Population dynamics mediate plasticity and genetic adaptation to metal stress in water fleas

Density-dependent selection represents one of the most fundamental interactions between ecology and evolution. Nevertheless, the role of density-dependent selection as a potential driver of observed evolutionary changes in populations subjected to metal stress remains untested. We found that increasing population density and copper selection pressure significantly influenced rapid evolutionary responses in the water flea <i>Ceriodaphnia cornuta</i> (family Daphniidae), as observed through artificial selection and relaxation experiments. Common garden experiments revealed that these selective pressures impacted fitness differently in the presence or absence of copper. This study demonstrates that an elevated population density accelerates adaptive changes in <i>C. cornuta</i> populations, enhancing survival and reproduction rates under copper selection. Genotypic analyses reveal shifts towards copper-tolerant genotypes under high-density conditions, but with reversion in the genotype frequencies following relaxation of the copper stress. Our findings highlight that population density fundamentally shapes adaptive responses, influencing the balance between phenotypic plasticity and genetic evolution under an environmental stress. Accounting for this variation at different densities of natural populations is crucial for accurately predicting species responses to anthropogenic change.