atlantic (WA), have served as pivotal examples of the role that geographic barriers play in process of speciation. Geminates also provide convenient temporal reference points for studies of molecular evolution and applications of the molecular clock to questions of evolutionary biology. Most studies involving geminate pairs, however, assume that gene flow between EP and WA populations terminated at the time of final seaway closure, ~3.5 million years ago (MYA). By building a molecular phylogeny for the bivalve family Arcidae and employing calibration points that are independent of geological information about seaway closure, I have found that all ark geminates split before the time of final seaway closure. For one geminate pair whose molecular-based divergence time is significantly greater than 10 MYA, morphometric analyses of Recent and fossil shells suggest both species were distinct at least 16 MYA. The geographic distribution of fossils also suggests that morphological forms associated with the Recent species had amphi-American distributions both before and after formation of the Central American Isthmus but are now geographically restricted to either side of the isthmus in the Recent fauna.