The widespread observation that heat tolerance is less variable than cold tolerance (‘cold‐tolerance asymmetry’) leads to the prediction that species exposed to temperatures near their thermal maxima should have reduced evolutionary potential for adapting to climate warming. However, the prediction is largely supported by species‐level global studies based on single estimates of both physiological metrics per taxon. We ask whether cold‐tolerance asymmetry holds for Iberian lizards after accounting for intraspecific variation in critical thermal maxima (CTₘₐₓ) and minima (CTₘᵢₙ). To do so, we quantified CTₘₐₓ and CTₘᵢₙ for 58 populations of 15 Iberian lizard species (299 individuals). Then, we randomly selected one population from each study species (population sample = 15 CTₘₐₓ and CTₘᵢₙ values), tested for differences between the variance of both thermal metrics across species, and repeated the test for thousands of population samples as if we had undertaken the same study thousands of times, each time sampling one different population per species (as implemented in global studies). The ratio of variances in CTₘₐₓ to CTₘᵢₙ across species varied up to 16‐fold depending on the populations chosen. Variance ratios show how much CTₘₐₓ departs from the cross‐species mean compared to CTₘᵢₙ, with a unitary ratio indicating equal variance of both thermal limits. Sampling one population per species was six times more likely to result in the observation of greater CTₘₐₓ variance (‘heat‐tolerance asymmetry’) than cold‐tolerance asymmetry. The probability of obtaining the data (given the null hypothesis of equal variance being true) was twice as likely for cases of cold‐tolerance asymmetry than for the opposite scenario. Range‐wide, population‐level studies that quantify heat and cold tolerance of individual species are urgently needed to ascertain the global prevalence of cold‐tolerance asymmetry. While broad latitudinal clines of cold tolerance have been strongly supported, heat tolerance might respond to smaller‐scale climatic and habitat factors hence go unnoticed in global studies. Studies investigating physiological responses to climate change should incorporate the extent to which thermal traits are characteristic of individuals, populations and/or species. A free Plain Language Summary can be found within the Supporting Information of this article.

中文翻译：

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在控制了种内变异后，伊比利亚蜥蜴的耐热性比寒冷的耐受性更具可变性

普遍观察到耐热性不如耐寒性（“耐寒性不对称性”）可变，因此预测暴露于接近其热最大值的温度的物种应该降低适应气候变暖的进化潜力。然而，该预测在很大程度上得到了基于对每个分类单元的两个生理指标的单一估计的物种级全球研究的支持。在考虑了临界热最大值 (CTₘₐₓ) 和最小值 (CTₘᵢₙ) 的种内变化后，我们询问了伊比利亚蜥蜴的耐寒不对称性是否成立。为此，我们量化了 15 种伊比利亚蜥蜴物种（299 只）的 58 个种群的 CTₘₐₓ 和 CTₘᵢₙ。然后，我们从每个研究物种中随机选择一个种群（种群样本 = 15 CTₘₐₓ 和 CTₘᵢₙ 值），测试了物种间两种热指标的方差之间的差异，并对数千个种群样本重复测试，就好像我们进行了数千次相同的研究一样，每次对每个物种采样一个不同的种群（如在全球研究中实施）。根据选择的种群，不同物种的 CTₘₐₓ 与 CTₘᵢₙ 的差异比率高达 16 倍。方差比显示与 CTₘᵢₙ 相比，CTₘₐₓ 偏离跨物种平均值的程度，单一比率表示两个热极限的方差相等。对每个物种采样一个种群，观察到更大的 CTₘₐₓ 方差（“耐热不对称”）的可能性是耐寒不对称的六倍。获得数据的概率（假设方差相等的原假设为真）对于冷耐受不对称的情况是相反情况的两倍。迫切需要量化单个物种的耐热和耐冷性的范围广泛的人口水平研究，以确定全球范围内的耐寒不对称性。虽然广泛的耐寒气候得到了广泛的支持，但耐热性可能会对较小规模的气候和栖息地因素做出反应，因此在全球研究中未被注意到。调查对气候变化的生理反应的研究应该包括热特征在多大程度上是个体、种群和/或物种的特征。可以在本文的支持信息中找到免费的普通语言摘要。