Selection favors context-dependent bias in altruism

Abstract

Altruism, in which individuals sacrifice some of their own reproduction to help others, can evolve if it is preferentially directed toward relatives. Organisms may recognize relatives through phenotypic similarity. Under the models originally studied by Hamilton, the threshold relatedness at which altruism becomes beneficial depends on the overall relatedness of the population. Implicit in this result is that natural selection may favor context-dependent strategies, in which donors judge their similarity to potential recipients relative to their similarity to the overall population when deciding whether to help. In this manuscript, we use a combination of simulations and theory to determine the circumstances under which context dependence is favored over simple strategies that do not depend on the observation of other interacting individuals. We find that a "plastic" strategy that uses the population context in its rule for donating consistently beats strategies that use only information about the potential recipient.