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.