  The problem of altruism is intimately connected with questions about the level at which natural selection acts. If selection acts exclusively at the individual level, favouring some individual organisms over others, then it seems that altruism cannot evolve, for behaving altruistically is disadvantageous for the individual organism itself, by definition. However, it is possible that altruism may be advantageous at the group level. A group containing lots of altruists, each ready to subordinate their own selfish interests for the greater good of the group, may well have a survival advantage over a group composed mainly or exclusively of selfish organisms. A process of between-group selection may thus allow the altruistic behaviour to evolve. Within each group, altruists will be at a selective disadvantage relative to their selfish colleagues, but the fitness of the group as a whole will be enhanced by the presence of altruists. Groups composed only or mainly of selfish organisms go extinct, leaving behind groups containing altruists. In the example of the Vervet monkeys, a group containing a high proportion of alarm-calling monkeys will have a survival advantage over a group containing a lower proportion. So conceivably, the alarm-calling behaviour may evolve by between-group selection, even though within each group, selection favours monkeys that do not give alarm calls.