Do environmental fluctuations during development affect trait variation? An experimental test with salinity

Human‐induced climate change is a driver of extinction, with extreme events occurring more frequently. It increases both the amount and fluctuations in environmental stress that organisms experience. In such environments, greater intra‐specific trait variation creates more potential for adaptation through natural selection. Many studies focus on changes in trait means between novel and historic environments but overlook changes in trait variation. Here we tested how salinisation – which currently affects 20–50% of freshwaters worldwide – alters trait variation in a freshwater fish, Gambusia holbrooki . We reared newborn fish in freshwater, stable‐salinity or fluctuating‐salinity water until maturation, and then compared variation in fitness‐related traits in each sex during early and late adulthood. Salinisation had stronger effects on young virgin adults than older mated adults, with sex‐specific impacts (female: gut development; male: age at maturity and body size). When we accounted for the mean trait value in each environment, salinisation also affected relative variation (i.e. the coefficient of variation) in female fecundity (egg size, offspring number). Notably, the fluctuating‐salinity treatment did not magnify the effect of the stable‐salinity treatment on trait variation but sometimes reversed its effect. Our findings suggest that researchers should pay closer attention to fluctuating environmental stressors, as they can differ from stable stressors in how they affect between‐individual variation in trait expression.