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ABSTRACT Cities impose unique selection pressures on wildlife and generate clines in phenotypic traits along urban–rural gradients. Roads are a widespread feature of human‐dominated landscapes and are known to cause direct wildlife mortality; however, whether they act as a selective force influencing phenotypic trait variation along urban–rural gradients remains unclear. This study tested the hypothesis that roads influence natural selection of coat color in the eastern gray squirrel (Sciurus carolinensis), a species with two distinct coat colors: a gray morph that is common in all areas and a melanic morph more prevalent in urban areas than in rural ones. Vehicular collisions are a significant cause of mortality in eastern gray squirrels, with the melanic morph more visually conspicuous on roads and more easily detected and avoided by drivers than the gray morph. Standardized road cruise surveys along an urbanization gradient in Syracuse, New York, USA, revealed that the prevalence of melanism among living squirrels in Syracuse was negatively related to distance from the city center, whereas there was no urban–rural cline in melanism among road‐killed individuals, with the melanic morph underrepresented among road‐killed squirrels by up to 30% along the urbanization gradient. An examination of the prevalence of each color morph on and off road surfaces in a range‐wide compilation of > 100,000 photographs ofS. carolinensisalso indicated that the melanic morph was underrepresented among road‐killed squirrels imaged. Our study highlights vehicular collisions as an important source of natural selection on phenotypic traits, suggesting a potential role in shaping patterns of urban evolution and contributing to the maintenance of urban–rural clines.
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This content will become publicly available on July 8, 2026
Urban refuge? Squirrel pigmentation clines maintained by strong natural selection beyond city limits
Abstract Urbanization creates heterogeneous selective landscapes that cause the evolution of urban-rural clines in phenotypic traits. Although cities can introduce novel selection pressures, little attention has been paid to the role of selection outside the city in maintaining urban-rural clines. We integrate whole genome sequencing, demographic modeling, and complementary models of selection to test how natural selection in both urban and rural environments shapes the evolution of an urban-rural cline in coat color in eastern gray squirrels (Sciurus carolinensis). Coat color polymorphism in this species, which presents as either gray or melanic, is primarily determined by a 24-bp deletion in the melanocortin-1 receptor gene (Mc1R). Melanic squirrels are more prevalent in urban environments but rare or absent in rural forests. Whole genome sequencing and demographic modeling revealed substantially greater urban-rural divergence at Mc1R than the genomic background, suggesting urban-rural clines in melanism are maintained by selection. We applied three separate approaches leveraging demographic and genomic data to estimate the selection coefficient against Mc1R alleles in each habitat, producing a surprising, yet consistent finding: strong selection against the melanic morph in the rural environment and near neutrality in the city. Our findings demonstrate that selection outside the city can be sufficient to maintain urban-rural clines, and urban environments can maintain genetic diversity that would otherwise be lost in rural landscapes. This study provides a rare opportunity to unravel both the spatial dynamics and the selective pressures shaping trait variation in a widespread vertebrate species, highlighting the complex and sometimes protective role of urban landscapes in evolutionary processes.
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- Award ID(s):
- 2018249
- PAR ID:
- 10654446
- Publisher / Repository:
- Research Square
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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