Social selection redefined: distinguishing features of social selection and the potential for runaway social evolution

Natural selection encompasses a range of evolutionary processes that lead to differential survival and reproduction via ecological, sexual and social selection pressures. We argue that, despite playing a demonstrably profound role in the evolution of social species, social selection has not been sufficiently integrated into the broader evolutionary paradigm as a driver of evolutionary changes. Here we review social selection, including its various descriptions in the literature, and propose a new, unifying definition: social selection comprises the selection pressures imposed by the social environment, which are determined by group characteristics and/or any interindividual social interactions that lead to differential fitness for an acting individual and/or their groupmates. We additionally review how self-reinforcing social selection can generate a runaway process, and provide a second definition for runaway social selection. We argue that social selection illuminates behavioural evolution, potentially driving the evolution of complex sociality and offering insights into social systems beyond those gained from ecological or sexual selection alone.