  The major weakness of group selection as an explanation of altruism, according to the consensus that emerged in the 1960s, was a problem that Dawkins (1976) called ‘subversion from within’; see also Maynard Smith 1964. Even if altruism is advantageous at the group level, within any group altruists are liable to be exploited by selfish ‘free-riders’ who refrain from behaving altruistically. These free-riders will have an obvious fitness advantage: they benefit from the altruism of others, but do not incur any of the costs. So even if a group is composed exclusively of altruists, all behaving nicely towards each other, it only takes a single selfish mutant to bring an end to this happy idyll. By virtue of its relative fitness advantage within the group, the selfish mutant will out-reproduce the altruists, hence selfishness will eventually swamp altruism. Since the generation time of individual organisms is likely to be much shorter than that of groups, the probability that a selfish mutant will arise and spread is very high, according to this line of argument. ‘Subversion from within’ is generally regarded as a major stumbling block for group-selectionist theories of the evolution of altruism.