What is Hamilton's rule?

Hamilton's rule states that a gene for altruism spreads when the relatedness between actor and recipient times the benefit to the recipient exceeds the cost to the actor, summarised as rb greater than c.

How can sterile worker insects be favoured by selection?

Workers that forgo their own reproduction can still pass on copies of their genes by helping a closely related queen produce many siblings, so helping can yield greater inclusive fitness than breeding alone.

Social Evolution and Cooperation

Cooperation and self-sacrifice seem to defy natural selection, yet helping relatives or trading favours can pay off in genetic terms, explaining the evolution of social life within an ecological context.

Definition

Social evolution and cooperation concern how interactions among individuals—cooperative, altruistic, or conflicting—evolve, with kin selection explaining behaviour that benefits relatives through shared genes and reciprocity explaining cooperation among non-kin.

Scope

This topic covers the ecological and evolutionary basis of social behaviour: kin selection and inclusive fitness, Hamilton's rule, reciprocal altruism, the evolution of cooperation and conflict, group living, eusociality and reproductive division of labour, and parent-offspring and sexual conflict. It treats the genetic relatedness and ecological conditions under which helping and sociality are favoured.

Core questions

How can altruistic behaviour evolve despite its cost to the actor?
When does Hamilton's rule predict that helping will spread?
How does reciprocal altruism sustain cooperation among non-relatives?
Why do eusocial societies with sterile helpers evolve?

Key theories

Kin selection and Hamilton's rule: An altruistic act is favoured when the benefit to the recipient, multiplied by the genetic relatedness between actor and recipient, exceeds the cost to the actor, so genes for helping relatives can spread through inclusive fitness.
Reciprocal altruism: Cooperation between unrelated individuals can evolve when help is reliably returned over repeated interactions, so that the long-term benefits of mutual aid outweigh the short-term cost of helping.

Mechanisms

Selection acts on genes through their total effect on copies of themselves, whether in the actor's own offspring or in relatives who share them; this inclusive-fitness accounting underlies Hamilton's rule, which weighs benefit and cost by relatedness. Among non-kin, cooperation can be stabilised by repeated interactions in which defectors lose future help, by reputation, or by partner choice. Eusociality, with sterile workers aiding a reproductive, arises where high relatedness and ecological benefits of group living make helping at the nest more profitable than independent breeding.

Clinical relevance

Understanding social evolution informs the management of social and group-living species, the conservation of cooperative breeders, control of social insects, and evolutionary perspectives on human cooperation. This is educational context, not management prescription.

History

Hamilton formalised inclusive fitness in 1964, transforming the study of social behaviour, and Trivers extended cooperation to non-kin through reciprocal altruism in 1971. Wilson's synthesis of sociobiology in 1975 brought these ideas to wide attention and controversy, and the evolution of eusociality remains intensely studied.

Debates

Kin selection versus group selection: The relative merits of inclusive-fitness theory and multilevel or group-selection accounts of social evolution have been sharply contested, especially over the origins of eusociality, though many researchers view the frameworks as complementary or formally equivalent.

Key figures

William D. Hamilton
Robert Trivers
John Maynard Smith
Edward O. Wilson

Seminal works

hamilton1964
trivers1971
davies2012

Frequently asked questions

What is Hamilton's rule?: Hamilton's rule states that a gene for altruism spreads when the relatedness between actor and recipient times the benefit to the recipient exceeds the cost to the actor, summarised as rb greater than c.
How can sterile worker insects be favoured by selection?: Workers that forgo their own reproduction can still pass on copies of their genes by helping a closely related queen produce many siblings, so helping can yield greater inclusive fitness than breeding alone.