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

To predict adaptive evolution, we need to understand the degree to which selection on one trait can constrain or redirect evolutionary responses in other traits. We used artificial selection on cuticle color in Drosophila melanogaster and D. simulans to investigate whether color and behavior evolve in tandem. We selected for light and dark thoracic colors for 16 generations in two populations per species and measured correlated responses in a suite of behavioral traits, including 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 both species and sexes, pointing to a modest but repeatable correlated response that must be interpreted in light of low and variable aggression in our assay. In contrast, patterns for geotaxis, sleep, and total activity evolved unpredictably, often varying across species and populations, and showed no clear or uniform association with color. Taken together, our results suggest that some behaviors may share a predictable relationship with color, whereas others behave largely independently under the conditions we examined. The persistence of correlated responses in aggression and basal activity in response to selection on color is consistent with a conserved genetic basis, such as pleiotropy or tight linkage, although our data do not, by themselves, distinguish among these mechanisms. Future work that identifies the loci underlying variation in both color and behavior and tests their effects experimentally will be important for clarifying how multivariate genetic covariance shapes the direction and pace of adaptive evolution.