Why Skew Selection, a Model of Parental Exploitation, Should Replace Kin Selection

Synopsis In his 1964 paper, William Hamilton wrote that inclusive fitness trumps direct fitness if, and only if, the effect of interactions among siblings on their parent’s fitness is ‘zero’. Kin selection models have succeeded only because they have ignored the fact that, if an altruist dies saving two siblings, the ‘zero impact on their parent’s fitness’ constraint is violated. Imagine a parent with three offspring. If two offspring drown, parental fitness is 1. On the other hand, if one altruistic offspring dies saving its two drowning siblings, parental fitness doubles to 2. Thus, direct fitness trumps inclusive fitness as an explanation for the evolution of altruism. In other words, parents that produce some portion of altruistic offspring willing to die to save some of their siblings (who would die without the intervention of the altruist) will realize greater fitness than parents producing no altruists. Skew selection, a bioeconomic extension of Michael Ghiselin’s (1974) parental exploitation model, is presented to explain the evolution of altruism from a direct fitness point of view.