Professor of Vertebrate Conservation Biology SUNY College of Environmental Science and Forestry, United States
Background/Question/Methods
Urbanization is the dominant trend of global land use change. The replicated nature of environmental change associated with urbanization should drive parallel evolution, yet insight into the repeatability of evolutionary processes in urban areas has been limited by a lack of multi-city studies. We used community science data on coat color in > 60,000 eastern gray squirrels (Sciurus carolinensis) across 43 North American cities to test for parallel clines in melanism, a genetically based trait associated with thermoregulation and crypsis. We used a linear mixed model to test for a relationship between melanism and impervious cover, and we included interaction effects between impervious cover and city size, forest cover, and winter temperature to test how these city-level characteristics affect cline shape.
Results/Conclusions
We show the prevalence of melanism was positively associated with urbanization as measured by impervious cover. Although clines in melanism tended to take the same form among cities, the strength of urban–rural clines varied. Urban–rural clines in melanism were strongest in the largest cities with extensive forest cover and weakest or absent in cities with warmer winter temperatures, where thermal selection likely limits the prevalence of melanism. Overall our results suggest that novel traits can evolve in a highly repeatable manner among urban areas, modified by factors intrinsic to individual cities, including their size, land cover, and climate.
