The members of a single population of organisms (animals, human-beings, plants, ) interact with each other pairwise. In each match each organism takes an action from a set B. The organisms do not consciously choose actions; rather, they either inherit modes of behavior from their forebears or assigned them by mutations. We assume that there is a function u that measures each organisms ability to survive: if an organism takes the action a when it faces distribution of actions in the population of its potential opponents, then its ability to survive is measured by expectation of u (a, b) under . This description corresponds to a two-player symmetric strategic game {1, 2}, (B, B), (ui) where u1 (a, b) = u (a, b) and u2 (a, b) = u (b, a). Let G = {1, 2}, (B, B), (ui) be a symmetric strategic game, where u1 (a, b) = u2 (b, a) = u (a, b) for some function u. An evolutionarily stable strategy (ESS) of G is an action b* B for which (b*, b*) is a Nash equilibrium of G and u (b, b) 2. The members of a single population of organisms (animals, human beings, plants, ...) interact with each other pairwise. In each match each organism takes an action from a set B. The organisms do not consciously choose actions, rather, they either inherit modes of behavior from their forebears or assigned them by mutations. We assume that there is a function u that measures each organism's ability to survive: if an organism takes the action a when it faces distribution of actions in the population of its potential opponents, then its ability to survive is measured by expectation of u (a, b) under B. This description corresponds to a two-player symmetric strategic game