Hamilton's inclusive fitness maintains heritable altruism polymorphism through <i>rb</i> = <i>c</i>.

Abstract

How can altruism evolve or be maintained in a selfish world? Hamilton's rule shows that the former process will occur when <i>rb</i> > <i>c</i>-the benefits to the recipients of an altruistic act <i>b</i>, weighted by the relatedness between the social partners <i>r</i>, exceed the costs to the altruists <i>c</i>-drives altruistic genotypes spreading against nonaltruistic ones. From this rule, we infer that altruistic genotypes will persist in a population by forming a stable heritable polymorphism with nonaltruistic genotypes if <i>rb</i> = <i>c</i> makes inclusive fitness of the two morphs equal. We test this prediction using the data of 12 years of study on a cooperatively breeding bird, the Tibetan ground tit <i>Pseudopodoces humilis</i>, where helping is performed by males only and kin-directed. Individual variation in ever acting as a helper was heritable (<i>h</i>² = 0.47), and the resultant altruism polymorphism remained stable as indicated by low-level annual fluctuation of the percentage of helpers among all adult males (24-28%). Helpers' indirect fitness gains from increased lifetime reproductive success of related breeders statistically fully compensated for their lifetime direct fitness losses, suggesting that <i>rb</i> = <i>c</i> holds. While our work provides a fundamental support for Hamilton's idea, it highlights the equivalent inclusive fitness returns to altruists and nonaltruists mediated by <i>rb</i> = <i>c</i> as a theoretically and realistically important mechanism to maintain social polymorphism.