EVOLUTIONARY CONSEQUENCES OF MUTATION AND SELECTION WITHIN AN INDIVIDUAL

Whether in sexual or asexual organisms, selection among cell lineages during development is an effective way of eliminating deleterious muta tions. Using a mathematical analysis, we find that relatively small di fferences in cell replication rates during development can translate i nto large differences in the proportion of mutant cells within the adu lt, especially when development involves a large number of cell divisi ons. Consequently, intraorganismal selection can substantially reduce the deleterious mutation rate observed among offspring as well as the mutation load within a population, because cells rather than individua ls provide the selective ''deaths'' necessary to stem the tide of dele terious mutations. The reduction in mutation rate among offspring is m ore pronounced in organisms with plastic development than in those wit h structured development. It is also more pronounced in asexual organi sms that produce multicellular rather than unicellular offspring. By e ffecting the mutation rate, intraorganismal selection may have broad e volutionary implications; as an example, we consider its influence on the evolution of ploidy levels, finding that cell-lineage selection is more effective in haploids and tends to favor their evolution.