Data from: Urbanization drives the evolution of parallel clines in plant populations

Urban ecosystems are an increasingly dominant feature of terrestrial landscapes. While evidence that species can adapt to urban environments is accumulating, the mechanisms through which urbanization imposes natural selection on populations are poorly understood. The identification of adaptive phenotypic changes (i.e. clines) along urbanization gradients would facilitate our understanding of the selective factors driving adaptation in cities. Here, we test for phenotypic clines in urban ecosystems by sampling the frequency of a Mendelian-inherited trait—cyanogenesis—in white clover (Trifolium repens L.) populations along urbanization gradients in four cities. Cyanogenesis protects plants from herbivores, but reduces tolerance to freezing temperatures. We found that the frequency of cyanogenic plants within populations decreased towards the urban centre in three of four cities. A field experiment indicated that spatial variation in herbivory is unlikely to explain these clines. Rather, colder minimum winter ground temperatures in urban areas compared with non-urban areas, caused by reduced snow cover in cities, may select against cyanogenesis. In the city with no cline, high snow cover might protect plants from freezing damage in the city centre. Our study suggests that populations are adapting to urbanization gradients, but regional climatic patterns may ultimately determine whether adaptation occurs.

城市生态系统已成为陆地景观中愈发占据主导地位的组成部分。尽管已有越来越多证据表明物种能够适应城市环境，但城市化对种群施加自然选择的具体机制仍未得到充分阐释。识别沿城市化梯度分布的适应性表型变化（即渐变群cline），将有助于我们理解驱动城市环境中适应性演化的选择因子。本研究以四座城市沿城市化梯度分布的白车轴草（Trifolium repens L.）种群为研究对象，通过统计其孟德尔遗传性状——生氰作用（cyanogenesis）的发生频率，对城市生态系统中的表型渐变群进行检验。生氰作用可帮助植物抵御植食性动物，但会降低其耐寒性。研究结果显示，四座城市中有三座的种群内，生氰植株的占比随向城市中心靠近而逐渐降低。田间实验表明，植食压力的空间差异无法解释此类渐变群的形成。反之，城市区域因积雪覆盖减少而较非城区更低的冬季最低地表温度，可能对生氰作用产生负选择压力。在未出现渐变群的那座城市中，中心城区较高的积雪覆盖度或许能够保护植物免受冻害。本研究表明，种群正逐步适应城市化梯度，但区域气候格局或最终决定适应性演化能否发生。