The physiological and molecular processes that are collectively known as ageing gradually increase the vulnerability and eventually lead to the death of organisms. This process of degeneration of an individual is seemingly contradictory to Darwin’s theory of Evolution. For how can natural selection favour a process that gradually increases the rate of mortality with age and correspondingly reduces the capacity to reproduce? Why did these so-called mechanisms of senescence evolve? This contradiction has been noticed by a number of scientists over the years and the Evolutionary theory of ageing is an attempt to resolve the question.
In nature, there is an increased reduction in the probability of an organism still being alive at an older age. This is due to predation, disease and accidental death, all of which may be unpredictable and age-invariant. It is believed that strategies, which result in a higher reproductive rate at a young age, but a relatively shorter overall lifespan, result in greater reproductive success and are therefore favoured by natural selection. This postulation resulted in the development of the Evolutionary theory of Ageing.
To help to better understand this theory, think of how in nature all organisms will inevitably die of diseases, accidents, predation, etc. This means that the genes that are beneficial early in life are favoured by natural selection over genes that are beneficial later in life. To visualise this, imagine a species with an average longevity of 2 years. There is not really much evolutionary advantage in possessing beneficial genes at age 10 because only a small percentage of the population will reach such an age. Genes that are beneficial at the age of 1 however, will be selected for by evolution. To take this reasoning to the next step, a gene that results in the death of an organism at the age of 20 will have a negligible impact on an individual who bears it since only a relatively small percentage of the population will reach such an age. The largest contribution to creating a new generation, therefore, comes from young, not old organisms and so the influence of natural selection fades with age, producing an environment that makes it possible for potentially damaging late-acting genes to exist.
The Evolutionary theory of ageing (developed by William Hamilton in association with several other researchers), is the classical theory of why humans live so long after their reproductive age is effectively over. The theory (as was previously discussed) proposes that animals generally die shortly after reproducing due to the fact that lingering for any longer would not lead to any greater numbers of surviving offspring – thought to be the only model for success evolutionarily speaking. Organisms (such as humans) that provide parental care, can, however, escape this unfortunate state of affairs temporarily because in those species natural selection has a reason to favour genes that promote post-reproductive longevity – a phenomenon that is generally known as the Grandmother effect.
The Evolutionary theory of ageing, therefore, defines ageing in terms of natural selection and its relationship with fertility.