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.

物种形成的基因流伴生分化模型（divergence-with-gene-flow model of speciation）拥有坚实的理论基础，且在自然界中已发现越来越多颇具说服力的实例，但该模型至今仍饱受争议。加拉帕戈斯群岛（Galápagos Archipelago）的达尔文雀（Darwin’s finches）在我们认知物种形成过程中扮演了关键角色。近期研究表明，该类群或许也能为理解基因流伴生分化下的物种形成提供全新视角。在圣克鲁斯岛（Santa Cruz）上，近期研究发现达尔文小地雀（Geospiza fuliginosa）在生态特征迥异的干旱与湿润生境间存在适应性分化的证据。尽管这些生境间的地理距离极短，但在低降水时期，强烈的歧化选择被认为能够产生并维持适应性分化。与之相反，在高降水时期，歧化选择被预测会减弱，适应性性状的种群分化也随之瓦解。由于低降水与高降水时期无规律交替，适应性分化的地理格局不仅会随之瓦解，更重要的是，还能在原位重新形成。本研究利用微卫星等位基因频率数据（microsatellite allele frequency data），评估了圣克鲁斯岛达尔文小地雀的种群遗传结构。我们在全岛布设21个采样点，覆盖4个生态区。我们拒绝了‘种群亚结构与采样点间的生态及地理差异相关’这一假说，转而支持圣克鲁斯岛的该物种为单一随机交配种群（panmictic population）。随机交配意味着岛内存在高水平的基因流，这表明相较于遗传漂变，选择是驱动该岛小地雀表型分化的更合理过程。我们还讨论了本研究结果如何支持经典适应性进化、表型可塑性（phenotypic plasticity）、生境匹配选择（matching habitat choice），或是这三种过程的任意组合。