A theory put forth by W. D. Hamilton to explain how altruism can evolve when it increases the fitness of relatives. The theory proposes that a social act is favored by natural selection if it increases the inclusive fitness of the performer. Inclusive fitness consists of the individual's own fitness as well as his effects on the fitness of any genetically related neighbors. The idea is that genetic alleles change in frequency in a population owing to effects on the reproduction of relatives of the individual in which the character is expressed, rather than on the personal reproduction of the individual. According to Hamilton, altruism is favored when k is greater than 1 divided by r, where r is the relatedness between individuals and k is the ratio of gain to loss of the behavior being studied. The Hamilton theory is often referred to as kin selection. For example, a mutation that affected the behavior of a sterile worker bee so that she fed her fertile queen but starved herself would increase the inclusive fitness of that worker because, while her own fitness decreased, her actions increased the fitness of a close relative. See Chronology, 1964, Hamilton.
Subjects: Genetics and Genomics.