Interacting Phenotypes and the Evolutionary Process. II. Selection Resulting from Social Interactions.

Social interactions often affect the fitness of interactants. Because of this, social selection has been described as a process distinct from other forms of natural selection. Social selection has been predicted to result in different evolutionary dynamics for interacting phenotypes, including rapid or extreme evolution and evolution of altruism. Despite the critical role that social selection plays in theories of social evolution, few studies have measured the force of social selection or the conditions under which this force changes. Here we present a model of social selection acting on interacting phenotypes that can be evaluated independently from the genetics of interacting phenotypes. Our model of social selection is analogous to covariance models of other forms of selection. We observe that an opportunity for social selection exists whenever individual fitness varies as a result of interactions with conspecifics. Social selection occurs, therefore, when variation in fitness due to interactions covaries with traits, resulting in a net force of selection acting on the interacting phenotypes. Thus, there must be a covariance between the phenotypes of the interactants for social selection to exist. This interacting phenotype covariance is important because it measures the degree to which a particular trait covaries with the selective environment provided by conspecifics. A variety of factors, including nonrandom interactions, behavioral modification during interactions, relatedness, and indirect genetic effects may contribute to the covariance of interacting phenotypes, which promotes social selection. The independent force of social selection (measured as a social selection gradient) can be partitioned empirically from the force of natural selection (measured by the natural selection gradient) using partial regression. This measure can be combined with genetic models of interacting phenotypes to provide insights into social evolution.