Data from: Islands within an island: repeated adaptive divergence in a single population

Physical barriers to gene flow were once viewed as prerequisites for adaptive evolutionary divergence. However, a growing body of theoretical and empirical work suggests that divergence can proceed within a single population. Here we document genetic structure and spatially-replicated patterns of phenotypic divergence within a bird species endemic to 250 km2 Santa Cruz Island, California, USA. Island scrub-jays (Aphelocoma insularis) in three separate stands of pine habitat had longer, shallower bills than jays in oak habitat, a pattern that mirrors adaptive differences between allopatric populations of the species’ mainland congener. Variation in both bill measurements was heritable, and island scrub-jays mated non-randomly with respect to bill morphology. The population was not panmictic; instead, we found a continuous pattern of isolation by distance across the east-west axis of the island, as well as a subtle genetic discontinuity across the boundary between the largest pine stand and adjacent oak habitat. The ecological factors that appear to have facilitated adaptive differentiation at such a fine scale—environmental heterogeneity and localized dispersal—are ubiquitous in nature. These findings support recent arguments that microgeographic patterns of adaptive divergence may be more common than currently appreciated, even in mobile taxonomic groups like birds.

基因流的物理屏障曾被视作适应性进化分化的必要前提。然而，日益增多的理论与实证研究表明，进化分化可在单一种群内部发生。

本研究针对栖息于美国加利福尼亚州250平方千米圣克鲁兹岛的特有鸟类岛丛鸦（Aphelocoma insularis），记录了其种群内的遗传结构与空间重复的表型分化模式。研究发现，栖息于三处独立松树林生境斑块的岛丛鸦，其喙部较栎树生境中的个体更长且更浅，这一模式与该物种大陆同属异域种群间的适应性差异高度吻合。

两项喙部形态指标的变异均具有可遗传性，且岛丛鸦的交配行为与喙部形态存在非随机匹配性。该种群并非泛交（panmictic）种群；相反，我们在岛屿东西轴向上观测到连续的距离隔离模式，同时在最大松树林斑块与邻近栎树生境的交界处，还存在细微的遗传间断性。

看似促成这一精细尺度适应性分化的核心生态因子——环境异质性与局域扩散——在自然界中广泛存在。本研究结果支持近期学术观点：即使在鸟类这类具有较强移动能力的分类类群中，适应性分化的微地理模式或许比当前学界认知的更为常见。