Urbanization and a green corridor do not impact genetic divergence in common milkweed (Asclepias syriaca)

Journal: Scientific Reports

Sophie Breitbart, Anurag Agrawal, Helene Wagner, Marc Johnson

Abstract

Urbanization is altering landscapes globally at an unprecedented rate. While ecological differences between urban and rural environments often promote phenotypic divergence among populations, it is unclear to what degree these trait differences arise from genetic divergence as opposed to phenotypic plasticity. Furthermore, little is known about how specific landscape elements, such as green corridors, impact genetic divergence in urban environments. We tested the hypotheses that: 1) urbanization, and 2) proximity to an urban green corridor influence genetic divergence in common milkweed (Asclepias syriaca) populations for phenotypic traits. Using seeds from 52 populations along three urban-to-rural subtransects in the Greater Toronto Area, Canada, one of which followed a green corridor, we grew ~1000 plants in a common garden setup and measured >20 ecologically-important traits associated with plant defense/damage, reproduction, and growth over four years. We found significant heritable variation for nine traits within common milkweed populations and weak phenotypic divergence among populations. However, neither urbanization nor an urban green corridor influenced genetic divergence in individual traits or multivariate phenotype. These findings contrast with the expanding literature demonstrating that urbanization promotes rapid evolutionary change and offer preliminary insights into the eco-evolutionary role of green corridors in urban environments.

Using this code

R script key

• functions.R: Contains functions used in analyses. Make sure the librarian package is installed before running this script.
• libraries.R: Contains libraries used in analyses.

Folder key

• data: Contains data collected during 2019, 2020, 2021, and 2022 field seasons.
• Figures_Tables: Contains figures and tables summarizing analyses.
• misc: Contains miscellaneous files associated with project.
• renv: Contains archived versions of packages used in analyses.
• scripts: Contains scripts used to clean data, generate haversine distances, and perform analyses.

Notes

• While analyzing the data, we had two questions guiding our analysis: Q1 and Q2. After drafting the paper, we created 3 questions instead (Q1, Q2, Q3). The Q1 and Q2 in this code refer to what is now Q2 and Q3 in the manuscript, respectively. The Q1 referred to in the manuscript was never given an official name.