A full understanding of how cities shape adaptation requires characterizing genetically-based phenotypic and fitness differences between urban and rural populations under field conditions. We used a reciprocal transplant experiment with the native plant common ragweed, (Ambrosia artemisiifolia), and found that urban and rural populations have diverged in flowering time, a trait that strongly affects fitness. Although urban populations flowered earlier than rural populations, plants growing in urban field sites flowered later than plants in rural field sites. This counter-gradient variation is consistent adaptive divergence between urban and rural populations. Also consistent with local adaptation, both urban and rural genotypes experienced stronger net selection in the foreign than in the local habitat, but this pattern was not significant for male fitness. Despite the evidence for local adaptation, rural populations had higher lifetime fitness at all sites, suggesting that selection has been stronger or more uniform in rural than urban populations. We also found that inter-population differences in both flowering time and fitness tended to be greater among urban than rural populations, which is consistent with greater drift or spatial variation in selection within urban environments. In summary, our results are consistent with adaptive divergence of urban and rural populations, but also suggest there may be greater environmental heterogeneity in urban environments which also affects evolution in urban landscapes.
|Original language||English (US)|
|Journal||Proceedings of the Royal Society B: Biological Sciences|
|State||Published - Jun 27 2018|
Bibliographical noteFunding Information:
Data accessibility. The raw data and scripts used to analyse data are archived on Dryad; (http://dx.doi.org/10.5061/dryad.3kv50) . Authors’ contributions. A.J.G., D.A.M. and P.T. conceived and designed the experiment and wrote the manuscript. A.J.G. conducted all seed collections, and conducted the field experiment. A.J.G. conducted all data analyses, with advice and assistance from D.A.M. and P.T. Competing interests. We declare we have no competing interests. Funding. A.J.G. was supported by an NSERC PGS-D. The seed collections and field experiment were supported by the following fellowships and research grants awarded to A.J.G.: Dayton Fellowship and Florence Rothman Fellowship (Bell Natural History Museum), EEB Research Award (University of Minnesota), Rosemary Grant Research Award (Society for the Study of Evolution). Acknowledgements. We would like to thank L. Bolin, E. Holton, L. Sampson, A. Waters for assistance with seed preparation and data collection in the field; C.G. Willis for assistance with data collection and land use data extraction; M. Lotsetter, L. Motle, S. White, T. Warnke, the Living Labs Program at the University of Minnesota and the Minnesota Department of Natural Resources for providing the field sites; M. Johnson and two anonymous reviewers for comments on the manuscript; R. Briscoe-Runquist, L. Burghardt, M. Kulbaba, and R.G. Shaw for assistance with data analysis; and members of the Moeller and Tiffin laboratories for fruitful discussions.
- Counter-gradient variation
- Local adaptation
- Reciprocal transplant
- Spatial heterogeneity
- Urban adaptation