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 of ...

(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 a game theory thought experiment involving two rational agents, each of whom can either cooperate for mutual benefit or betray their partner ("defect") for individual gain. The dilemma arises from the fact that while def ...

, is

Tit-for-tat Tit for tat is an English saying meaning "equivalent retaliation". It is an alternation (linguistics), alternation of ''wikt:tip#Noun 3, tip for wikt:tap#Verb 2, tap'' "blow for blow", first recorded in 1558. It is also a highly effective strat ...

(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