Green-beard effect

The green-beard effect is a thought experiment used in evolutionary biology to explain selective altruism among individuals of a species. Altruistic behaviour is paradoxical when viewed in the light of old ideas of evolutionary theory that emphasised the role of competition. The evolution of altruism is better explained through the gene-centered view of evolution, which emphasizes an interpretation of natural selection from the point of view of the gene which acts as an agent that has the metaphorical "selfish goal" of maximizing its own propagation. A gene for (behavioral) selective altruism can be favored by (natural) selection if the altruism is primarily directed at other individuals who share the gene. Since genes are invisible, such an effect requires perceptible markers for altruistic behaviour to occur.

A green-beard effect occurs when an allele, or a set of linked alleles, produce three expressed (or phenotypic) effects:

The carrier of the gene (or a specific allele) is essentially recognizing copies of the same gene (or a specific allele) in other individuals. Whereas kin selection involves altruism to related individuals who share genes in a non-specific way, green-beard alleles promote altruism toward individuals who share a gene that is expressed by a specific phenotypic trait. Some authors also note that the green-beard effects can include "spite" for individuals lacking the "green-beard" gene. This can have the effect of delineating a subset of organisms within a population that is characterized by members who show greater cooperation toward each other, this forming a "clique" that can be advantageous to its members who are not necessarily kin.

Green-beard effect could increase altruism on green-beard phenotypes and therefore its presence in a population even if genes assist in the increase of genes that are not exact copies; all that is required is that they express the three required characteristics. Green-beard alleles are vulnerable to mutations that produce the perceptible trait without the helping behaviour.

The idea of a green-beard allele was proposed by William D. Hamilton in his articles of 1964, and got the name from the example used by Richard Dawkins ("I have a green beard and I will be altruistic to anyone else with green beard") in The Selfish Gene (1976).

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