Theories for the evolution of aging rest on the assumption that at least so
me deleterious mutations have effects that are limited to certain ages. Man
y mutation accumulation studies have tried to measure the number and magnit
ude of deleterious mutations, but few studies have tried to determine the e
xtent to which the effects of mutations are limited to particular ages. Her
e we estimate the age-specific effect of deleterious mutations on mortality
rate in an outbred population of the fruit fly, Drosophila melanogaster. W
e used the 'middle class neighborhood' approach to accumulation mutations i
n populations of flies that had recently been obtained from the wild. There
are mutations that increase mortality rates, but whose effects are limited
to specific ages. The age-specificity of mutational effects differs betwee
n the sexes, between virgin and mated flies, and over time. After 10 and 20
generations of mutation accumulation, there were clear age-specific effect
s of mutations. After 30 generations, however, the degree of age-specificit
y decreased. In addition, mutation accumulation led to a steady increase in
larval mortality and a small but significant increase in the sex ratio of
eclosing flies. We discuss the implications of these results for models of
aging, and suggest approaches that future studies should take to obtain acc
urate information on the age-specificity of novel mutations.