Evolutionarily stable strategy

In game theory, an evolutionarily stable strategy (or ESS; also evolutionary stable strategy) is a strategy which, if adopted by a population, cannot be invaded by any competing alternative strategy. An example of such a strategy would be a sexual display that is a serious handicap, as this will prevent the invasion of the populations by "cheating" organisms in poor physical condition that fake an impressive display. This concept is particularly important in the study of the evolution of altruism.

The stable strategy concept is an equilibrium refinement to a Nash equilibrium, which is a strategy that prevents any organism in a population from gaining an advantage by adopting a new strategy. The difference between a Nash equilibrium and an ESS is that a Nash equilibrium may sometimes exist due to rational foresight preventing players from adopting an alternative strategy with short term advantages, but long-term disadvantages. An ESS is defined to exclude such equilibria, and assumes that natural selection is the only force which selects against strategies with lower payoffs.

The term was introduced and defined by John Maynard Smith and George R. Price in a 1973 Nature paper and is central to Maynard Smith's (1982) book Evolution and the Theory of Games. The concept was derived from R.H. MacArthur and W.D. Hamilton's work on sex ratios, especially Hamilton's (1967) concept of an unbeatable strategy. The idea can be traced back to Ronald Fisher and Charles Darwin (1859).