The Role of Deleterious and Beneficial Mutations in the Evolution of Ploidy Levels

All sexual organisms experience a haploid and a diploid phase during their life cycle, yet the extent of each phase is remarkably variable. We explore the evolution of a locus that alters the timing of meiosis, hence altering the proportion of the life cycle spent in either the haploid or diploid phase. Evolution ofthe life cycle will occur in response to viability selection acting at fitness loci even when selection does not directly act on the locus modifying the life cycle. Both selection against deleterious mutations and for beneficial mutations are considered. It is found that even when diploids mask deleterious alleles and reveal beneficial ones, the diploid phase of the life cycle will not necessarily increase. Conditions under which each phase is expected to increase are developed. These theoretical results may, in part, explain observed life cycle variation.