Project: Evaluating Marine Clines to Predict Larval Retention

Despite great analytical advances for inferring gene flow among marine populations, it is still difficult to identify the populations or mechanisms that maintain spatial variation in demography or connectivity in species with planktonic larvae. How novel diversity is retained in what appear to be well-mixed oceanic systems is poorly understood, particularly the balances between selection and dispersal that influence patterns of genetic variation in these species. This research explores effects of asymmetric dispersal on marine diversity, in general, and on the barnacle Notochthamalus scabrosus along the Chilean coast, in particular. Essentially, the study focuses on how currents influence geogeographic and phylogeograhic patterns of benthic invertebrates with planktonic larvae. In this region, there is a genetic cline of this species that may represent performance-based differences between two distinct lineages of the barnacle, and a transition in nearshore oceanography and intertidal ecology. The mechanisms that allow the cline to persist in the face of larval dispersal will be tested quantitatively. The research would improve the spatial, temporal, and genomic characterization of the cline in N. scabrosus, examine potential selection gradients, and compare indirect estimators of asymmetric gene flow with environmental isolation models that link physical oceanography and phylogeography. The study would be the first empirical exploration of predictions made by recent biogeographic and population-genetic models, developed by the PIs, of the conditions necessary for genetic structure to persist in the coastal ocean. The originality of this project is that instead of relying solely on phylogeographic inference to predict connectivity of populations under various untested assumptions, a combination of physical and biological data would be used to identify the specific mechanisms that maintain coastal diversity. This study would link the dynamics maintaining the N. scabrosus cline to patterns in other marine species actively studied by colleagues in Chile, and would help validate current modeling efforts of larval dispersal along that coast. Moreover, working with Chilean colleagues would be efficient for obtaining and analyzing data needed for this research. The focus on the Chilean coast has a number of benefits, including its intriguing similarity to the well-studied Oregon and California coasts, potentially allowing for generalizations from the results of this work. Most importantly, this work would push source-sink dynamics from the theoretical literature into basic research of how demographic patterns are distributed and maintained across a species' range, with implications for understanding the structure and dynamics of range limits themselves. The work would identify likely source populations along the Chilean coast based on demographic and gene flow analysis, in concert with oceanographic characteristics, and provide a framework transferable to similar efforts along any coast (e.g., Pacific coast of the US), towards clarifying general concepts of population and species range limits.