MUTATION AND SELECTION WITHIN THE INDIVIDUAL

Selection within the individual may have played a critical and creativ e role in evolution, boosting the survival chances of mutations benefi cial to the cell and the individual, hindering the spread of deleterio us mutations, and reducing the genetic load imposed on the population. We review the literature and present new results to describe the effe cts of cell-lineage selection on the rate and fixation probability of new mutations. Cell-lineage selection can alter these quantities by se veral orders of magnitude. Cell-lineage selection is especially import ant in the case of rare recessive mutations, which are hidden from sel ection at the individual level but may be exposed to selection at the cellular level. Because selection within the individual acts as a siev e eliminating deleterious mutations and increasing the frequency of be neficial ones, mutations observed among progeny will have been pre-sel ected and are more likely to increase cell proliferation than would ra ndomly generated mutations. Although many authors have focused on the potential conflict between selection at the cellular and individual le vels, it must be much more common that the two levels act concordantly . When selection at the cell and individual levels act in a cooperativ e manner, increased rather than decreased opportunity for germline sel ection will be favored by evolution.