Sockeye salmon are evolving through natural selection to deal with a warming climate, according to a study by researchers at the National Oceanic and Atmospheric Administration.
In recent decades, scientists have observed that salmon in the Columbia River are starting their migration earlier in the year. The fish now migrate upstream an average of 10 days earlier than they did in the 1940s.
Researchers knew that the change was associated with warming water temperatures, but an important question remained. Is this a behavioral response in reaction to warmer water, or are the fish evolving modified behavior through mortality and natural selection?
Using 60 years of water temperature records and data on salmon mortality during migration, researchers led by Lisa Crozier have determined that the latter has played an important role.
According to their model, up to two-thirds of the 10-day advance in spawning is explained by natural selection, with a behavioral response to changes in river flow explaining the rest.
"Evidence of an evolutionary response in Columbia River sockeye salmon is good news, because it appeared to reduce their exposure to potentially lethal river temperatures in recent years," said Crozier. "This study gives managers insight into the multiple processes that help salmon persist in the face of a changing environment, and augments our toolbox for predicting how other species might respond to similar changes."
Lisa G. Crozier, Mark D. Scheuerell, Richard W. Zabel, "Using Time Series Analysis to Characterize Evolutionary and Plastic Responses to Environmental Change: A Case Study of a Shift toward Earlier Migration Date in Sockeye Salmon." http://www.jstor.org/stable/10.1086/662669
“Environmental change can shift the phenotype of an organism through either evolutionary or nongenetic processes,” says the report’s abstract. “Despite abundant evidence of phenotypic change in response to recent climate change, we typically lack sufficient genetic data to identify the role of evolution. We present a method of using phenotypic data to characterize the hypothesized role of natural selection and environmentally driven phenotypic shifts (plasticity). We modeled historical selection and environmental predictors of interannual variation in mean population phenotype using a multivariate state-space model framework. Through model comparisons, we assessed the extent to which an estimated selection differential explained 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. We found that an evolutionary response to thermal selection was capable of explaining up to two-thirds of the phenotypic trend. Adaptive plastic responses to June river flow explain most of the remainder. This method is applicable to other populations with time series data if selection differentials are available or can be reconstructed. This method thus augments our toolbox for predicting responses to environmental change.”