Background
In order for ageing to evolve in response to a declining strength of selection with age, a genetic architecture that allows for mutations with age-specific effects on organismal performance is required. Our understanding of how selective effects of individual mutations are distributed across ages is however poor. Established evolutionary theories assume that mutations causing ageing have negative late-life effects, coupled to either positive or neutral effects early in life. New theory now suggests evolution of ageing may also result from deleterious mutations with increasing negative effects with age, a possibility that has not yet been empirically explored.

Results
To directly test how the effects of deleterious mutations are distributed across ages, we separately measure age-specific effects on fecundity for each of 20 mutations in Drosophila melanogaster. We find that deleterious mutations in general have a negative effect that increases with age, and that the rate of increase depends on how deleterious a mutation is early in life.

Conclusions
Our findings suggest that ageing does not exclusively depend on genetic variants assumed by the established evolutionary theories of ageing. Instead, ageing can result from deleterious mutations with negative effects that amplify with age. If increasing negative effect with age is a general property of deleterious mutations, the proportion of mutations with the capacity to contribute towards ageing may be considerably larger than previously believed.