Historical effects during experimental evolution of multicellularity in <i>Saccharomyces cerevisiae</i>

Natural selection is the basis of adaptive evolution, and any adaptations that arise are contingent upon the genetic background of a population. Because the genetic background is a product of prior evolution, adaptive evolution is also contingent upon the evolutionary history of a population. Here, we show the scope for historically contingent outcomes across several selection experiments involving alternating adaptations for size. Previously, replicate laboratory yeast populations rapidly evolved multicellularity via settling selection and then reverted to unicellularity during selection in a spatially structured environment. In this study, we show that genetic recombination via selfing regenerates multicellularity from some secondarily unicellular genotypes, and those same genotypes give rise to populations that rapidly re-evolved multicellularity under settling selection. We also observe that some secondarily derived multicellular phenotypes had different cellular architectures across populations. Because source lineages in our study varied in their degree of common ancestry, we can identify the depth of historical contingency. Our results show that the earliest adaptive changes in a lineage have substantial, persistent evolutionary consequences, demonstrating historically contingent outcomes even when evolutionary history is only measured in hundreds of generations.