# Kin selection ## Microsim (three.js) <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Kin_selection.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin"></iframe> </div> *A costly good deed can spread a gene — as long as it lands on kin. Sitting in the GENOMICS hub beside Reciprocal altruism and [[Evolutionarily_stable_strategy]], this microsim turns the shared ancestry and pedigree math of relatives into one cost–benefit inequality you can tune by hand.* > Kin selection is evolution's answer to a puzzle: how can self-sacrificing, "altruistic" behaviour evolve if it lowers the helper's own survival or breeding? The answer is that helping close relatives passes on shared copies of the same genes, so a costly good deed can still spread through a population. W. D. Hamilton captured this in a single inequality — Hamilton's rule — that weighs the benefit to the recipient against the cost to the actor, discounted by how closely the two are related. In this microsim you set the cost *C* and the benefit *B*, trigger an altruistic act, and auto-play generations to watch whether the helpful gene wins or vanishes. ## About this microsim The simulation exposes two sliders and three buttons — there is no free camera or hidden menu. The **Cost to actor, C** slider (0–10) sets the direct fitness the helper gives up, and the **Benefit to recipient, B** slider (0–10) sets the fitness gained by the individual being helped, both in units of offspring-equivalents. **Perform Altruistic Act** applies one round of helping at your chosen *C* and *B*; **Auto-Play Generations** repeats the act across successive generations so you can watch the altruistic gene's frequency climb or crash; **Reset** returns the population to its starting state. Relatedness *r* — the third variable in Hamilton's rule — is not adjustable in this build; with *r* held fixed, the sim isolates how cost and benefit alone push the altruistic gene toward fixation or loss. ## Related microsims - Reciprocal altruism — cooperation between non-relatives, the other main route to altruism: help now, repaid later. - [[Evolutionarily_stable_strategy]] — the game-theory test for whether a cooperative or selfish strategy resists invasion. - Mendelian inheritance — where the coefficient of relatedness *r* actually comes from. - Genetics — genes as the unit that selection ultimately counts. - [[Quorum_sensing]] — cooperation and cheating among microbes, kin selection at the cellular scale. ## Links (Wikipedia order) <!-- injected from _registry/childlinks/Kin_selection.json (2026-07-30T02:09:12Z) --> `Abiogenesis` · `Abortion` · `Accident-proneness` · `Acquired_characteristic` · [[Adaptation]] · `Adaptationism` · `Adaptive_radiation` · `Adaptive_unconscious` · 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`Theory_of_mind_in_animals` · `Thorson's_rule` · `Thrips` · `Transgenerational_epigenetic_inheritance` · `Transitional_fossil` · `Transmutation_of_species` · `Trivers–Willard_hypothesis` · `Unconscious_cognition` · `Uniformitarianism` · `Unilineal_evolution` · `Unit_of_selection` · `University_of_Montpellier` · `Variability_hypothesis` · `Vervet_monkey` · `Vespidae` · `Vicar_of_Bray_(scientific_hypothesis)` · `Viral_evolution` · `Visual_perception` · `Vitalism` · `Von_Baer's_laws_(embryology)` · `W._D._Hamilton` · `War_in_ants` · `Wason_selection_task` · `Westermarck_effect` · `Willi_Hennig` · `Worker_policing` · `Zygote` ## Overview Kin selection is a form of natural selection that favours behaviours increasing the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. It is the leading explanation for the evolution of altruism and social cooperation, from alarm calls in Belding's ground squirrels to the sterile worker castes of ants, bees, and wasps. The idea is usually expressed through **inclusive fitness** — the sum of an individual's own (direct) reproduction and its effect on relatives' (indirect) reproduction, each relative weighted by genetic relatedness. J. B. S. Haldane anticipated it with his quip that he would lay down his life for two brothers or eight cousins; W. D. Hamilton formalised it mathematically in 1964, and John Maynard Smith named it "kin selection." ## The mechanism: Hamilton's rule At the centre of kin selection is **Hamilton's rule**, which states that an altruistic gene spreads when $rB > C$ where $C$ is the fitness cost to the actor, $B$ is the fitness benefit to the recipient, and $r$ is the **coefficient of relatedness** — the probability that the two individuals share a given gene by common descent. Relatedness acts as an exchange rate: the more closely kin are related, the smaller the benefit needed to justify a given cost. | Relationship | Coefficient of relatedness, *r* | |---|---| | Identical twin / clone | 1.0 | | Parent–offspring | 0.5 | | Full siblings | 0.5 | | Half siblings | 0.25 | | Grandparent–grandchild | 0.25 | | First cousins | 0.125 | | Full sisters, haplodiploids (Hymenoptera) | 0.75 | With full siblings ($r = 0.5$), the rule becomes $0.5\,B > C$: a helper should sacrifice one offspring-equivalent only if the sibling gains more than two — Haldane's "two brothers" in algebra. The unusually high relatedness of haplodiploid sisters ($r = 0.75$) was long invoked to explain why eusociality, complete with sterile helpers, arose repeatedly in ants, bees, and wasps. ## Controls -> what each maps to | Control | Maps to | Range / values | Meaning | |---|---|---|---| | Cost to actor, C | The cost term $C$ in $rB > C$ | Slider, 0–10 | Direct fitness (offspring-equivalents) the altruist loses by helping | | Benefit to recipient, B | The benefit term $B$ | Slider, 0–10 | Fitness the recipient gains from being helped | | Perform Altruistic Act | One application of the rule | Button | Runs a single helping event at the current *C* and *B* | | Auto-Play Generations | Iterated selection | Button | Repeats the act over successive generations to show gene-frequency change | | Reset | Initial state | Button | Restores the starting population and gene frequency | ## Learning objective By adjusting cost and benefit, you should be able to predict — and then watch — when Hamilton's rule $rB > C$ lets an altruistic gene spread through a population of relatives rather than die out. ## Limits and connections Hamilton's rule assumes additive, independent costs and benefits and a well-defined relatedness; real systems add reciprocity between non-kin, spite, greenbeard genes, and ecological limits on dispersal. Inclusive-fitness accounting is mathematically equivalent to multilevel (group) selection, and it complements rather than replaces Reciprocal altruism as a route to cooperation. Because *r* flows from ordinary Mendelian inheritance, the same pedigree logic that predicts eye colour also predicts when helping evolves. ## Poster & source <div class="microsim-fallback"> <!-- poster image pending backfill --> <p><em>Live microsim · <a href="https://wikitube-3d-microsims.netlify.app/Kin_selection.html">open full</a> · source: Microsims for Dissemination/GENOMICS_ThreeJS_Microsims/Kin_selection.html</em></p> </div> <!-- CRAFT-LINK:START g12 --> *Built to the [[WT!Three_js_Microsim_Master_Class|three.js Master Class]].* <!-- CRAFT-LINK:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Kin_selection) : [Wikitube](https://en.wikitube.io/wiki/Kin_selection) ## Previous hub tags Tree parents: [[Emergence]] · [[Game_theory]]. Legacy hubs: `GENOMICS`. --- *Sources: 1 legacy note. Minted wave 1, 2026-07-30 (v1.6 order).*