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地区，山地区域一直是探究物种分化过程的核心研究对象。与海洋孤岛类似，这类“天空之岛（sky islands）”的物种多样性，或可归因于山顶的地理隔离效应。然而，天空之岛——尤其是高纬度地区的天空之岛——曾经历冰川旋回等极端气候事件，这类事件推动了物种分布的海拔范围与地理格局发生改变，因此动态拓殖过程也可能是驱动物种分化的潜在因素。本研究以一种无翅山地蚱蜢——俄勒冈黑蝗（Melanoplus oregonensis）——为研究对象，该物种是北美落基山脉辐射演化的多样类群成员之一，旨在检验地理隔离与拓殖这两种对立假说。研究采用近似贝叶斯计算（approximate Bayesian computation, ABC）与考虑物种分布空间异质性及时空动态变化的空间显式模拟，结果显示：基于避难所种群拓殖天空之岛的模型，相较于天空之岛间地理隔离的模型，能更显著地拟合实证遗传数据。此外，无论将个体移动建模为扩散过程，还是基于景观中栖息地适宜性差异进行参数设定，对拓殖模型的支持均保持稳定。本研究通过验证分析确认模型对数据具备良好拟合度，并开展了模型通用性与性能评估。本项研究不仅丰富了关于山地生物多样性背后复杂动态的相关研究成果，还基于物种分化过程的显式模型——而非通过物种多样性模式推断物种多样化驱动因素——为两种对立假说提供了亟需的实证评估。