Thompson Ken A, Renaudin Marie, Johnson Marc T J
Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada M5S 3B2
Department of Biology, University of Toronto Mississauga, Mississauga, Ontario, Canada L5L 1C6.
Proc Biol Sci. 2016 Dec 28;283(1845). doi: 10.1098/rspb.2016.2180.
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.
城市生态系统日益成为陆地景观的主要特征。虽然物种能够适应城市环境的证据不断积累，但城市化对种群施加自然选择的机制却鲜为人知。识别沿城市化梯度的适应性表型变化（即渐变群）将有助于我们理解驱动城市适应的选择因素。在此，我们通过在四个城市中沿着城市化梯度对白花三叶草（Trifolium repens L.）种群中一种孟德尔遗传性状——生氰作用的频率进行采样，来测试城市生态系统中的表型渐变群。生氰作用可保护植物免受食草动物侵害，但会降低对低温的耐受性。我们发现，在四个城市中的三个城市，种群中生氰植物的频率朝着城市中心降低。一项田间试验表明，食草作用的空间变化不太可能解释这些渐变群。相反，城市中积雪覆盖减少导致城市地区冬季地面最低温度比非城市地区更低，这可能不利于生氰作用。在没有渐变群的城市中，高积雪覆盖可能保护市中心的植物免受冻害。我们的研究表明，种群正在适应城市化梯度，但区域气候模式可能最终决定是否会发生适应。
