Here's my essay about whether senescence is an adaptation. The text can be found on the web - at: --> http://alife.co.uk/misc/adaptive_senescence/ <-- Those who have me pinned as a /panadaptionist/ can probably guess at the answer I give - and they probably would not be far wrong - since I conclude that senescence does have an adaptative element, though other parts of it are most likely maladaptive, and are (e.g.) the consequence of deleterious mutatitons. I dodge the question of to what extent high-level selection processes are responsible for the effect - on the grounds that the question can be answered without dealing with that issue. It seems like a good time to address the question of the utility of senescence - since there still seems to be a lot of debate about the subject - and it is relevant to a current issue regarding whether aging should best be regarded as a disease process - as some anti-aging researchers have asserted. Adaptive senescence ------------------- Is aging a disease? ------------------- The question of whether aging is a disease is a common one. 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? --------------------------- Most previous writers on this subject have assumed that aging is maladaptive from the point of view of the individual. 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. 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 ------------------- In the light of aging's adaptive qualities, I am inclined to agree with conventional medicine on the point of whether aging is a disease. 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 it is actually a natural and adaptive process. The fact that aging is adaptive also suggests that - after a certain point - government funding of anti-aging medicine will not make much economic sense - with it often being less expensive to simply replace the senesced individual "components" than attempting to repair them. -- __________ |im |yler http://timtyler.org/ [email protected] Remove lock to reply.
