  A major aim of evolutionary biology is to explain the respective roles of
adaptive versus non-adaptive changes in the evolution of complexity. While
selection is certainly responsible for the spread and maintenance of complex
phenotypes, this does not automatically imply that strong 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 of adaptive and non-adaptive processes: as population size
decreases, selection weakens and genetic drift grows in importance. Because of
this relationship, many theories invoke a role for population size in the
evolution of complexity. Such theories are difficult to test empirically
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 asexual populations tend to evolve greater complexity. We find that
both small and large---but not intermediate-sized---populations are favored to
evolve larger genomes, which provides the opportunity for subsequent increases
in phenotypic complexity. However, small and large populations followed
different evolutionary paths towards these novel traits. Small populations
evolved larger genomes by fixing slightly deleterious insertions, while large
populations fixed rare beneficial insertions that increased genome size. These
results demonstrate that genetic drift can lead to the evolution of complexity
in small populations and that purifying selection is not powerful enough to
prevent the evolution of complexity in large populations.
