 To see why the traditional game theoretic solution concept of a Nash equilibrium is too weak to capture the notion of evolutionary stability, consider the game of figure 1. There are two Nash equilibria in pure strategies: \( (S_1, S_1) \) and \( (S_2, S_2) \). Since a Nash equilibrium is a set of mutual best responses, no player can improve their payoff by adopting a different strategy, but a Nash equilibrium allows for the possibility that a player who deviates from their equilibrium strategy receives the same payoff. This is the case for the \((S_2, S_2)\) equilibrium. And that is why a Nash equilibrium does not suffice for evolutionary stability: it allows for the possibility of drift away from the equilibrium, eventually leading to the replacement of the incumbent strategy.