There is a body of theories that collectively provide an explanation behind the causes and mechanisms of aging from an evolutionary perspective. These are called the evolutionary theories and they also form part of a greater body of theories of aging explaining the purpose, reason, and mechanisms behind senescence and the aging process. In general, these evolutionary theories are based on the Darwinian concept of natural selection.
The Evolutionary Theories of Aging
1. The Programmed Theory of Aging
German evolutionary biologist August Weismann proposed the first widely publicized programmed death theory of aging in 1881 based on the mechanism of Darwinian evolution. He argued that aging is an evolutionary phenomenon stemming from the need to replace the old to make room for the newer generation.
Programmed death is a component of natural selection based on the premise of Weismann. The supposed phenomenon allegedly promotes evolution and maintains the survival of an entire species. However, the problem with the theory is that it merely explains the purpose of aging but fails to provide the specific mechanisms that would explain in details the causes of aging.
Weisman later dropped the programmed theory of aging. His 1892 publication included a discussion that suggested a link between lifespan and limitation in cell division. Take note that this same idea received drastic development from other researchers and it is now known as the Hayflick Limit theory of aging.
2. Mutation Accumulation Theory
There are other evolutionary theories of aging. For example, the mutation accumulation theory proposed by Nobel-prize-winning British zoologist Peter Medawar in 1952 claims that aging results from the declining effectiveness of natural selection with age. The theory explains why genetic mutations that are lethal in early life do not persist or are not prevalent within in a population because their carriers usually die even before they reproduce. Examples include rare genetic disorders such as progeria that manifests at an early age.
On the other hand, genetic mutations expressed later in life persist because their carriers are able to produce offspring. Over the evolutionary timeline, these mutations accumulate in the populations of newer generations and are expressed as signs of aging. In other words, the mutation accumulation theory of aging or Medawar’s hypothesis on mammalian aging suggests that aging and its signs simply represent an assortment of genetic diseases.
For individuals with lethal genetic mutations that are expressed earlier in their life, the theory also asserts that they have fewer chances to reproduce because they die at a young age.
Advances in the understanding of how gene expression works have uncovered a fundamental problem with the mutation accumulation theory. Modern genetics explains that gene expression transpires in specific tissues at specific times. A genetic program activates different genes at different times across the entire lifespan of an organism.
3. Antagonist Pleiotropy Hypothesis
Another theory is the antagonistic pleiotropy hypothesis first introduced by American evolutionary biologist George C. Williams in 1957. There are two key assumptions under this theory. First, a particular gene may affect not one trait but several traits of an organism. Take note that this is called pleiotropy. Second, the multiple effects of a single gene or the so-called pleiotropic effects may affect individual fitness and survival in opposite or antagonistic ways.
The theory essentially asserts that natural selection favors the youth over old age whenever a conflict of interest arises. It describes the presence of genetic traits that are beneficial to the fitness of an organism early in life but causes functional decline through aging later in life. Hence, there is a tradeoff between reproductive capacity and longevity.
Note that the antagonistic pleiotropy hypothesis traces its roots from observations of patients with an inherited disease Huntington’s disease. It explains why the disease persists in the population despite being inevitably lethal. Furthermore, these theories also explain why genetic variants such as the gene p53 and BRCA1 that increase fertility early in life also increase cancer risk later in life.
4. Disposable Soma Theory
English biologist Thomas Kirkwood proposed the disposable soma theory of aging in 1977. Similar to other evolutionary theories of aging, the concept generally explains the link between reproduction and longevity.
To be specific, however, the theory claims that aging results from the body compromising the allocation of energy across different functions. Kirkwood noted that the body uses energy for metabolism, reproduction, and repair and maintenance. There is an alleged evolutionary tradeoff between these functions. The body of an organism invests more energy on growth and reproduction and less on repair and maintenance, especially once it arrives at a period ripe for reproduction.
Nature supposedly predisposes an organism to invest more in reproduction because this function is essential to the survival of an entire species. Repair and maintenance functions do not have direct positive evolutionary consequence. Nevertheless, lesser energy allocation repair and maintenance causes gradual deterioration of bodily systems.
There are arguments and studies disproving the disposable soma theory. Based on the theory, a shortage in the food supply would make the compromise or tradeoff more severe. However, several experiments revealed different conclusions that do not coincide with the theory. For examples, studies showed that subjects that underwent calorie-restricted diets lived healthier and longer lives. Other studies also did not find a correlation between dietary restriction and increased lifetime reproductive success.
FURTHER READINGS AND REFERENCES
- Kirkwood, T. B. (1977). “Evolution of Aging.” Nature. 270: 301-304. DOI: 10.1038/270301a0
- Weismann, A. 1889. Essays Upon Heredity and Kindred Biological Problems. Oxford: Clarendon Press
- Weismann, A. 1892. Über Leben und Tod. Jena, Germany: Gustav Fischer Verlag
- Williams, G. C. 1957. Pleiotropy, Natural Selection, and the Evolution of Senescence. Evolution. 11: 398-411. DOI: 10.2307/2406060