Data from: Panmixia supports divergence with gene flow in Darwin’s small ground finch, Geospiza fuliginosa, on Santa Cruz, Galápagos Islands

The divergence-with-gene-flow model of speciation has a strong theoretical basis with a growing number of plausible examples in nature, but remains hotly debated. Darwin’s finches of the Galápagos Archipelago have played an important role in our understanding of speciation processes. Recent studies suggest that this group may also provide insights into speciation via divergence with gene flow. On the island of Santa Cruz, recent studies found evidence for adaptive divergence in Darwin’s small ground finch, Geospiza fuliginosa, between ecologically contrasting arid and humid zones. Despite the short geographical distance between these zones, strong disruptive selection during low rainfall periods is expected to generate and maintain adaptive divergence. Conversely, during high rainfall periods, when disruptive selection is predicted to be weakened, population divergence in adaptive traits is expected to break down. Because periods of low and high rainfall irregularly alternate, the geographical pattern of adaptive divergence can be assumed to break down and, importantly, regenerate in situ. Here, we use microsatellite allele frequency data to assess the genetic population structure of G. fuliginosa on Santa Cruz. We sample 21 sites and four ecological zones across the island. We reject hypotheses of population substructure linked to ecological and geographical differences among sites in favour of a single panmictic population. Panmixia implies high levels of gene flow within Santa Cruz, which favours selection over genetic drift as a valid process generating phenotypic divergence in G. fuliginosa on Santa Cruz. We discuss how our findings may support classic adaptation, phenotypic plasticity, matching habitat choice or any combination of these three processes.物种形成的基因流动分歧模型（divergence-with-gene-flow model of speciation）具备坚实的理论基础，且在自然界中涌现出日益增多的可信实证案例，但该模型至今仍饱受学界热议。加拉帕戈斯群岛（Galápagos Archipelago）的达尔文雀，在我们对物种形成过程的认知中发挥了关键作用。近期研究表明，该类群或许也能为理解伴随基因流动的物种形成过程提供新视角。在圣克鲁斯岛（Santa Cruz）上，近期研究在达尔文小地雀（Geospiza fuliginosa）中发现了适应性分歧的证据，该分歧存在于生态特征迥异的干旱与湿润生境带之间。尽管这些生境带的地理距离极近，但在低降水时期，强烈的歧化选择（disruptive selection）被认为能够催生并维持适应性分歧。反之，在高降水时期，歧化选择预计会减弱，此时适应性性状的种群分歧有望消解。由于低降水与高降水时期的交替并无规律，适应性分歧的地理分布模式可能会消解，且重要的是，还能在原地重新形成。本研究利用微卫星等位基因频率（microsatellite allele frequency）数据，对圣克鲁斯岛上达尔文小地雀（Geospiza fuliginosa）的种群遗传结构进行评估。我们在全岛范围内布设了21个采样点，覆盖4个生态带。我们拒绝了‘种群亚结构与各采样点的生态、地理差异相关’这一假说，转而支持该岛小地雀属于单一随机交配种群（panmictic population）的结论。随机交配意味着圣克鲁斯岛上的小地雀种群间存在高水平的基因流动，这表明驱动该种群表型分歧的有效过程是选择而非遗传漂变（genetic drift）。本研究还讨论了我们的发现如何支持经典适应性进化、表型可塑性（phenotypic plasticity）与生境匹配选择这三种过程，或是三者的任意组合。