On Tue, 12 Aug 2003 00:10:48 +0000 (UTC), "Alan Wright"
 wrote:

>
>"dkomo"  wrote in message
>news:bh4fov$r8e$1{at}darwin.ediacara.org...
>> Ron Okimoto wrote:
>> >
>> > dkomo wrote:
>> >
>> > > Suppose that animals remained at the age of peak fitness
and sexual
>> > > maturity indefinitely.  They wouldn't grow old and die
from old age.
>> > > And they would also be able to mate and produce offspring without
>> > > regard to how old they were.  Would such a situation
quickly lead to
>> > > ecological disaster, and is this the reason that
senesence evolved in
>> > > the first place?
>> > >
>> > > Well, it's possible that under such an assumption the
natural world
>> > > wouldn't look too much different from the way it does now.  In the
>> > > wild few animals have the luxury of growing old.  They die from
>> > > environmental causes long before they can die from
senescence.  They
>> > > die from disease, starvation, thirst, predation, climatic changes,
>> > > dominance and territorial disputes, natural disasters
like flood and
>> > > fire, and so.  In zoos many animals often live twice as
long on the
>> > > average as they do in the wild, and it is only in zoos
that we can see
>> > > them die from the diseases of old age.
>> > >
>> > > Surprisingly, the age distribution of a population of
"immortal"
>> > > animals has the same exponential tail as a
"survival chart" of aging
>> > > animals does.  That is, plot the number of animals
versus their ages
>> > > and you'll see an exponential drop off as age increases.  It's
>> > > counter-intuitive, but if the death rate of a population
is *constant*
>> > > without regard to age, then the age distribution will be
a decreasing
>> > > exponential.
>> > >
>> > > This can be shown quite easily.  Let r be the death rate
expressed as
>> > > probability per unit time.  Take a large sample of
ageless animals all
>> > > of the same age.  Then in a small time interval dt the number of
>> > > animals that die is
>> > >
>> > >            dN = -N r dt
>> > >
>> > > N is the number of animals at time t.  The minus sign indicates a
>> > > decrease in N -- dN is negative in other words.  I'm
using Mathematica
>> > > convention where two variables separated by a space means
>> > > multiplication:
>> > >
>> > >                 A B = A*B
>> > >
>> > > So  dN/N = -r dt  and integrating both sides we get
>> > >
>> > >     log N = -K1 r t     where K1 is an arbitrary constant
>> > >                         of integration
>> > >
>> > > and finally    N = N0 exp(-r t)  where N0 is initial size of the
>> > > sample at t = 0.
>> > >
>> > > This is exactly the same equation that as that for
radiactive decay.
>> > > It also governs the decrease in the population of any
set of things
>> > > that are removed at a rate r, for example a population
of glasses in a
>> > > bar that break at a rate r.
>> > >
>> > > Thus if we looked at a population of immortal animals that
>> > > nevertheless get zapped by their environment at a
constant rate, we'd
>> > > see a lot of young animals, fewer older ones and *very*
few really old
>> > > ones.  In other words, the age distribution doesn't look much
>> > > different from that of a normal population.
>> > >
>> > > And this analysis leads to a mystery: why did senescence evolve at
>> > > all?  From this perspective, there seems to be little use for it.
>> > >
>> > > Well, it turns out that the above analysis has a serious
flaw in it.
>> > > Can you tell what the flaw is?  Hint: it has nothing to
do with the
>> > > mathematical derivation.  Once this flaw is carefully
analyzed, we can
>> > > begin to understand why senescence in the plant and
animal worlds is
>> > > virtually universal.  Senescence evolved because it's
critical to the
>> > > survival of life.
>> > >
>> > >     --dkomo{at}cris.com
>> >
>> > You do not get the same distribution of older and younger birds.  The
>> > current distribution is a truncation distribution.  Immortals
would skew
>> > the distribution to the high end.  Natural selection in a
fairly uniform
>> > environment would quickly skew the distribution to older birds of
>> > selected genotypes and then you would get less of a skew once the
>younger
>> > birds were as uniform as the older birds.  You just have to
think of the
>> > fact that if the animal survives for three years it probalby has a
>better
>> > chance of surviving three more than some untested genotype.  Natural
>> > selection will winnow the population so that the older birds are more
>> > uniform genetically, this will make the population more genetically
>> > uniform and decrease the the populations ability to adapt to changing
>> > conditions.  If the environment was highly variable then you would
>expect
>> > to see decreased genetic uniformity and the older birds would
not become
>> > genetically uniform.  They would be subject to the random
changes of the
>> > environment and it could be possible that no genotype would be selected
>> > for.  But for environments that are fairly stable like rain forests
>> > specific niche adaptations would take place rapidly and stagnate.
>> > Species would more likely specialize and big rocks falling from the sky
>> > would more likley wipe them out.
>> >
>>
>> I pretty much agree with this analysis.  I believe that senesence is
>> an evolutionary adaptation to increase the genetic variability (the
>> hot term nowadays is "evolvability") of a species so that it can
>> better survive insults from the environment.
>>
>> And you hit upon what I hinted was the flaw in my original analysis:
>> the assumption that the death r remains constant with age.  In fact,
>> plausible arguments can be made that r would actually *decrease* with
>> age.  Older animals that remained in peak condition would be better
>> survivors than the younger animals.  The evolutionary reason for
>> senescence is to prevent something like this from happening.
>>
>>
>>     --dkomo{at}cris.com
>
>I wish I had noticed this thread earlier. The above seems to fly in the
>face of most of what I've read lately about aging (primarily stuff about
>mammals, such as by Austad).
>
>If my understanding is correct, aging must largely be regarded as a
>*lack* of adaptations to prevent a variety of cell damaging processes.
>This is thought to occur simply because there is no selective advantage
>to the organism to outlive its reproductive viability (including support
>roles).
>
>In addition, genetic damage is certainly a part of aging, but many of
>the (purportedly most important) mechanisms or proposed mechanisms
>do not seem to directly affect the genes (e.g. oxidation, glycosylation
>occur primarily outside the nucleus).
>
>So I would appreciate pointers to papers or books which discuss how
>(if I understand this correctly) acquired genetic damage associated with
>aging increases genetic variability in some evolutionarily important way.
>
>Alan
>
>
The above flies in the face of a lot of things, including fact.
Very few oganisms have death rates independent of age. 
Very few organisms have exponential survivorship curves.
See any introductory book in population ecology.



[moderator's note: do you guys HAVE to quote the ENTIRE article,
some 140 lines, to add a measly 4 lines of comment? Don't any of
you remember how to do Usenet?  Don't be so bloody lazy that you
can't clip out the irrelevant lines! You can do it, people!  You
can be RESPONSIBLE internet users. - JAH]
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