## Definition **Kin selection** is the evolutionary mechanism by which alleles can increase in frequency by causing organisms to favour the survival and reproduction of relatives, who are statistically likely to carry copies of the same allele. **Inclusive fitness** is the expanded measure of Darwinian fitness that counts not only an individual's own reproductive success but also its contribution to the reproduction of genetic relatives, weighted by their degree of relatedness. The framework was formalised by W.D. Hamilton (1964) and popularised by Dawkins as the gene-centred explanation for apparent altruism. ## Hamilton's Rule An altruistic act (cost $c$ to the actor, benefit $b$ to the recipient) is favoured by selection when: $ r \cdot b > c $ where $r$ is the **coefficient of relatedness** — the probability that a gene present in the actor is also present in the recipient by common descent. The rule states that a gene promoting self-sacrifice can spread if the benefit it confers on relatives, discounted by relatedness, exceeds the direct cost to the carrier. ## Coefficients of Relatedness | Relationship | $r$ | |---|---| | Identical twins | 1.00 | | Parent–offspring | 0.50 | | Full siblings | 0.50 | | Grandparent–grandchild | 0.25 | | Half-siblings | 0.25 | | Uncle/aunt–nephew/niece | 0.25 | | First cousins | 0.125 | These figures apply to the *non-universal* portion of the genome — the ~10% of genes that vary within a species. All members of a species share roughly 90% of their DNA; it is the varying fraction where relatedness becomes meaningful for selection. ## Why Mothers and Fathers Differ Parent–offspring relatedness is $r = 0.5$, but the certainty of parenthood differs between sexes. A mother in internally fertilising species is always certain her offspring are hers; a father may not be. This asymmetry, Dawkins argues, explains the widespread pattern where females invest more in offspring care than males. In human cultures where female infidelity is common, the maternal uncle — who knows with certainty he shares 25% of genes with nephews — may invest more in those nephews than the putative father does. ## Altruism Toward Non-Kin Kin selection explains altruism within families. Apparent altruism among non-kin is explained by: - **Reciprocal altruism** (Trivers 1971): repeated interactions allow tit-for-tat strategies to evolve (see [[Evolutionarily Stable Strategy]]). - **Selfish herd effects:** joining a group reduces individual predation risk without requiring genuine altruism. - **Manipulation:** signals that mimic kin cues can exploit kin-recognition systems. ## Connection to Gene-Centred View Kin selection is the mechanism that makes the gene-centred view concrete for social behaviour. The gene "does not care" which body it inhabits; it promotes the survival of any body that is likely to carry it. Relatives are the most reliable statistical proxies for gene sharing available to an organism. ## Related - [[Gene-Centred View of Evolution]] - [[The Selfish Gene]] - [[Evolutionarily Stable Strategy]] ## Sources - [[The Extended Selfish Gene (Dawkins 2016)]]