Positive selection of senescence through increased evolvability: ageing is not a by-product of evolution.

The possibility of ageing being directly selected through evolution has been discussed for the past hundred years. As ageing is occurring, by definition, only late in life - i.e. after the organismal development is finalized -, many think that it cannot be actively selected for as a process. In addition, by decreasing an individual's fitness, it is thought unlikely to be selected for. In order to explain the observation of its broad presence in the realm of life, numerous theories have been proposed in the past 75 years, in agreement with this view. Here, building upon a simple life-history trait model that we recently introduced and that summarizes the life of an organism to its two core abilities - reproduce and thrive -, we discuss the possibility of ageing being selected for through evolution. Our model suggests that senescence can be positively selected through evolution thanks to the higher evolvability it confers to organisms, not through a given mechanism but through a function 'ageing', limiting organismal maintenance and ability to reproduce. It provides an elegant explanation for the apparent tradeoff between longevity and fertility that led to the disposable soma theory without requiring an energy tradeoff while confirming the substrate for mutation accumulation and antagonistic pleiotropy theories. In addition, it predicts that the Lansing effect should be present in organisms showing rapid post-reproductive senescence. This formal and numerical modeling of ageing evolution also provides new hints to test the validity of existing theories. ### Competing Interest Statement The authors have declared no competing interest.