Climate structures genetic variation across a species' elevation range: a test of range limits hypotheses

Gene flow may influence the formation of species range limits, yet little is known about the patterns of gene flow with respect to environmental gradients or proximity to range limits. With rapid environmental change it is especially important to understand patterns of gene flow to inform conservation efforts. Here we investigate the species range of the selfing, annual plant, Mimulus laciniatus, in the California Sierra Nevada. We assessed genetic variation, gene flow, and population abundance across the entire elevation-based climate range. Contrary to expectations, within-population plant density increased towards both climate limits. Mean genetic diversity of edge populations was equivalent to central populations, however all edge populations exhibited less genetic diversity than neighboring interior populations. Genetic differentiation was fairly consistent and moderate among all populations and no directional signals of contemporary gene flow were detected between central and peripheral elevations. Elevation-driven gene flow (isolation by environment), but not isolation by distance was found across the species range. These findings were the same towards high- and low-elevation range limits and were inconsistent with two common center-edge hypotheses invoked for the formation of species range limits: 1) decreasing habitat quality and population size; 2) swamping gene flow from large, central populations. This pattern demonstrates that climate, but not center-edge dynamics, is an important range-wide factor structuring M. laciniatus populations. To our knowledge this is the first empirical study to relate environmental patterns of gene flow to range limits hypotheses. Similar investigations across a wide variety of taxa and life histories are needed.

基因流（gene flow）可影响物种种群分布边界的形成，但目前针对环境梯度或距分布边界远近的基因流模式的研究仍较为匮乏。在环境快速变化的当下，明晰基因流模式可为物种保护工作提供关键依据，这一点尤为重要。本研究以分布于加利福尼亚州内华达山脉的一年生自交植物裂叶沟酸浆（Mimulus laciniatus）的分布范围为研究对象，对其整个基于海拔的气候分布范围内的遗传变异、基因流及种群丰度进行了评估。与预期相悖的是，种群内植株密度在两类气候边界处均呈上升趋势；边缘种群的平均遗传多样性与核心种群相当，但所有边缘种群的遗传多样性均低于邻近的内部种群。所有种群间的遗传分化程度整体较为稳定且处于中等水平，未在核心与外围海拔区域间检测到当代基因流的定向信号。本研究在整个物种分布范围内发现了海拔驱动的基因流（即环境隔离，isolation by environment），而非距离隔离（isolation by distance）效应。上述研究结果在高、低海拔分布边界处均保持一致，且与解释物种种群分布边界形成的两类主流中心-边缘假说相悖：其一为生境质量与种群规模递减假说；其二为来自大规模核心种群的基因流淹没假说。该结果表明，气候而非中心-边缘动态，是塑造裂叶沟酸浆种群分布的全域性关键因素。据我们所知，本研究是首个将基因流的环境模式与分布边界假说相关联的实证研究。未来需针对各类群生物与不同生活史类型开展类似研究。