Pronounced Fixation, Strong Population Differentiation and Complex Population History in the Canary Islands Blue Tit Subspecies Complex

Evolutionary molecular studies of island radiations may lead to insights in the role of vicariance, founder events, population size and drift in the processes of population differentiation. We evaluate the degree of population genetic differentiation and fixation of the Canary Islands blue tit subspecies complex using microsatellite markers and aim to get insights in the population history using coalescence based methods. The Canary Island populations were strongly genetically differentiated and had reduced diversity with pronounced fixation including many private alleles. In population structure models, the relationship between the central island populations (La Gomera, Tenerife and Gran Canaria) and El Hierro was difficult to disentangle whereas the two European populations showed consistent clustering, the two eastern islands (Fuerteventura and Lanzarote) and Morocco weak clustering, and La Palma a consistent unique lineage. Coalescence based models suggested that the European mainland forms an outgroup to the Afrocanarian population, a split between the western island group (La Palma and El Hierro) and the central island group, and recent splits between the three central islands, and between the two eastern islands and Morocco, respectively. It is clear that strong genetic drift and low level of concurrent gene flow among populations have shaped complex allelic patterns of fixation and skewed frequencies over the archipelago. However, understanding the population history remains challenging; in particular, the pattern of extreme divergence with low genetic diversity and yet unique genetic material in the Canary Island system requires an explanation. A potential scenario is population contractions of a historically large and genetically variable Afrocanarian population, with vicariance and drift following in the wake. The suggestion from sequence-based analyses of a Pleistocene extinction of a substantial part of North Africa and a Pleistocene/Holocene eastward re-colonisation of western North Africa from the Canaries remains possible.

针对岛屿辐射演化类群的进化分子研究，可为解析异域分化、奠基事件、种群规模及遗传漂变在种群分化进程中的作用提供洞见。本研究利用微卫星标记（microsatellite markers），评估了加那利群岛蓝山雀亚种复合群的种群遗传分化与等位基因固定程度，并旨在通过基于溯祖理论的方法（coalescence based methods）揭示其种群历史。

加那利群岛种群呈现出强烈的遗传分化，遗传多样性降低，且存在显著的等位基因固定现象，包含大量私有等位基因（private alleles）。在种群结构模型中，中部岛屿种群（拉戈梅拉岛、特内里费岛与大加那利岛）与耶罗岛之间的演化关系难以厘清；而两个欧洲种群呈现出稳定的聚类结果，两个东部岛屿（富埃特文图拉岛与兰萨罗特岛）种群与摩洛哥种群的聚类效果较弱，拉帕尔马岛种群则形成了独立且稳定的独特演化支系。

基于溯祖理论的模型结果显示，欧洲大陆种群为非洲-加那利种群的外类群（outgroup）；西部岛屿类群（拉帕尔马岛与耶罗岛）与中部岛屿类群之间存在分化事件，且三个中部岛屿之间、两个东部岛屿与摩洛哥种群之间均存在近期的分化事件。显而易见，强烈的遗传漂变与种群间极低的同步基因流，共同塑造了该群岛内复杂的等位基因固定模式与频率偏倚分布。

然而，阐明该类群的种群历史仍具挑战；尤为关键的是，加那利群岛种群系统中存在“遗传多样性低下却拥有独特遗传物质”的极端分化模式，这一现象亟需合理解释。一种潜在的情景为：历史上规模庞大且遗传多样性丰富的非洲-加那利种群发生了种群收缩，随后伴随异域分隔事件与遗传漂变的持续作用。此外，基于序列分析提出的“更新世（Pleistocene）时期北非大部分区域发生种群灭绝，且在更新世/全新世（Holocene）时期从加那利群岛向东重新拓殖西北非”这一假说，仍具备可行性。