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Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to their own survival and/or reproduction. The classic example is a eusocial insect colony, in which sterile females act as workers to assist their mother in the production of additional offspring.

The earliest expressions of the basic concepts were by R.A. Fisher in 1930,[1] J. B. S. Haldane in 1955,[2] but it was W. D. Hamilton who truly formalized the concept, in works published in 1963[3] and—most importantly—in 1964.[4] The term "kin selection" may first have been coined by John Maynard Smith in 1964 when he wrote:

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.[5]

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 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 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; this is the fundamental principle behind the theory of kin selection. According to the 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).


Hamilton's rule

Formally, such genes should increase in frequency when

\quad rB > C


r = the genetic relatedness of the recipient to the actor, often 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 inequality 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. Altruistic acts are those that benefit the recipient but harm the actor. The phrase Kin selection, however, was coined by John Maynard Smith.

Originally, the definition for relatedness (r) in Hamilton's rule was explicitly given as Sewall Wright's coefficient of relationship: the probability that at a random locus, the alleles there will be identical by descent (Hamilton 1963, American Naturalist, p. 355). Subsequent authors, including Hamilton, sometimes reformulate this with a regression, which, 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 (Hamilton 1970, Nature & Grafen 1985 Oxford Surveys in Evolutionary Biology). This has been invoked to explain the evolution of spiteful behaviours. Spiteful behavior defines an act (or acts) that results in harm, or loss of fitness, to both the actor and the recipient.

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 would lay down my life for two brothers or eight cousins".[6] 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 a single identical twin of his or more than two full siblings.


An altruistic case is one where the instigating individual suffers a fitness loss while the receiving individual benefits by a fitness gain. The sacrifice of one individual to help another is an example of altruism.

Hamilton (1964) outlined two ways in which kin selection altruism could be favoured.

Kin Recognition: Firstly, if individuals have the capacity to recognize 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 genetic similarity and that this similarity is not necessarily caused by genealogical closeness (kinship).

Viscous Populations: Secondly, even indiscriminate altruism may be favoured in so-called viscous populations, i.e. those characterized 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—spatial proximity serves as a rudimentary form of discrimination. This suggests a rather general explanation for altruism. Directional selection will always favor those with higher rates of fecundity within a certain population. Social individuals can often ensure the survival of their own kin by participating in, and following the rules of a group (assuming the implied faculties for group discrimination).

These mechanisms explain a relatively high r between interacting individuals. Absolute genetic similarity is not a measure of r; rather, r shows the “excess” relatedness between an actor and a recipient compared with the relatedness between an actor and a random member of the population. Thus, in a clonal population with 100% genetic similarity, r = 0 (as strange as that may sound). This is because there can be no correlation between genetic similarity and interaction strengths if genetic similarity is constant.

It has often been observed that altruism cannot be maintained in a population of randomly interacting individuals (see Michod [1982][7] and references therein). In such a population, the correlation between genetic similarity and interaction strength is necessarily absent, thus r = 0 and rB < C for any C > 0. This is why mechanisms such as spatial structure and kin recognition are so important for the long-term stability of altruistic traits, and why measures such as "population-wide average r" are meaningless in the absence of such mechanisms.

Kin Selection in evolutionary psychology

Evolutionary psychologists have attempted to explain prosocial behavior through kin selection by stating that “behaviors that help a genetic relative are favored by natural selection.” Human beings have developed a tendency over time to frame and interpret their actions as an avenue to the survival of their genetic material, making kin selection not a completely altruistic form of prosocial behavior and is perhaps better described as a component of social exchange theory. This theory does not necessarily imply that people “compute” genetic benefit when helping others, but there is an indication that those who behave in such a way are more likely to pass on their genes to future generations.[8]


Experiment about Kin Selection

Eusociality (true sociality) is used to describe social systems with three characteristics: one is an overlap in generations between parents and their offspring, two is cooperative brood care, and the third characteristic is specialized castes of nonreproductive individuals.[9] Social insects are an excellent example of organisms that display presumed kin selected traits. The workers of some species are sterile, a trait that would not occur if individual selection was the only process at work. The relatedness coefficient r is abnormally high between the worker sisters in a colony of Hymenoptera due to haplodiploidy, and Hamilton's rule is presumed to be satisfied because the benefits in fitness for the workers are believed to exceed the costs in terms of lost reproductive opportunity, though this has never been demonstrated empirically. There are competing hypotheses, as well, which may also explain the evolution of social behavior in such organisms (see Eusociality).

Alarm calls in ground squirrels are another example. While they may alert others of the same species to danger, they draw attention to the caller and expose it to increased risk of predation. Paul Sherman, of Cornell University, studied the alarm calls of ground squirrels. He observed that they occurred most frequently when the caller had relatives nearby.[10] In a similar study, John Hoogland was able to follow individual males through different stages of life. He found that the male prairie dogs modified their rate of calling when closer to kin. These behaviors show that self-sacrifice is directed towards close relatives and that there is an indirect fitness gain.[9]

Alan Krakauer of University of California, Berkeley has studied kin selection in the courtship behavior of wild turkeys. Like a teenager helping her older sister prepare for prom night, a subordinate turkey may help his dominant brother put on an impressive team display that is only of direct benefit to the dominant member.[11]

Recent studies provide evidence that even certain plants can recognize and respond to kinship ties. Using sea rocket for her experiments, Susan Dudley at McMaster University in Canada compared the growth patterns of unrelated plants sharing a pot to plants from the same clone. She found that unrelated plants competed for soil nutrients by aggressive root growth. This did not occur with sibling plants.[12]

In human fertilization, some sperm cells consume their acrosome prematurely on the surface of the egg cell, facilitating for surrounding sperms, having on average 50% genome similarity, to penetrate the egg cell.[13]

In the wood mouse (Apodemus sylvaticus), aggregates of spermatozoa form mobile trains, some of the spermatozoa undergo a premature acrosome reactions that correlate to improved mobility of the mobile trains towards the female egg for fertilization. This association is thought to proceed as a result of a "green beard effect" in which the spermatozoa perform a kin-selective altruistic act after identifying genetic similarity with the surrounding spermatozoa.[14]

The theory of Kin selection has had a profound impact on interpretations of genetic evolution of eusociality but it has been recently criticized by Martin Nowak and Edward O. Wilson, striking a blow in an increasingly heated debate about evolution of eusociality. The author argues that "Inclusive fitness theory is not a simplification over the standard approach. It is an alternative accounting method, but one that works only in a very limited domain. Whenever inclusive fitness does work, the results are identical to those of the standard approach. Inclusive fitness theory is an unnecessary detour, which does not provide additional insight or information."[15]

See also


  1. ^ Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Oxford: Clarendon Press. 
  2. ^ Haldane, J. B. S. (1955). [Expression error: Unexpected < operator "Population Genetics"]. New Biology 18: 34–51. 
  3. ^ Hamilton, W. D. (1963). [Expression error: Unexpected < operator "The evolution of altruistic behavior"]. American Naturalist 97: 354–356. doi:10.1086/497114. 
  4. ^ Hamilton, W. D. (1964). [Expression error: Unexpected < operator "The Genetical Evolution of Social Behavior"]. Journal of Theoretical Biology 7 (1): 1–52. doi:10.1016/0022-5193(64)90038-4. 
  5. ^ Smith, J. M. (1964). [Expression error: Unexpected < operator "Group Selection and Kin Selection"]. Nature 201 (4924): 1145–1147. doi:10.1038/2011145a0. 
  6. ^ Kevin Connolly and Margaret Martlew, ed (1999). "Altruism". Psychologically Speaking: A Book of Quotations. BPS Books. pp. 10. ISBN 1-85433-302-X.  (see also: Haldane's Wikiquote entry)
  7. ^ Michod, R. E. (1982). [Expression error: Unexpected < operator "The Theory of Kin Selection"]. Annual Review of Ecology and Systematics 13: 23–55. doi:10.1146/ 
  8. ^ Aronson, W. A.; et al. (2007). Social Psychology (6th ed.). Upper Saddle River, NJ: Pearson Prentice-Hall. ISBN 0132382458. 
  9. ^ a b Freeman, Scott; Herron, Jon C. (2007). Evolutionary Analysis (4th ed.). Upper Saddle River, NJ: Pearson, Prentice Hall. p. 460. ISBN 0132275848. 
  10. ^ Milius, Susan (1998). "The Science of Eeeeek!". Science News (Science News, Vol. 154, No. 11) 154 (11): 174–175. doi:10.2307/4010761. Retrieved 2008-07-02. 
  11. ^ In the mating game, male wild turkeys benefit even when they do not get the girl, Robert Sanders
  12. ^ Smith, Kerri (2007). "Plants can tell who's who". Nature News. doi:10.1038/news070611-4. 
  13. ^ Angier, Natalie (2007-06-12). "Sleek, Fast and Focused: The Cells That Make Dad Dad". New York Times. 
  14. ^ Moore, Harry; et al. (2002). [Expression error: Unexpected < operator "Exceptional sperm cooperation in the wood mouse"]. Nature 418 (6894): 174–177. doi:10.1038/nature00832. PMID 12110888. 
  15. ^ Nowak, M. A.; Tarnita, CE; Wilson, EO (2010). "The Evolution of Eusociality". Nature 466 (7310): 1057–1062. doi:10.1038/nature09205. PMID 20740005. 

Further reading

  • Hamilton, W.D. (1964). [Expression error: Unexpected < operator "The Genetical Evolution of Social Behaviour. I"]. Journal of Theoretical Biology 7 (1): 1–16. doi:10.1016/0022-5193(64)90038-4. PMID 5875341. 
  • Hamilton, W.D. (1964). [Expression error: Unexpected < operator "The Genetical Evolution of Social Behaviour. II"]. Journal of Theoretical Biology 7 (1): 17–52. doi:10.1016/0022-5193(64)90039-6. PMID 5875340. 
  • Lucas, J.R.; Creel, S.R.; Waser, P.M. (1996). [Expression error: Unexpected < operator "How to Measure Inclusive Fitness, Revisited"]. Animal Behaviour 51 (1): 225–228. doi:10.1006/anbe.1996.0019. 
  • Madsen, E.A.; Tunney, R.J.; Fieldman, G.; Plotkin, H.C.; Dunbar, R.I.M.; Richardson, J.M.; McFarland, D. (2007). [Expression error: Unexpected < operator "Kinship and Altruism: a Cross-Cultural Experimental Study"]. British Journal of Psychology 98 (2): 339–359. doi:10.1348/000712606X129213. PMID 17456276. 
  • Queller, D.C. & Strassman, J.E. (2002) Quick Guide: Kin Selection. Current Biology,12,R832.
  • West, S.A., Gardner, A. & Griffin, A.S. (2006) Quick Guide: Altruism. Current Biology,16,R482-R483.

Simple English

Kin selection or kin altruism is a form of natural selection. Some animals cooperate with relatives, even if this brings risk to themselves. The classic example of this is seen in the family life of mammals, or in colonial insects such as ants.

Many mammals and birds raise alarms to warn others of danger. Others cooperate in tasks, such as scrub jays help each other with to build nests. In all these cases where animals cooperate, the question is whether there is any biological benefit to themselves. It is now clear that there is benefit if the animals are closely related. This is because related organisms have (to a degree) a shared genetic inheritance.

The first to write about the concept were by R.A. Fisher in 1930,[1] and J.B.S. Haldane in 1955,[2] but it was W.D. Hamilton who truly formalized the concept.[3][4] The actual term kin selection was probably coined by John Maynard Smith, when he wrote:

"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.[5]

By cooperating, relatives influence each other's fitness. Under natural selection, a gene which improves the fitness of individuals will increase in frequency. A gene which lowers the fitness of individuals will become rare.

However, behaviour which enhances the fitness of relatives but lowers that of the actor,[6] may nonetheless increase in frequency. Relatives do, by definition, carry many of the same genes. This is the fundamental principle behind the theory of kin selection. According to the theory, the enhanced fitness of relatives may more than compensate for the fitness loss of the helpers (individuals displaying the behaviour).

This is a special case of a more general model, called inclusive fitness.

Hamilton's equation

Hamilton's equation describes whether or not a gene for helping behaviour will spread in a population.[7] The gene will spread if rxb is greater than c:

rb > c \


  • c \ is the reproductive cost to the helper,
  • b \ is the reproductive benefit to the receiver, and
  • r \ is the probability, above the population average, of the individuals sharing an altruistic gene[8] – the "degree of relatedness".


  1. Fisher, R.A. (1930). The genetical theory of natural selection. Oxford: Clarendon Press. 
  2. Haldane, J.B.S. (1955). [Expression error: Unexpected < operator "Population genetics"]. New Biology 18: 34–51. 
  3. Hamilton, W.D. (1963). [Expression error: Unexpected < operator "The evolution of altruistic behavior"]. American Naturalist 97: 354–356. doi:10.1086/497114. 
  4. Hamilton, W.D. (1964). [Expression error: Unexpected < operator "The genetical evolution of social behavior"]. Journal of Theoretical Biology 7 (1): 1–52. doi:10.1016/0022-5193(64)90038-4. 
  5. Maynard Smith, John (1964). [Expression error: Unexpected < operator "Group selection and kin selection"]. Nature 201 (4924): 1145–1147. doi:10.1038/2011145a0. 
  6. Individual displaying the behaviour.
  7. Hamilton W.D. 1996. Narrow roads of geneland: the collected papers of W.D. Hamilton, vol 1. Freeman, Oxford.
  8. Gene(s) which promote helping behaviour.

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