The rise of the Central American Isthmus (CAI) and the gradual closure of the seaway connecting the tropical Eastern Pacific (EP) and Western Atlantic (WA) oceans has long served as a model for understanding the formation of new species in the sea. Given geological information that dates the final stage of seaway closure at 3.1-3.5 million years ago, researchers have hypothesized that hundreds of morphologically similar pairs of species found today on either side of the isthmus (called "geminate" or twin species) are believed to represent the descendants of single ancestral species broadly distributed throughout the tropical EP and WA oceans prior to the isthmus formation. Recent evidence, however, raises questions about how closure of the CAI actually affected patterns of species diversity in tropical America. Given that extinction rates may have been as high as 70% for some groups following isthmus formation, researchers think that sister-taxa that survived in both oceans should be rare. How can the apparent survival of so many living geminate species in both the EP and WA be reconciled with the developing paleontological picture of massive Caribbean biotic turnover? To answer this question, Dr. Peter Marko of the University of North Carolina will conduct a combined molecular and paleontological analysis of speciation and extinction among geminate species complexes in the bivalve mollusk family Arcidae. Living species will be discriminated with a combination of phylogeographic surveys of nuclear and mitochondrial gene variation along with discriminant analysis of morphological traits. Species discrimination in the fossil record will then be accomplished by extending statistical tests of morphological differences among genetically distinct species in the living fauna to specimens from the fossil record. The research will provide financial support and training in field collections, morphometrics, and molecular phylogenetics for two graduate students plus funds to send students to a national meeting to gain experience presenting research results. Funds will also offer opportunities for undergraduate education and training in molecular research and the PI will actively recruit individuals from traditionally underrepresented groups. Although not directly funded by this proposal, the molecular phylogenetic work will also be integrated with a phylogenetic effort conducted by a morphological systematist in the UK.
