Abstract
In the past decade, numerous studies have explored how urbanization affects the mean phenotypes of populations, but it remains unknown how urbanization impacts phenotypic variation, a key target of selection that shapes, and is shaped by, eco-evolutionary processes. Our review suggests that urbanization may often increase intraspecific phenotypic variation through several processes, a conclusion aligned with results from our illustrative analysis on tit morphology across 13 European city/forest population pairs. Urban-driven changes in phenotypic variation will have immense implications for urban populations and communities, particularly through urbanization's effects on individual fitness, species interactions, and conservation. We call here for studies that incorporate phenotypic variation in urban eco-evo research alongside advances in theory.