Temporal variation in natural selection is predicted to strongly impact the evolution and demography of natural populations, with consequences for the rate of adaptation, evolution of plasticity, and extinction risk. Most of the theory underlying these predictions assumes a moving optimum phenotype, with predictions expressed in terms of the temporal variance and autocorrelation of this optimum. However, empirical studies seldom estimate patterns of fluctuations of an optimum phenotype, precluding further progress in connecting theory with observations. To bridge this gap, we assess the evidence for temporal variation in selection on breeding date by modeling a fitness function with a fluctuating optimum, across 39 populations of 21 wild animals, one of the largest compilations of long-term datasets with individual measurements of trait and fitness components. We find compelling evidence for fluctuations in the fitness function, causing temporal variation in the magnitude, but not the direction of selection. However, fluctuations of the optimum phenotype need not directly translate into variation in selection gradients, because their impact can be buffered by partial tracking of the optimum by the mean phenotype. Analyzing individuals that reproduce in consecutive years, we find that plastic changes track movements of the optimum phenotype across years, especially in bird species, reducing temporal variation in directional selection. This suggests that phenological plasticity has evolved to cope with fluctuations in the optimum, despite their currently modest contribution to variation in selection.

据预测，自然选择的时间变化会强烈影响自然种群的进化和人口统计，从而对适应率、可塑性进化和灭绝风险产生影响。这些预测背后的大多数理论都假设了一个移动的最佳表型，并以该最佳值的时间方差和自相关来表示预测。然而，实证研究很少估计最佳表型的波动模式，从而排除了将理论与观察联系起来的进一步进展。为了弥合这一差距，我们通过对 21 种野生动物的 39 个种群中具有波动最优值的适应度函数进行建模来评估繁殖日期选择的时间变化的证据，长期数据集的最大汇编之一，具有对特征和适应度组件的单独测量。我们发现了适应度函数波动的有力证据，导致幅度的时间变化，而不是选择的方向。然而，最佳表型的波动不需要直接转化为选择梯度的变化，因为它们的影响可以通过平均表型对最佳值的部分跟踪来缓冲。分析连续几年繁殖的个体，我们发现塑料变化跟踪最佳表型的运动，尤其是在鸟类中，减少了方向选择的时间变化。这表明物候可塑性已经进化以应对最佳波动，