Mortality Evidence and Various Aging Theories

Evidence And Various Aging Hypotheses

“Aging” is defined as the age-related fitness decline, alias increasing mortality, observed for many species in the wild [Finch 1990] [Ricklefs 1998].
In this definition, the early stages of life (development and growth of the individual), which for various reasons usually have a high mortality not necessarily related to the adult mortality, are excluded.

The life table of a species that shows aging phenomenon is roughly described by Weibull’s equation [Ricklefs 1998]:
m_t = m₀ + α ⋅ t^β,
where: t is the time; m_t is the overall mortality rate at time t; m₀ is the mortality at its initial lowest value (at the time defined 0 after the early stages of life); α and β are two constants and α ⋅ t^β is defined intrinsic mortality (m_i) [Fig. 1].

FIGURE 1 – Aging phenomenon.

Aging is not at all a universal pattern of life table for any species.

IMPORTANT NOTE:
For species not showing a progressive age-related increasing mortality in wild conditions the term aging will not be used in this page and in the related pages to avoid misunderstandings.

Many hypotheses have been proposed to explain aging. A classification of them is the following:
1) Wear and Tear Hypotheses and Stochastic Hypothesis
2) Mutation Accumulation Hypothesis
3) Antagonistic Pleiotropy Hypothesis
4) Disposable Soma Hypothesis
5) Historical Hypothesis
6) Adaptive Hypothesis

Only Adaptive Hypothesis make predictions that are consistent with empirical evidence.

Historical Hypothesis does not appear a plausible explanation of aging but it should be carefully considered as an inertial factor that limits the transformations from a life table pattern to another.

Wear and Tear Hypotheses, Stochastic Hypothesis, Mutation Accumulation Hypothesis, Antagonistic Pleiotropy Hypothesis, Disposable Soma Hypothesis do not appear plausible explanations of aging but their arguments are important for topics related to aging.

References:
- Finch, C.E. (1990) Longevity, Senescence, and the Genome. University of Chicago Press, Chicago. [Google Scholar]
- Ricklefs, R.E. (1998) Evolutionary Theories of Aging: Confirmation of a Fundamental Prediction, with Implications for the Genetic Basis and Evolution of Life Span. Am. Nat. 152, 24-44. [PubMed] [Google Scholar]