Greenbeard Mechanism and the Evolution of Cooperation

Abstract

Explaining the origin and maintenance of cooperation through Darwin's theory of biological evolution by natural selection is one of the most challenging but now widely studied problems in evolutionary biology. A genetic mechanism that enhances the inclusive fitness of a cooperative gene, proposed by Hamilton and later popularised by Dawkins as the greenbeard mechanism, suggests that cooperation can evolve even without genealogical relatedness (between nonkin) if the social partners share greenbeard genes. These genes encode a phenotypic marker (metaphorically, a green beard) and cause their bearers to cooperate with those who have the marker (cooperators) and avoid or harm those who do not (defectors). The marker and the cooperative trait are encoded either by a single gene or by a group of linked genes that co‐inherit. Thus, cooperation is maintained by direct and indirect reciprocity between individuals that are unrelated elsewhere in the genome but possess copies of the greenbeard genes. Key Concepts:

The evolution of cooperation implies the evolution of a trait (or behaviour) that is costly to the actor but beneficial to the recipient; starvation‐induced fruiting body formation in Dictyostelium , for example, involves the altruistic death of the amoebae that form the stalk. Since natural selection works by selecting individuals that survive longer and reproduce more than their competitors, how do cooperative behaviours, which reduce the chances of survival and reproduction of individuals who cooperate, evolve through natural selection? In 1964, Hamilton proposed a genetic mechanism (now called the greenbeard mechanism, after Dawkins' colourful analogy) for the evolution of altruistic cooperation. In the greenbeard mechanism, a gene or a group of linked genes encodes for a cooperative trait, a perceptible marker (the hypothetical green beard) and the ability to recognise this marker in other individuals and behave cooperatively towards those with the marker or harm those without the marker. The greenbeard mechanism can mediate cooperation between genealogically unrelated individuals (no kinship), such as those from different species, as long as they share a greenbeard gene (kind selection). Depending on their effect on the recipient, the greenbeard genes can be helping or harming types, and each can be either facultative or obligatory. The greenbeard effect can be mediated by a single gene or by multiple genes. The greenbeard genes can be monomorphic or polymorphic and they can cause the greenbeard effect directly (e.g. by encoding cell adhesion proteins) or indirectly (e.g. by activating the downstream behavioural genes). Individuals with falsebeards can evolve and reap the benefits of cooperation without paying the costs of cooperation. The long‐term stability of greenbeard‐mediated cooperation, therefore, requires polychromatic or diverse greenbeards.