How altruism evolves: assortment and synergy

If one defines altruism strictly at the population level such that carriers of the altruistic genotype are required to experience, on average, a net fitness cost relative to average population members, then altruism can never evolve. This is simply because a genetically encoded trait can only increase in a population (relative to alternative traits) if the mean fitness of individuals carrying this genotype is higher than the population average fitness. This is true whether the genotype of interest encodes a self-serving behaviour such as enhanced predator avoidance, or an altruistic behaviour in which the actor enhances the fitness of those it interacts with more than its own. The paradox in the evolution of altruism is that carriers that are, on average, at a local disadvantage (i.e. compared to those they interact with) can still have higher fitness than the population average and hence can increase overall. The most fundamental explanation for how altruism (defined by local interactions) increases in a population requires that there be assortment in the population such that the benefit from others falls sufficiently often to carriers (and at the same time nonaltruists are stuck interacting more with each other). Nonadditivity if present can play a similar role: when collective cooperation yields synergistic benefits (positive nonadditivity) altruistic behaviour can evolve even in the absence of positive assortment, and when there are diminishing returns for cooperation (negative nonadditivity) the evolution of altruism is hindered (Queller, 1985; Hauert et al., 2006). In their target article Lehmann & Keller (2006) use a form of Hamilton’s rule (1964, 1975) to classify different mechanisms by which helping behaviours can evolve. However, the version they develop tends to obscure the fundamental roles that assortment and nonadditivity play. Their framework also confuses local and population-wide definitions of altruism in making distinctions between nonrelatives and relatives, and what they label as mere ‘cooperation’ vs. true ‘altruism’. We argue that a previous generalization of Hamilton’s rule developed by Queller (1985) makes clear the roles played by assortment and nonadditivity and therefore serves as a better starting point for classifying various proposed models and mechanism of how altruistic traits can evolve.