Hamilton's inclusive fitness maintains heritable altruism polymorphism throughrb=c

Abstract

<p id="d7476079e167">Why should some individuals help others at the expense of their own fitness? Hamilton’s elegant formula <i>rb</i> &gt; <i>c</i> resolves the major paradox in social evolution and has become a golden rule of sociobiology. However, <i>rb</i> &gt; <i>c</i> only tells part of the story, namely how altruistic genotypes expand. Theoretically, altruistic genotypes can persist by coexisting stably with nonaltruistic ones relying on <i>rb</i> = <i>c</i>, which may let both genotypes have equal inclusive fitness. We present evidence for this prediction using long-term data on a species of bird. Our work suggests that altruism should be understood beyond <i>rb</i> &gt; <i>c</i>, given that <i>rb</i> = <i>c</i> has the potential to explain widespread altruism polymorphisms in nature. </p><p class="first" id="d7476079e201">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> &gt; <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. </p>