CLUSTERS OF NEW IDENTICAL MUTANTS AND THE FATE OF UNDERDOMINANT MUTATIONS

Given favorable environmental and demographic conditions, premeiotic c lusters of identical mutations can produce a broad distribution of the initial frequency of underdominant alleles. Because of these clusters , new underdominant mutations may not necessarily be as rare in a popu lation as previously assumed. The fixation of underdominant mutations, especially those with low heterozygous fitness, is increased when mut ations appear in a cluster due to a genetic change that occurred befor e germline differentiation. Most restrictions on the fixation of under dominant mutations in a single population, such as strong genetic drif t, weak selection against mutant heterozygotes, isolated population st ructure, inbreeding, meiotic drive, and selection in favor of mutant h omozygotes can be relaxed or even dropped. Instead, the fate of strong underdominant mutations is determined mainly by ecological and geneti c factors that affect the cluster size distribution of new premeiotic mutations. Accumulation of reproductive isolation by the fixation of u nderdominant mutations becomes more feasible with clusters, and mutati on is not always the weakest force during this evolutionary process. T he large mean and variance of reproductive success in many multicellul ar species make it possible that even underdominant mutations with ver y low heterozygous fitness could contribute substantially to reproduct ive isolation.