 To see this, suppose that a population of individuals all followed the strategy \(S_2\). If a mutant appeared who played the strategy \(S_1\), the payoff of the \(S_1\)-mutant would be the same as the rest of population, and hence there would be no selection pressure against the mutant. If a second mutant appeared, the payoff earned by an \(S_1-S_1\) interaction would yield a fitness to both greater than the average fitness of the population. This would allow the \(S_1\) mutant to spread and eventually take over the rest of the population.