Kin selection
Kin selection refers to changes in gene frequency across generations that are driven at least in part by interactions between related individuals, and this 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 the fitness of each other. 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 relatives but lowers that of the individual displaying the behavior (altruism), may nonetheless increase in frequency, because relatives often carry the same genes. The enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour.
Hamilton's rule
Formally, such genes should increase in frequency when
where
- 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.
- 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 equation is known as Hamilton's rule after W. D. Hamilton who published, in 1964, the first formal quantitative treatment of kin selection to deal with the evolution of apparently altruistic acts. The phrase Kin selection, however, was coined by John Maynard Smith.
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 possible 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.
In the 1930s J.B.S. Haldane had full grasp of the basic quantities and considerations that play a role in kin selection. He famously said that, "I'd lay down my life for two brothers or eight cousins".[1] Kin altruism is the term for altruistic behaviour whose evolution is supposed to have been driven by kin selection.
Haldane's remark alluded to the fact that if an individual loses its life to save two siblings, four nephews, or eight cousins, it is a "fair deal" in evolutionary terms, as siblings are on average 50% identical by descent, nephews 25%, and cousins 12.5% (in a diploid population that is randomly mating and previously outbred). But Haldane also joked that he would truly die only to save more than one identical twins or more than two full siblings.
Mechanism
Hamilton (1964) outlined two ways in which kin selected altruism could be favoured.
Firstly, if individuals have the capacity to recognise kin (kin recognition) and to adjust their behaviour on the basis of kinship (kin discrimination), then the average relatedness of the recipients of altruism could be high enough for this to be favoured. Because of the facultative nature of this mechanism, it is generally regarded that kin recognition and discrimination are unimportant except among 'higher' forms of life (although there is some evidence for this mechanism among protozoa). A special case of the kin recognition / discrimination mechanism is the hypothetical 'green beard', where a gene for social behaviour also causes a distinctive phenotype that can be recognised by other carriers of the gene. Hamilton's discussion of greenbeard altruism serves as an illustration that relatedness is a matter of genetical similarity and that this similarity is not necessarily caused by genealogical closeness (kinship).
Secondly, even indiscriminite altruism may be favoured in so-called viscous populations, i.e. those characterised by low rates or short ranges of dispersal. Here, social partners are typically genealogically-close kin, and so altruism may be able to flourish even in the absence of kin recognition and kin discrimination faculties. This suggests a rather general explanation for altruism.
See also
References
- ^ Kevin Connolly and Maragaret Martlew, ed. (1999). "Altruism". Psychologically Speaking: A Book of Quotations. BPS Books. p. 10. 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.
- Queller, D.C. & Strassman, J.E. (2002) Quick Guide: Kin Selection. Current Biology,12,R832.