Data from: Selection on an extreme weapon in the frog legged leaf beetle (Sagra femorata)

Biologists have been fascinated with the extreme products of sexual selection for decades. However, relatively few studies have characterized patterns of selection acting on ornaments and weapons in the wild. Here, we measure selection on a wild population of weapon-bearing beetles (frog legged leaf beetles: Sagra femorata) for two consecutive breeding seasons. We consider variation in both weapon size (hindleg length), and in relative weapon size (deviations from the population average scaling relationship between hindleg length and body size), and provide evidence for directional selection on weapon size per se and stabilizing selection on a particular scaling relationship in this population. We suggest that whenever growth in body size is sensitive to external circumstance such as nutrition, then considering deviations from population-level scaling relationships will better reflect patterns of selection relevant to evolution of the ornament or weapon than will variation in trait size per se. This is because trait-size versus body-size scaling relationships approximate underlying developmental reaction norms relating trait growth with body condition in these species. Heightened condition-sensitive expression is a hallmark of the exaggerated ornaments and weapons favored by sexual selection, yet this plasticity is rarely reflected in the way we think about – and measure – selection acting on these structures in the wild.

数十年来，生物学家始终对性选择所催生的极端性状着迷不已。然而，针对野外环境中性装饰与性武器所受选择模式的系统性表征研究仍相对匮乏。本研究针对两个连续繁殖季的野生携性武器甲虫种群——蛙腿叶甲（Sagra femorata）——开展了选择压力量化分析。我们同时考量了武器大小（后足长度）与相对武器大小（即偏离后足长度-体长种群平均缩放比例关系的偏差）的变异，并为该种群中武器大小本身所受的定向选择，以及特定缩放比例关系所受的稳定选择提供了实证依据。我们提出，当体型生长受营养等外部环境条件影响时，相较于仅考量性状大小本身的变异，考虑种群水平缩放比例关系的偏差将更能反映与性装饰或性武器演化相关的选择模式。这是因为在这类物种中，性状大小与体型的缩放比例关系近似于潜在的发育反应规范，该规范关联了性状生长与个体身体状况。性选择所青睐的夸张型性装饰与性武器，其典型特征便是高度依赖个体身体状况的表达模式，但这类表型可塑性却极少被纳入我们对野外环境中这些结构所受选择的认知与量化框架中。