why do we die?

Ron Grunwald grun at acpub.duke.edu
Tue Nov 7 18:21:30 EST 1995


In article <47m4aq$4eo at threed.uchc.edu>, rwilson at panda.uchc.edu (rwilson) wrote:

>   It seems to me that theres no fundamental law of biology  which
> excludes the possibility of a multicellular organism living a very,
> very long time.  There are trees which are thousands of years old and I
> can imagine vegetative fungi living for tens of thousands of years or
> longer. 

>    Second, it would be disadvantageous for the long term survival of
> the species.  To avoid a population explosion a very-longed-lived
> species would have to compensate with a very low reproductive rate.
> This in itself would be bad because they'd be more succeptable to

Be careful. Long term individual survival coupled to low reproductive 
rates is a perfectly reasonable life history and should not be
characterized as being intrinsic 'bad'. The examples you site in the first
paragraph are examples, but even humans fit the definition as compared to,
say, flies and annual weeds.

> enviromental catastropheses but theres more:  sex and reproduction
> allow for new combinations of genes which provide the raw material for
> evolution. So a long lived species would evolve slowly and have trouble
> adapting to new enviroments.

I don't think that sex and reproduction should be thought of as mechanisms
for introducing variation into a population for the purposes of adapting
to future changes. Such a 'pre-adaptation' argument is very telological
and generally difficult to reconcile with an individual selection
formulation of natural selection. How does an organism 'anticipate' a
catastrophy? Sex is primarily a mechanism for correcting DNA errors and as
such is generally conservative. 

>   I guess theres a balance between having a rapid generation time, fast
> evolution and by neccesity, knocking off the oldsters to make room for
> the young fertile ones, and having a slower generation time which
> allows the oldsters to care for and 'teach' the younger generation   

This is quite true. It is exactly that trade off that favors low
reproductive rates in some environments (K-selection) vs fast rates in
other environments (r-selection). The literature on K vs r selection is
enormous and provides an indication of what would be required for there to
selection for longer life-spans. 

This, in effect, is exactly what should be considered when discussing the
increase in human lifespan in more affluent societies (= K-selective
environment). While the extension of human lifespans and the decrease in
reproductive rates may be a legitimate topic for social debate, you cannot
make an evolutionary argument that it is a general and inevitable trend
(as some other writers in this thread have) nor that it is 'bad' is some
evolutionary sense.

-- 
Ron Grunwald                             



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