Data from: Distributional shifts – not geographic isolation – as a probable driver of montane species divergence

As biodiversity hotspots, montane regions have been a focus of research to understand the divergence process. Like their oceanic counterparts, the diversity of the “sky islands” might be ascribed to geographic isolation of mountaintops. However, because the sky islands, and especially those in northern latitudes, are subject to extreme climatic events such as the glacial cycles that drove both altitudinal and geographical shifts in species’ distributions, the dynamic colonization process is also a possible factor driving divergence. Here we test these two hypotheses (i.e., isolation versus colonization) in a flightless montane grasshopper, Melanoplus oregonensis, which is a member of a diverse group that radiated across the Rocky Mountains of North America. Using approximate Bayesian computation (ABC) and spatially explicit simulations that account for spatial heterogeneity and temporal shifts in species distributions, we show that a colonization model of the sky islands from refugial populations provides a significantly better fit to the empirical genetic data than a model of the geographic isolation among sky islands. Moreover, support for the colonization model holds irrespective of whether the movement of individuals was modeled as a diffusion process or was informed by differences in habitat suitabilities across the landscape. With validation analyses to confirm the models provide a good fit to the data, as well as general power and quality analyses, the research not only adds to a growing body of work on the complex dynamics underlying montane biodiversity, but it also provides much needed evaluation of competing hypotheses based on explicit models of the divergence process, as opposed to inferences about diversification drivers from species diversity patterns.

作为生物多样性热点区域，山地生态系统一直是探究物种分化过程的核心研究对象。与海洋中的对应生境类似，这类“天空岛 (sky islands)”的生物多样性或许可归因于山顶的地理隔离效应。然而，这些天空岛——尤其是高纬度地区的天空岛——会遭受冰期循环等极端气候事件的影响，这类事件会驱动物种分布的海拔范围与地理边界发生变化，因此动态定殖过程也可能是推动物种分化的潜在驱动因素。本研究以无飞行能力的山地蚱蜢物种俄勒冈黑蝗 (Melanoplus oregonensis) 为研究材料，该类群隶属于在北美落基山脉地区经历辐射演化的多样类群，以此检验两种相互竞争的假说：即地理隔离假说与定殖假说。本研究采用近似贝叶斯计算 (ABC) 以及兼顾物种分布空间异质性与时间动态变化的空间显式模拟方法，结果表明：基于避难所种群的天空岛定殖模型，相较于天空岛间地理隔离模型，能够更显著地拟合实证遗传数据。此外，无论将个体运动建模为扩散过程，还是基于景观中不同区域的生境适宜性差异来设置运动模式，对定殖模型的支持强度均保持稳定。通过验证分析确认模型对实证数据具备良好拟合效果，结合模型的通用效力与质量评估分析，本研究不仅为日益增长的山地生物多样性复杂动态相关研究补充了新的实证证据，还基于明确的分化过程模型，对相互竞争的假说开展了亟需的评估——而非仅通过物种多样性格局来推断分化驱动因子。