Longer life span evolves under high rates of condition-dependent mortality.

Aging affects nearly all organisms, but how aging evolves is still unclear. The central prediction of classic theory is that high extrinsic mortality leads to accelerated aging and shorter intrinsic life span. However, this prediction considers mortality as a random process, whereas mortality in nature is likely to be condition dependent. Therefore, the novel theory maintains that condition dependence may dramatically alter, and even reverse, the classic pattern. We present experimental evidence for the evolution of longer life span under high condition-dependent mortality. We employed an experimental evolution design, using a nematode, Caenorhabditis remanei, that allowed us to disentangle the effects of mortality rate (high versus low) and mortality source (random versus condition dependent). We observed the evolution of shorter life span under high random mortality, confirming the classic prediction. In contrast, high condition-dependent mortality led to the evolution of longer life span, supporting a key role of condition dependence in the evolution of aging. This life-span extension was not the result of a trade-off with reproduction. By simultaneously corroborating the classic results [8-10] and providing the first experimental evidence for the novel theory, our study resolves apparent contradictions in the study of aging and challenges the traditional paradigm by demonstrating that condition-environment interactions dictate the evolutionary trajectory of aging.