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Thesis Format

Integrated Article

Degree

Master of Science

Program

Applied Mathematics

Supervisor

Bertram, Jason

Abstract

Current research continues to debate the influence of fluctuating selection on genetic diversity within populations. In most previous models of fluctuating selection for studying genetic diversity, the distribution of selection coefficients is assumed to be symmetrical, meaning that the chances of having positive and negative selection coefficients are identical over time. These models predict that selective fluctuations reduce genetic diversity similar to the stochastic influence of genetic drift. Using stochastic simulations and analytical approaches based on diffusion approximations, we analyze the impact of fluctuating selection on genetic diversity when the distribution of selection coefficients over time is not symmetric, but is instead shifted to negative values. This captures the fact that new mutations are more likely to be deleterious. We show that, unlike the symmetric case, selective fluctuations can greatly increase genetic variation when new mutations are deleterious on average. We show that this phenomenon occurs because deleterious mutations that would be kept at low frequency in constant environment are able to transiently attain high frequencies in a changing environment. Our findings suggest that fluctuating selection could be an important force for generating genetic diversity even if it does not lead to long-term coexistence of alternate alleles.

Summary for Lay Audience

Current research continues to explore how changing environmental conditions affect genetic diversity within populations. Genetic diversity refers to the variety of different genes within a species. One key question in evolutionary biology is how the changes in the environment impact this diversity. In most previous models that studied genetic diversity, the environmental changes are symmetrical, meaning that positive and negative impacts on genes occurred equally over time. Under this assumption, the models predicted that environmental fluctuations reduce genetic diversity, which is the same behavior observed with the genetic drift (randomness of changes in allele frequencies in a finite population due to the unpredictable nature of reproduction and survival). However, our research takes a different approach where we study what happens when the average of environmental changes is not symmetrical but instead tends to be negative. This means that new mutations are more likely to be harmful (deleterious) rather than beneficial. Our findings show that when environmental changes predominantly cause harm, the results are quite different from the symmetric case. Specifically, environmental fluctuations can actually increase genetic diversity when new mutations are deleterious on average. In essence, our study finds when new mutations are on average harmful, environmental changes, is a powerful force in creating genetic diversity. Our research highlights the importance of considering asymmetrical environmental impacts in understanding how genetic diversity is maintained and generated in natural populations.

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