Quantifying the Impact of Population Size on Natural Selection Across the Genome

In a world of rapid anthropogenic change understanding factors influencing the pace with which a population can respond to natural selection has become a necessity for conservation. Often when considering risk of extinction we take population size into account. However, understanding evolutionary implications of that population size may also be important. Most mechanisms involved in evolution act within a population, such as selection, mutation, migration, and genetic drift. Notably, these processes are not static across species or populations and are often influenced by dynamic aspects such as population size. By comparing among groups that naturally show difference in population size we can better understand the impacts of population size pressures on genetic diversity and the effectiveness of natural selection. In this comparative study of five co-distributed Alaskan mammals—Collared Pikas, Hoary Marmots, Brown Lemmings, Artic Ground Squirrels, and Singing Voles—we tested whether population size influences the ability of a population to respond to selection using genomic data and predictive simulations. Our results show species with larger population sizes display more genomic variation, suggesting that selection may be working more effectively in larger populations. Meanwhile, smaller populations are more likely to lose advantageous loci by chance alone. In conclusion, evolution is not a static process, but rather, as we found, selection response is dynamic in relation to population size.