 In the years following the original work of Maynard Smith and Price, alternate analytic solution concepts have been proposed for evolutionary game theory. One such alternative is the idea of an evolutionarily stable set (see Thomas 1984, 1985a,b). Consider, by way of motivation, the game shown in figure 4. In that game, there are no evolutionarily stable strategies, since both \(S_1\) and \(S_2\) receive the same payoff when played against each other. However, any population containing a mix of \(S_1\) and \(S_2\) is, in a sense, stable: although drift will certainly occur regarding the exact proportion of \(S_1\) and \(S_2\), regardless of the specific mix the population will still tend to drive out any \(S_3\) or \(S_4\) mutants due to the differences in expected fitness.