Urbanization promotes genetic divergence of functional traits in Asclepias syriaca (common milkweed)

Background/Question/Methods

Urbanization is radically altering global landscapes at an unprecedented rate. The resulting ecological differences between urban and rural environments often promote differences in observable characteristics among populations (i.e., phenotypic divergence). However, it is unclear how often and why these differences are due to organisms’ genetics vs. a mechanism which allows organisms to alter their phenotypes in response to environmental variation (i.e., phenotypic plasticity). We tested the hypothesis that urbanization influences genetic divergence in Asclepias syriaca (common milkweed) populations for ecologically-important traits. We collected seeds from 52 populations along an urban-to-rural transect in the Greater Toronto Area, then grew ~1000 plants in a common garden. We measured 20+ traits related to plant reproduction, defense, and growth over three growing seasons and used (general) linear mixed models to test for the effect of distance from city center as a proxy for urbanization.

Results/Conclusions

Urbanization influenced genetic divergence in multiple traits. For instance, the most urban populations were shorter and produced higher concentrations of secondary metabolites (cardenolides) than the most rural populations. These findings demonstrate that urban environments can promote genetic divergence in phenotypic traits among urban and rural A. syriaca populations. A future study will determine whether the observed genetic divergence is due to local adaptation to urban environments.