'Nothing in biology makes sense except in the light of evolution'
- Theodosius Dobzhansky (1900 - 1975)
Ageing is usually thought of as the degradation of an individual's body as the tissues, cells, and DNA have worn down, a process that will continue to do so to the point where the body simply stops functioning. While this is somewhat true, it does not really explain why ageing happens. The breakdown in function of the body is a result of ageing, not the cause. Ageing cannot be fully explained on the level of an individual; but, as with almost all biological processes, it must be seen in an evolutionary context.
Technically speaking, ageing is a consequence of the decline in the force of natural selection acting upon age-specific deleterious alleles in a population in between the age that an organism first reproduces, and the end of its reproductive life. Don't panic, this entry will try to explain.
Alleles are different forms of a gene within a population. So, for example, if there is one gene for eye colour, the different forms of that gene that code for blue eyes or brown eyes are different alleles of that gene. The blue allele is responsible for blue eye colour. Mutating genes often cause diseases, and a mutant gene is known as an allele of the normal gene.
Natural selection is pretty well understood. Survival of the fittest and all that. But when we talk about ageing, it's the force of natural selection that's important. If you possess an allele that will kill you then the force is very strong, but if you have an allele that just gives you a slight fitness disadvantage, then the force of natural selection will act weakly on that allele.
As with all evolutionary questions, you have to approach it from a population point of view. That is, individuals within populations1 possess different alleles upon which natural selection works. Natural selection determines the frequency of alleles in the next generation of the population. And that is, in essence, evolution.
So Then, What Is Ageing?
Imagine there exists an allele that manifests itself only at a specific age. This allele causes the individual that carries it to drop down stone-cold dead right there and then. Natural selection will take care of that allele, right? Anyone that carries that allele will be selected right out of the population, removing the allele from the population, right? Well, not quite.
As it happens, whether this allele is selected out of the population or not depends on the age of the individual. If this deadly gene acts at an age before the organism first reproduces then it will be instantly selected against - the force of natural selection will be at 100% because the deadly allele has sabotaged its own reproduction. Conversely, a lethal allele that acts in late-life, when reproduction has ended, will not be selected out of the population whatsoever, as it has already been passed on - the force of natural selection is at 0%. A gene that kills after reproduction has no bearing upon its own reproduction therefore selection is absent. In between these two extremes is a steady decline in the force of natural selection from 100% to 0%, as the population reproduces. This decline is known as ageing. Simply put, as we reproduce, natural selection cares less and less about us.
Natural selection is the mechanism by which deleterious alleles are removed from the population. If ageing is the decline in the force of natural selection acting upon individuals then more and more degenerative alleles will persist (ie, not be selected against) as individuals get older and pass on more and more of their genes via reproduction (including these 'bad' genes). Ageing results in the accumulation of such age-specific deleterious alleles, up to the point where they kill us.
Ageing is an inescapable degenerative genetic disorder that we all suffer. Diseases such as cancer, heart disease and other illnesses that tend to strike after the age of 50 in humans, are a result of the alleles responsible for them (or responsible for the susceptibility to them) not being selected against because they have already been passed on.
Ageing Is Something That Has Evolved
Organisms that reproduce by binary fission (such as bacteria and other simple animals like amoeba and hydra that reproduce by simply splitting into two, or budding) do not age. If there were no other factors in the environment killing these things they would simply live forever. Animals, plants and other 'higher' multi-cellular lifeforms have evolved from these simple creatures, therefore ageing must be something that has also evolved.
What is it about certain organisms that make them age? The answer lies within their life histories. Organisms that have a juvenile (ie, non-reproductive) stage that is different from the adult (reproductive) stage of life exhibit signs of ageing. Those in which there is no differentiation between any stage of life, as in fissile organisms, do not age. Again, the reason lies with the force of natural selection. If there is no difference between a young and an old organism, and it is equally capable of reproducing throughout all its life history, then the force of natural selection will not be different throughout its lifetime. However, if there is a difference between the reproductive capacity of the the young and the old, such as with organisms that reproduce with eggs or seeds, then the force of natural selection declines, and ageing occurs.
Aging is a consequence of distinct parents and offspring. When the two are indistinguishable, the force of natural selection will not change through the individual's life so ageing will not happen. Experimentation with freshwater asexually-reproducing oligochaetes2 showed that species that reproduced via binary fission did not show the same decline in survival that was observed in oligochaete species that reproduced via asexual egg-laying. The former have identical offspring, whereas the latter have a juvenile stage.
This is Not Just a Theoretical Thing
Experiments done on the geneticist's organism of choice, the fruit fly Drosophila melanogaster, validates this idea. You can extend the life of a population of fruit flies by forcing their first age of reproduction to be later and later (by removing eggs laid before a certain time). When you delay the flies' first age of reproduction, the force of natural selection is higher for a longer period of time, and any age-specific deleterious alleles are now being selected against with greater force. The fruit flies live longer and are younger for longer, as the whole ageing process has been shifted in time.
'This is great!' the eugenicists in the audience might proclaim, 'Why don't we do this for humans?' The funny thing is that we already have, albeit indirectly and unconsciously. Animals which have certain characteristics that somewhat protect themselves against natural selection are longer-lived. Such characteristics include shells, wings, venom and intelligence. Natural selection applies less to us than to other animals because we are smart enough to avoid predation, and we are even smart enough to prevent and treat disease with technologies such as sanitation and medicine, things that have only really come about in the last 200 years.
While the fruits of our intelligence play a part in shielding us from the ravages of natural selection, another important reason often overlooked is that, just like the flies in the experiment, humans are reproducing at later and later ages. Pre-Industrial Revolution, it was not unknown for 13-year-olds to be married, bearing children while still in their early teens. Nowadays, people often have their first children after the age of 30. This effect happens on a long timescale over generations, so remember, just because you had your first child late, doesn't mean you will live longer3.
We are getting older because, as a population, the force of natural selection acting upon us is lesser because of our intelligence but also because we are postponing the waning of natural selection that arrives with reproduction.
A gene or allele that kills or even just reduces the Darwinian fitness, of its host before the host has chance to pass it on to the next generation will be, by its very nature, swiftly selected out of the population. An allele that kills or reduces the fitness of an individual after that individual has already passed on its genes will not be selected against whatsoever.
In between these two extremes the force of natural selection will decline, and deleterious alleles will accumulate in the population in older individuals. These alleles are responsible for degenerative disorders such as cancers, strokes, Alzheimer's, and heart diseases and others. The progressive reduction of natural selection acting upon these alleles, otherwise known as ageing, dooms us all to a fate of slow degenerative death.
As the great evolutionary geneticist Dobzhansky noted, problems that seem intractable at the level of the individual organism solve themselves elegantly if you take a step back and look at them from a population, or evolutionary, perspective. The question of ageing, which seems impossibly complex at the level of the individual, is no exception.
1 Populations are also known as 'gene pools'.
2 Oligochaetes are worm-like animals.
3 Guys, you'd be better off getting castrated as castrated males live five to six years longer - drastic but true.