What is Hamilton's rule?

Hamilton's rule states that altruistic behaviour is favoured by selection when the benefit to the recipient, weighted by the genetic relatedness between actor and recipient, is greater than the cost to the actor, written rb greater than c.

Why do worker bees not reproduce?

Sterile workers raise their queen's offspring, who are their close relatives; by helping rear highly related sisters they can propagate copies of their own genes, which kin selection shows can be favoured over reproducing themselves.

Social Behavior and Kin Selection

Many animals live in groups and even help others at a cost to themselves; kin selection explains how such apparent altruism can evolve when it benefits relatives who share the helper's genes.

Definition

Social behavior is interaction among members of the same species living together, and kin selection is the evolutionary process by which behaviours that aid genetic relatives are favoured because they increase the actor's inclusive fitness.

Scope

This topic covers the evolution of social living and cooperation, with particular attention to altruistic behaviour, in which an animal reduces its own reproduction to benefit others. It introduces inclusive fitness and Hamilton's rule, the kin-selection account of altruism, and applies these ideas to phenomena such as alarm calling, cooperative breeding, and the extreme sociality of social insects. It also notes reciprocity and the ongoing debate over the levels at which selection acts.

Core questions

How can behaviour that lowers an individual's own reproduction evolve?
What does Hamilton's rule state about when altruism is favoured?
How does kin selection explain alarm calls, helping at the nest, and social insects?
What roles do reciprocity and mutual benefit play in cooperation?

Key theories

Inclusive fitness and Hamilton's rule: An animal's inclusive fitness includes effects on the reproduction of relatives weighted by relatedness; altruism is favoured when the relatedness-weighted benefit to recipients exceeds the cost to the actor, expressed as rb greater than c.
Pathways to cooperation: Cooperative and altruistic behaviour can be favoured through kin selection among relatives and through reciprocity and mutual benefit among non-relatives, providing complementary explanations for the evolution of social living.

Mechanisms

Because relatives share genes by common descent, a gene promoting help to kin can spread if the help raises the reproduction of those who likely carry copies of the same gene. Hamilton's rule formalises this: an altruistic act is favoured when the benefit to the recipient, multiplied by the coefficient of relatedness between actor and recipient, exceeds the cost to the actor. This logic explains alarm calls given to warn relatives, helpers that forgo breeding to raise siblings, and the sterile worker castes of social insects, whose unusual genetics can make workers highly related to the offspring they rear. Among unrelated individuals, cooperation can instead be maintained by reciprocity, where help is repaid over time.

Clinical relevance

The theory of social evolution informs the conservation of group-living and cooperatively breeding species, the management of social insects important in pollination and pest control, and broader understanding of cooperation in biological systems. This is educational context, not clinical advice.

History

W. D. Hamilton's 1964 theory of inclusive fitness provided the genetic basis for the evolution of altruism, resolving a long-standing puzzle posed by Darwin over sterile insect castes. Robert Trivers extended the framework with reciprocal altruism in 1971, and E. O. Wilson's synthesis of sociobiology in 1975 brought these ideas to the study of social behaviour across animals, sparking lasting research and debate.

Debates

Kin selection versus group and multilevel selection: Although inclusive-fitness theory is widely used, the best framework for explaining cooperation, especially the evolution of eusociality, has been debated between kin-selection and group- or multilevel-selection accounts.

Key figures

W. D. Hamilton
Robert Trivers
E. O. Wilson
George Williams

Seminal works

hamilton1964
alcock2019

Frequently asked questions

What is Hamilton's rule?: Hamilton's rule states that altruistic behaviour is favoured by selection when the benefit to the recipient, weighted by the genetic relatedness between actor and recipient, is greater than the cost to the actor, written rb greater than c.
Why do worker bees not reproduce?: Sterile workers raise their queen's offspring, who are their close relatives; by helping rear highly related sisters they can propagate copies of their own genes, which kin selection shows can be favoured over reproducing themselves.