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

Climate change is a driver of extinction, with extreme events occurring more frequently. It increases both the amount and variability in environmental stress that organisms experience. In novel environments, greater intra-specific trait variation increases the opportunity for natural selection as individuals with fortuitously advantageous trait expression may thrive. Many studies focus on changes in trait means between novel and historic environments but overlook changes in trait variation. 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 than old adults, with sex-specific impacts (female: gut development; male: age at maturity and body size). When we accounted for mean differences in trait expression between environments, salinisation also affected relative variation in female fecundity (egg size, offspring number). Notably, fluctuations in salinity did not magnify the effect of stable salinity, but sometimes reversed its effect. Our findings suggest that researchers should pay closer attention to environmental fluctuations: changes in trait variation in a population can alter its adaptive potential.