Kin selection was first suggested by Darwin as an explanation of the sterile castes of social insects and has later been mathematically defined by W. D. Hamilton as a mechanism for the evolution of apparently altruistic acts. Kin altruism is the technical term for altruistic behaviour that has been shown, or is theoretically supposed, to be explained by kin selection.

Under natural selection a gene that causes itself to increase in frequency should become more common in the population. Since identical copies of genes may be carried in relatives, a gene in one organism that prompts behaviour which aids another organism carrying the same gene may become more successful provided that

where

R = the genetical relatedness of the recipient to the actor, usually defined as the probability that a gene picked randomly from each at the same locus is identical by descent (i.b.d).

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 is known as Hamilton's rule.

This leads to the concept that an individual should sacrifice itself in order to save "two siblings, four nephews or eight cousins," since siblings share 50% of an individual's genes, nephews 25% and cousins 12.5% (in a diploid, randomly mating and outbred population).

For kin-selection to occur all that is necessary is for individuals to behave nepotistically towards their kin. Usually this requires kin recognition either due to innate cognition of character traits associated with relatedness or due to recognition of specific individuals with whom they have grown up. This nepotism is not always clearly altruistic or even necessarily requiring genuine kin recognition, for instance most young Plains Spadefoot Toads are detritivorous and congregate with kin but occasionally one will eat a shrimp and then become carnivorous, such individuals live more solitarily and at least when satiated prefer to eat non-kin than kin reducing the damage they might otherwise do to the survivorship of their relatives.

Kin selection has been used to explain the evolution of humanity's social structure, social insects such as ants and termites, and even the evolution of multicellular animals.

Technically, the correct definition for relatedness (R) in Hamilton's rule describes it as a regression measure. Regressions, unlike probabilities, can be negative, and so it is not implausible for there to be negative relatedness between two individuals. Negative relatedness simply means that two individuals are less genetically alike than average, and this can lead to the evolution of spiteful behaviours.

See also

• Selfish gene