Kin Selection
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Kin selection is the
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary strategy that favours the
reproductive success Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves. Reproduct ...
of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution is driven by kin selection. Kin selection is an instance of
inclusive fitness In evolutionary biology, inclusive fitness is one of two metrics of evolutionary success as defined by W. D. Hamilton in 1964: * Personal fitness is the number of offspring that an individual begets (regardless of who rescues/rears/supports them ...
, which combines the number of offspring produced with the number an individual can ensure the production of by supporting others, such as siblings.
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
discussed the concept of kin selection in his 1859 book, ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'', where he reflected on the puzzle of sterile social insects, such as
honey bees A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the current cosmo ...
, which leave reproduction to their mothers, 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 Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who ...
in 1930 and
J.B.S. Haldane John Burdon Sanderson Haldane (; 5 November 18921 December 1964), nicknamed "Jack" or "JBS", was a British-Indian scientist who worked in physiology, genetics, evolutionary biology, and mathematics. With innovative use of statistics in biolog ...
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 William Donald Hamilton (1 August 1936 – 7 March 2000) was a British evolutionary biologist, recognised as one of the most significant evolutionary theorists of the 20th century. Hamilton became known for his theoretical work expounding a ...
popularised the concept and the major advance in the mathematical treatment of the phenomenon by
George R. Price George Robert Price (October 6, 1922 – January 6, 1975) was an American population geneticist. Price is often noted for his formulation of the Price equation in 1967. Originally a physical chemist and later a science journalist, he moved ...
which has become known as Hamilton's rule. In the same year,
John Maynard Smith John Maynard Smith (6 January 1920 – 19 April 2004) was a British theoretical and mathematical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics und ...
used the 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. Hamilton proposed two mechanisms for kin selection. First,
kin recognition Kin recognition, also called kin detection, is an organism's ability to distinguish between close genetic kin and non-kin. In evolutionary biology and psychology, such an ability is presumed to have evolved for inbreeding avoidance, though animals d ...
allows individuals to be able to identify their relatives. Second, in viscous populations, populations in which the movement of organisms from their place of birth is relatively slow, local interactions tend to be among relatives by default. The viscous population mechanism makes kin selection and social cooperation possible in the absence of kin recognition. In this case,
nurture kinship The concept of nurture kinship in the anthropological study of human social relationships (kinship) highlights the extent to which such relationships are brought into being through the performance of various acts of nurture between individuals. A ...
, the treatment of individuals as kin as a result of living together, is sufficient for kin selection, given reasonable assumptions about population dispersal rates. Note that kin selection is not the same thing as
group selection Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene. Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behav ...
, where
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
is believed to act on the group as a whole. In humans, altruism is both 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 monkey The vervet monkey (''Chlorocebus pygerythrus''), or simply vervet, is an Old World monkey of the family Cercopithecidae native to Africa. The term "vervet" is also used to refer to all the members of the genus ''Chlorocebus''. The five distinct ...
s use
allomothering Allomothering, allomaternal infant care/handling, or non-maternal infant care/handling is performed by any group member other than the mother. Alloparental care is provided by group members other than the genetic father or the mother and thus is di ...
, where related females such as older sisters or grandmothers often care for young, according to their relatedness. The social shrimp ''
Synalpheus regalis ''Synalpheus regalis'' is a species of snapping shrimp that commonly live in sponges in the coral reefs along the tropical West Atlantic. They form a prominent component of the diverse marine cryptofauna of the region. For the span of their en ...
'' protects juveniles within highly related colonies.


Historical overview

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
was the first to discuss the concept of kin selection (without using that term). In ''On the Origin of Species'', he wrote about the conundrum represented by
altruistic Altruism is the principle and moral practice of concern for the welfare and/or happiness of other human beings or animals, resulting in a quality of life both material and spiritual. It is a traditional virtue in many cultures and a core asp ...
sterile
social insects Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping genera ...
that: 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 Edward Osborne Wilson (June 10, 1929 – December 26, 2021) was an American biologist, naturalist, entomologist and writer. According to David Attenborough, Wilson was the world's leading expert in his specialty of myrmecology, the study of an ...
's ''
Sociobiology Sociobiology is a field of biology that aims to examine and explain social behavior in terms of evolution. It draws from disciplines including psychology, ethology, anthropology, evolution, zoology, archaeology, and population genetics. Within t ...
''. The earliest mathematically formal treatments of kin selection were by
R.A. Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who ...
in 1930 and
J.B.S. Haldane John Burdon Sanderson Haldane (; 5 November 18921 December 1964), nicknamed "Jack" or "JBS", was a British-Indian scientist who worked in physiology, genetics, evolutionary biology, and mathematics. With innovative use of statistics in biolog ...
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 Most generally, in the lineal kinship system used in the English-speaking world, a cousin is a type of familial relationship in which two relatives are two or more familial generations away from their most recent common ancestor. Commonly, " ...
". 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 Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
population that is randomly mating and previously
outbred Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity. Outcrossing can be a usefu ...
). 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:
W. D. Hamilton William Donald Hamilton (1 August 1936 – 7 March 2000) was a British evolutionary biologist, recognised as one of the most significant evolutionary theorists of the 20th century. Hamilton became known for his theoretical work expounding a ...
, in 1963 and especially in 1964 popularised the concept and the more thorough mathematical treatment given to it by George Price.
John Maynard Smith John Maynard Smith (6 January 1920 – 19 April 2004) was a British theoretical and mathematical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics und ...
may have coined the actual term "kin selection" in 1964: Kin selection causes changes in
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
frequency across generations, driven by interactions between related individuals. This dynamic forms the conceptual basis of the theory of
sociobiology Sociobiology is a field of biology that aims to examine and explain social behavior in terms of evolution. It draws from disciplines including psychology, ethology, anthropology, evolution, zoology, archaeology, and population genetics. Within t ...
. Some cases of
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
by
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
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 Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
; 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 behaviour, 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 In evolutionary biology, inclusive fitness is one of two metrics of evolutionary success as defined by W. D. Hamilton in 1964: * Personal fitness is the number of offspring that an individual begets (regardless of who rescues/rears/supports them ...
". 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 Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping genera ...
and "gene-culture co-evolution" for the underlying mechanics of
sociobiology Sociobiology is a field of biology that aims to examine and explain social behavior in terms of evolution. It draws from disciplines including psychology, ethology, anthropology, evolution, zoology, archaeology, and population genetics. Within t ...
. Kin selection is contrasted with
group selection Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene. Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behav ...
, 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.


Hamilton's rule

Formally, genes should increase in frequency when :rB > C where :''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 Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * ''Locus'' (magazine), science fiction and fantasy magazine ** ''Locus Award' ...
is identical by descent. :''B'' = the additional reproductive benefit gained by the recipient of the
altruistic Altruism is the principle and moral practice of concern for the welfare and/or happiness of other human beings or animals, resulting in a quality of life both material and spiritual. It is a traditional virtue in many cultures and a core asp ...
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 The coefficient of relationship is a measure of the degree of consanguinity (or biological relationship) between two individuals. The term coefficient of relationship was defined by Sewall Wright in 1922, and was derived from his definition of th ...
parameter (''r'') in Hamilton's rule was introduced in 1922 by
Sewall Wright Sewall Green Wright FRS(For) Honorary FRSE (December 21, 1889March 3, 1988) was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. He was a founder of population genetics alongsi ...
as a
coefficient of relationship The coefficient of relationship is a measure of the degree of consanguinity (or biological relationship) between two individuals. The term coefficient of relationship was defined by Sewall Wright in 1922, and was derived from his definition of th ...
that gives the
probability Probability is the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...
that at a random
locus Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * ''Locus'' (magazine), science fiction and fantasy magazine ** ''Locus Award' ...
, the
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s there will be
identical by descent A DNA segment is identical by state (IBS) in two or more individuals if they have identical nucleotide sequences in this segment. An IBS segment is identical by descent (IBD) in two or more individuals if they have inherited it from a common an ...
. A 2014 review of many lines of evidence for Hamilton's rule found that its predictions were confirmed in a wide variety of social behaviours across a broad phylogenetic range of birds, mammals and insects, in each case comparing social and non-social taxa. Among the experimental findings, a 2010 study used a wild population of
red squirrels The red squirrel (''Sciurus vulgaris'') is a species of tree squirrel in the genus ''Sciurus'' common throughout Europe and Asia. The red squirrel is an arboreal, primarily herbivorous rodent. In Great Britain, Ireland, and in Italy numbers ...
in Yukon, Canada. Surrogate mothers adopted related orphaned squirrel pups but not unrelated orphans. The cost of adoption was calculated 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. Females always adopted orphans when ''rB'' was greater than ''C'', but never adopted when ''rB'' was less than ''C'', supporting Hamilton's rule.Further detail of Hamilton's rule is available a
Simulating the Evolution of Sacrificing for Family
Discovering the specific definitions of r, B, and C, and at
Hamilton’s Rule and Its Discontents
Why the general definitions of the variables always applies, but one specific definition can fail.


Mechanisms

Altruism Altruism is the principle and moral practice of concern for the welfare and/or happiness of other human beings or animals, resulting in a quality of life both material and spiritual. It is a traditional virtue in many cultures and a core as ...
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:


Kin recognition and the green beard effect

First, if individuals have the capacity to recognise kin and to discriminate (positively) on the basis of
kinship In anthropology, kinship is the web of social relationships that form an important part of the lives of all humans in all societies, although its exact meanings even within this discipline are often debated. Anthropologist Robin Fox says that ...
, 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. 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, those 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 Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
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: Hamilton's later clarifications often go unnoticed. Stuart West and colleagues have countered the long-standing assumption that kin selection requires innate powers of kin recognition. Another doubtful assumption is that social cooperation must be based on limited dispersal and shared developmental context. Such ideas have obscured the progress made in applying kin selection to species including humans, on the basis of cue-based mediation of social bonding and social behaviours.Holland, Maximilian. (2012) ''Social Bonding and Nurture Kinship: Compatibility between Cultural and Biological Approaches''. North Charleston: Createspace Press.


Special cases


Eusociality

Eusociality Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping genera ...
(true sociality) is used to describe social systems with three characteristics: an overlap in generations between parents and their offspring, cooperative brood care, and specialised castes of non-reproductive 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 Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...
due to
haplodiploidy Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky. Haplodiploidy determines the sex ...
. 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. Competing hypotheses have been offered to explain the evolution of social behaviour in such organisms. The eusocial shrimp ''
Synalpheus regalis ''Synalpheus regalis'' is a species of snapping shrimp that commonly live in sponges in the coral reefs along the tropical West Atlantic. They form a prominent component of the diverse marine cryptofauna of the region. For the span of their en ...
'' protects juveniles in the colony. By defending the young, the large defender shrimp can increase its inclusive fitness.
Allozyme Alloenzymes (or also called allozymes) are variant forms of an enzyme which differ structurally but not functionally from other allozymes coded for by different alleles at the same locus. These are opposed to isozymes, which are enzymes that perfo ...
data demonstrated high relatedness within colonies, averaging 0.50. This means that colonies represent close kin groups, supporting the hypothesis of kin selection.


Allomothering

Vervet monkey The vervet monkey (''Chlorocebus pygerythrus''), or simply vervet, is an Old World monkey of the family Cercopithecidae native to Africa. The term "vervet" is also used to refer to all the members of the genus ''Chlorocebus''. The five distinct ...
s utilise
allomothering Allomothering, allomaternal infant care/handling, or non-maternal infant care/handling is performed by any group member other than the mother. Alloparental care is provided by group members other than the genetic father or the mother and thus is di ...
, parenting by group members other than the actual mother or father, where the allomother is typically an older female sibling or a grandmother. Individuals act aggressively toward other individuals that were aggressive toward their relatives. The behaviour implies kin selection between siblings, between mothers and offspring, and between grandparents and grandchildren.


In humans

Whether or not
Hamilton's rule Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution ...
always applies, relatedness is often important for human altruism, in that humans are inclined to behave more altruistically toward kin than toward unrelated individuals.Cartwright, J. (2000). ''Evolution and human behavior: Darwinian perspectives on human nature.'' Massachusetts: MIT Press. Many people choose to live near relatives, exchange sizeable gifts with relatives, and favour relatives in wills in proportion to their relatedness.


Experimental studies, interviews, and surveys

Interviews of several hundred women in Los Angeles showed that 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.


Observational studies

A study of food-sharing practices on the West Caroline islets of
Ifaluk Ifalik (also spelled Ifaluk) is a coral atoll of four islands in the central Caroline Islands in the Pacific Ocean, and forms a legislative district in Yap State in the Federated States of Micronesia. Ifalik is located approximately east of Wolea ...
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 maximise 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 was correlated with a higher degree of investment in the children, with more food, health care, and clothing. Relatedness was also associated with the regularity of a child's visits to local medical practitioners and with the highest grade the child had completed in school, and negatively associated with children being behind in school for their age. Observation of the
Dolgan Dolgans (; Dolgan: , , (Sakha); Yakut: ) are an ethnic group who mostly inhabit Krasnoyarsk Krai, Russia. They are descended from several groups, particularly Evenks, one of the indigenous peoples of the Russian North. They adopted a Turk ...
hunter-gatherers of northern Russia suggested that there are larger and more frequent asymmetrical transfers of food to kin. Kin are more likely to be welcomed to non-reciprocal meals, while non-kin are discouraged from attending. Finally, when reciprocal food-sharing occurs between families, these families are often closely related, and the primary beneficiaries are the offspring. Violence in families is more likely 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 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.


Human social patterns

Evolutionary psychologists Evolutionary psychology is a theoretical approach in psychology that examines cognition and behavior from a modern evolutionary perspective. It seeks to identify human psychological adaptations with regards to the ancestral problems they evolv ...
, following early human sociobiologists' 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, many evolutionary psychologists recognise that this common shorthand formulation is inaccurate: 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. Consideration of the demographics of the typical evolutionary environment of any species is crucial to understanding the evolution of social behaviours. As Hamilton himself put it, "Altruistic or selfish acts are only possible when a suitable social object is available. In this sense behaviours are conditional from the start". 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.Sherman et al (1997) ''Recognition Systems''. In ''Behavioural Ecology'', edited by J. R. Krebs and N. B. Davies. Oxford: Blackwell Scientific. That is, it is the context that mediates the development of the bonding process and the expression of the altruistic behaviours, not genetic relatedness as such. This interpretation is compatible with the cross-cultural ethnographic data and has been called
nurture kinship The concept of nurture kinship in the anthropological study of human social relationships (kinship) highlights the extent to which such relationships are brought into being through the performance of various acts of nurture between individuals. A ...
.


In plants


Observations

Though originally thought unique to the animal kingdom, evidence of kin selection has been identified in the
plant kingdom Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
. Competition for resources between developing zygotes in plant ovaries increases when seeds had been pollinated with male gametes from different plants. How developing
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...
s differentiate between full siblings and half-siblings in the ovary is undetermined, but genetic interactions are thought to play a role. Nonetheless, competition between
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...
s in the ovary is detrimental to the
reproductive success Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves. Reproduct ...
of the (female) plant, and fewer zygotes mature into seeds. As such, the reproductive traits and behaviors of plants suggests the
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
of behaviors and characteristics that increase the genetic relatedness of fertilized eggs in the plant ovary, thereby fostering kin selection and cooperation among the seeds as they develop. These traits differ among plant species. Some species have evolved to have fewer
ovule In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the fe ...
s per ovary, commonly one ovule per ovary, thereby decreasing the chance of developing multiple, differently fathered seeds within the same ovary. Multi-ovulated plants have developed mechanisms that increase the chances of all ovules within the ovary being fathered by the same parent. Such mechanisms include dispersal of
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
in aggregated packets and closure of the
stigmatic Stigmata ( grc, στίγματα, plural of , 'mark, spot, brand'), in Roman Catholicism, are bodily wounds, scars and pain which appear in locations corresponding to the crucifixion wounds of Jesus Christ: the hands, wrists, and feet. Stigm ...
lobes after pollen is introduced. The aggregated pollen packet releases pollen gametes in the ovary, thereby increasing likelihood that all ovules are fertilized by pollen from the same parent. Likewise, the closure of the ovary pore prevents entry of new pollen. Other multi-ovulated plants have evolved mechanisms that mimic the evolutionary adaption of single-ovulated ovaries; the ovules are fertilized by pollen from different individuals, but the mother ovary then selectively aborts fertilized ovules, either at the zygotic or
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
nic stage. After seeds are dispersed, kin recognition and cooperation affects root formation in developing plants. Studies have found that the total root mass developed by ''Ipomoea hederacea'' (morning glory shrubs) grown next to kin is significantly smaller than those grown next to non-kin; shrubs grown next to kin thus allocate less energy and resources to growing the larger root systems needed for competitive growth. When seedlings were grown in individual pots placed next to kin or non-kin relatives, no difference in root growth was observed. This indicates that kin recognition occurs via signals received by the roots. Further, groups of ''I. hederacea'' plants are more varied in height when grown with kin than when grown with non-kin. The evolutionary benefit provided by this was further investigated by researchers at the
Université de Montpellier The University of Montpellier (french: Université de Montpellier) is a public research university located in Montpellier, in south-east of France. Established in 1220, the University of Montpellier is one of the oldest universities in the wor ...
. They found that the alternating heights seen in kin-grouped
crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydroponic ...
s allowed for optimal light availability to all plants in the group; shorter plants next to taller plants had access to more light than those surrounded by plants of similar height. The above examples illustrate the effect of kin selection in the equitable allocation of light, nutrients, and water. The evolutionary emergence of single-ovulated ovaries in plants has eliminated the need for a developing seed to compete for nutrients, thus increasing its chance of survival and
germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...
. Likewise, the fathering of all ovules in multi-ovulated ovaries by one father, decreases the likelihood of competition between developing seeds, thereby also increasing the seeds' chances of survival and germination. The decreased root growth in plants grown with kin increases the amount of energy available for reproduction; plants grown with kin produced more seeds than those grown with non-kin. Similarly, the increase in light made available by alternating heights in groups of related plants is associated with higher fecundity. Kin selection has also been observed in plant responses to herbivory. In an experiment done by Richard Karban et al., leaves of potted ''Artemisia tridentata'' (sagebrushes) were clipped with scissors to simulate herbivory. The gaseous
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term ' ...
emitted by the clipped leaves were captured in a plastic bag. When these volatiles were transferred to leaves of a closely related sagebrush, the recipient experienced lower levels of herbivory than those that had been exposed to volatiles released by non-kin plants. Sagebrushes do not uniformly emit the same volatiles in response to herbivory: the chemical ratios and composition of emitted volatiles vary from one sagebrush to another. Closely related sagebrushes emit similar volatiles, and the similarities decrease as relatedness decreases. This suggests that the composition of volatile gasses plays a role in kin selection among plants. Volatiles from a distantly related plant are less likely to induce a protective response against herbivory in a neighboring plant, than volatiles from a closely related plant. This fosters kin selection, as the volatiles emitted by a plant will activate the herbivorous defense response in related plants only, thus increasing their chance of survival and reproduction. Kin selection may play a role in plant-pollinator interactions, especially because pollinator attraction is influenced not only by floral displays, but by the spatial arrangement of plants in a group, which is referred to as the “magnet effect”. For example, in an experiment performed on '' Moricandia moricandioides'', Torices et al. demonstrated that focal plants in the presence of kin show increased advertising effort (defined as total petal mass of plants in a group divided by the plant biomass) compared to those in the presence of non-kin, and that this effect is greater in larger groups. ''M. moricandioides'' is a good model organism for the study of plant-pollinator interactions because it relies on pollinators for reproduction, as it is self-incompatible. The study design for this experiment included planting establishing pots of ''M. moricandioides'' with zero, three or six neighbors (either unrelated or half-sib progeny of the same mother) and advertising effort was calculated after 26 days of flowering. The exact mechanism of kin recognition in ''M. moricandioides'' is unknown, but possible mechanisms include above-ground communication with volatile compounds, or below-ground communication with root exudates.


Mechanisms in plants

The ability to differentiate between kin and non-kin is not necessary for kin selection in many animals. However, because plants do not reliably germinate in close proximity to kin, it is thought that, within the plant kingdom, kin recognition is especially important for kin selection there, but the mechanism remains unknown. One proposed mechanism for kin recognition involves communication through roots, with secretion and reception of root exudates. This would require exudates to be actively secreted by roots of one plant, and detected by roots of neighboring plants. The root exudate allantoin produced by rice plants, ''
Oryza sativa ''Oryza sativa'', commonly known as Asian rice or indica rice, is the plant species most commonly referred to in English as ''rice''. It is the type of farmed rice whose cultivars are most common globally, and was first domesticated in the Yan ...
'', has been documented to be in greater production when growing next to cultivars that are largely unrelated. High production levels of Allantoin correlated to up regulation of auxin and auxin transporters, resulting in increased lateral root development and directional growth of their roots towards non kin, maximizing competition. This is mainly not observed in ''Oryza Sativa'' when surrounded by kin, invoking altruistic behaviors to promote inclusive fitness. However the root
receptors Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
responsible for recognition of kin exudates, and the pathway induced by receptor activation, remain unknown. The
mycorrhiza   A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...
associated with roots might facilitate reception of exudates, but again the mechanism is unknown. Another possibility is communication through
green leaf volatiles Green leaf volatiles (GLV) are volatile organic compounds that are released when plants suffer tissue damage. Specifically, they include aldehydes, esters, and alcohols of 6-carbon compounds released after wounding. These compounds are very quickly ...
. Karban et al. studied kin recognition in sagebrushes, ''
Artemisia tridentata ''Artemisia tridentata'', commonly called big sagebrush,MacKay, Pam (2013), ''Mojave Desert Wildflowers'', 2nd ed., , p. 264. Great Basin sagebrush or (locally) simply sagebrush, is an aromatic shrub from the family Asteraceae, which grows in ari ...
''. The volatile-donating sagebrushes were kept in individual pots, separate from the plants that received the volatiles, finding that plants responded to
herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
damage to a neighbour's leaves. This suggests that root signalling is not necessary to induce a protective response against herbivory in neighbouring kin plants. Karban et al. suggest that plants may be able to differentiate between kin and non-kin based on the composition of volatiles. Because only the recipient sagebrush's leaves were exposed the volatiles presumably activated a
receptor protein In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems. These signals are typically chemical messengers which bind to a recept ...
in the plant's leaves. The identity of this receptor, and the signalling pathway triggered by its activation, both remain to be discovered.


Objections

The theory of kin selection has been criticised by W. J. Alonso (in 1998) and by Alonso and C. Schuck-Paim (in 2002). 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 maximise 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 ideas were mostly ignored until they were put forward again in a series of controversial papers by
E. O. Wilson Edward Osborne Wilson (June 10, 1929 – December 26, 2021) was an American biologist, naturalist, entomologist and writer. According to David Attenborough, Wilson was the world's leading expert in his specialty of myrmecology, the study of an ...
,
Bert Hölldobler Berthold Karl Hölldobler (born 25 June 1936) is a German sociobiologist and evolutionary biologist who studies evolution and social organization in ants. He is the author of several books, including ''The Ants'', for which he and his co-author, ...
,
Martin Nowak Martin Andreas Nowak (born April 7, 1965) is an Austrian-born professor of Mathematical and theoretical biology, mathematical biology, at Harvard University since 2003. He is one of the leading researchers in the field that studies the role of co ...
and Corina Tarnita. Nowak, Tarnita and Wilson argued that They, like Alonso and Schuck-Paim, argue for a multi-level selection model instead. This aroused a strong response, including a rebuttal published in ''
Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...
'' from over a hundred researchers.


See also

*
Darwinian anthropology Darwinian anthropology describes an approach to anthropological analysis which employs various theories from Darwinian evolutionary biology. Whilst there are a number of areas of research that can come under this broad description (Marks, 2004) so ...
*
Group selection Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene. Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behav ...
*
Price equation In the theory of evolution and natural selection, the Price equation (also known as Price's equation or Price's theorem) describes how a trait or allele changes in frequency over time. The equation uses a covariance between a trait and fitness, ...
* ''
The Selfish Gene ''The Selfish Gene'' is a 1976 book on evolution by the ethologist Richard Dawkins, in which the author builds upon the principal theory of George C. Williams's ''Adaptation and Natural Selection'' (1966). Dawkins uses the term "selfish gene ...
'' * Genomic imprinting


Notes


References

{{DEFAULTSORT:Kin Selection Selection Evolutionary biology Evolutionary psychology Sociobiology