Data from: Quantification and decomposition of environment-selection relationships

In nature, selection varies across time in most environments, but we lack an understanding of how specific ecological changes drive this variation. Ecological factors can alter phenotypic selection coefficients through changes in trait distributions or individual mean fitness, even when the trait-absolute fitness relationship remains constant. We apply and extend a regression-based approach in a population of Soay sheep (Ovis aries) and suggest metrics of environment-selection relationships that can be compared across studies. We then introduce a novel method which constructs an environmentally-structured fitness function. This allows calculation of full (as in existing approaches) and partial (acting separately through the absolute fitness function slope, mean fitness, and phenotype distribution) sensitivities of selection to an ecological variable. Both approaches show positive overall effects of density on viability selection of lamb mass. However, the second approach demonstrates that this relationship is largely driven by effects of density on mean fitness, rather than on the trait-fitness relationship slope. If such mechanisms of environmental dependence of selection are common, this could have important implications regarding the frequency of fluctuating selection, and how previous selection inferences relate to longer-term evolutionary dynamics.在自然界中，多数环境下的自然选择随时间动态变化，但我们仍未明确特定生态变化如何驱动这一变异。即便性状与绝对适合度（absolute fitness）之间的关联保持恒定，生态因子仍可通过改变性状分布（trait distributions）或个体平均适合度（individual mean fitness），来调控表型选择系数（phenotypic selection coefficients）。我们在索艾羊（Soay sheep, *Ovis aries*）种群中应用并拓展了一种基于回归的分析方法（regression-based approach），并提出了可跨研究比较的环境-选择关系（environment-selection relationships）度量指标。随后我们提出一种全新方法，该方法可构建环境结构化适合度函数（environmentally-structured fitness function），借此可计算自然选择对某一生态变量（ecological variable）的整体敏感性（如现有方法所示）与分维度敏感性——后者分别通过绝对适合度函数斜率、平均适合度及表型分布三条路径发挥作用。两种分析方法均表明，种群密度对羔羊体重的存活选择（viability selection）具有正向整体效应。但第二种方法则揭示，该关联主要由密度对平均适合度的调控所驱动，而非源于性状-适合度关联的斜率变化。倘若这类环境依赖的自然选择机制普遍存在，那么其将对波动选择（fluctuating selection）的发生频率，以及过往自然选择推断与长期进化动态（evolutionary dynamics）之间的关联模式，产生重要启示。