Department of Environmental Science and Policy, George Mason University, Fairfax, Virginia, 22030.
Current Address: Department of Natural Resources and the Environment, University of New Hampshire, Durham, New Hampshire, 03824.
Evolution. 2022 Jul;76(7):1481-1494. doi: 10.1111/evo.14538. Epub 2022 Jun 26.
Adaptation to local environments is common in widespread species and the basis of ecological speciation. The song sparrow (Melospiza melodia) is a widespread, polytypic passerine that occurs in shrubland habitats throughout North America. We examined the population structure of two parapatric subspecies that inhabit different environments: the Atlantic song sparrow (M. m. atlantica), a coastal specialist, and the eastern song sparrow (M. m. melodia), a shrubland generalist. These populations lacked clear mitochondrial population structure, yet coastal birds formed a distinct nuclear genetic cluster. We found weak overall genomic differentiation between these subspecies, suggesting either recent divergence, extensive gene flow, or a combination thereof. There was a steep genetic cline at the transition to coastal habitats, consistent with isolation by environment, not isolation by distance. A phenotype under divergent selection, bill size, varied with the amount of coastal ancestry in transitional areas, but larger bill size was maintained in coastal habitats regardless of ancestry, further supporting a role for selection in the maintenance of these subspecies. Demographic modeling suggested a divergence history of limited gene flow followed by secondary contact, which has emerged as a common theme in adaptive divergence across taxa.
适应本地环境在广泛分布的物种中很常见，也是生态物种形成的基础。歌雀（Melospiza melodia）是一种分布广泛、多型的雀形目鸟类，分布于北美的灌木栖息地。我们研究了两种栖息在不同环境中的近缘亚种的种群结构：大西洋歌雀（M. m. atlantica），一种沿海特化种，和东部歌雀（M. m. melodia），一种灌木地广布种。这些种群的线粒体种群结构不明显，但沿海鸟类形成了一个独特的核遗传群。我们发现这些亚种之间的整体基因组分化较弱，这表明它们要么是最近才分化的，要么是基因交流广泛，或者两者兼有。在过渡到沿海生境的过程中，存在着陡峭的遗传梯度，这与环境隔离而不是与距离隔离一致。一个表现出分歧选择的表型，喙的大小，随着过渡区沿海祖先数量的变化而变化，但无论祖先如何，在沿海生境中都保持着较大的喙大小，这进一步支持了选择在维持这些亚种中的作用。种群动态模型表明，有限的基因流和随后的二次接触导致了分歧历史，这已成为跨类群适应性分歧的一个共同主题。
