Kin selection
Kin selection is a process whereby natural selection favours a trait due to its positive effects on the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin selection can lead to the evolution of altruistic behaviour. It is related to inclusive fitness, which combines the number of offspring produced with the number an individual can ensure the production of by supporting others. A broader definition of kin selection includes selection acting on interactions between individuals who share a gene of interest even if the gene is not shared due to common ancestry.
Charles Darwin discussed the concept of kin selection in his 1859 book, On the Origin of Species, where he reflected on the puzzle of sterile social insects, such as honey bees, which leave reproduction to their mothers, arguing that a selection benefit to related organisms would allow the evolution of a trait that confers the benefit but destroys an individual at the same time. J.B.S. Haldane in 1955 briefly alluded to the principle in limited circumstances, and R.A. Fisher mentioned a similar principle even more briefly in 1930. However, it was not until 1964 that W.D. Hamilton generalised the concept and developed it mathematically that it began to be widely accepted. The mathematical treatment was made more elegant in 1970 due to advances made by George R. Price. The term "kin selection" was first used by John Maynard Smith in 1964.
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 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 interaction between related individuals, simply as a result of living in each other's proximity, is sufficient for kin selection, given reasonable assumptions about population dispersal rates. Kin selection is not the same thing as group selection, where natural selection 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 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.
Historical overview
was the first to discuss the concept of kin selection. In On the Origin of Species, he wrote about the conundrum represented by altruistic sterile social insects 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's Sociobiology.
Kin selection was briefly referred to by R.A. Fisher in 1930 and J.B.S. Haldane in 1932 and 1955. J.B.S. Haldane grasped the basic quantities 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%. 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, in 1963 and especially in 1964 generalised the concept and developed it mathematically, showing that it holds for genes even when they are not rare, deriving Hamilton's rule and defining a new quantity known as an individual's inclusive fitness. He is widely credited as the founder of the field of social evolution. A more elegant mathematical treatment was made possible by George Price in 1970.
John Maynard Smith may have coined the actual term "kin selection" in 1964:
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 sociobiology. 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 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". 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. Inclusive fitness theory is still generally accepted however, as demonstrated by the publication of a rebuttal to Wilson's claims in Nature from over a hundred researchers.
Kin selection is contrasted 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.
Hamilton's rule
Formally, genes for a particular behavior should increase in frequency whenwhere
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 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. Modern formulations of the rule use Alan Grafen's definition of relatedness based on the theory of linear regression.
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 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.
Mechanisms
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 outlined two ways in which kin selection altruism could be favoured: