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，生物学家一直对性选择（sexual selection）所催生的极端性状着迷不已。然而，目前针对野外环境中装饰结构与武器结构所受选择模式的系统性表征研究仍相对稀缺。本研究针对两个连续繁殖季的野生携武器甲虫种群——蛙腿叶甲（frog legged leaf beetles，学名Sagra femorata）——开展了选择压力的量化分析。本研究同时考量了武器尺寸（后足长度）的变异，以及相对武器尺寸（即偏离种群平均的后足长度-体型缩放关系的偏差值），并为该种群中武器尺寸本身的定向选择（directional selection）以及特定缩放关系的稳定选择（stabilizing selection）提供了实验证据。我们提出，当体型生长受营养等外部环境因素影响时，相较于仅考量性状尺寸本身的变异，考虑种群水平缩放关系的偏差值，能更准确地反映与装饰或武器演化相关的选择模式。这是因为在这类物种中，性状尺寸-体型缩放关系近似于关联性状生长与身体条件（body condition）的潜在发育反应规范（reaction norms）。性选择青睐的夸张装饰与武器结构，其标志性特征之一便是高度依赖身体条件的表达，但这种表型可塑性（phenotypic plasticity）极少体现在我们对野生种群中这些结构所受选择的认知与量化方式中。