Figure 1.

Natural selection favours selfish individuals who do not cooperate. Consider a population of wild-type cooperators (‘W’) in which an uncooperative, selfish cheater (‘c’) arises through mutation or migration. In a mixed population, the selfish cheater benefits from the cooperative behaviour of the cooperators, without paying the cost. Consequently, the selfish cheater has a higher fitness than the cooperators and spreads through the population, despite the fact that this leads to a decline in mean fitness (redrawn from Nowak 2006; West et al. 2007b). This figure illustrates the problem of cooperation—our main aim in this paper is to discuss how this problem can be exploited for medical intervention strategies.