Given that species diverge mostly in geographical isolation, the dynamics of local diversity is governed by the time it takes for these species to co-occur (secondary sympatry). Based on mitochondrial DNA and morphological evidence, some echinoderms seem able to achieve secondary sympatry an order of magnitude faster than other groups of marine invertebrates. The proposed research seeks to investigate whether these patterns could alternatively be caused by introgressive hybridization masking an earlier divergence, or reflect high levels of intraspecific polymorphism obscuring similar rates of secondary sympatry. To tease apart these hypotheses, the parameters of the isolation-with-migration (IM) model will be estimated from a multilocus dataset in a complex of sea cucumbers comprised of four putative, recently diverged, species. Furthermore, computer simulations will be used to determine whether the history of some loci deviates from this model, suggesting introgressive hybridization.

Results will contribute to our understanding of species limits in challenging groups, and diversification in the richest marine biogeographic region. They will also generate hypotheses on the mechanisms involved in the generation of this diversity. In particular, the estimation of gene flow and population sizes will lead to predictions on the role of these demographic parameters in speciation. Furthermore, these estimates will help improve stock delineation and estimation of sea cucumbers, a major marine resource in tropical coastal communities.

Project Report

Local diversity results from the accumulation in a given location of species that are reproductively isolated. Because speciation is most commonly initiated in allopatry, understanding how quickly species can co-occur after diverging in geographical isolation is key to gain insights into the temporal dynamics of local diversity. With this project, we investigated whether the signal of rapid diversification suggested by mitochondrial DNA among three sympatric putative species of sea cucumbers could have been the result of other confounding factors (introgression, polymorphism). We developped molecular resources to address this question, and use the multi-species coalescent framework to test alternative hypotheses of species limits. We demonstrated that the three putative species are indeed reproductively isolated, and diverged less than 1 million years ago. This contrasts with other species which typically can co-occur only 10 million years after the speciation event. During the course of this project we developed molecular resources that will be useful for the research community to address population genetics questions for sea cucumbers. Outreach activities for this project included the sensibilization of high-school students to the issues of biodiversity science. We taught these students how to collect and process marine invertebrates to be included in natural history collections. Additionally, they used DNA barcoding to identify species they collected.

National Science Foundation (NSF)
Division of Environmental Biology (DEB)
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Samuel M. Scheiner
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University of Florida
United States
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