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Kin selection as the key to altruism: its rise and fall.

Article Excerpt
ONE OF THE ENDURING UNSETTLED ISSUES OF EVOLUTIONARY BIOLOGY is the paradox of collateral altruistic behavior--that is, when some individuals subordinate their own interests and those of their immediate offspring in order to serve the interests of a larger group beyond offspring (Wilson, 1975). How might such behavior evolve if the genes promoting it are at such a disadvantage in competition with genes that oppose it?

Charles Darwin saw that the paradox was dangerous to his theory of evolution by natural selection. He was particularly concerned by the social behavior of ants. Not only do flagrantly selfless individuals exist, but they form distinct worker castes, which in some species are subdivided further into specialized subcastes--for example, large, aggressive (but sterile) soldiers and small nurses and foragers. How could such creatures come into existence if they never reproduce? Darwin solved the dilemma to his own satisfaction and that of other biologists for nearly a hundred years by noting that if the combined offspring of the queen ant formed a colony that allowed her to produce more offspring than could an otherwise comparable solitary female, sterile castes would evolve as part of the variation of a single hereditary type. That hereditary type, not the plastic forms it produces, is therefore the unit of selection. The altruistic castes, he said, are like the well-flavored vegetable part in a single crop strain produced by selective breeding (Darwin, 1859).

In 1932 and again in 1955 J. B. S. Haldane, one of the founders of the modern genetic theory of evolution, put a new twist on the altruism problem (Haldane, 1932; 1955). He pointed out how selflessness could evolve even if individuals are not organized into societies. His solution later came to be known as kin selection. Your genes, Haldane said, can be multiplied in a population even if you never reproduce, providing your actions favor the differential survival and reproduction of collateral relatives, such as siblings, nieces, and cousins, to sufficient degree. Suppose, he argued, you see a relative drowning, and if in rescuing him you have a one-tenth chance of drowning yourself. Your genes, including those predisposing you to perform this act of altruism, will nevertheless be increased in the population if such actions increase the number of offspring of the relative by more than...