Hamilton's rule and the causes of social evolution

Abstract

Hamilton's rule is a central theorem of inclusive fitness (kin selection) theory and predicts that social behaviour evolves under specific combinations of relatedness, benefit and cost. This review provides evidence for Hamilton's rule by presenting novel syntheses of results from two kinds of study in diverse taxa, including cooperatively breeding birds and mammals and eusocial insects. These are, first, studies that empirically parametrize Hamilton's rule in natural populations and, second, comparative phylogenetic analyses of the genetic, life-history and ecological correlates of sociality. Studies parametrizing Hamilton's rule are not rare and demonstrate quantitatively that (i) altruism (net loss of direct fitness) occurs even when sociality is facultative, (ii) in most cases, altruism is under positive selection via indirect fitness benefits that exceed direct fitness costs and (iii) social behaviour commonly generates indirect benefits by enhancing the productivity or survivorship of kin. Comparative phylogenetic analyses show that cooperative breeding and eusociality are promoted by (i) high relatedness and monogamy and, potentially, by (ii) life-history factors facilitating family structure and high benefits of helping and (iii) ecological factors generating low costs of social behaviour. Overall, the focal studies strongly confirm the predictions of Hamilton's rule regarding conditions for social evolution and their causes.

Go High or Go Low? Adaptive Evolution of High and Low Relatedness Societies in Social Hymenoptera

“…The effects of genetic relatedness are demonstrable across multiple contexts (Bourke, 2011(Bourke, , 2014. Likewise, genetic diversity is often an advantageous group-level trait Fewell, 2007, 2008), that can produce social heterosis (Nonacs and Kapheim, 2007).…”

“…The three panels show the effects on allele distribution as selection goes from favoring monandry and high relatedness (h = 0.50), to favoring polyandry, drifting and low relatedness (h = 1.00). (Bourke, 2014). Indeed with constant benefits and costs, Hamilton's rule predicts causative relationships between high relatedness and the propensity to evolve cooperation (Boomsma, 2009).…”

“…Furthermore, across these species there is also great variability in levels of relatedness ranging from 100% clonal societies to societies that are near random collections of kin and non-kin. Historically, multiple aspects of social Hymenoptera behavior and life history are considered as the best examples of nepotistic kin selection in action (Bourke, 2014), while simultaneously genetic diversity within groups has been found to significantly increase both group survival and reproduction across a variety of taxa (see multiple examples reviewed in Nonacs and Kapheim, 2007). As a FIGURE 1 | Hamilton's rule for cooperation in relation to relatedness.…”

There is no fitness but fitness, and the lineage is its bearer

“…0, with b and c defined as additive fitness effects and r the pairwise relatedness. Most debate over empirical quantification of Hamilton's rule is about how to measure r, b and c properly [43]. We argue that a more fundamental problem is the assumption that the canonical Hamilton's rule applies without considering whether its assumptions apply.…”

“…While some authors (e.g. [43]) do explicitly recognize the specific assumptions behind the canonical Hamilton's rule, they also assert that this form of Hamilton's rule nonetheless represents a good approximation to many interactions in nature. This assertion, to our knowledge, remains untested empirically.…”