Local adaptation in widely-dispersed marine species may result from strong selection on multiple life history traits that differ among populations. This results in unique sets of traits that maximize fitness of individuals within populations along the species' range. The Atlantic silverside, Menidia menidia, is a fish that exhibits high gene flow potential, but differences in growth and sexual differentiation among populations demonstrate strong local adaptation. To better understand the spatial pattern of local adaptation in this species, I compared the form of sex determination, which shifts from genetic to environmental sex determination, across this species' range and quantified the rate of gonad development across populations. Strong local selection in this species drives genetic differences among populations, but few of the mechanisms that control these differences have been studied. Therefore, I also quantified the activity of a major sex-determining gene during gonad differentiation in populations with different forms of sex determination and assessed the role of a major growth protein in structuring growth differences among populations with unique life-history traits. Finally, widely-dispersed marine species come into contact with many anthropogenic stressors within their ranges, and there may be differential interactions between these stressors and local populations. I addressed how locally-adapted differences in sex determination contribute to population-level susceptibility to estrogenic contaminants in the wild.