Figure 1.
Without any mechanism for the evolution of cooperation, natural selection favors defectors. In a mixed population defectors, D, have a higher payoff (=fitness) than cooperators, C. Therefore, natural selection continuously reduces the abundance, i, of cooperators until they are extinct. The average fitness of the population also declines under natural selection. The total population size is given by N. There are i cooperators and N − i defectors. The fitness of cooperators and defectors is respectively given by fC = b(i − 1)/(N − 1) − c and fD = bi/(N − 1). The average fitness of the population is given by f̅ = (b − c)i/N.