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Kin Selection is the phrase used to refer to changes in gene frequency driven by natural selection that can only be understood by looking at how biological relatives influence the fitness of each other.

Hamilton's Rule[]

Under natural selection, a gene encoding a trait that enhances the fitness of each individual carrying it should increase in frequency in the population and, conversely, a gene that lowers the individual fitness of its carriers should be eliminated. However, a gene that prompts behaviour which enhances the fitness of biological relatives but lowers that of the individual displaying the behavior, may nonetheless increase in frequency relative to genes that do not, because biological relatives often carry the same genes and the enhanced fitness of biological relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour. More formally, such genes should increase in frequency when


R = the genetical relatedness of the recipient to the actor, usually defined as the probability that a gene picked randomly from each at the same locus is identical by descent (i.b.d).
B = the additional reproductive benefit gained by the recipient of the 'altruistic' act,
C = the reproductive cost to the individual of performing the act.

This is known as Hamilton's rule after W. D. Hamilton who published in 1964 the first formal quantitative treatment of biological kin selection to deal in particular with the evolution of apparently altruistic acts.

Kin altruism[]

The phrase Kin selection, however, was coined by John Maynard Smith, and already in the 1930s J.B.S. Haldane had full grasp of the basic quantities and considerations that play a role in biological kin selection. He famously said that, "I'd lay down my life for two brothers or eight cousins".[1] . Kin altruism is the technical term for altruistic behaviour whose evolution is supposed to have been driven by biological kin selection. Note, however, that in a very recent contribution, B.J. Williams showed that when mating pairs are rigidly monogamous and altruistic acts lower the fitness of both spouses, Hamilton's rule is not correct and it is much harder for altruism to evolve except when reproductive rates are very high. However, rigid monogamy is not common in nature and unfortunately Williams did not try to relax this assumption.

Haldane's remark alluded to the fact that an individual that loses its life to save two siblings, four nephews, or eight cousins, is actually accepting a fair deal in evolutionary terms since biological siblings are on average 50% identical by descent, biological nephews 25%, and biological cousins 12.5% (in a diploid population that is randomly mating and previously outbred). And the individual would be getting an even better deal if it died to save, e.g., two identical twins or three full siblings.

But the concept of biological kin selection was first invoked by Darwin as an explanation of the sterile castes of social insects. In 1859 he wrote in Chapter 7 of The Origin of Species about a "special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind." - - Despite Mendelian genetics not being known at the time, Darwin commented insightfully:

"...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. Thus, a well-flavoured vegetable is cooked, and the individual is destroyed; but the horticulturist sows seeds of the same stock, and confidently expects to get nearly the same variety; breeders of cattle wish the flesh and fat to be well marbled together; the animal has been slaughtered, but the breeder goes with confidence to the same family..."

In these examples the "family" stands for the biological kin group, the slaughtered cattle and cooked vegetables are biological kin individuals tested by selection to ascertain their having or not the desired trait, and the testing results in enhanced reproduction of that biologial family that produced the individuals which did show the desired trait; in the same way as the performance of sterile ant workers boosts the fitness of those ant queens that produce sterile workers and thus the fitness of the genes which enable queens to generate sterile workers that help the colony.

For biological kin-selection to occur individuals have to behave nepotistically (i.e., have to favor their biologial kin). Often this involves recognizing biological kin thanks to innate reactions to traits which signal relatedness or to imprinted features which distinguish the individuals one grew up with (which are more likely to be biological kin). But such nepotism need not be purely altruistic nor involve genuine biological kin recognition. For instance Tiger Salamanders produce a non-cannibalistic larval morph that cannot prey on salamander eggs and other young salamanders. This reduces the damage the larvae inflict on each other and thus on their own biological kin. This morph switch is biological kin-imprinted since the cannibalistic morph is triggered when the eggs develop in presence of non-biological kin. However, this non-cannibalism protects all eggs and young salamanders in a pond rather than only those of the same biological kin.

Biological kin selection has been invoked to explain the evolution of social insects such as ants and termites, of multicellularity, and even of humanity's social structure. However, the role of biological kin selection in the formation of human social systems is highly disputed.

Technically, the correct definition for relatedness (R) in Hamilton's rule describes it as a regression measure. Regressions, unlike probabilities, can be negative, and so it is not implausible for individuals to be negatively related, which simply means that two individuals can be less genetically alike than two random ones on average. This has been invoked to explain the evolution of spiteful behaviours.

See also[]

Also, applications to the study of human behavior:


  • ^  (1999) "Altruism" Kevin Connolly and Maragaret Martlew Psychologically Speaking: A Book of Quotations, 10, BPS Books. ISBN 185433302X. (see also: Haldane's Wikiquote entry)
  • Hamilton, W.D. (1964). The genetical evolution of social behaviour I and II. — Journal of Theoretical Biology 7: 1-16 and 17-52. pubmed I pubmed II
  • Lucas, J.R., Creel, S.R. & Waser, P.M. (1996) How to measure inclusive fitness, revisited, Animal Behaviour, 51, 225-228.
  • "'Williams, B.J."' (2005). Kin selection in human populations: Theory reconsidered,"Human Biology", "'77"', 421-431.

Further reading[]

  • Mace, R. and Sear, R. (2005). Are humans cooperative breeders? In E. Voland, A. Chasiotis & W. Schiefenhoevel (Eds.), Grandmotherhood: the Evolutionary Significance of the Second Half of Female Life. Rutgers University Press, Piscataway. pp 143-159 Full text
  • Sear, R. & Mace, R. (working paper) Evolution and the human family: a review of the demographic evidence. Full text


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