Behavior evolves as a correlated response to selection on cuticle color in Drosophila melanogaster and D. simulans

We selected for light and dark thoracic colors for 16 generations in two populations per species and measured correlated responses in aggression, basal activity, total activity, sleep, and geotaxis. Dark selected individuals consistently showed higher aggression and basal activity than light selected or control flies across species and sexes, indicating a modest, repeatable correlated response. Geotaxis, sleep, and total activity evolved unpredictably, varying across species and populations with no clear color association. Within-population correlations between color and aggression were significant but weak, indicating that color is a poor predictor of individual aggressive behavior. Populations with higher aggression evolved higher basal activity, suggesting a correlated response rather than a direct color-activity link. These persistent correlated responses are consistent with a conserved genetic basis (pleiotropy or linkage disequilibrium), though our data do not distinguish between them. Identifying loci underlying variation in color and behavior and experimentally testing their effects will clarify how multivariate genetic covariance shapes adaptive evolution.