Dissertation Research: Molecular mechanisms underlying rapid adaptive divergence in the Swamp Sparrow

由于自然选择推动了对当地环境的适应，生活在不同地理区域的单一物种的种群往往会出现分化。在这个环境迅速变化的当代，理解分歧和局部适应的过程特别重要。北美沿海盐沼为研究快速适应分化提供了极好的系统，因为这些栖息地是在过去15,000年内形成的，对其居民来说是新的选择性环境。在许多独立的鸟类谱系中，有一种强烈的模式，即在殖民沿海盐沼后进化出更大的喙和更深的羽毛，这表明这些特征提供了选择性优势。该项目将有助于确定这些性状差异的遗传基础，并为了解局部适应的进化机制提供了一个窗口。该项目还将为研究生和本科生提供实践培训，并将为保护沿海鸟类种群提供有用的信息。沼泽麻雀有两个在美国东北部适应当地的亚种：一种是棕色的内陆麻雀，有一张小嘴，另一种是深灰色的海岸麻雀，有一张大嘴。初步DNA序列(DdRAD)数据显示，这两种形式非常相似，但基因组的几个区域在它们之间存在高度差异。沿海沼泽麻雀的自然历史表明，强烈的定向选择可能影响了这些高度分化地区的序列变化。为了验证这一假设，将通过为每个分化区域生成额外的DNA序列数据来调查全基因组范围内的分歧模式；这些数据将被统计检查以寻找选择性扫描的分子签名。该项目是对这一新兴统计方法的重要检验。

Populations of a single species that live in different geographic regions often diverge as natural selection drives adaptation to the local environment. Understanding the processes of divergence and local adaptation is of particular importance in this contemporary era of rapidly changing environments. The coastal salt marshes of North America provide excellent systems in which to study rapid adaptive divergence, given that these habitats have formed within the last 15,000 years and represent novel selective environments for their inhabitants. Within many independent lineages of birds, there is a strong pattern of evolving larger bills and darker plumage after colonizing coastal salt marshes, suggesting that these traits offer a selective advantage. This project will allow identification of the genetic basis of divergence in these traits, and provide a window into the evolutionary mechanisms underlying local adaptation. This project will also provide hands-on training for graduate and undergraduate students, and will provide useful information for the conservation of coastal bird populations. Two locally adapted subspecies of the Swamp Sparrow exist in the northeastern US: A brown inland form with a small bill, and a dark grey coastal form with a large bill. Preliminary DNA sequence (ddRAD) data reveal that the two forms are very similar, but several regions of the genome are highly differentiated between them. The natural history of coastal Swamp Sparrows suggests that strong directional selection may have influenced sequence variation in these highly differentiated regions. To test this hypothesis, patterns of genome-wide divergence will be surveyed by generating additional DNA sequence data for each differentiated region; these will be statistically examined for molecular signatures of selective sweeps. This project provides an important test of this emerging statistical methodology.