1 Department of Biology, University of Toronto Mississauga, Mississauga, ON, Canada.
2 Centre for Urban Environments, University of Toronto Mississauga, Mississauga, ON, Canada.
3 Department of Biology, University of North Carolina, Chapel Hill, NC, USA.
4 Department of Biology, University of Louisiana, Lafayette, LA, USA.
5 Department of Biology, Queen's University, Kingston, ON, Canada.
6 Department of Biology, Concordia University, Montreal, QC, Canada.
7 Department of Biological Sciences, DePaul University, Chicago, IL, USA.
8 Department of Biology, DePauw University, Greencastle, IN, USA.
9 Department of Urban Design and Planning, University of Washington, Seattle, WA, USA.
10 Colegio de Ciencias Biológicas y Ambientales, Universidad San Francisco de Quito USFQ, Quito, Ecuador.
11 De

Urbanization transforms environments in ways that alter biological evolution. We examined whether urban environmental change drives parallel evolution by sampling 110,019 white clover plants from 6169 populations in 160 cities globally. Plants were assayed for a Mendelian antiherbivore defense that also affects tolerance to abiotic stressors. Urban-rural gradients were associated with the evolution of clines in defense in 47% of cities throughout the world. Variation in the strength of clines was explained by environmental changes in drought stress and vegetation cover that varied among cities. Sequencing 2074 genomes from 26 cities revealed that the evolution of urban-rural clines was best explained by adaptive evolution, but the degree of parallel adaptation varied among cities. Our results demonstrate that urbanization leads to adaptation at a global scale.