DISSERTATION RESEARCH: Investigating Genomic Signatures of Range Shifts and Demographic Change in Migratory Hummingbirds

范围的变化和种群规模的变化被认为是动物进化的共同特征，部分原因是大多数物种比最近的冰河时代更古老，并且在全球气候和土地覆盖的剧烈变化中持续存在。在这种背景下，候鸟提供了特别有趣的案例研究：它们能够跨越广阔的地理区域寻找新的适宜栖息地，但它们适应新地区的潜力可能受到决定迁徙方向和时间的遗传线索的限制。最近对各种气候变化情景下物种分布范围的预测表明，北美的大多数鸟类将需要改变它们的分布范围，以维持它们在过去一个世纪所经历的气候条件。因此，了解范围变化的动态及其对物种进化潜力的长期影响是21世纪生物学家的一项重要任务。该项目的重点是揭示蜜蜂蜂鸟的人口和范围变化的时空动态，蜜蜂蜂鸟是一种年轻的辐射，包括在北美温带繁殖的所有迁徙蜂鸟。蜂鸟是研究范围和人口统计学的好对象，因为它们极端的生理限制、食性饮食和高度的传播能力表明它们对区域气候和物候的变化很敏感。此外，该项目的两个重点物种——Selasphorus rufus和Calypte anna——在过去的100年里经历了巨大的范围变化，为研究当代的殖民和扩张过程提供了一个机会。该项目采用减少代表性DNA测序文库的种群基因组分析，通过(1)确定上个世纪在新范围内建立种群的奠基者效应和基因流动的作用，(2)测试遗传差异和跨迁徙分界线减少的基因流动，以及(3)沿着纬度梯度在多物种比较框架中比较更新世气候变化的人口统计学响应，来研究对环境变化的响应。

Range shifts and changes in population size are thought to be common features of evolution in animals, in part because most species are older than recent ice ages and have persisted through dramatic changes in global climate and land cover. Migratory birds present particularly interesting case studies in this context: they are capable of crossing huge geographic areas in search of newly suitable habitats, but their potential for adaptation to new areas may be limited by genetic cues determining the direction and timing of migration. Recent efforts to forecast species' ranges under an assortment of climate change scenarios suggest that most bird species in North America will need to shift their ranges in order to persist in the climatic conditions they experienced during the past century. Understanding the dynamics of range shifts and their long-term impacts on species' evolutionary potential is thus an important task for biologists in the 21st century. This project is focused on uncovering the temporal and spatial dynamics of shifts in demography and range in the Bee hummingbirds, a young radiation including all migratory hummingbirds breeding in temperate North America. Hummingbirds are good subjects for studies of range and demography because their extreme physiologic constraints, nectivorous diet, and high dispersal abilities suggest that they are sensitive to shifts in regional climate and phenology. In addition, two of the project's focal species - Selasphorus rufus and Calypte anna - have experienced dramatic range shifts over the last 100 years, offering a chance to study the process of colonization and expansion in contemporary time. This project applies population-genomic analyses of reduced-representation DNA sequencing libraries to examine response to environmental change by (1) determining the role of founder effects and gene flow in establishing populations in novel ranges over the last century, (2) testing for genetic divergence and reduced gene flow across migratory divides, and (3) comparing demographic responses to Pleistocene climate change in a multispecies comparative framework along a latitudinal gradient.