Old Wine in New Bottles: A Review of Reaction Norms in Salmonids

Genetic variability in reaction norms reflects differences in the ability of individuals, populations, and ultimately species to respond to environmental change. Studies of such genetic differentiation serve to refine our capacity to predict how populations will respond to natural and human-induced changes to the environment, including those resulting from fisheries and climate change. Given the extraordinary variability in morphology, behaviour, and life history in salmonids, one might anticipate the research milieu on reaction norms in these fishes to be empirically rich and intellectually engaging. Here, I undertake a review of genetic variability in reaction norms in salmonids. Although in its infancy from a publication perspective, there is taxonomically broad evidence of genetic differentiation in continuous, threshold, and bivariate reaction norms among individuals, families, and populations (including inter-population hybrids and backcrosses) for traits as divergent as embryonic development, age and size at maturity, and gene expression. There is compelling inferential evidence that plasticity is heritable and that population differences in reaction norms can reflect adaptive responses, by natural selection, to local environments. As a stimulus for future work, research questions are identified that focus on reaction norm variability, selection, costs and constraints, demographic and conservation consequences, and genetic markers and correlates of phenotypic plasticity.