DELETERIOUS MUTATION AND THE EVOLUTION OF GENETIC LIFE-CYCLES

It is often proposed that the ability of diploids to mask deleterious mutations lends to an evolutionary advantage over haploidy. In this pa per, we studied the evolution of the relative duration of haploid and diploid phases using a model of recurrent deleterious mutations across the entire genome. We found that a completely diploid life cycle is f avored under biologically reasonable conditions, even when prolonging the diploid phase reduces a populations mean fitness. A haploid cycle is favored when there is complete linkage throughout the genome or whe n mutations are either highly deleterious or partially dominant. These results hold when loci interact multiplicatively and for synergistic epistasis. The strength of selection generated on the life cycle can b e substantial because of the cumulative effect of selection against mu tations across many loci, We did nor find conditions that support cycl es that retain both phases, such as those found in some plants and alg ae. Thus, selection against deleterious mutations may be an important force in the evolution of life cycles but may not be sufficient to exp lain all the patterns of life cycles seen in nature.