Anthropogenic factors explain wing colour variation in Colias (Lepidoptera: Pieridae) across the Toronto urbanization gradient

Abstract

Hybridization, or interbreeding between two previously diverged populations, is increasing due to human influences on the environment. Rates of hybridization might be increasing particularly quickly among urban species, especially those that are known to be sensitive to environmental disturbances. Butterflies are one such taxa, as some species are commonly found in urban areas, despite being sensitive indicators. For example, Colias philodice and C. eurytheme (Lepidoptera: Pieridae) hybridize when their ranges are in contact or overlap and are commonly found in large cities throughout North America. However, it is an open question how variation in urbanization might affect variation in hybridization. Using wild Colias across a gradient of urbanization in Toronto, Ontario, we tested how the variation in Colias colour, as an indicator of hybridization, relates to human disturbance and urbanization. We created an objective numeric classification for both C. eurytheme and C. philodice, against which we categorized Colias samples. We found that some of the urbanization metrics, including distance to road, Julian date, and number of pedestrians affect the variation in Colias colour, suggesting that urbanization affects Colias colouration, and potentially rates of hybridization, in Toronto. This means that human disturbance of the environment could be affecting Colias rates of hybridization.