Evolutionary Stable Strategy

An evolutionarily stable strategy (ESS) is a strategy that, when fixed in a population, cannot be invaded by any rare mutant alternative under the dynamics of evolutionary dynamics. Introduced by Maynard Smith and Price in 1973, the ESS concept was intended to solve a puzzle: how can natural selection maintain behaviors like restrained aggression or cooperation when defectors would seem to outcompete them? An ESS must be a Nash equilibrium — no invader does better against the resident strategy than the resident does against itself — but the ESS condition adds a stability requirement. The concept is powerful but limited: many real evolutionary systems never settle on an ESS, instead exhibiting persistent cycles, drift, or repeated transitions between metastable states. The static equilibrium framing may be less useful than dynamic alternatives like adaptive dynamics for understanding long-term evolution.