Comparing Evolutionary and Non-Genetic Hypotheses to Explain Phenotypic Responses to Environmental Change: a Case Study of a Shift Toward Earlier Migration Date in Sockeye Salmon

Environmental change can shift organisms’ phenotypes through either evolutionary or non-genetic processes.  Despite abundant evidence of phenotypic change in response to recent climate change, we typically lack sufficient genetic data to specify the role of evolution. We present a method based on phenotypic data for characterizing the hypothesized role of natural selection and environmentally-driven phenotypic shifts (plasticity). We model historical selection and environmental predictors of interannual variation in mean population phenotype in a multivariate, state-space model framework. Through model selection, we assess the extent to which the hypothesized evolutionary process explains observed variation better than environmental factors alone. We applied the method to a 60-year trend toward earlier migration in Columbia River sockeye salmon Oncorhynchus nerka, producing estimates of annual selection differentials, average realized heritability, and relative cumulative effects of selection and plasticity. Although we do not use quantitative genetic methods, our estimates compare favorably with results from traditional evolutionary techniques. We found patterns consistent with an evolutionary response to thermal selection, potentially explaining ¹/₃ to ²/₃ of the phenotypic trend. Adaptive plastic responses to trends in mean June river flow explain the remainder. This method can be readily applied to other populations with time series data if selection differentials are available or can be reconstructed, augmenting our toolbox for predicting responses to environmental forcing such as climate change.