What is Hamilton's rule?

Hamilton's rule states that an altruistic behavior is favored by selection when the genetic relatedness between the helper and the recipient, times the reproductive benefit to the recipient, exceeds the reproductive cost to the helper.

Why do worker bees give up their own reproduction?

Because their high relatedness to the offspring they help raise means that, through kin selection, they can propagate copies of their genes more effectively by assisting relatives than by breeding independently.

Kin Selection and Social Evolution

Kin selection explains the evolution of cooperation and altruism by recognizing that helping relatives can propagate shared genes, formalized by Hamilton's rule.

Definition

Kin selection is natural selection acting on an individual's effect on the reproduction of its relatives, weighted by relatedness. Social evolution is the broader study of how cooperative, altruistic, and conflicting behaviors evolve among interacting individuals.

Scope

This topic covers inclusive fitness and Hamilton's rule, the evolution of altruism and cooperation, reciprocal altruism among unrelated individuals, the special case of eusociality in insects and other taxa, and the role of relatedness and conflict in social systems.

Core questions

How does Hamilton's rule predict when altruism will evolve?
What is inclusive fitness and how does it differ from individual reproductive success?
How can cooperation evolve among unrelated individuals through reciprocity?
Why has eusociality, with sterile helper castes, evolved repeatedly?

Key theories

Inclusive fitness and Hamilton's rule: An altruistic act spreads when the relatedness between actor and recipient, multiplied by the benefit to the recipient, exceeds the cost to the actor, so genes can promote their own copies in relatives.
Reciprocal altruism: Cooperation between unrelated individuals can be favored when help is reliably returned over time, making mutual aid stable even without shared genes.

Mechanisms

Kin selection operates because relatives share genes by common descent, so an allele causing its bearer to help kin can increase its own frequency even at a cost to the bearer's direct reproduction. Hamilton's rule, relatedness times benefit greater than cost, specifies the condition. Inclusive fitness sums an individual's direct reproduction and its effects on relatives' reproduction. Among non-relatives, reciprocity and enforcement can stabilize cooperation. Eusociality, with non-reproductive helpers, is favored by high relatedness and ecological factors that make helping more productive than independent breeding, though conflicts of interest persist within social groups.

Clinical relevance

Social-evolution theory illuminates cooperation and conflict among cells and microbes, including the evolution of cancer as a breakdown of cellular cooperation and cooperative behaviors among pathogens that affect virulence.

History

Hamilton's 1964 papers introduced inclusive fitness and kin selection, transforming the study of social behavior. Trivers extended cooperation to reciprocal altruism and parental investment in the early 1970s, and decades of work on insects, birds, and microbes have tested and refined the theory.

Debates

Inclusive fitness versus alternative formulations: A prominent debate concerns whether inclusive-fitness theory is the best framework for cooperation or whether multilevel-selection and other models offer superior explanations.

Key figures

W. D. Hamilton
Robert Trivers
George C. Williams
Edward O. Wilson

Seminal works

hamilton1964
daviesKrebsWest2012
futuyma2017

Frequently asked questions

What is Hamilton's rule?: Hamilton's rule states that an altruistic behavior is favored by selection when the genetic relatedness between the helper and the recipient, times the reproductive benefit to the recipient, exceeds the reproductive cost to the helper.
Why do worker bees give up their own reproduction?: Because their high relatedness to the offspring they help raise means that, through kin selection, they can propagate copies of their genes more effectively by assisting relatives than by breeding independently.