A Multi-City Assessment of Genomic Evolution in the Native Wildflower Impatiens capensis

Urbanization is a major driver of environmental change that shapes the evolution of populations. However, the environmental differences amongst cities and their effects on neutral and adaptive evolution are less well understood. We investigated the contribution of city-level variation to patterns of genetic evolution in Impatiens capensis, a native wildflower found in parks and green spaces in many cities across eastern North America. While the mixed mating system and flexible pollination requirements of I. capensis likely contribute to its resilience to urbanization, microenvironmental differences among cities may shape how this species evolves in cities. We used genotype-by-sequencing to evaluate genetic variation, contemporary demographic history, and genetic signatures of local adaptation in plants sampled from urban and rural sites across 10 cities in Ontario, Canada. Urbanization and city size shaped the amount of genetic diversity present at sites and were associated with fine-scale spatial genetic structure. We identified a signal of repeated population bottlenecks across all cities, corresponding to the timing of rapid urban expansion in the region. City size was the environmental predictor most strongly associated with multilocus selection, highlighting the contribution of city variation to adaptive genomic evolution. Our findings provide one of the first examples of parallel demographic shifts in response to urbanization in plants and offer insights into why a native wildflower like I. capensis may be particularly resilient to urbanization. Taken together, our results emphasize the role that urban parks can play in maintaining genetic diversity and facilitating adaptation, suggesting that prioritizing greenspace conservation is critical for promoting urban biodiversity.