“演化岛”的隔离与稳定解析西南山地山雀类鸟类的遗传多样性

China's Southwest Mountainous Region is a 'biodiversity hotspot' of global interest for conservation. It contains both centers of endemic avian species and genera to China. The dramatic topography of the Southwest Mountainous Region resulting from the tectonic uplift during the late Pliocene leads to dramatic ecological stratification, which creates physical barriers to migration and isolates organisms into different subregions and mountain systems. Unlike other regions at this latitude, the Southwest Mountainous Region was largely unaffected by the Pleistocene glaciations. Genetically isolated populations could thus be maintained throughout the Pleistocene in these rather stable montane environments. Using a comparative phylogeography based on newly developed genome-wide resequencing method, we aim to investigate how the current genetic diversity of the Southwest Mountainous Region is shaped by the combination of tectonic activities in the Pliocene and stable habitats during the Pleistocene glaciations in two tits, great tit (Parus major) and green-backed tit (Parus monticolus). We compare patterns of genetic differentiation in the Southwest Mountainous Region with populations of the same species occupying an area with a very different geological history: the adjacent Central China Region. This region, more or less at the same latitude as the Southwest Mountainous Region, comprises lowland and a mosaic of low mountains. Fossils and palynological evidences suggest that the vegetation in the Central China Region experienced dramatically climate-induced changes in the Pleistocene. The Central China Region thus offers an excellent area for comparisons of population structure in organisms that occur also in the Southwest Mountainous Region. By comparing genetic structure and historical demography of these populations that have experienced different geological events and climatic changes, we test if long-term persistence and stability of "evolutionary islands" explain high genetic diversity in the Southwest Mountainous Region.

高山地区具有较高的物种多样性。以鸟类为例，西南山地是唯一包含中国鸟类特有属、种分布中心的地区。该地区山脉南北纵横、海拔高差大。通过对部分鸟类的谱系地理研究发现：地形的复杂性与环境的异质性阻碍了鸟类种群的扩散，形成"演化岛"状的隔离效应，从而导致遗传分化。在第四纪冰期，这些"演化岛"环境相对稳定，使得鸟类在冰期时得以生存并保持稳定种群，进一步积累了遗传分化。为了探讨西南山地"演化岛"对鸟类遗传多样性的影响，我们应用基因组重测序技术对两种不同生态习性、海拔分布的近缘山雀（大山雀与绿背山雀）的西南山地与华中地区种群进行比较谱系地理研究。华中地区地势平坦，在第四纪冰期时期曾被冰川所覆盖，能够为西南山地种群提供可对比的例证。通过比较不同地质事件影响下的谱系结构、种群历史，我们拟探讨西南山地"演化岛"的隔离与稳定在当地鸟类遗传多样性格局形成过程中的作用，即进一步验证此前提出的"演化岛"假说。

西南山地山脉、河流南北纵横、高差很大，垂直变化明显。地形的复杂性与异质化的环境阻碍了鸟类种群的扩散，形成“生态岛”的隔离效应，使得这些物种在生态岛长期隔离过程中产生了遗传分化。在第四纪冰期中，西南山地低海拔地区对鸟类提供了相对稳定的生态环境，使得这些物种在冰期时期得以生存并保持稳定，没有经历瓶颈效应以及冰后期种群扩张所带来的遗传混合，进一步积累了遗传分化，促进类物种间的分化。可见，西南山地的“生态隔离岛”对鸟类遗传多样性的形成与积累起到极为重要的作用（cradle and museum effects）。..我们对西南山地鸟类的遗传特有性与另一个鸟类特有化中心，台湾岛做了比较。台湾岛形成岛屿的时间与西南山地隆起的时间大致相似，但台湾岛鸟类的特有化程度比较低，大多是集中在特有种、亚种水平，而西南山地与祁连山地的特有化则主要集中在特有属、特有种水平。蕴含有高分类阶元的特有类群反应该地区长期独立的进化历史。台湾岛是相对隔离的岛屿，而西南山地与外界相互连接，因此前者应该比后者具有更长独立进化历史，更高分类阶元的特有类群，但事实却相反。前期的谱系研究由于方法的限制，只发现台湾岛鸟类的遗传特有性低，但不能解释其原因。我们通过比较研究发现：台湾岛与大陆鸟类在更新世冰期时发生第二次地理接触，产生基因交流，由此减弱了台湾岛鸟类的遗传特有化程度。在更新世冰期，北半球被冰川所覆盖，但西南山地与祁连山地的低海拔地区没有冰川覆盖，提供给当地鸟类比较稳定的环境，保存了当地鸟类的遗传特有性。而台湾岛在冰期时期海平面下降，通过大陆桥与内陆地区连接，两岸鸟类产生基因交流，从而减弱了遗传特有性。因此，同是鸟类的特有化区，但由于青藏高原第四纪相对稳定的冰期保存了当地鸟类的遗传特有性，从而造成了台湾岛与西南山地不同的特有化格局。