The Effect of Biogeographic Origin on the Adaptive Trajectories of Plants in Urban Environments

Abstract

Parallel adaptation is a hallmark of urban evolution, yet whether plant responses to urbanization are globally consistent or influenced by biogeographic origin remains unclear. We investigated pollinator communities, floral display and reproductive allocation in Trifolium repens along urbanization gradients in 11 European urban areas spanning boreal, continental, and Mediterranean biogeographic regions. We found strong differences, with boreal and continental gradients showing no consistent floral trait shifts, whereas Mediterranean gradients showed parallel reductions in flower and inflorescence size with increased urbanization. This pattern persisted under common-garden conditions, indicating a genetic basis. Pollinator assemblage, lacking consistent variation along the gradients, did not explain phenotypic changes. Instead, in the Mediterranean, we found an increased asexual investment, suggesting an adaptive trade-off with floral display. These findings highlight that urban evolutionary trajectories can be modulated by biogeographic origin, with semi-arid Mediterranean environments emerging as hotspots of urban-driven adaptation and particularly vulnerable to future urban expansion.