Coevolutionary cycling in allele frequencies and the evolution of virulence

Abstract Coevolutionary cycling in allele frequencies due to negative frequency-dependent selection—sometimes referred to as Red Queen Dynamics—is a key potential outcome of host-parasite coevolution. While many theoretical studies have focused on understanding the consequences of coevolutionary cycling for the evolution of sex and recombination, little is known about the impact of coevolutionary cycling on the evolution of other life history traits. It is therefore currently unknown how coevolutionary cycling in allele frequencies affects the evolution of key disease characteristics, such as virulence. Here, we combine population genetic and quantitative genetic approaches to theoretically determine the impacts of coevolutionary cycling in allele frequencies on the evolution of virulence in a free-living parasite. By varying the level of genetic specificity required for infection while controlling for the average infection rate, we induce coevolutionary cycles and examine their effects on virulence evolution. We show that coevolutionary cycling does indeed have a strong impact on virulence evolution, with more specific infection genetics and higher allelic diversity generally driving larger and more rapid cycles in allele frequencies, leading to selection for higher virulence. Our research provides new fundamental insights into the relationship between coevolutionary cycling and the evolution of virulence. Competing Interest Statement The authors have declared no competing interest. Footnotes Data availability Source code is available in the Supplementary Material and at https://github.com/dxanielkm/Ashby_and_Kim_2025/tree/main. Funding We acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC). Nous remercions le Conseil de recherches en sciences naturelles et en genie du Canada (CRSNG) de son soutien. Conflict of interest The authors declare no conflicts of interest. Minor revisions to the text and equations for clarity