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Why do we age?

Daryl Shanley discusses the mechanisms of ageing

Ageing can be described as a progressive loss of function accompanied by decreasing fertility and increasing mortality. Such a trait that impairs survival and fertility is clearly bad for the individual, so why has it evolved? In this video, Dr Daryl Shanley from Newcastle University asks: why do we age and can we understand this process from an evolutionary point of view?

Daryl discusses the following major evolutionary theories of ageing:

Programmed death August Wisemann (1899)	This theory suggests that animals are programmed to have a limited life-span so that the older generation can make room for the next generation.
Mutation accumulation Peter Medawar (1952)	Harmful mutations that are expressed later in life (such as Huntington’s disease) are not selected against and are passed on. Over successive generations, these late-acting harmful mutations will accumulate, leading to an increase in mortality rates later in life.
Antagonistic pleiotropy George Williams (1957)	This theory points to the existence of pleiotropic genes – genes that demonstrate favorable effects on fitness at a young age and harmful effects at old age – as an explanation for the ageing process.
Disposable soma theory Tom Kirkwood (1977)	This theory states that every living organism must budget its energy amongst different physiological processes. In early life, most resources go into growth, but on maturity, there is a trade-off between self-maintenance and reproduction. Ageing is a result of this trade-off.

These theories are not mutually exclusive and can provide complementary explanations for why ageing occurs. Do you think these theories are a helpful way to explain the ageing process?