Plasticity cannot fully compensate evolutionary differences in heat tolerance across fish species.

Understanding how evolution and phenotypic plasticity contribute to variation in heat tolerance is crucial to predict responses to warming. Here we analyze 272 thermal death time curves of 53 fish species acclimated to different temperatures and quantify their relative contributions. Analyses show that evolution and plasticity account, respectively, for 80.5 % and 12.4 % of the variation in elevation across curves, whereas their slope remained invariant. Evolutionary and plastic adaptive responses differ in magnitude, with heat tolerance increasing 0.54 ºC between species and 0.32 ºC within species for every 1 ºC increase in environmental temperatures. After successfully predicting critical temperatures under ramping conditions to validate these estimates, we show that fish populations can only partly ameliorate the impact of warming waters via thermal acclimation and this deficit in plasticity could increase as the warming accelerates.