Inclusive Fitness

The idea of inclusive fitness, or kin selection, is an important component of contemporary evolutionary theory. Haldane (1955) understood the process, as indicated by his remark that he would lay down his life for two brothers or eight cousins. However, the idea was mainly popularized by Hamilton (1964a, 1964b).

The main theoretical yield of the concept of inclusive fitness is that it explains (or perhaps explains away) altruism. Genes are selfish, and a person is merely a gene's "survival machine" (Dawkins, 1989). However, a gene's fondest wish (to anthropomorphize a bit) is to be passed into the next generation. This can occur if the person carrying the gene survives to reproduce--but it can also occur if the person's relatives (who carry many of the same genes) survive to reproduce. Thus, a lapwing will fake injury to distract a hawk from its young, thereby acting altruistically toward its offspring (Maynard Smith, 1995).

Sometimes the altruistic act benefits non-relatives. For example, members of many species will take care of youngsters, even if the youngsters are not their own. This may be because the evolved mechanism is not sensitive enough to make fine discriminations, opening the altruist up to exploitation. People who take care of pets, for example, are having their caretaking mechanism exploited (Maynard Smith, 1995).


Dawkins, R. (1989). The selfish gene. New York: Oxford University Press.

Haldane, J. B. S. (1955). Population genetics. New Biology, 18, 34-51.

Hamilton, W. D. (1964). The genetical evolution of social behavior: I. Journal of Theoretical Biology, 7, 1-16.

Hamilton, W. D. (1964). The genetical evolution of social behavior: II. Journal of Theoretical Biology, 7, 17-52.

Maynard Smith, J. (1995). The theory of evolution. New York: Cambridge University Press.

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