dkomo  wrote:

> Tim Tyler wrote:
> > 
> > dkomo  wrote:
> > 
.....
> > : And this analysis leads to a mystery: why did senescence evolve at
> > : all?  From this perspective, there seems to be little use for it.
> > 
> > Things do not /need/ to be useful to exist.  Look at cancer, for
> > example. It is not functional.  It is a malfunction.
> > 
> > It's is what happens when somatic cells are damaged by their
> > environment, poisoned from within, or are the subject of hostile
> > takeovers.
> >
> 
> Interestingly enough cancer is primarily, although not exclusively, a
> disease of old age, at least in humans.  That indicates to me that it
> is one of the effects of senescence, along with all the other diseases
> of old age.  Cancer may be just another way of removing from the scene
> organisms that have fulfilled their purpose, i.e. produced a
> sufficient number of progeny.  Nature intends for cancer to happen.

Comments like that are what gives adaptationist explanation a bad name.
Cancer occurs because errors accrue in genes during somatic
differentiation, leading to a failure of apoptotic genes. Noise happens
in communication unless steps are taken to eliminate it, and errors
happen in evolution unless selection occurs to prevent it. There is no
"intention" for things to fail, just reasons for things not to fail if
possible.
> 
> > : Senescence evolved because it's critical to the survival of life.
> > 
> > If senescense is so important, then why do organisms which exhibit
> > negligible senescence exist?
> 
> I'm not sure we yet know the answer to that question.  But let me turn
> the question around: if senescence is mostly just stochastic
> degradation that all multicellular organisms must eventually undergo
> rather than, as I believe, a precise genetic program which serves a
> vital evolutionary function, then why do organisms which exhibit
> negligible senescence exist?  And why is there such a huge variation
> in the lifespans of different species, even closely related species? 
> Would we not expect to see organisms degrade at approximately the same
> rate?
> 
We do. In the case of "immortal" cell lines of protists, for example,
errors accrue all the time but some few lines do not accrue fatal
errors. this is rather different to saying that senescence is adaptive
in some but not other organisms.
-- 
John Wilkins - wilkins.id.au
[I]magine a puddle waking up one morning and thinking, "...interesting
hole I find myself in - fits me rather neatly, doesn't it? ...
must have been made to have me in it." Douglas Adams, Salmon of Doubt
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