Garcia, R. A. (2017). Kin Selection. (V. Zeigler-Hill, & T. K. Shackelford, Eds.) Encyclopedia
of Personality and Individual Differences, pp. 1-2. doi:10.1007/978-3-319-28099-8_1541-1
Title of entry: Kin Selection
Synonyms: kin altruism; inclusive fitness
Definition: Kin Selection is a view of natural selection that incorporates the role of shared genes
in an organism’s decision making.
If an organism’s primary objective is the propagation of its own genes, why would such a selfish
organism perform actions that benefit its competitors – incurring a cost to its own reproductive
success, while increasing the reproductive success of others? These altruistic behaviors, while
seemingly in direct opposition to natural selection, are often quite rational and in accordance
with the essence of natural selection. This is particularly the case when one considers that the
competitors that often reap the benefits of these acts are close relatives (e.g., brothers, sisters,
sons, daughters, cousins). Relatives share a more-than-average amount of the same genes, and
while the sharing is not 100% (except in the case of monozygotic ‘identical’ twins), by helping a
relative’s reproductive success, you are indirectly increasing the likelihood that some of your
shared genes are represented in the next generation. This expansion of the principles of natural
selection to include the effect of actions on the reproductive success of relatives and one’s own
direct reproductive success (collectively known as inclusive fitness) is known as kin selection.
The term first coined by Maynard Smith in 1964, kin selection is an extension of natural
selection that incorporates how indirect sources of reproductive success (fitness of relatives)
affect an organism’s actions to optimize its own fitness. Kin selection has been discussed as early
as Darwin’s The Origin of Species (Darwin, 1859) with much of its popularity coming from
Hamilton’s mathematical treatment (Hamilton, 1964). Hamilton’s rule captures the essence of
kin selection; kin selection should occur when:
𝑟𝐵 > 𝐶,
= the genetic relatedness between the actor and the beneficiary,
= the fitness benefit to
the beneficiary, and
= the fitness cost to the actor. In other words, kin selection should occur
when the indirect fitness gain of one outcome (
) is likely to be greater than the direct fitness
gain of the alternative outcome (
). In honeybees (Apis mellifera), for example, workers will
attack intruders to defend their colony, even at the expense of their own life. This is, in part, due
to the fact that the colony is comprised of the worker’s relatives; the indirect reproductive benefit
) outweighs the reproductive cost (
). While conceptually powerful, it is difficult to quantify
the costs and benefits that result from different decision outcomes. Thus, the first successful
instance of empirical support for Hamilton’s rule to date involving wild animals was not carried
out until 2010 (Gorrell, et al., 2010).
Kin selection is unlikely to occur unless a means of determining genetic relatedness exists. Two
mechanisms proposed by Hamilton (1964) are: kin recognition and viscous populations. In the
case of kin recognition, specialized kin-detection brain processes evolve over time to allow
individuals to discriminate among other members of the same species. As a molecular
assessment of genetic relatedness is evolutionarily novel, this mechanism uses the high
correspondence between phenotype and genotype to infer the degree of relatedness between the
self and other members of the species. Viscous populations, on the other hand, use the
correspondence between geographic dispersal and genetic relatedness. In groups with little
geographic dispersal, it can be reasonably inferred that those closer in space are more genetically
related than those farther away. While both mechanisms were proposed by Hamilton, the role of
kin recognition has since been downplayed in favor of a viscous populations explanation
While kin selection appears to present a strong explanation of altruistic behavior, its strictest
form has largely been debunked outside of a narrow set of situations. A softer form of kin
selection depicts shared genetic relatedness as secondary to developmental cues that mediate the
development of social bonds and altruistic behaviors (Holland, 2012). The multilevel selection
model, on the other hand, presents an alternate framing of kin selection away from an
individual’s inclusive fitness to a group’s fitness – stating that selection of altruistic behaviors
happens as a consequence of increased group fitness due to cooperation (Nowak, Tarnita, &
Conclusion: As noted by Nowak, Tarnita, & Wilson (2010): “Inclusive fitness theory is not a
simplification over the standard approach. It is an alternative accounting method…” Kin
selection is a way of framing natural selection to incorporate the role relatives play in an
organism’s decision-making process. Kin selection is said to occur when an organism reaps the
benefit of indirect fitness gains rather than those of direct fitness gains. That is, kin selection
occurs when an organism selfishly sacrifices its personal reproductive success to increase the
reproductive fitness of its relatives.
Cross-references: reciprocal altruism; social cooperation; social selection; sociobiology
Darwin, C. (1859). On the origin of species by means of natural selection, or the preservation of
favoured races in the struggle for life. London: John Murray.
Gorrell, J. C., McAdam, A. G., Coltman, D. W., Humphries, M. M., Boutin, S., Jamieson, C.
(2010). Adopting kin enhances inclusive fitness in asocial red squirrels. Nature
Communications, 1(22), 1-4.
Hamilton, W. D. (1964). The genetical evolution of social behavior. Journal of Theoretical
Biology, 7(1), 1-16.
Hamilton, W. D. (1987). Discriminating nepotism: Expectable, common and overlooked. In D.
J. C. Fletcher & C. D. Michener (Eds.), Kin Recognition in Animals. New York: Wiley.
Holland, M. (2012). Social bonding and nurture kinship: Compatibility between cultural and
biological approaches. North Charleston: Createspace Press.
Nowak, M., Tarnita, C., & Wilson, E. O. (2010). The evolution of eusociality. Nature, 466,