Is gene flow from pelagic larval dispersal important in the adaptation and evolution of marine invertebrates?

Pelagic larvae provide a potent means of dispersal among conspecific populations of marine organisms. Whether the gene flow resulting from this dispersal confers a short-term, adaptive advantage by damping genetic responses of local populations to ephemeral conditions, and whether this translates into a long-term, evolutionary stability of genetic cohesiveness among widespread populations, however, are open questions. Differentiation of local, conspecific populations, despite larval dispersal, appears to be the rule rather than the exception for such organisms. Geographic patterns of electrophoretically detectable allozyme variation in the lobster Homarus and the barnacle Balanus glandula provide examples of genetic divergence on both macro- and microgeographic scales. For the lobster, differentiation of Gulf of St. Lawrence and Atlantic populations with respect to allozyme and reproductive physiological traits may be due to a previously unsuspected barrier to successful larval production on the Scotian shelf. For the barnacle, significant microgeographic genetic heterogeneity is found despite a qualitative homogeneity over broad regions as expected from larval dispersal; selection after settlement rather than heterogeneity of recruits appears to explain local divergence in the face of substantial gene flow. Thus, differentiation of populations of marine invertebrates having pelagic larvae can arise from either physical or biological barriers to larval dispersal or from differential survival or fecundity of immigrants. Where diversifying selection follows coarse-grained patterns of environmental heterogeneity, gene flow by larval dispersal would appear to be maladaptive. If the adaptive significance of the pelagic larva lies not in the benefits of gene flow, it may reside instead in the benefits of migrating to nursery habitats. Long-term, evolutionary consequences are more difficult to evaluate. The hypothesis that distributions of planktotrophic versus lecithotrophic fossil molluscs differ because of high gene flow in the former and low gene flow in the latter is not supported by neontological population genetic evidence. Also, the widespread existence of cryptic sibling species cautions against relying upon estimates of speciation and extinction rates for fossil taxa that are morphologically defined.