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Hamilton's Rule: When Does Altruism Evolve?

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Hamilton's Rule r × B > C
Common Relationships
Genetic Relatedness (r)
0.50
The proportion of genes shared between the altruist and the recipient
Benefit to Recipient (B)
5.0
The fitness benefit gained by the recipient of the altruistic act
Cost to Altruist (C)
2.0
The fitness cost incurred by the individual performing the altruistic act
Will this altruistic behavior evolve?
✓
r × B - C = 0.50 × 5.0 - 2.0 = 0.50
The altruistic behavior will be favored by natural selection.
Examples from Nature
Explanation
Connection to "The Selfish Gene"

Hamilton's Rule and Kin Selection

Hamilton's Rule, formulated by W.D. Hamilton in the 1960s, provides a mathematical explanation for the evolution of altruistic behaviors in nature. It states that an altruistic behavior will be favored by natural selection when the cost to the altruist (C) is less than the benefit to the recipient (B) multiplied by the genetic relatedness (r) between them.

The rule is expressed as: r × B > C

This elegant formula helps explain many puzzling behaviors in nature:

  • Worker sterility in social insects: Worker bees, ants, and wasps give up reproduction to help the queen reproduce. Due to their unusual genetics (haplodiploidy), they share 75% of their genes with their sisters, making it genetically advantageous to help raise more sisters.
  • Alarm calls: When a ground squirrel spots a predator, it gives an alarm call that alerts relatives but also draws attention to itself. The risk is worth it because it protects genetic relatives.
  • Cooperative breeding: In some bird species, individuals help raise siblings or other relatives instead of reproducing themselves.

The key insight is that genes causing altruistic behavior can spread if they help copies of themselves in other individuals. This mechanism is called "kin selection" and it explains how seemingly selfless behaviors can evolve through "selfish genes."

Kin Selection in "The Selfish Gene"

In "The Selfish Gene," Richard Dawkins builds on Hamilton's work to explain how altruism can evolve from a gene-centered perspective. Chapter 6, "Genesmanship," explores the concept of kin selection in depth.

Dawkins writes: "What natural selection favors is genes that succeed in the gene pool. In most cases this will mean that it favors genes that cause their bodies to behave altruistically towards other bodies containing the same gene."

This perspective solves the apparent paradox of altruism. From the gene's eye view, helping a relative isn't truly altruistic—it's a form of genetic self-interest. A gene that causes an organism to help relatives is effectively helping copies of itself.

Dawkins introduces the concept of the "extended phenotype," suggesting that genes influence not just the individual body they inhabit but can have effects that extend beyond the individual's body, including influencing the behavior of relatives.

He emphasizes: "When we talk about a gene 'for' altruism, we are talking about a gene that affects the behavior of an individual towards others in such a way that they indirectly increase the chance of the gene being passed into future generations through relatives."

This gene-centered view revolutionized our understanding of social behaviors in nature and helped explain phenomena that were previously difficult to reconcile with traditional natural selection theory.

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