Nobody really believes, not even I, that the green-beard effect, in this ultra-simple form, will ever be found in nature. In nature, genes discriminate in favour of copies of themselves by means of less specific but more plausible labels than green beards. Kinship is just such a label. 'Brother' or, in practice, something like 'he who has just hatched in the nest from which I have just fledged', is a statistical label. Any gene that makes individuals behave altruistically towards bearers of such a label has a good statistical chance of aiding copies of itself: for brothers have a good statistical chance of sharing genes. Hamilton's theory of kin selection can be seen as one way in which a green-beard type of effect can be made plausible. Remember, by the way, that there is no suggestion here that genes 'want' to help copies of themselves. It is just that any gene that happens to have the effect of helping copies of itself will tend, willy nilly, to become more numerous in the population.
