Anthropogenic Landscape Alteration, but Not Urbanization, Influences Non-Adaptive Evolution in Common Milkweed...

Urbanization can alter mating and dispersal, with consequences for non-adaptive evolution in populations. Potential outcomes vary widely due to the heterogeneity of urban landscapes and the diverse life history strategies of taxa. Furthermore, it is unclear how plants, which are significantly understudied in this context, are impacted. To better understand how urbanization influences non-adaptive evolution in a native plant of conservation importance, we analyzed patterns of neutral genetic variation in common milkweed (Asclepias syriaca). From 256 individuals sampled across 122 locations throughout the Greater Toronto Area, Canada, we created two datasets of 2,835 and 972 single nucleotide polymorphisms through genotype-by-sequencing. Genetic diversity and effective population size N were mostly consistent between urban and rural habitats. Genetic differentiation between urban and rural habitats was low, and samples originated from a single genetic population. Demographic analysis indicated that N decreased by > 99% within the past 800 years, with the rate of loss accelerating over time. These findings suggest that this A. syriaca population was little affected by the transition from rural to urban habitat; rather, anthropogenic activity prior to urbanization, such as precontact Indigenous inhabitation and colonial settlement, had observable effects on population demography. This study demonstrates how anthropogenic factors can modify the degree to which urbanization impacts evolution and emphasizes the importance of contextualizing results with demographic, ecological, and cultural histories.