A new paper in Nature (440:663-666), by Vincent Jansen and Minus van Baalen re-examines the plausibility of a green beard type of mechanism for altruism between individuals who have similar phenotypic "tags". The authors use simulations of Prisoner's Dilemma games in a spatial setting.
Abstract: The evolution of altruism, a behaviour that benefits others at one's own fitness expense, poses a darwinian paradox. The paradox is resolved if many interactions are with related individuals so that the benefits of altruism are reaped by copies of the altruistic gene in other individuals1, a mechanism called kin selection2. However, recognition of altruists could provide an alternative route towards the evolution of altruism1, 3, 4, 5. Arguably the simplest recognition system is a conspicuous, heritable tag, such as a green beard1, 3. Despite the fact that such genes have been reported6, 7, 8, the 'green beard effect'3 has often been dismissed because it is unlikely that a single gene can code for altruism and a recognizable tag1, 3, 9. Here we model the green beard effect and find that if recognition and altruism are always inherited together, the dynamics are highly unstable, leading to the loss of altruism. In contrast, if the effect is caused by loosely coupled separate genes, altruism is facilitated through beard chromodynamics in which many beard colours co-occur. This allows altruism to persist even in weakly structured populations and implies that the green beard effect, in the form of a fluid association of altruistic traits with a recognition tag, can be much more prevalent than hitherto assumed.
The last few sentences are the most interesting:
"Our analysis leads to the testable hypothesis that diversity in recognition tags inversely correlates with average relatedness. This suggests that relatively easily observed tag diversity can serve as an indicator for the nature of the underlying social interactions. Third, to detect the green beard effect one should look for cases where tag and trait can dynamically associate8, 16. A tag that functions as a green beard in one instance need not be associated with altruism in another population or at another instance in time, which obviously has consequences for our capacity to detect green beards."
One can imagine that this would have important implications in terms of the evolution of cooperation, phenotype matching, phenotypic diversity, and reproductive bahavior.