Natural selection, larval dispersal, and the geography of phenotype in the sea

Populations evolve generalist, specialist, and plastic strategies in response to environmental heterogeneity. Describing such within-species variation in phenotype and how it arises is central to understanding a variety of ecological and evolutionary topics. The literature on phenotypic differences among populations is highly biased; for every one article published on a marine species, at least 10 articles are published on a terrestrial species and eight focus on terrestrial plants. Here, I outline what we know from the marine literature about geographic variation in phenotype in the sea, with a principal focus on local adaptation. The theory of environmental "grain" predicts that the most likely evolutionary response (e.g., local adaptation, phenotypic plasticity, generalism, and balanced polymorphism) depends on the spatial scale of environmental variation relative to the distance that an organism disperses. Consistent with these predictions, phenotypic plasticity is stronger among invertebrates with geographically broad dispersal versus restricted dispersal (i.e., planktonic-dispersers versus direct-developers). However, contrary to predictions, the relative frequency, and spatial scale of local adaptation is not consistently greater among direct-developers relative to planktonic disperers. This indicates that the likelihood of local adaptation depends on other organismal or environmental traits. Two of the most vexing issues that remain include (1) predicting the extent to which barriers to dispersal are a cause versus consequence of phenotypic differentiation and (2) delineating the relative importance of evolutionary forces that favor or impede local adaptation. Understanding the mechanistic basis of the geography of phenotypic differences, or phenogeography, has gained recent momentum because of a need to predict impacts of global climatic change, anthropogenic disturbances, and dispersal of organisms to non-native habitats.