Kin Selection

According to Hamilton’s kin selection theory (also known as “inclusive fitness” theory), kin selection is the process by which social evolution occurs in nature. The theory extends the genetical theory of natural selection to social behaviors and finds that their evolution is affected by the likelihood that individuals share genes (relatedness). In biology, a social behavior occurs when one individual (the actor) behaves so as to affect the direct fitness (number of offspring) of itself and another individual (the recipient). For example, altruism occurs when the actor’s behavior decreases the actor’s direct fitness and increases the recipient’s direct fitness. Conversely, selfishness occurs when the actor’s behavior increases the actor’s direct fitness and decreases the recipient’s. Social behaviors are widespread in nature. A classic example is the altruism shown by the sterile workers of social insects such as ants, which sacrifice their own reproduction in order to rear the queen’s offspring. At first sight, altruism poses a problem for the genetical theory of natural selection, which seems to preclude the spread of a gene for reduced reproduction. Kin selection was devised by William Hamilton in the early 1960s to address this “problem of altruism.” The basic principle behind kin selection had been hinted at by Darwin, Fisher, and Haldane, but it was Hamilton who provided the first general model. Hamilton called his idea “inclusive fitness” theory, and it was later dubbed “kin selection” by Maynard Smith in 1964. For most purposes, the two can be considered identical, although inclusive fitness theory technically includes kin selection theory because the relatedness it invokes need not involve kin (genealogical relatives). Kin selection theory solved the problem of altruism by showing that a gene for altruism can spread if altruism is directed at individuals likely to bear the same gene. By definition, kin are likely to share genes. So, a gene for altruism can spread if altruism is directed at kin and the loss of gene copies through the actor’s decreased reproduction is more than offset by the gain in gene copies through the increased reproduction of the recipient. The algebraic version of this condition is termed “Hamilton’s rule.” Although kin selection theory was devised to explain altruism, it also applies to the other forms of social behavior such as selfishness. The theory is therefore now widely used to investigate and explain many kinds of social behavior in living organisms as diverse as bacteria and human beings.