The effects of environmental heterogeneity within a city on the evolution of clines

There is increasing evidence that environmental change associated with urbanization can drive rapid adaptation. However, most studies of urban adaptation have focused on coarse urban vs. rural comparisons or sampled along a single urban-rural environmental gradient, thereby ignoring the role that within-city environmental heterogeneity might play in adaptation to urban environments. In this study, we examined fine-scale variation in the presence of hydrogen cyanide (HCN)-a potent anti-herbivore defence-and its two underlying genes (Ac and Li) between park green spaces and surrounding suburban habitats for five city parks in the Greater Toronto Area. We show that fine-scale urbanization has driven the formation of micro-clines in HCN on a scale of <2 km, though the presence and strength of micro-clines varied across parks. Interestingly, these micro-clines were driven by lower HCN frequencies inside park green spaces, and are therefore in the opposite direction to that predicted based on previously described patterns of HCN frequency change along urban-rural gradients. Synthesis: These results suggest larger scale, adaptive urban-rural clines occur across a complex matrix of environmental heterogeneity within cities that drives fine-scale adaptive microclines of varying strengths and directions.