Cooperative interactions within the family enhance the capacity for evolutionary change in body size.

Classical models of evolution seldom predict the rate at which populations evolve in the wild. One explanation is that the social environment affects how traits change in response to natural selection. Here, we determine how social interactions between parents and offspring, and among larvae, influence the response to experimental selection on adult size. Our experiments focus on burying beetles (), whose larvae develop within a carrion nest. Some broods exclusively self-feed on the carrion while others are also fed by their parents. We found populations responded to selection for larger adults but only when parents cared for their offspring. We also found populations responded to selection for smaller adults too, but only by removing parents and causing larval interactions to exert more influence on eventual adult size. Comparative analyses revealed a similar pattern: evolutionary increases in species size within the genus are associated with the obligate provision of care. Synthesising our results with previous studies, we suggest that cooperative social environments enhance the response to selection whereas excessive conflict can prevent further directional selection.