Adaptive senescence

Discussion in 'Health and medical' started by Tim Tyler, Mar 12, 2004.

  1. Tim Tyler

    Tim Tyler Guest

    Here's my essay about whether senescence is an adaptation.

    The text can be found on the web - at:

    --> <--

    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

    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

    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

    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

    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.
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