MINI REVIEW Spite and the scale of competition

Social behaviours can be categorized according to the direct fitness consequences they entail for the actor and recipient (Fig. 1; Hamilton, 1964, 1970, 1971). A behaviour increasing the direct fitness of the actor is mutualistic if the recipient also benefits, and selfish if the recipient suffers a loss. It is easy to see how such behaviours can be favoured by natural selection. Behaviours which reduce the direct fitness of the actor – altruism if the recipient enjoys a benefit, spite if the recipient suffers a loss – are less easy to explain. Hamilton (1963, 1964) introduced the concept of inclusive fitness and showed that while certain behaviours are detrimental to the individual, they may result in a net increase in the actor’s genes in the population. Altruism can be favoured by natural selection despite a direct fitness cost (C) to the actor if the benefit (B) accruing to the recipient is sufficiently large and if the genetic relatedness (R) of the recipient to the actor is sufficiently positive. Specifically, when Hamilton’s (1963, 1964) rule, RB > C, is satisfied. A spiteful behaviour, entailing a negative benefit (B < 0) to the recipient and a positive cost (C > 0) to the actor, is similarly favoured if RB > C, which would require a negative relatedness (R < 0) between actor and recipient.