Potentiated Plastic and Genetic Adaptation to Copper Pollution in High-Density Populations of Water Fleas

Rapid evolution is frequently observed in response to environmental stressors, but its trajectory is tightly modulated by population density, which influences both the magnitude and pace of phenotypic plasticity and genetic adaptation. Using experimental evolution with copper selection and subsequent relaxation, we investigated how density-dependent processes interact with contaminant stress in shaping adaptive responses of the freshwater zooplankton Ceriodaphnia cornuta. Elevated population densities significantly accelerated adaptive evolution to copper, as evidenced by increased population growth rates and reduced plasticity under stress. These changes were accompanied by shifts in multilocus genotype composition, indicating selection for copper-tolerant genotypes. Copper-adapted populations retained their fitness advantage after the stressor was removed, likely due to the fixation of tolerance alleles with low fitness costs and the canalization of adaptive traits through reduced plasticity. Our findings show that population density not only alters the speed of evolution but also shapes the evolutionary pathway and persistence of adaptive change. By integration of ecological and evolutionary perspectives, this study underscores the importance of density-dependent selection in mediating species responses to anthropogenic stressors.