Understanding how populations adapt to heterogeneous thermal regimes is essential for comprehending how latitudinal gradients in species diversification are formed, and how taxa will respond to ongoing climate change. Adaptation can occur by innate genetic factors, by phenotypic plasticity, or by a combination of both mechanisms. Yet, the relative contribution of such mechanisms to large-scale latitudinal gradients of thermal tolerance across conspecific populations remains unclear. We examine thermal performance in 11 populations of the intertidal copepod Tigriopus californicus, ranging from Baja California Sur (Mexico) to British Columbia (Canada). Common garden experiments show that survivorship to acute heat-stress differs between populations (by up to 3.8°C in LD50 values), reflecting a strong genetic thermal adaptation. Using a split-brood experiment with two rearing temperatures, we also show that developmental phenotypic plasticity is beneficial to thermal tolerance (by up to 1.3°C), and that this effect differs across populations. Although genetic divergence in heat tolerance strongly correlates with latitude and temperature, differences in the plastic response do not. In the context of climate warming, our results confirm the general prediction that low-latitude populations are most susceptible to local extinction because genetic adaptation has placed physiological limits closer to current environmental maxima, but our results also contradict the prediction that phenotypic plasticity is constrained at lower latitudes.

Survivorship data to acute heat stress.temp= temperature of acute heat stress. pop= population of origin according to Suppl. Table 1; C demarks split broods reared in Control conditions (20C) and T demarks split broods reared in Treatment conditions (25C). alive/dead= number of individuals that were alive or dead 3 days after heat stress. prop= proportion of survivors.raw_data_all_pops.xlsSummary statistics on heat tollerance.Mean and SE estimated for LD50, LD10, and delta-LD50.Summary_stats.xlsxraw_data_all_popsdata matrix for R analyses. stress_temp= heat-stress temperature; population= population; dev_temp= rearing condition; survived= number of individuals surviving to heat-stress after 3 days; died= number of individuals not surviving to heat-stress after 3 days; prop= proportion of survivorship.glm_data_all_temps.txt