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.