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 , 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 LD 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.

Differences in population vulnerability to warming are defined by spatial patterns in thermal adaptation. These patterns may be driven by natural selection over spatial environmental gradients, but can also be shaped by gene flow, especially in marine taxa with high dispersal potential. Understanding and predicting organismal responses to warming requires disentangling the opposing effects of selection and gene flow. We begin by documenting genetic divergence of thermal tolerance and developmental phenotypic plasticity. Ten populations of the widespread copepod Acartia tonsa were collected from sites across a large thermal gradient, ranging from the Florida Keys to Northern New Brunswick, Canada (spanning over 20° latitude). Thermal performance curves (TPCs) from common garden experiments revealed local adaptation at the sampling range extremes, with thermal tolerance increasing at low latitudes and decreasing at high latitudes. The opposite pattern was observed in phenotypic plasticity, which was strongest at high latitudes. No relationship was observed between phenotypic plasticity and environmental variables. Instead, the results are consistent with the hypothesis of a trade-off between thermal tolerance and the strength of phenotypic plasticity. Over a large portion of the sampled range, however, we observed a remarkable lack of differentiation of TPCs. To examine whether this lack of divergence is the result of selection for a generalist performance curve or constraint by gene flow, we analyzed cytochrome oxidase I mtDNA sequences, which revealed four distinct genetic clades, abundant genetic diversity, and widely distributed haplotypes. Strong divergence in thermal performance within genetic clades, however, suggests that the pace of thermal adaptation can be relatively rapid. The combined insight from the laboratory physiological experiments and genetic data indicate that gene flow constrains differentiation of TPCs. This balance between gene flow and selection has implications for patterns of vulnerability to warming. Taking both genetic differentiation and phenotypic plasticity into account, our results suggest that local adaptation does not increase vulnerability to warming, and that low-latitude populations in general may be more vulnerable to predicted temperature change over the next century.

Many species face extinction risks owing to climate change, and there is an urgent need to identify which species' populations will be most vulnerable. Plasticity in heat tolerance, which includes acclimation or hardening, occurs when prior exposure to a warmer temperature changes an organism's upper thermal limit. The capacity for thermal acclimation could provide protection against warming, but prior work has found few generalizable patterns to explain variation in this trait. Here, we report the results of, to our knowledge, the first meta-analysis to examine within-species variation in thermal plasticity, using results from 20 studies (19 species) that quantified thermal acclimation capacities across 78 populations. We used meta-regression to evaluate two leading hypotheses. The climate variability hypothesis predicts that populations from more thermally variable habitats will have greater plasticity, while the trade-off hypothesis predicts that populations with the lowest heat tolerance will have the greatest plasticity. Our analysis indicates strong support for the trade-off hypothesis because populations with greater thermal tolerance had reduced plasticity. These results advance our understanding of variation in populations' susceptibility to climate change and imply that populations with the highest thermal tolerance may have limited phenotypic plasticity to adjust to ongoing climate warming.

8Differences in population vulnerability to warming are defined by spatial patterns in thermal 9 adaptation. These patterns may be driven by natural selection over spatial environmental 10 gradients, but can also be shaped by gene flow, especially in marine taxa with high dispersal 11 potential. Understanding and predicting organismal responses to warming requires disentangling 12 the opposing effects of selection and gene flow. We begin by documenting genetic divergence of 13 thermal tolerance and developmental phenotypic plasticity. Ten populations of the widespread 14 copepod Acartia tonsa were collected from sites across a large thermal gradient, ranging from 15 the Florida Keys to Northern New Brunswick, Canada (spanning over 20 degrees latitude). 16Thermal performance curves from common garden experiments revealed local adaptation at the 17 sampling range extremes, with thermal tolerance increasing at low latitudes and decreasing at 18 high latitudes. The opposite pattern was observed in phenotypic plasticity, which was strongest 19 at high latitudes. Over a large portion of the sampled range, however, we observed a remarkable 20 lack of differentiation of thermal performance curves. To examine whether this lack of 21 divergence is the result of selection for a generalist performance curve or constraint by gene 22 flow, we analyzed cytochrome oxidase I mtDNA sequences, which revealed abundant genetic 23 2 diversity and widely-distributed haplotypes. Strong divergence in thermal performance within 24 genetic clades, however, suggests that the pace of thermal adaptation can be relatively rapid. The 25 combined insight from the laboratory physiological experiments and genetic data indicate that 26 gene flow constrains differentiation of thermal performance curves. This balance between gene 27 flow and selection has implications for patterns of vulnerability to warming. Taking both genetic 28 differentiation and phenotypic plasticity into account, our results suggest that local adaptation 29 does not increase vulnerability to warming, and that low latitude populations in general may be 30 more vulnerable to predicted temperature change over the next century. 31