Data from: An empirical test for a zone of canalization in thermal reaction norms

Theoretical models on the evolution of phenotypic plasticity predict a zone of canalization where reaction norms cross and genetic variation is minimized in the environment a population most frequently encounter. Empirical tests of this prediction are largely missing, in particular for life-history traits. We addressed this prediction by quantifying thermal reaction norms of three life-history traits (somatic growth rate, age and size at maturation) of a Norwegian population of Daphnia magna and testing for the occurrence of an intermediate temperature (Tm) at which genetic variance in the traits is minimized. Size at maturation changed relatively little with temperature compared to the other traits, and there was no genetic variance in the shape of the reaction norm. Consequently, age at maturation and somatic growth rate were strongly negatively correlated. Both traits showed a strong genotype-environment interaction and the estimated Tm was 14°C for both age at maturation and growth rate. This value of Tm corresponds well with mean summer temperatures experienced by the population and suggests that the population has evolved under stabilizing selection in temperatures that fluctuate around this mean temperature. These results suggest local adaptation to temperature in the studied population and allow predicting evolutionary trajectories of thermal reaction norms under changing thermal regimes.关于表型可塑性（phenotypic plasticity）演化的理论模型预测存在一类渠化（canalization）区域：在此区域内反应规范（reaction norms）发生交叉，且种群最常遭遇的环境中遗传变异被最小化。目前该预测的实证检验大多仍付阙如，针对生活史性状（life-history traits）的相关检验尤为匮乏。我们以挪威大型溞（Daphnia magna）种群为研究对象，通过量化其3种生活史性状——体细胞生长率、成熟年龄与成熟体型——的温度反应规范（thermal reaction norms），并检验是否存在可使这些性状的遗传方差（genetic variance）降至最低的中间温度（Tm），以此验证该预测。结果显示，与其余两种性状相比，成熟体型随温度的变化幅度相对较小，且反应规范的形状不存在遗传变异。由此可见，成熟年龄与体细胞生长率呈显著负相关。两种性状均表现出强烈的基因型-环境互作（genotype-environment interaction），且成熟年龄与生长率的估算中间温度Tm均为14℃。该Tm值与该种群所经历的夏季平均温度高度吻合，表明该种群在围绕该平均温度波动的温度条件下经历了稳定选择（stabilizing selection）演化。上述结果表明所研究种群存在温度相关的本地适应（local adaptation），并可用于预测变化热环境格局下温度反应规范的演化轨迹。