Why do we age?
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?
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