The Evolution of Social Behavior by Kin Selection

Kin-selection theory (Hamilton's "genetical theory") explains how aid that is self-sacrificing (in terms of classical individual fitness), or "altruism," can evolve if sufficiently beneficial to relatives. It is discussed here in order to clarify the meaning of kin selection and inclusive fitness (the total reproductive valve of an individual, both its production of offspring and effects on the reproduction of relatives). Hamilton's condition K > 1/r, the relationship of benefit/cost and relatedness necessary for advantageous altruism, is reformulated so as to be applicable to altruism by descendents, and from the point of view of any member of a population (e.g., affected parties other than the altruist). A General expression is derived which defines inclusive fitness in terms of a classical and a kinship component. A unit of inclusive fitness-"offspring equivalents"-is defined. An index of the liklihood that altruism will occur in different social and ecological situations. K1, is employed to evaluate conflicts of interest among the members of social groups. Specific cases of altruism are discussed with attention to costs and benefits in orden to show how kin selection can operate even among quite distant relatives. The probability of altruism is increased if the beneficiary stands to gain a great deal (e.g., in emergencies), if the cost is low (e.g., if the altruist is excluded from reproduction on his own or is in control of an abundant resource), or both; and if the donor is particularly efficient at giving aid or if the beneficiary is particularly efficient at using it, as in the case of the specialized workers and queens of social insects, or if both situation obtain. Phenomena discussed include social responses to food shortages in insects and primates; anti-predator responses of ungulates in variously structured social groups; social grooming and solicitude toward infants in primates; adoption of orphans in a phylogenetically diverse set of animals; "helpers" among birds, mammals, and insects; alarm calls of vertebrates; and dominance-subordinance interactions in vertebrates and invertebrates. Subordinant behavior among primates and other animals living in groups of relatives may sometimes represent a kind of altruism that is advantageous (in terms of inclusive fitness) to the subordinant individual, providing the subordinant individual is a reproductively inferior relative of the dominant individual and contributes sufficiently to the dominant individual's reproduction. Mutualism (reciprocity and cooperation) and parental manipulation may produce beneficent behavior resembling that produced by kin selection. Mutually beneficent behavior can be maintained by reciprocal-altruistic selection, parental parental imposition, or the selfish advantageousness of acts incidentally benefiting neighbors, as well as by kin selection. Reciprocal altruism-temporary altruism with the expectation of more than compensating future aid (reciprocation) on the part of the beneficiary-requires meticulous contemporaneous controls on cheating and is therefore probably restricted to intelligent animals, the only documented example being in man. A synthesis of current ideas on the evolution of insect sociality shows how mutualism, parental manipulation, and kin selection could all have operated, either in conjunction or independently, to produce extreme altruism (worker sterility) starting with different kinds of primitice groups. A kin-selection interpretation of insect sociality is given which differs from that of Hamilton in not relying on extraordinarily high relatedness among the members of a colony. The evolution of a reproductive division of Labor in insects probably involved differences in reproductive capacity among adults in primitively social groups of relatives, making it profitable, in terms of inclusive fitness, for some (namely, the reproductively inferior individuals) to become altruistic helpers. Kin-selection theory outlines certain limits to selfishness as well as the conditions under which altruism is advantageous. Inclusive fitness, because it includes the effects of all selfish and social traits on the reproductice value of an individual, is capable of evaluating the selective significance (biological function) of any social act, whether selfish, altruistic, reciprocal, cooperative, or destructive in nature. Thus, it provides an approach which could serve as the basis for a general and comprehensive theory of social behavior.