Inclusive fitness

In evolutionary biology inclusive fitness theory is a model for the evolution of social behaviors (traits), first set forward by W. D. Hamilton in 1963 and 1964. Instead of a trait's frequency increase being thought of only via its average effects on an organism's direct reproduction, Hamilton argued that its average effects on indirect reproduction, via identical copies of the trait in other individuals, also need to be taken into account. Hamilton's theory, alongside reciprocal altruism, is considered one of the two primary mechanisms for the evolution of social behaviors in natural species.

From the gene's point of view, evolutionary success ultimately depends on leaving behind the maximum number of copies of itself in the population. Until 1964, it was generally believed that genes only achieved this by causing the individual to leave the maximum number of viable direct offspring. However, in 1964 W. D. Hamilton showed mathematically that, because other members of a population may share identical genes, a gene can also increase its evolutionary success by indirectly promoting the reproduction and survival of such individuals. The most obvious category of such individuals is close genetic relatives, and where these are concerned, the application of inclusive fitness theory is often more straightforwardly treated via the narrower kin selection theory.