Kin selection

Hamilton's rule states that an altruistic gene will spread if the cost to the actor (C) is less than the benefit to the recipient (B) multiplied by the coefficient of relatedness (r) between them: rB > C.¹ The coefficient of relatedness measures the probability that a gene in one individual is identical by descent to a gene in another. For full siblings in diploid organisms, r = 0.5; for parent and offspring, r = 0.5; for identical twins, r = 1.0. Hamilton's rule is a central component of inclusive fitness theory, which sums an individual's own reproductive success (direct fitness) and the reproductive success of relatives, weighted by relatedness (indirect fitness).² This framework allows evolutionary biologists to predict when altruistic behaviors, such as alarm calling or cooperative breeding, are likely to evolve.

Kin selection operates through mechanisms such as kin recognition, which allows individuals to discriminate between relatives and non-relatives, often using cues like scent or visual signals.³ In many social insects, such as ants, bees, and wasps, haplodiploidy results in sisters being more closely related to each other (r = 0.75) than to their own offspring (r = 0.5), which is thought to have facilitated the evolution of eusociality, where workers forgo reproduction to help raise their mother's offspring.⁴ In vertebrates, kin selection explains cooperative breeding in species like the Florida scrub jay, where offspring stay with their parents to help raise younger siblings, and alarm calling in ground squirrels, where individuals warn relatives of predators at personal risk.⁵

Kin selection has faced critiques, notably from proponents of group selection, who argue that altruism can evolve through benefits to the group rather than to individuals. However, inclusive fitness theory has been defended as a mathematically rigorous framework that can be reconciled with group selection models.⁶ A major refinement is the green-beard effect, where a gene can cause an individual to recognize other carriers of the same gene and behave altruistically toward them, even if they are not close relatives.⁷ Another refinement is the concept of supergenes, such as in fire ants, where a single genetic region controls colony queen number, affecting relatedness and social behavior.

Kin selection extends beyond animals to microorganisms. For example, in the social amoeba Dictyostelium discoideum, individual cells aggregate to form a fruiting body, with some cells sacrificing themselves to form a stalk that aids spore dispersal; kin recognition ensures that this altruism is directed toward relatives.³ In humans, kin selection has been invoked to explain behaviors such as nepotism and inheritance patterns, though cultural factors also play a significant role.⁵ A notable edge case is the evolution of spite, where an individual harms itself to harm a non-relative, which can be favored if it benefits relatives indirectly. The concept of kin selection has also been applied to explain the evolution of genomic imprinting, where genes are expressed differently depending on parent of origin, as seen in the conflict between maternal and paternal genes in offspring.

Inclusive fitness

The sum of an individual's own reproductive success and the reproductive success of relatives, weighted by relatedness.

Hamilton's rule

A mathematical condition (rB > C) for the evolution of altruistic traits.

Coefficient of relatedness (r)

The probability that a gene in one individual is identical by descent to a gene in another.

Haplodiploidy

A sex-determination system where females are diploid and males are haploid, common in Hymenoptera.

Green-beard effect

A gene that causes an individual to recognize and behave altruistically toward other carriers of the same gene.