Green-beard Effect

A green-beard effect occurs when a gene, or linked genes, produce three phenotypic effects:

1. a perceptible trait — the hypothetical "green beard";
2. recognition of this trait in others; and
3. preferential treatment to those recognized.

So this gene is directly recognizing copies of itself, regardless of average relatedness. Whereas most alleles that are favored by kin selection spread by promoting altruism towards those likely to be carrying the same allele, green-beard alleles would rise to frequency by promoting altruism toward individuals certain to be carrying the same allele.

Green-beard altruism could increase the presence of green-beard phenotypes in a population even if genes are assisting other genes that are not exact copies of themselves in a molecular sense; all that is required is that they produce the three phenotypic characteristics described above. Green beard genes are vulnerable to mutant genes arising that produce the perceptible trait without the helping behaviour.

The idea of a green-beard gene was proposed by William D. Hamilton in his articles of 1964, and named as "Green Beard" by Richard Dawkins in The Selfish Gene (1976).