SPECIATION AND POPULATION GENETIC-STRUCTURE IN TROPICAL PACIFIC SEA-URCHINS

Unlike populations of many terrestrial species, marine populations oft en are not separated by obvious, permanent barriers to gene Bow. When species have high dispersal potential and few barriers to gene flow, a llopatric divergence is slow. Nevertheless, many marine species are of recent origin, even in tars with high dispersal potential. Tn underst and the relationship between genetic structure and recent species form ation in high dispersal taxa, we examined population genetic structure among four species of sea urchins in the tropical Indo-West Pacific t hat have speciated within the past one to three million years. Despite high potential for gene Bow, mtDNA sequence variation among 200 indiv iduals of four species in the urchin genus Echinometra shows a signal of strong geographic effects, These effects include (1) substantial po pulation heterogeneity; (2) lower genetic variation in peripheral popu lations; and (3) isolation by distance. These geographic patterns are especially strong across scales of 5000-10,000 km, and are weaker over scales of 2500-5000 km. As a result, strong geographic patterns would not have been readily visible except over the wide expanse of the tro pical Pacific. Surface currents in the Pacific do not explain patterns of gene Bow any better than do patterns of simple spatial proximity. Finally, populations of each species tend to group into large mtDNA re gions with similar mtDNA haplotypes, but these regional boundaries are not concordant in different species. These results show that all four species have accumulated mtDNA differences over similar spatial and t emporal scales but that the precise geographic pattern of genetic diff erentiation varies for each species. These geographic patterns appear much less deterministic than in other well-known coastal marine system s and may be driven by chance and historical accident.