No detectable effect of urbanization on genetic drift or g

What this paper is about

383 milkweed insects were collected from urban and rural sites in Toronto and five nearby cities, and sequenced to see whether city landscapes had isolated them.

No detectable effect on genetic drift or gene flow in any of the species tested, including monarchs. These insects move more than the hypothesis assumed.

Summarised here from the abstract below. Not the authors’ words.

Abstract

Urbanization is hypothesized to isolate populations and restrict dispersal, leading to reduced genetic diversity and increased genetic differentiation. We tested this hypothesis in specialist herbivorous insects of milkweed, positing that higher dispersal ability would mitigate the negative effects of urbanization on genetic drift and gene flow, and that these effects would vary with city size. In this study, we collected 383 milkweed insects from urban and rural sites in Toronto, Canada, and five surrounding cities. Using ddRADseq, we generated 145,000 SPNs for monarchs, 10,000 SNPs for beetles, 6,000 SNPs for weevils to quantify genetic diversity, demographic history and population genetic structure. Contrary to our hypotheses, our results indicated no effect of urbanization or dispersal ability on diversity or genetic differentiation. Genetic diversity, measured as π, varied between 0.0013 and 0.0044 across species, with no urban vs. rural component, but with monarchs having >2 X higher diversity compared to beetles and weevils. Similarly, genetic differentiation was generally low, FST varying between 0.01 and 0.28, but there are no consistent trends among urban vs. rural samples for any of the three species. However, demographic analyses revealed a consistent decline in effective population size for all three sampled species, beginning around the last glacial maximum and intensifying over the past 1,000 years. Our findings suggest that both urbanization and dispersal ability have not been a major factor in reducing gene flow or increasing genetic drift among milkweed's herbivorous insect populations. Instead, historical events such as climatic change since the last glacial maximum, and large-scale anthropogenic disturbance in general, have had a more pronounced impact on demography. These results highlight the importance of considering the combined effects of natural and anthropogenic long-term historical processes when studying population genetics in the context of urbanization.

Abstract reproduced from the published paper, which is licensed CC BY. Copyright remains with the authors.

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Milkweed USA. (2026). No detectable effect of urbanization on genetic drift or gene flow in specialist herbivorous insects of milkweed (listing). Retrieved DATE, from https://milkweedusa.com/research/no-detectable-effect-of-urbanization-on-genetic-drift-or-2025/

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"No detectable effect of urbanization on genetic drift or gene flow in specialist herbivorous insects of milkweed (listing)." Milkweed USA, 2026, https://milkweedusa.com/research/no-detectable-effect-of-urbanization-on-genetic-drift-or-2025/. Accessed DATE.

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Milkweed USA. "No detectable effect of urbanization on genetic drift or gene flow in specialist herbivorous insects of milkweed (listing)." Last modified 2026-08-01. Accessed DATE. https://milkweedusa.com/research/no-detectable-effect-of-urbanization-on-genetic-drift-or-2025/.

BibTeX

@misc{milkweedusa_researchnodetectableeffectof,
  title  = {No detectable effect of urbanization on genetic drift or gene flow in specialist herbivorous insects of milkweed (listing)},
  author = {{Milkweed USA}},
  year   = {2026},
  url    = {https://milkweedusa.com/research/no-detectable-effect-of-urbanization-on-genetic-drift-or-2025/},
  note   = {Accessed DATE}
}