Data from: Morphological clines and weak drift along an urbanization gradient in the butterfly, Pieris rapae

Urban areas are increasing globally, providing opportunities for biodiversity researchers to study the process in which species become established in novel, highly disturbed habitats. This ecological process can be understood through analyses of morphological and genetic variation, which can shed light on patterns of neutral and adaptive evolution. Previous studies have shown that urban populations often diverge genetically from non-urban source populations. This could occur due to neutral genetic drift, but an alternative is that selection could lead to allele frequency changes in urban populations. The development of genome scan methods provides an opportunity to investigate these outcomes from samples of genetic variation taken along an urbanization gradient. Here we examine morphological variation in wing size and diversity at neutral amplified fragment length polymorphisms in the butterfly Pieris rapae L. (Lepidoptera, Pieridae) sampled from the center to the periphery of Marseille. We utilize established and novel environmental correlation approaches to scan genetic variation for evidence of selection. We find significant morphological differences in urban populations, as well as weak genetic structure and decreased genetic diversity in urban versus non-urban sites. However, environmental correlation tests provide little support for selection in our dataset. Our comparison of different methods and allele frequency clines suggests that loci identified as significant are false positives. Although there is some indication that selection may be acting on wing size in urban butterflies, genetic analyses suggest P. rapae are undergoing neutral drift.全球范围内城市区域持续扩张，为生物多样性研究者提供了研究物种在全新且受高度干扰的生境中定殖过程的契机。这类生态过程可通过形态学与遗传变异分析得以解析，为中性进化与适应性进化的模式提供阐释视角。既往研究表明，城市种群往往在遗传层面与非城市来源种群产生分化。这种分化可能源于中性遗传漂变，但也存在另一种可能：选择作用可导致城市种群的等位基因频率发生改变。基因组扫描（genome scan）方法的发展，为基于城市化梯度下获取的遗传变异样本探究上述分化成因提供了可能。本研究以菜粉蝶（Pieris rapae L.，鳞翅目（Lepidoptera）、粉蝶科（Pieridae））为研究对象，采集马赛市从市中心到市郊的种群样本，分析其翅型相关形态变异，以及中性扩增片段长度多态性（amplified fragment length polymorphism, AFLP）位点的遗传多样性。本研究采用成熟与新兴的环境关联分析方法，对遗传变异进行扫描以寻找选择作用的证据。研究结果显示，城市种群存在显著的形态学差异；相较于非城市样点，城市种群的遗传结构较弱，遗传多样性亦有所降低。然而，环境关联分析检验并未为本次数据集的选择作用提供显著支持证据。通过对不同分析方法与等位基因频率梯度的比较，本研究发现被判定为显著性的基因座实为假阳性结果。尽管有迹象显示城市菜粉蝶的翅型可能受到选择作用，但遗传分析结果表明，菜粉蝶种群正经历中性遗传漂变过程。