Strength and tempo of directional selection in the wild

Directional selection is a major force driving adaptation and evolutionary change. However, the distribution, strength, and tempo of phenotypic selection acting on quantitative traits in natural populations remain unclear across different study systems. We reviewed the literature (1984–1997) that reported the strength of directional selection as indexed by standardized linear selection gradients (β). We asked how strong are viability and sexual selection, and whether strength of selection is correlated with the time scale over which it was measured. Estimates of the magnitude of directional selection (|β|) were exponentially distributed, with few estimates greater than 0.50 and most estimates less than 0.15. Sexual selection (measured by mating success) appeared stronger than viability selection (measured by survival). Viability selection that was measured over short periods (days) was typically stronger than selection measured over longer periods (months and years), but the strength of sexual selection did not vary with duration of selection episodes; as a result, sexual selection was stronger than viability selection over longer time scales (months and years), but not over short time scales (days).

定向选择（directional selection）是推动生物适应性演化与进化改变的核心驱动力之一。然而，针对不同研究体系，自然种群中作用于数量性状（quantitative traits）的表型选择（phenotypic selection）的分布特征、作用强度与作用节奏，迄今仍未得到明确阐释。

我们对1984年至1997年间的相关文献开展了综述，这些文献以标准化线性选择梯度（β）作为定向选择强度的量化指标，报道了定向选择的作用强度。本综述旨在解答两大科学问题：一是存活选择（viability selection）与性选择（sexual selection）的作用强度分别处于何种水平；二是选择强度是否与其测量所用的时间尺度存在相关性。

研究结果显示，定向选择强度的绝对值（|β|）的估计值呈指数分布：仅有极少数估计值大于0.50，绝大多数估计值则小于0.15。以交配成功率为测量指标的性选择，其作用强度似乎高于以存活率为衡量标准的存活选择。

针对短期（数日）测量的存活选择，其作用强度通常高于长期（数月乃至数年）测量的存活选择；但性选择的强度并未随选择事件的持续时长发生显著变化。由此产生的结果是：在更长的时间尺度（数月与数年）下，性选择的强度显著高于存活选择；而在短期尺度（数日）下则并非如此。