Correlated Contemporary Evolution in New Zealand Salmon: Implications for PMRN Inferences

In salmon and other organisms, size and growth rate often act as cues for maturation and thus age at maturity may or may not evolve independently of these features.  Recent concerns surrounding the potential phenotypic responses of populations facing anthropogenic selection, such as harvest, place a premium on understanding the evolutionary genetic basis for contemporary changes in size at age and age at maturity. Here, we use Chinook salmon (Oncorhynchus tshawytscha) populations introduced to New Zealand as a model system to assess the relative roles of independent versus correlated evolution as the basis for contemporary divergence in size at age and age at maturity.  Using a common garden approach, we found consistent evidence of contemporary evolution of differences among populations in both size at age and age at maturity, often corresponding to patterns observed in the wild.  Populations diverged in size and growth profiles, even when accounting for correlated shifts in age at maturation.  By contrast, most (but not all) instances of divergence in age at maturity were accounted for by correlated divergence in growth profiles, rather than by differences in the threshold relationship linking growth rate and probability of maturation.  These findings suggest that the probabilistic maturation reaction norm (PMRN) approach, commonly used to infer evolution of age at maturity in harvested populations, may often conceal the primary mode by which age at maturity initially evolves in nature.