Kinship, Evolutionary Theory of

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Kinship, Evolutionary Theory of


Altruistic behaviorself-sacrificial behavior performed for the benefit of others (Wilson 1975, p. 578)has long been a paradox for evolutionary theorists, because natural selection favors traits that contribute to the reproductive success (individual fitness) of those who possess or perform them. Hence, altruism, which by definition contributes to the reproductive success of competitions at the expense of performers, ought not to evolve by natural selection. Yet it is widespread in nature. The sterile worker castes in bees, ants, and wasps provide prime examples (Wilson 1975). In 1964 William Hamilton, a graduate student at the University of London, saw that biological altruism can evolve, even though it reduces the individual reproductive success of the helps donor, if it aids the donors genetic kin, some of whom inherit the helping allele from a common ancestor of the donor and recipient.

Consider C, the cost to a donor of a helping act, and B, the benefit of the act to a recipient. Hamiltons statement can be expressed as Br 1 > Cr 2 where r 1 equals the probability that the recipients offspring has a copy of a helping allele inherited from a common ancestor of the donor and recipient, such as a parent or grandparent; and r 2 equals the probability that the donors own offspring would have a copy of the same helping allele inherited from a common ancestor, such as a parent or grandparent. These probabilities are called genetic correlations or coefficients of relatedness between individuals.

As an example, consider a donor who inherited a helping allele from his mother for giving up one offspring to help a half-sibling produce three additional offspring. This allele was inherited via the common parent of the half siblings, their mother. When an organism reproduces, only one of each pair of chromosomes is passed to its gamete (Mendels law of segregation), which unites with a gamete from another individual to produce the zygote that grows into an adult. Therefore, the probability that any particular allele at a locus on a chromosome in a parent is passed on to its offspring is 0.5. There are two gamete-producing reproductive events between the donor and any of his or her three additional half nieces or nephews produced because of the help: One produces the half-sibling that is being helped; the other produces each of the three offspring of the half-sibling who is being helped (the half-nieces and half-nephews of the donor) because of the help. Therefore, there is a 0.5 × 0.5 = 0.25 chance that any one of them inherits a copy of the helping allele possessed by the donor, their uncle or aunt. Putting these numbers into the above equation produces 3(0.25) > 1(0.50); 0.75 > 0.50.

Hence, natural selection can favor the evolution of psychological mechanisms for producing helping behaviors directed to genetic relatives. Note that both sides of the equation refer to changes in the helpers reproductive success because of helping acts. The right side, Cr 2, indicates the direct reproductive cost to the donor through offspring not produced, whereas the left side, Br 1, indicates the indirect reproductive benefits to the donor through additional offspringa genetic relative produced because of the help. This reasoning leads to replacing the concept of individual fitness with the concept of inclusive fitness, the reproductive success of an individual adjusted by the help it gives and receives from genetic relatives. Kin selection refers to natural selection acting via inclusive fitness rather than via individual fitness. Finally, note that those desiring an evolutionary explanation for true altruism, in which a donor receives no benefit for a helping act, will not find it in Hamiltons equation.

Hamiltons paper (1964) is one of the most important scientific papers of the twentieth century. First, it enables evolutionary explanations for a wide range of social interactions in organisms (Alcock 2005; Wilson 1975) and humans (Burnstein 2005; Buss 2004). Second, it shifted the focus in evolutionary thinking from the reproduction of individuals to the replication of genes. Albert Einstein gave us relativity theory when he imagined how the universe would look if he were riding on a beam of light; Hamilton gave us a new way of thinking about evolution when he imagined how natural selection would seem to work if he were riding on a gene as it crossed generations.

SEE ALSO Altruism; Darwin, Charles; Evolutionary Psychology; Hamiltons Rule; Natural Selection; Sociobiology


Alcock, John. 2005. Animal Behavior: An Evolutionary Approach. 8th ed. Sunderland, MA: Sinauer Associates.

Burnstein, Eugene. 2005. Altruism and Genetic Relatedness. In The Handbook of Evolutionary Psychology, ed. David M. Buss, 528551. Hoboken, NJ: Wiley.

Buss, David M. 2004. Evolutionary Psychology: The New Science of the Mind. 2nd ed. New York: Pearson.

Hamilton, William D. 1964. The Genetical Evolution of Social Behavior, I and II. Journal of Theoretical Biology 7: 152.

Wilson, E. O. 1975. Sociobiology: The New Synthesis. Cambridge, MA: Harvard University Press.

Charles Crawford

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