A major aim of evolutionary biology is to explain the respective roles of adaptive versus nonadaptive changes in the evolution of complexity. While selection is certainly responsible forthe spread and maintenance of complex phenotypes, this does not automatically imply thatstrong selection enhances the chance for the emergence of novel traits, that is, the origination of complexity. Population size is one parameter that alters the relative importance ofadaptive and non-adaptive processes: as population size decreases, selection weakensand genetic drift grows in importance. Because of this relationship, many theories invoke arole for population size in the evolution of complexity. Such theories are difficult to testempirically because of the time required for the evolution of complexity in biological populations. Here, we used digital experimental evolution to test whether large or small asexualpopulations tend to evolve greater complexity. We find that both small and large—but notintermediate-sized—populations are favored to evolve larger genomes, which provides theopportunity for subsequent increases in phenotypic complexity. However, small and largepopulations followed different evolutionary paths towards these novel traits. Small populations evolved larger genomes by fixing slightly deleterious insertions, while large populationsfixed rare beneficial insertions that increased genome size. These results demonstrate thatgenetic drift can lead to the evolution of complexity in small populations and that purifyingselection is not powerful enough to prevent the evolution of complexity in large populations.
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