Bacteria becoming antibiotic-resistant, insects becoming pesticide-resistant, and organisms adapting to hostile environments are all examples of adaptive evolution through natural selection. Each adaptive event leaves its mark in the organism’s DNA; beneficial alleles become fixed and can be detected in the majority of the population. By understanding this process, population geneticists and evolutionary biologists figure out how species changed over time and what events, such as diseases and environmental changes, threatened their survival. 

During adaptation, an advantageous allele increases its frequency in the population due to positive selection. Consequently, nearby linked alleles on the chromosome can “hitchhike” along with it to high frequency, thereby reducing genetic variation in the genomic vicinity of the advantageous allele. This process is called a selective sweep. New research published in GENETICS implemented a new simulation model to understand how population structure affects signatures of selective sweep in population genomic data. 

There are already several methods available to detect selective sweeps; most of these methods are based on the classic selective sweep model, which assumes that populations are homogeneously mixed and individuals mate randomly (so-called “panmixia”). However, many populations in the real world inhabit larger geographic ranges, where individuals typically mate with individuals who live nearby and disperse only locally, rather than randomly across the whole range, thereby violating the assumption of the classic sweep model that populations are homogeneously mixed.

To better understand how such geographical population structure affects sweep signatures and dynamics, Chotai et al. used individual-based simulations of selective sweeps in populations whose individuals inhabit a two-dimensional geographic range with local dispersal and mating. Their results showed the following important differences in selective sweep dynamics compared to the classic sweep model: a) limited dispersal of alleles can slow down the spread of an adaptive allele and thereby lower its fixation probability compared to expectations in a panmictic population of equal size, b) differences in spatial dynamics can alter the signatures left by a selective sweep in surrounding genetic regions, c) low-dispersal causes an enrichment of intermediate frequency single-nucleotide polymorphisms compared to the panmictic scenario, and d) low dispersal can alter the haplotype structure observed around a sweep locus, with characteristic changes in the haplotype frequency spectrum.

Additionally, the researchers also investigated whether the degree of spatial structure required for these effects to emerge could be relevant to real-world populations. By comparing dispersal rates and selection coefficients of known sweeps in nature, such as lactose tolerance in humans, color pigmentation in Monarch butterflies, and pesticide resistance in Anopheles mosquitoes, the researchers conclude that these effects may indeed be observable in real-world selection events.

Overall, this study suggests that current methods for studying selective sweeps should consider the effect of spatial structure on their inferences, especially when the populations involved inhabit larger geographic ranges with limited dispersal.

References

  • Signatures of selective sweeps in continuous-space populations

    Meera Chotai, Xinzhu Wei, Philipp W Messer

    GENETICS. November 2025. 231(3).

    DOI: 10.1093/genetics/iyaf183

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Sejal Davla is a freelance science writer and data scientist with expertise in neuroscience and genetics. She is a motivated storyteller and works on projects at the intersection of science, data, and policy.

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