Kin Selection

Kin selection refers to apparent strategies in evolution that favour the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of kin "group" selection. In the "The Origin of Species", he wrote clearly about altruistic sterile social insects that

This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. Breeders of cattle wish the flesh and fat to be well marbled together. An animal thus characterized has been slaughtered, but the breeder has gone with confidence to the same stock and has succeeded"

In this passage "the family" and "stock" stand for a kin group. These passages and others by Darwin about "kin selection" are highlighted and justly celebrated in D.J. Futuyma's textbook of reference "Evolutionary Biology" and in E.O Wilson's "Sociobiology" .

The earliest mathematically formal treatments of kin selection were by R.A. Fisher in 1930 and J. B. S. Haldane in 1932 and 1955. Later on, in works published in 1963 and—most importantly—in 1964, W. D. Hamilton popularized the concept and the more thorough mathematical treatment given to it by George Price. The term "kin selection" may first have been coined by John Maynard Smith in 1964:

These processes I will call kin selection and group selection respectively. Kin selection has been discussed by Haldane and by Hamilton. … By kin selection I mean the evolution of characteristics which favour the survival of close relatives of the affected individual, by processes which do not require any discontinuities in the population breeding structure.

Kin selection refers to changes in gene frequency across generations that are driven at least in part by interactions between related individuals, and this dynamic forms much of the conceptual basis of the theory of social evolution. Indeed, some cases of evolution by natural selection can only be understood by considering how biological relatives influence one another's fitness. Under natural selection, a gene encoding a trait that enhances the fitness of each individual carrying it should increase in frequency within the population; and conversely, a gene that lowers the individual fitness of its carriers should be eliminated. However, a hypothetical gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behavior, may nonetheless increase in frequency, because relatives often carry the same gene. According to this fundamental principle of kin selection theory, the enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour. As such, this is a special case of a more general model, called inclusive fitness (in that inclusive fitness refers simply to gene copies in other individuals without requiring that they be kin). However, the validity of this analysis has recently been challenged. Wilson writes that "the foundations of the general theory of inclusive fitness based on the theory of kin selection have crumbled" and now relies on the theory of eusociality and "gene-culture co-evolution" for the underlying mechanics of sociobiology.

"Kin group selection" should not be confused with the concept of "group selection": a theory that a genetic trait can become prevalent within a group because it benefits the group as a whole, regardless of any benefit to individual organisms.