Genealogies under purifying selection

Selection against deleterious mutations, called purifying selection, plays a central role in evolution and acts in all populations. It is known that the genetic patterns observed in genomic regions undergoing purifying selection differ from those resulting from neutral evolution. However, a comprehensive understanding of the underlying mechanisms shaping those patterns is still lacking. In the present work, we use simulations combined with a genealogical approach to identify the effect of purifying selection on the ancestry and thus on the genetic diversity. Our analysis relies on the postulate that the genealogy belongs to the universality class of Beta-coalescents. Under this assumption, we derive statistics measuring the distortion of the genealogy. This approach allows us to consider a wide range of regimes (i.e. arbitrary selection and mutation strengths) and uncover a rich phase diagram. We find that, for strong selection, the limiting genealogy is given by Kingman’s coalescent on a polynomial timescale. As selection gets weaker, Muller’s ratchet starts operating, setting off the emergence of multiple mergers in the genealogical structures. Our results show that while multiple-merger coalescents are often interpreted as the signature of selective sweeps in rapidly adapting populations, these structures can also appear in the context of Muller’s ratchet.