Adaptive senescence

Is aging a disease?

On one hand, the medical profession tends to regard aging and disease separate processes - with aging being normal and natural - and disease representing deviations from the normal state.

However - now that aspects of the aging process are showing signs of being susceptible to anti-aging interventions - some researchers seem to be keen to rebrand aging as a collection of disease states.

However, is aging really best characterised as a disease? This essay examines this premise.

Is senescence a adaptation?

It is not immediately obvious how it can benefit an individual to have their body disintegrate from under them.

Consequently most previous authors who have claimed that senescence represents an adaptation have invoked high-level selection processes to explain the existence of the phenomenon.

It is plausible that a rapid turnover in members of the population could help with cases where organisms are engaged in battles with parasites. Older individuals tend to have more parasites - and at some stage they become a hazzard to other members of the species - as walking reservoirs of infectious agents. It would benefit the species to have such individuals die - taking their load of parasites out of circulation.

Similarly, many species use sexual recombination as a defense against rapidly evolving parasites. To work, this mechanism needs a constant influx of new individuals - and thus the removal of elderly ones.

Supporters of these ideas cite genes which affect lifespan, heritable differences in lifespans among closely related creatures, the influence of hormones on aging - and cases of "catastrophic" aging as evidence that aging is indeed an adaptive process.

However, a significant problem with these explanations is that they offer no individual- level benefit to aging.

Instead they invoke species-level selection (or other high level selection processes). Such selection processes have a poor history of explaining things in biology - and many doubt whether such explantions are viable.

There are other explanations of the aging process that don't require high level selection processes to work:

• Disposable soma
  • This states that reproductive and maintenance processes compete for resources. Reproducing early clearly has many advantages - and is consequently somatic tissue maintenance programs do not receive sufficient investment to support indefinite survival.
• Antagonistic pleiotropy
  • This proposes that genes that delay the expression of other deleterious genes are favoured.
    More generally, it suggests that alleles may be favoured if they have beneficial early effects but deleterious later effects.

Under these theories, aging is not necessarily seen as being adaptive. In particular, it is sometimes seen as being the product of evolutionary indifference - rather than something actively selected for.

If a deleterious gene arises through mutation, and another gene is selected to delay the expression of its effects, then the resulting senescence isn't really an adaptation - since another organisms without the deleterious gene in the first place would be fitter.

Rather senescence there would be seen as a side effect of deleterious mutations while they are in the process of being neutralised and before they are completely disabled or removed from the population.

To the extent that such mechanisms are responsible for aging, it is not adaptive - and really is worthy of being regarded as disease to be cured - at least as much as other diseases caused by inherited, wholly-deleterious mutations.

However, the more general case of the antagonistic pleiotropy theory - and the disposable soma theory - don't rule out the possibility that aging is adaptive.

Antagonistic pleiotropy suggests that failure to turn off developmental processes may be partially responsible for senescence. If building an off switch for such processes is more expensive than allowing them to persist, then the resulting senescence must be seen as being selectively favoured.

Similarly, senescence could be the result of a failure to invest in maintenance and repair equipment, due to the competition for resources within organisms between survival and reproductive systems being won by the reproductive system.

In either of these scenarios, then senescence may well be selectively advantageous. It might not be possible (beyond a certain point) to modify the organism so that they are more long-lived without compromising their fitness.

There is good evidence for the "disposable soma" idea - that organisms face resource tradeoffs between reproductive and maintenance activities.

So - I think that even if we totally reject the idea that senescence benefits the species - we must still accept the notion that elements of senescence are the direct consequence of adaptations that influence its rate; that senescing individuals may well have a higher fitness than ones that attempt to prolong their lifespans indefinitely - and thus that aging is part of nature's plan, rather than representing a deviation from it.

Anti-aging medicine

Some parts of aging probably are maladaptive - but there are others which are better considered to be a normal and natural part of living organisms - and thus need not be regarded as something necessarily requiring rectifying.

I suspect that those who are labelling aging as a disease have economic motives. For instance, they appear to think that it will be easier for them to locate funding to find a "cure" for aging if it is popularly regarded as as disease. They are quite likely correct about this - but that doesn't justify labelling aging as a disease process if parts of it are actually natural and adaptive processes.

The fact that some elements of aging are adaptive also suggests that - after a certain point - government funding of anti-aging medicine will not make much economic sense.

After a certain point it will be less expensive to simply replace the senesced individual "components" than it would be to attempt to repair them.

Advocacy

Aging isn't a disease, so obviously it can't be "cured".

Call it what you like. Aging is a biological phenomenon that causes debilitation and death. These things are undesirable. Thus, it seems pretty reasonable to me that something which stops getting older from causing debilitation and death can be called a cure for aging.

- http://www.sens.org/cred.htm#curing

The problem with this is that aging cannot be viewed in isolation. It represents (among other things) pleitropic side effects of devoting energy to reproduction. So you cannot simply assume that death is undesirable - that would be assuming what you are trying to prove.

As to the suggestion that it doesn't matter whether aging is classified as a disease or not - IMO much confusion is caused by thinking of aging as analogous to a disease that is in need of fixing.

Reliability theory suggests that aging is a natural part of complex systems.

In my view, this suggests that attemping to extend lifespans beyond a certain point will prove to be misguided.

Often old things really do need throwing out. That is not necessarily a cause for lamentation - rather it is part of the natural order of things.