A weak evolutionarily stable strategy (WESS) is a more broad form of

evolutionarily stable strategy An evolutionarily stable strategy (ESS) is a strategy (or set of strategies) that is ''impermeable'' when adopted by a population in adaptation to a specific environment, that is to say it cannot be displaced by an alternative strategy (or set o ...

(ESS). Like ESS, a WESS is able to defend against an invading "mutant" strategy. This means the WESS cannot be entirely eliminated from the population. The definition of WESS is similar to ESS. Any strategy ''s'' is a weakly evolutionarily stable strategy (WESS) if for any strategy s*≠s: (i) u(s, s) > u(s*, s) or (ii) u(s, s) = u(s*, s) and u(s, s*) ≥ u(s*, s*). One example of WESS, in a

prisoner's dilemma The Prisoner's Dilemma is an example of a game analyzed in game theory. It is also a thought experiment that challenges two completely rational agents to a dilemma: cooperate with their partner for mutual reward, or betray their partner ("def ...

, is

Tit-for-tat Tit for tat is an English saying meaning "equivalent retaliation". It developed from "tip for tap", first recorded in 1558. It is also a highly effective strategy in game theory. An agent using this strategy will first cooperate, then subseque ...

(a strategy that cooperates in the first interaction and then reciprocates the other player's action from the previous turn in all other iterations).

References

{{reflist Evolutionary game theory