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Weak evolutionarily stable strategy
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<p>A weak evolutionarily stable strategy (WESS) is a more broad form of <a href="/facts/Evolutionarily_stable_strategy/6LTjs9QB">evolutionarily stable strategy</a> (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. </p><p>The definition of WESS is similar to ESS. Any strategy <i>s</i> is a weakly evolutionarily stable strategy (WESS) if for any strategy s*≠s: </p><p>(i) u(s, s) > u(s*, s) or </p><p>(ii) u(s, s) = u(s*, s) and u(s, s*) ≥ u(s*, s*). </p><p>One example of WESS, in a <a href="/facts/Prisoner%27s_dilemma/44kueSBQ">prisoner's dilemma</a>, is <a href="/facts/Tit-for-tat/YQ2HshrX">Tit-for-tat</a> (a strategy that cooperates in the first interaction and then reciprocates the other player's action from the previous turn in all other iterations). </p>