Reviewing the State of Evolutionary Theories of Aging
We live in a world in which near all species exhibit degenerative aging, yet some few species exhibit negligible aging until very late life, and a very much smaller number of species appear not to age at all. Aging isn't inevitable, yet it is near universal. Why has evolution produced this outcome? While there is a consensus answer to this question centered around the concept of antagonistic pleiotropy, the evolution of aging is a field of research characterized by continual debate, an ever changing sea of novel ideas that come and go from year to year. In part this is because it is challenging to prove any given theory definitively right or definitively wrong, but also in part because we live in an age of biotechnology, in the midst of a flood of new data on the biochemistry of aging, any piece of which might be argued to change the bigger picture in some way.
Ageing is generally regarded as a non-adaptive by-product of evolution. Based on this premise three classic evolutionary theories of ageing have been proposed. These theories have dominated the literature for several decades. Despite their individual nuances, the common thread which unites them is that they posit that ageing results from a decline in the intensity of natural selection with chronological age. Empirical evidence has been identified which supports each theory. However, a consensus remains to be fully established as to which theory best accounts for the evolution of ageing.
A consequence of this uncertainty are counter arguments which advocate for alternative theoretical frameworks, such as those which propose an adaptive origin for ageing, senescence, or death. Given this backdrop, this review has several aims. Firstly, to briefly discuss the classic evolutionary theories. Secondly, to evaluate how evolutionary forces beyond a monotonic decrease in natural selection can affect the evolution of ageing. Thirdly, to examine alternatives to the classic theories. Finally, to introduce a pluralistic interpretation of the evolution of ageing. The basis of this pluralistic theoretical framework is the recognition that certain evolutionary ideas will be more appropriate depending on the organism, its ecological context, and its life history.