Urbanization drives parallel adaptive clines in plant populations

Abstract

Urban areas are a new and increasingly dominant feature of terrestrial landscapes that dramatically alter environments. It is unclear whether wild populations can adapt to the unique challenges presented by urbanization. To address this problem, we sampled the frequency of a Mendelian-inherited trait--cyanogenesis--in white clover (Trifolium repens L.) plants along urbanization gradients in four large cities. Cyanogenesis protects plants from herbivores, but also reduces freezing tolerance. Plants evolved reduced cyanogenesis with increasing proximity to the urban center in three of the four cities. In an experiment, we demonstrate that gradients in herbivore pressure do not cause these clines. Instead, urban areas experience relatively cold minimum winter ground temperatures because of reduced snow cover within cities, which selects against cyanogenesis. Together, our study demonstrates that wild populations exhibit parallel adaptive evolution in response to urbanization, which likely facilitates the persistence of these plants and promotes pollinator abundance and diversity.