Data from: Genetic and phenotypic divergence in an island bird: isolation by distance, by colonisation or by adaptation?

Discerning the relative roles of adaptive and non-adaptive processes in generating differences among populations and species, as well as how these processes interact, are fundamental aims in biology. Both genetic and phenotypic divergence across populations can be the product of limited dispersal and gradual genetic drift across populations (isolation by distance), of colonisation history and founder effects (isolation by colonisation) or of adaptation to different environments preventing migration between populations (isolation by adaptation). Here we attempt to differentiate between these processes using island populations of Berthelot's pipit (Anthus berthelotii), a passerine bird endemic to three Atlantic archipelagos. Using microsatellite markers and approximate Bayesian computation, we reveal that the northwards colonisation of this species ca 8,500 years ago resulted in genetic bottlenecks in the colonised archipelagos. We then show that high levels of genetic structure exist across archipelagos, and that these are consistent with a pattern of isolation by colonisation, but not with isolation by distance or adaptation. Finally, we show that substantial morphological divergence also exists and that this is strongly concordant with patterns of genetic structure and bottleneck history, but not with environmental differences or geographic distance. Overall our data suggest that founder effects are responsible for both genetic and phenotypic changes across archipelagos. Our findings provide a rare example of how founder effects can persist over evolutionary timescales, and suggest that they may play an important role in the early stages of speciation.

辨别适应性与非适应性过程在种群和物种间差异产生中的相对作用，以及这些过程如何相互作用，是生物学的核心研究目标。种群间的遗传分化与表型分化，既可能源于有限扩散与种群间渐进遗传漂变（距离隔离，isolation by distance），也可能由定殖历史与奠基者效应（定殖隔离，isolation by colonisation）导致，或是因适应不同环境阻碍种群间基因交流而产生（适应性隔离，isolation by adaptation）。本研究借助分布于三大大西洋群岛的特有雀形目鸟类——伯特鹨（Anthus berthelotii）的岛屿种群，尝试区分上述三种过程。通过微卫星标记（microsatellite markers）与近似贝叶斯计算（approximate Bayesian computation），我们发现该物种约8500年前的北向定殖事件，引发了被定殖群岛的遗传瓶颈（genetic bottlenecks）。后续分析证实，群岛间存在高度遗传结构（genetic structure），且该结构符合定殖隔离模式，而非距离隔离或适应性隔离。最后我们发现，该类群存在显著的表型分化（phenotypic divergence），且其与遗传结构及瓶颈历史高度契合，但与环境差异或地理距离无关。综合来看，本研究数据表明奠基者效应（founder effects）是群岛间遗传与表型变化的主要成因。我们的发现提供了一个罕见案例，展示了奠基者效应如何在进化时间尺度上持续存在，并提示其可能在物种形成（speciation）的早期阶段发挥重要作用。