Selection in the city: Rapid and fine-scale evolution of urban eastern water dragons

Oceanic archipelagos have long been treated as a Petri dish for studies of evolutionary and ecological processes. Like archipelagos, cities exhibit similar patterns and processes, such as the rapid phenotypic divergence of a species between urban and nonurban environments. However, on a local scale, cities can be highly heterogenous, where geographically close populations can experience dramatically different environmental conditions. Nevertheless, we are yet to understand the evolutionary and ecological implications for populations spread across a heterogenous cityscape. To address this, we compared neutral genetic divergence to quantitative trait divergence within three native riparian and four city park populations of an iconic urban adapter, the eastern water dragon. We demonstrated that selection is likely acting to drive divergence of snout-vent length and jaw width across native riparian populations that are geographically isolated and across city park populations that are geographically close yet isolated by urbanization. City park populations as close as 0.9 km exhibited signs of selection-driven divergence to the same extent as native riparian populations isolated by up to 114.5 km. These findings suggest that local adaptation may be occurring over exceptionally small geographic and temporal scales within a single metropolis, demonstrating that city parks can act as archipelagos for the study of rapid evolution.