Urbanization drives parallel shifts in morphology and colouration of Pieris rapae across three cities

Abstract

Urbanization is a major driver of global environmental change, yet the evolutionary consequences for natural populations remain poorly understood. The cabbage white butterfly, Pieris rapae, is one of few butterfly species that thrives in cities, offering a unique opportunity to investigate whether urbanization drives parallel phenotypic evolution. In this study, we quantified phenotypic divergence between urban and non-urban populations across three independent metropolitan regions (Cleveland, Philadelphia, and Pittsburgh, U.S.), measuring body size, flight morphology, and wing pigmentation and reflectance in >500 field-caught individuals from 19 sites. Across regions, we found repeated reductions in overall body size and relative thorax mass in urban butterflies of both sexes, along with reduced relative forewing area in urban males, suggesting sex-specific impacts on mobility. Wing pigmentation showed limited urban-associated variation, but we detected novel sexual dimorphism in near-infrared reflectance within urban populations. Together, these results demonstrate repeatable divergence between urban and non-urban butterfly populations across multiple trait axes and suggest that plastic responses to urban stressors previously documented in experimental work manifest as consistent phenotypic shifts in natural populations.