Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Kin altruism is altruistic behaviour whose evolution is driven by kin selection. Kin selection is an instance of inclusive fitness, which combines the number of offspring produced with the number an individual can produce by supporting others, such as siblings.
Charles Darwin discussed the concept of kin selection in his 1859 book, The Origin of Species, where he reflected on the puzzle of sterile social insects, such as honey bees, which leave reproduction to their sisters, arguing that a selection benefit to related organisms (the same "stock") would allow the evolution of a trait that confers the benefit but destroys an individual at the same time. R.A. Fisher in 1930 and J.B.S. Haldane in 1932 set out the mathematics of kin selection, with Haldane famously joking that he would willingly die for two brothers or eight cousins. In 1964, W.D. Hamilton popularized the concept and the major advance in the mathematical treatment of the phenomenon by George R. Price which has become known as "Hamilton's rule". In the same year John Maynard Smith used the actual term kin selection for the first time.
According to Hamilton's rule, kin selection causes genes to increase in frequency when the genetic relatedness of a recipient to an actor multiplied by the benefit to the recipient is greater than the reproductive cost to the actor. The rule is difficult to test but was verified experimentally in 2010 by observing adoption of orphans by surrogate mothers in a wild red squirrel population. Hamilton proposed two mechanisms for kin selection: kin recognition, where individuals are able to identify their relatives, and viscous populations, where dispersal is rare enough for populations to be closely related. The viscous population mechanism makes kin selection and social cooperation possible in the absence of kin recognition. Nurture kinship, the treatment of individuals as kin when they live together, is sufficient for kin selection, given reasonable assumptions about dispersal rates. Kin selection is not the same thing as group selection, where natural selection acts on the group as a whole.
In humans, altruism is more likely and on a larger scale with kin than with unrelated individuals; for example, humans give presents according to how closely related they are to the recipient. In other species, vervet monkeys use allomothering, where related females such as older sisters or grandmothers often care for young, according to their relatedness. The social shrimp Synalpheus regalis protects juveniles within highly related colonies.
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.—Darwin
In this passage "the family" and "stock" stand for a kin group. These passages and others by Darwin about "kin selection" are highlighted 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. J.B.S. Haldane fully grasped the basic quantities and considerations in kin selection, famously writing "I would lay down my life for two brothers or eight cousins". 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. In 1955 he clarified:
Let us suppose that you carry a rare gene that affects your behavior so that you jump into a flooded river and save a child, but you have one chance in ten of being drowned, while I do not possess the gene, and stand on the bank and watch the child drown. If the child's your own child or your brother or sister, there is an even chance that this child will also have this gene, so five genes will be saved in children for one lost in an adult. If you save a grandchild or a nephew, the advantage is only two and a half to one. If you only save a first cousin, the effect is very slight. If you try to save your first cousin once removed the population is more likely to lose this valuable gene than to gain it. … It is clear that genes making for conduct of this kind would only have a chance of spreading in rather small populations when most of the children were fairly near relatives of the man who risked his life.
John Maynard Smith may have coined the actual term "kin selection" 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 causes changes in gene frequency across generations, driven by interactions between related individuals. This dynamic forms the conceptual basis of the theory of social evolution. Some cases of evolution by natural selection can only be understood by considering how biological relatives influence each other'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 principle, the enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour, making kin selection possible. This is a special case of a more general model, "inclusive fitness". This analysis has been challenged, Wilson writing that "the foundations of the general theory of inclusive fitness based on the theory of kin selection have crumbled"  and that he now relies instead on the theory of eusociality and "gene-culture co-evolution" for the underlying mechanics of sociobiology.
"Kin selection" should not be confused with "group selection" according to which a genetic trait can become prevalent within a group because it benefits the group as a whole, regardless of any benefit to individual organisms. All known forms of group selection conform to the principle that an individual behaviour can be evolutionarily successful only if the genes responsible for this behaviour conform to Hamilton's Rule, and hence, on balance and in the aggregate, benefit from the behaviour.
Formally, genes should increase in frequency when
- 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 performing the act.
This inequality is known as Hamilton's rule after W. D. Hamilton who in 1964 published the first formal quantitative treatment of kin selection.
The relatedness parameter (r) in Hamilton's rule was introduced in 1922 by Sewall Wright as a coefficient of relationship that gives the probability that at a random locus, the alleles there will be identical by descent. Subsequent authors, including Hamilton, sometimes reformulate this with a regression, which, unlike probabilities, can be negative. A regression analysis producing statistically significant negative relationships indicates that two individuals are less genetically alike than two random ones (Hamilton 1970, Nature & Grafen 1985 Oxford Surveys in Evolutionary Biology). This has been invoked to explain the evolution of spiteful behaviour consisting of acts that result in harm, or loss of fitness, to both the actor and the recipient.
There has been little empirical support for Hamilton's rule in wild animals, as it is hard to quantify the costs and benefits of different behaviours. The first study to test Hamilton's rule successfully was in 2010, involving a wild population of red squirrels in Yukon, Canada. The researchers found that surrogate mothers would adopt related orphaned squirrel pups but not unrelated orphans. The researchers calculated the cost of adoption by measuring a decrease in the survival probability of the entire litter after increasing the litter by one pup, while benefit was measured as the increased chance of survival of the orphan. The degree of relatedness of the orphan and surrogate mother for adoption to occur depended on the number of pups the surrogate mother already had in her nest, as this affected the cost of adoption. The study showed that females always adopted orphans when rB > C, but never adopted when rB < C, providing strong support for Hamilton's rule.
Altruism occurs where the instigating individual suffers a fitness loss while the receiving individual experiences a fitness gain. The sacrifice of one individual to help another is an example.
Hamilton (1964) outlined two ways in which kin selection altruism could be favoured:
The selective advantage which makes behaviour conditional in the right sense on the discrimination of factors which correlate with the relationship of the individual concerned is therefore obvious. It may be, for instance, that in respect of a certain social action performed towards neighbours indiscriminately, an individual is only just breaking even in terms of inclusive fitness. If he could learn to recognise those of his neighbours who really were close relatives and could devote his beneficial actions to them alone an advantage to inclusive fitness would at once appear. Thus a mutation causing such discriminatory behaviour itself benefits inclusive fitness and would be selected. In fact, the individual may not need to perform any discrimination so sophisticated as we suggest here; a difference in the generosity of his behaviour according to whether the situations evoking it were encountered near to, or far from, his own home might occasion an advantage of a similar kind." (1996 , 51)
Kin recognition: First, if individuals have the capacity to recognize kin and to discriminate (positively) on the basis of kinship, then the average relatedness of the recipients of altruism could be high enough for kin selection. Because of the facultative nature of this mechanism, kin recognition and discrimination are expected to be unimportant except among 'higher' forms of life such as the fish Neolamprologus pulcher (although there is some evidence for it among protozoa). Note also that kin recognition may be selected for inbreeding avoidance, and little evidence indicates that 'innate' kin recognition plays a role in mediating altruism. A thought experiment on the kin recognition/discrimination distinction is the hypothetical 'green beard', where a gene for social behaviour is imagined also to cause a distinctive phenotype that can be recognised by other carriers of the gene. Due to conflicting genetic similarity in the rest of the genome, there would be selection pressure for green-beard altruistic sacrifices to be suppressed, making common ancestry the most likely form of inclusive fitness.
Viscous populations: Secondly, even indiscriminate altruism may be favoured in "viscous" populations with low rates or short ranges of dispersal. Here, social partners are typically genealogically close kin, and so altruism can flourish even in the absence of kin recognition and kin discrimination faculties — spatial proximity and circumstantial cues serving as a rudimentary form of discrimination. This suggests a rather general explanation for altruism. Directional selection always favours those with higher rates of fecundity within a certain population. Social individuals can often enhance the survival of their own kin by participating in and following the rules of their own group.
Hamilton later modified his thinking to suggest that an innate ability to recognise actual genetic relatedness was unlikely to be the dominant mediating mechanism for kin altruism:
But once again, we do not expect anything describable as an innate kin recognition adaptation, used for social behaviour other than mating, for the reasons already given in the hypothetical case of the trees.(Hamilton 1987, 425)
Hamilton's later clarifications often go unnoticed, and because of the long standing assumption that kin selection requires innate powers of kin recognition, some theorists have tried to clarify the position in recent work:
In his original papers on inclusive fitness theory, Hamilton pointed out a sufficiently high relatedness to favour altruistic behaviours could accrue in two ways — kin discrimination or limited dispersal ( Hamilton, 1964, 1971,1972, 1975). There is a huge theoretical literature on the possible role of limited dispersal reviewed by Platt & Bever (2009) and West et al. (2002a), as well as experimental evolution tests of these models (Diggle et al., 2007; Griffin et al., 2004; Kümmerli et al., 2009 ). However, despite this, it is still sometimes claimed that kin selection requires kin discrimination (Oates & Wilson, 2001; Silk, 2002 ). Furthermore, a large number of authors appear to have implicitly or explicitly assumed that kin discrimination is the only mechanism by which altruistic behaviours can be directed towards relatives... [T]here is a huge industry of papers reinventing limited dispersal as an explanation for cooperation. The mistakes in these areas seem to stem from the incorrect assumption that kin selection or indirect fitness benefits require kin discrimination (misconception 5), despite the fact that Hamilton pointed out the potential role of limited dispersal in his earliest papers on inclusive fitness theory (Hamilton, 1964; Hamilton, 1971; Hamilton, 1972; Hamilton, 1975). (West et al. 2010, p.243 and supplement)
The assumption that kin recognition must be innate, and that cue-based mediation of social cooperation based on limited dispersal and shared developmental context are not sufficient, has obscured significant progress made in applying kin selection and inclusive fitness theory to a wide variety of species, including humans, on the basis of cue-based mediation of social bonding and social behaviours.
Evolutionary psychologists, following Darwinian anthropologists' interpretation of kin selection theory initially attempted to explain human altruistic behaviour through kin selection by stating that "behaviors that help a genetic relative are favored by natural selection." However, most Evolutionary psychologists recognize that this common shorthand formulation is inaccurate;
[M]any misunderstandings persist. In many cases, they result from conflating "coefficient of relatedness" and "proportion of shared genes," which is a short step from the intuitively appealing—but incorrect—interpretation that "animals tend to be altruistic toward those with whom they share a lot of genes." These misunderstandings don’t just crop up occasionally; they are repeated in many writings, including undergraduate psychology textbooks—most of them in the field of social psychology, within sections describing evolutionary approaches to altruism. (Park 2007, p860)
As with the earlier sociobiological forays into the cross-cultural data, typical approaches are not able to find explanatory fit with the findings of ethnographers insofar that human kinship patterns are not necessarily built upon blood-ties. However, as Hamilton's later refinements of his theory make clear, it does not simply predict that genetically related individuals will inevitably recognise and engage in positive social behaviours with genetic relatives: rather, indirect context-based mechanisms may have evolved, which in historical environments have met the inclusive fitness criterion (see above section). Consideration of the demographics of the typical evolutionary environment of any species is crucial to understanding the evolution of social behaviours. As Hamilton himself puts it, "Altruistic or selfish acts are only possible when a suitable social object is available. In this sense behaviours are conditional from the start." (Hamilton 1987, 420).
Under this perspective, and noting the necessity of a reliable context of interaction being available, the data on how altruism is mediated in social mammals is readily made sense of. In social mammals, primates and humans, altruistic acts that meet the kin selection criterion are typically mediated by circumstantial cues such as shared developmental environment, familiarity and social bonding. That is, it is the context that mediates the development of the bonding process and the expression of the altruistic behaviours, not genetic relatedness per se. This interpretation thus is compatible with the cross-cultural ethnographic data and has been called nurture kinship.
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. The social insects provide good examples of organisms with what appear to be 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. 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.
Another example is preference among individuals in sun-tailed monkey communities. A study showed that maternal kin (kin related to by mothers) favoured each other, but that with relatives more distant than half-siblings, this bias dropped significantly.
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 studied the alarm calls of ground squirrels, observing that the calls occurred most frequently when the caller had relatives nearby. In a similar study, John Hoogland followed individual males through different stages of life. Male prairie dogs modified their rate of calling when closer to kin. These behaviours show that self-sacrifice is directed towards close relatives, and that there is an indirect fitness gain.
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 a party, a subordinate turkey may help his dominant brother put on an impressive team display that is only of direct benefit to the dominant member.
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.
In the wood mouse (Apodemus sylvaticus), aggregates of spermatozoa form mobile trains, some of the spermatozoa undergo 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.
Whether or not Hamilton's rule always applies, studies have demonstrated that relatedness is often important for human altruism in that humans are inclined to behave more altruistically toward kin than toward unrelated individuals. Many people choose to live near relatives, exchange sizable gifts with relatives, and favour relatives in wills in proportion to their relatedness.
Experimental studies, interviews, and surveys
A study interviewed several hundred women in Los Angeles to study patterns of helping between kin versus non-kin. While non-kin friends were willing to help one another, their assistance was far more likely to be reciprocal. The largest amounts of non-reciprocal help, however, were reportedly provided by kin. Additionally, more closely related kin were considered more likely sources of assistance than distant kin. Similarly, several surveys of American college students found that individuals were more likely to incur the cost of assisting kin when a high probability that relatedness and benefit would be greater than cost existed. Participants’ feelings of helpfulness were stronger toward family members than non-kin. Additionally, participants were found to be most willing to help those individuals most closely related to them. Interpersonal relationships between kin in general were more supportive and less Machiavellian than those between non-kin.
In one experiment, the longer participants (from both the UK and the South African Zulus) held a painful skiing position, the more money or food was presented to a given relative. Participants repeated the experiment for individuals of different relatedness (parents and siblings at r = .5, grandparents, nieces, and nephews at r = .25, etc.). The results showed that participants held the position for longer intervals the greater the degree of relatedness between themselves and those receiving the reward.
A study of food-sharing practices on the West Caroline islets of Ifaluk determined that food-sharing was more common among people from the same islet, possibly because the degree of relatedness between inhabitants of the same islet would be higher than relatedness between inhabitants of different islets. When food was shared between islets, the distance the sharer was required to travel correlated with the relatedness of the recipient—a greater distance meant that the recipient needed to be a closer relative. The relatedness of the individual and the potential inclusive fitness benefit needed to outweigh the energy cost of transporting the food over distance.
Humans may use the inheritance of material goods and wealth to maximize their inclusive fitness. By providing close kin with inherited wealth, an individual may improve his or her kin’s reproductive opportunities and thus increase his or her own inclusive fitness even after death. A study of a thousand wills found that the beneficiaries who received the most inheritance were generally those most closely related to the will’s writer. Distant kin received proportionally less inheritance, with the least amount of inheritance going to non-kin.
A study of childcare practices among Canadian women found that respondents with children provide childcare reciprocally with non-kin. The cost of caring for non-kin was balanced by the benefit a woman received—having her own offspring cared for in return. However, respondents without children were significantly more likely to offer childcare to kin. For individuals without their own offspring, the inclusive fitness benefits of providing care to closely related children might outweigh the time and energy costs of childcare.
Family investment in offspring among black South African households also appears consistent with an inclusive fitness model. A higher degree of relatedness between children and their caregivers frequently correlated with a higher degree of investment in the children, with more food, health care, and clothing being provided. Relatedness between the child and the rest of the household also positively associated with the regularity of a child’s visits to local medical practitioners and with the highest grade the child had completed in school. Additionally, relatedness negatively associated with a child’s being behind in school for his or her age.
Observation of the Dolgan hunter-gatherers of northern Russia suggested that, while reciprocal food-sharing occurs between both kin and non-kin, there are larger and more frequent asymmetrical transfers of food to kin. Kin are also more likely to be welcomed to non-reciprocal meals, while non-kin are discouraged from attending. Finally, even when reciprocal food-sharing occurs between families, these families are often very closely related, and the primary beneficiaries are the offspring.
Other research indicates that violence in families is more likely to occur when step-parents are present and that "genetic relationship is associated with a softening of conflict, and people's evident valuations of themselves and of others are systematically related to the parties' reproductive values".
Numerous other studies suggest how inclusive fitness may work amongst different peoples, such as the Ye’kwana of southern Venezuela, the Gypsies of Hungary, and the doomed Donner Party of the United States.
Vervet monkeys display kin selection between siblings, mothers and offspring, and grandparent-grandchild. These monkeys utilize allomothering, where the allomother is typically an older female sibling or a grandmother. Other studies have shown that individuals will act aggressively toward other individuals that were aggressive toward their relatives.
Synalpheus regalis is a eusocial shrimp that protects juveniles in the colony. By defending the young, the large defender shrimp can increase its inclusive fitness. Allozyme data revealed that relatedness within colonies is high, averaging 0.50, indicating that colonies in this species represent close kin groups.
The theory of kin selection was criticized in two studies, one published in 1998 and another in 2002 in PNAS. Alonso and Schuck-Paim argue that the behaviours which kin selection attempts to explain are not altruistic (in pure Darwinian terms) because: (1) they may directly favour the performer as an individual aiming to maximize its progeny (so the behaviours can be explained as ordinary individual selection); (2) these behaviours benefit the group (so they can be explained as group selection); or (3) they are by-products of a developmental system of many "individuals" performing different tasks (like a colony of bees, or the cells of multicellular organisms, which are the focus of selection). They also argue that the genes involved in sex ratio conflicts could be treated as "parasites" of (already established) social colonies, not as their "promoters", and, therefore the sex ratio in colonies would be irrelevant to the transition to eusociality. Those papers were mostly ignored until they were re-edited by Martin Nowak, Corina Tarnita, and E. O. Wilson. These latter authors argue 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.—Nowak, Tarnita, and Wilson
They, like Alonso (1998) and Alonso and Schuck-Paim (2002) earlier, argue for a multi-level selection model instead. This aroused a strong response, including a rebuttal published in Nature from over a hundred researchers.
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