Re: The ultimate cause of aging
- From: dkomo <dkomo871@xxxxxxxxxxx>
- Date: Tue, 11 Mar 2008 18:00:07 -0700
Remus wrote:
On Mar 4, 10:47 am, dkomo <dkomo...@xxxxxxxxxxx> wrote:
Don't get too excited. I'm using the term "ultimate" here in the sense
of "evolutionary":
http://en.wikipedia.org/wiki/Proximate_and_ultimate_causation
The terms "ultimate" and "proximate" causation have been
enthusiastically adopted by scientists who study human and animal
behavior/psychology. But these terms were originally coined by Ernst
Mayr and meant to be applied to evolutionary biology as a whole.
"The basic reason why survival and reproduction decline with age is that
selection acts more weakly on later ages. Imagine an organism that does
not senesce, that is, that maintains itself with the same rate of
survival and reproduction indefinitely. It will still suffer accidental
mortality and so, on average, reproducing early will produce more
offspring than deferring reproduction until later; death might strike
first. Therefore, natural selection acts more strongly on variations
that act early in life and acts more and more weakly on late-acting
acting variations (see Box 20.2). If, as a result, senescence starts to
evolve, then there will be a feedback, so that selection on later ages
becomes even weaker. In the extreme case, where an organism such as a
salmon reproduces only once, there is absolutely no selection on
subsequent survival."
This is a rather simple analysis of the phenomenon
and not very complete.
It is, however, the de facto "ultimate" explanation of senescence. Do you think I make these things up? Check into any undergraduate or graduate evolution textbook (not just a general biology textbook) and you'll find an evolutionary explanation for aging quite similar to the one I gave.
I provided the explanation without editorial comment. I'm not totally gung ho about it. I think it has problems, but not for the reasons you give below. For one thing, it is too sketchy. You have to go beyond evolutionary explanations and dive into the "proximate" mechanisms to really understand the "how" of senescence, as opposed to the "why".
The basic problem with it, however, is that there is evidence of senescence effector and suppressor genes that control the rate of aging in some organisms. In other words, there seems to be some sort of a genetic aging program. This cannot be explained by the idea above that "natural selection acts more strongly on variations that act early in life and acts more and more weakly on late-acting variations." A genetic aging program would have to have evolved over the entire life history of individual organisms. I think group selection may have something to do with it.
If an organism had no senescence, then it could
throughout time propagate its beneficial genes
throughout the population to a much greater
extent than it could if it aged and died for
a short period of time.
Did you miss the part where the shmoo has a yearly death rate of 10%? Dying by growing old is far from being the only way living things die. A living thing could be biologically immortal but only survive a few years if it lived in a hazardous environment.
If it were short lived but had highly beneficial
traits in its genes, then it would have to propagate
those genes solely through its progeny, then
again throughout the generations, for those
genes to spread.
If it lived for a very long time, however, it could
keep generating those beneficial traits again
and again, at least until the amount of progeny
that it generated made those genes no longer
unique. Then cross-generational conflict
might mean that the genes would propogate
through progeny rather than the original
producer of the gene.
I think that the real problem, however, like
with sex, has to do with the potential for
piggybacking bad recessive genes, that take
advantage of the good ones, in the entire set
of genes and chromosomes within the organism.
If the chromosomes and the genes are
constantly being interchanged from generation
to generation, then bad recessive genes might
become dominant in some chromosome
combinations. Those organisms would not
be fit and would die. The net result would be
a better gene pool.
The truth of the matter is that there are a lot
of organisms that reproduce asexually just fine.
They don't evolve very quicky, and gradually
get selected against due to that reproduction
method, but its still at least somewhat
common among some of the lower animal
species. Overall, I would think that you
would expect a lot more immortal species
out there than you generally tend to find
if it were solely a matter of reproduction
and selection strategies among the
populations of various species.
I would guess that ultimately, the real
reason for senescence is that it is a
cheap and easy way for multicellular
organisms to suppress cancer production.
If a cell dies after a certain number of
divisions, and it starts deciding to go
on the warpath and reproduce with extreme
rapidity, then a counter that tells the cell
to die after a certain number of divisions is
a very easy way to make sure that the
cell is not going to destroy the whole organism
for the short term benefit of the progeny of
the single cell line.
If it were solely a matter of reproduction and
evolution, then you would think that there
would be a lot more exceptions to the rule
in the plant and animal kingdoms. If it
were at least partially based upon the
foundations for the machinery of cellular
regulation, which allows large sized
multicellular organisms to exist to begin
with, however, then the reason for the
existence of senescence, across so many
different species of the plant and animal
kingdoms, becomes a lot more apparent.
I agree that cancer suppresssion is one of the better proximate explanations for senescence. However, there are many others:
http://en.wikipedia.org/wiki/Aging#Theories
When it comes to proximate versus
ultimate reasons, it sounds nice to create
one 'great plan' or 'ultimate reason' why
so many beings are preprogrammed by
nature to constantly die off and be
dissipated into nothingness.
Making a vast number of cells reproduce,
grow, and specialize their functions, so that
they all can produce a vast aggregate capable
of growing and acting in specialized ways,
that will optimally extract resources from the
environment, however, is not a trivial
phenomenon.
Probably the main reason that there are very
few non-senescent species is that a foundation
based in multicellular regulation, is slightly
selected in favor, by evolution, due to the
additional property of reducing the propagation
of bad recessives, in comparison with the
selection of progeny. Both these factors have
acted in concert to suppress the generation of
immortal species.
--dkomo@xxxxxxxx
.
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- The ultimate cause of aging
- From: dkomo
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