Urbanization rivals regional climate as an evolutionary driver of white clover

Urbanization and climate change are globally progressing, driving evolution in many species. However, their relative importance as drivers of evolution remains poorly understood because they are typically studied separately and lacking quantitative comparison of their effects. Here, we focused on the evolution of hydrogen cyanide (HCN) production and its components (cyanogenic glycosides and the hydrolytic enzyme) in white clover (Trifolium repens L.). To elucidate the relative effects of micro- and macro-scale environmental changes (i.e., urbanization-induced environmental change within a city, and regional environmental change such as climates, respectively) on evolution, we collected 5589 white clover plants from 234 populations in four cities with different climate conditions. We examined the effects of micro-scale environmental factors such as sky openness and impervious surface cover, as well as the effects of macro-scale environmental factors such as longitude and latitude, along with herbivory pressure, on the spatial variation in the frequency of plants producing HCN and its components. We found that both micro- and macro-scale environmental changes affected the frequency of plants producing HCN and cyanogenic glycosides. Specifically, HCN frequency increased with higher temperatures at both micro- and macro-scale, while cyanogenic glycoside frequency decreased with higher impervious surface cover. Furthermore, micro-scale environmental change contributes with an effect size comparable to that of macro-scale environmental change. This study demonstrates that the evolutionary effects of micro-scale environmental change can scale to macro-scale environmental change, highlighting that a city can serve as a proxy for anticipating biological responses to future climate change.