T
Tim Tyler
Guest
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.
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.