Plasticity and heritability of morphological variation within and between parapatric stickleback demes

The threespine stickleback (Gasterosteus aculeatus) has emerged as an important model organism in evolutionary ecology, largely due to the repeated, parallel evolution of divergent morphotypes found in populations having colonized freshwater habitats. However, morphological divergence following colonization is not a universal phenomenon. We explore this in a large-scale estuarine ecosystem inhabited by two parapatric stickleback demes, each physiologically adapted to divergent osmoregulatory environments (fresh vs. saline waters). Using geometric morphometric analyses of wild-caught individuals, we detected significant differences between demes, in addition to sexual dimorphism, in body shape. However, rearing full-sib families from each deme under controlled, reciprocal salinity conditions revealed no differences between genotypes and highly significant environmental effects. It is also noteworthy that fish from both demes were fully plated, whether found in the wild or reared under re...