Kin selection is a hypothesis developed by W.D. Hamilton to explain altruism among close relatives. An altruistic behavior decreases the fitness (survival and/or reproductive success) of the individual performing the act (the actor) while increasing the fitness of other individuals (the recipients). Individuals not only pass on their genes through their own reproduction (individual fitness) but also by helping relatives survive to reproduce. Since close relatives have a higher probability of sharing genes through common descent than do more distant ones, altruistic acts performed preferentially toward kin will favor the very genes that code for the behavior. Hamilton termed this phenomenon inclusive fitness. Kin selection thus provides a mechanism by which some genes can increase in population frequency even though they decrease the actor's fitness.
Whether or not an altruistic behavior will be favored by selection depends on (1) the fitness cost to the actor, (2) the fitness benefit to the receiver(s), (3) the closeness of the relationship between individuals involved in the altruistic interaction, and (4) the number of individuals involved. Hamilton quantified this relationship with a mathematical equation now known as Hamilton's Rule. In its simplest form, the Rule states that altruistic acts should be favored whenever
rB > C
where r is the coefficient of relatedness, B is the fitness benefit gained by the recipient as a result of the altruistic act, and C is the fitness cost to the actor. Costs and benefits are generally measured in terms of changes in the probability of survival or the number of offspring produced. Thus, altruistic behavior is most likely to evolve when many close relatives receive a large fitness gain as the result of an altruistic act that is of small cost to the actor. At the opposite extreme, selfish behavior, which increases the fitness of the actor, is expected when a very costly act provides but small benefit to a few distantly related recipients.
Behavioral patterns that are well-explained by kin selection include some forms of alarm calling, where an animal alerts the other members of its group to the presence of danger, thereby making itself more vulnerable; the association between aposematic (warning) coloration and gregarious behavior; and the evolution of sterile worker castes in social insects such as ants.