evolutionary stable strategy

In sociobiology, an assemblage of behavioural or physical characters (collectively termed a ‘strategy’) of a population that is resistant to replacement by any forms bearing new traits, because the new traits will not be capable of successful reproduction.

ESS analysis is based on game theory and can be applied both to genetically determined physical characters (such as horn length), and to learned behavioural responses (for example, whether to fight or retreat from an opponent). An ESS may be conditional on the context, as in the rule ‘fight if the opponent is smaller, but retreat if the opponent is larger’.