The evolutionarily stable strategy under individual condition and tactic frequency.

A proof is presented to show that, when fitnesses from alternative tactics within a population depend on both their frequency and the phenotypic condition of individuals, there will be a unique ESS switchpoint s* that determines both the condition at which an individual will switch between tactics and the resulting frequency of the tactics in the population. For an individual at the ESS s*, the fitnesses of the alternative tactics will be equal. When fitness is averaged over the population, however, the average fitnesses of the alternative tactics will not be equal.