--- title: "Kin Selection, Relatedness and When Helping Pays" description: "Kin selection concerns reproductive effects on relatives. In a one-recipient additive model, a heritable helping trait is favored when relatedness times the recipient's reproductive benefit exceeds th" canonical: https://lightmysky.com/learn/science/kin-selection-relatedness-and-when-helping-pays-mt_scVf8G34WI source: https://lightmysky.com/learn/science/kin-selection-relatedness-and-when-helping-pays-mt_scVf8G34WI.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Kin Selection, Relatedness and When Helping Pays Kin selection concerns reproductive effects on relatives. In a one-recipient additive model, a heritable helping trait is favored when relatedness times the recipient's reproductive benefit exceeds the helper's direct reproductive cost. Pedigree calculations and matching fitness units make the comparison meaningful. Subject: Science · Area: Ecology & Conservation · Ages 20 to 22 Page: https://lightmysky.com/learn/science/kin-selection-relatedness-and-when-helping-pays-mt_scVf8G34WI ## Ready when they can - Calculates expected relatedness in stated outbred diploid pedigrees and applies Hamilton's rule. - Explains why the same costly act can satisfy Hamilton's rule for a full sibling but not for a recipient assigned r = 0. - Separates kin selection from cooperation between unrelated individuals. ## Lesson: When helping family pays Kin selection is natural selection through effects on relatives' reproductive success. An allele is a version of a gene. Here r is expected pedigree relatedness: the expected fraction of autosomal allele copies shared by descent from recent common ancestors in an outbred diploid pedigree. It is not overall DNA similarity. Unrelated individuals can share alleles; r = 0 means no extra relatedness relative to the population background used in this model. For these outbred diploid pedigrees, use 1/2 for each parent-to-child link. Multiply along each path through a shared ancestor, then add the paths. Parent and child: r = 1/2. Half siblings share one parent: r = (1/2)^2 = 1/4. Full siblings share two parents: r = 2 times 1/4 = 1/2. First cousins share two grandparents, with four links per path: r = 2 times (1/2)^4 = 1/8. The ancestors are unrelated and non-inbred. *(drawing: Expected relatedness under the stated pedigree assumptions. These values do not measure overall DNA similarity.)* Hamilton's rule is r times B > C. B is the recipient's gain in expected reproductive success; C is the helper's loss in its own expected reproductive success. Use matching fitness units. For a heritable helping trait in this one-recipient additive model, multiply r by B, then compare with C. If rB > C, helping is favored; if rB = C, this comparison gives no net advantage. Relatedness alone does not settle the result. **Example.** For a full sibling, let r = 0.5, B = 10 and C = 3 in matching fitness units. The weighted benefit is 0.5 times 10 = 5. Since 5 > 3, helping is favored in this model. For the same benefit and cost with r = 0, the weighted benefit is 0 times 10 = 0. Hamilton's rule is not met because 0 cannot exceed 3. This does not rule out other explanations of cooperation. A later return of help is reciprocity. Cooperation between nonrelatives can also bring an immediate shared benefit or benefits through reputation. Kin selection concerns reproductive effects through relatives. Familiarity does not determine r: a stranger may be a relative. Human motives, such as empathy, are separate from reproductive effects. A person may help without expecting repayment. Hamilton's rule does not tell people whom they ought to help. Direct fitness concerns one's own reproduction. A nonreproducing worker can gain indirect fitness by helping the queen's related offspring survive and reproduce. Such helping can be favored when relatedness-weighted gains exceed direct fitness costs. Workers need not calculate these payoffs. Worker reproduction and relatedness vary among colonies and species. Do not assume the diploid sibling values above apply unchanged to every ant or bee. **Recap.** Calculate r under stated pedigree assumptions. In the additive model, helping is favored when r times B > C in matching fitness units. This prediction does not establish a person's motive or duty. *This lesson, its questions and its flashcards were written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Sexual selection, mate choice and parental investment](https://lightmysky.com/learn/science/sexual-selection-mate-choice-and-parental-investment-mt_6oTPGFw_p5) ## Optional connections - [Meiosis and Why Gametes Are Haploid](https://lightmysky.com/learn/science/meiosis-and-why-gametes-are-haploid-mt_hvu0bczcfX)