From: rbradbur@hardy.u.washington.edu (Robert Bradbury)
Newsgroups: sci.bio
Subject: Re: The Evolution of Death
Summary: Variations of causes of death in species and cells.
Keywords: death, aging, apoptosis, evolution, natural selection
Date: 16 Apr 92 20:15:39 GMT
Organization: University of Washington, Seattle

In article <1992Apr10.124355.11441@newcastle.ac.uk> Mark.OLeary@newcastle.ac.uk (M.D. O'Leary) writes:
>   This does however remind me of other aspects of death and evolution...
>Biological death is obvio-usly programmed in to the organism (ie genetic) and
>since a long-lived adult will compete for resources with its offspring, 
>lifespan is also subject to a complex selection pressure. I guess there must
>be a trade off between the older members of a community protecting and
>teaching the young as a positive survival trait, and the burden to the group
>of maintaining otherwise non-productive individuals on the negative side (for
>social apes at least...) In non-social populations, it must be straight 
>competition for resources- mates, territory, food, nest sites etc.

In salmon the selective pressure to reproduce results in a massive hormone
release to get the fish back upstream.  This hormone 'overdose' results
in death.  For octopi(?) which reproduce once and then die one might argue that
there is competition for good living space so the parent makes room for the
offspring.  For insects which consume their mate to provide protein to
complete egg maturation reproduction is terminal for half the species.

>But then, I have heard it suggested that death is neccessary to allow 
>recombination of the gene pool (ie by allowing the offspring to live): Any
>population that had biological immortality would be wiped out by the first 
>significant shift in the environment that required a genetically based 
>adaptation.        

It is not that "death is neccessary", but that it is *very* difficult
to select for "longevity".  Once you reach the age of reproduction
(which may include raising your offspring) the selective pressure is
removed.  In order to select for "longevity" you must maintain your
genes as a constant percentage of the species gene pool.  That requires
an ever increasing production of offspring.  This has evolved with
some trees (sequoia) and fish (sturgeon) where older (larger) organisms
produce more seeds/eggs than younger organisms.  For organisms which
must make a large investment in each offspring (time/protein/reduced mobility)
this strategy will not work.

It is worth noting that the experiments which have lengthened the genetic
lifespan of Drosophila/C. elegans work not by selecting the children of
the longest lived parents but by selecting the last born children of
the oldest parents.  The net effect this has on the species is one of
delaying the onset of fertility and extending the lifespan.

> Now, suppose a species was subject to such intense inter-specific competition 
>and predation that individuals NEVER died of 'old age'. If you took such an
>organism to a more gentle environment, would it not have a VERY long life
>span? Maybe immortality?

There is no such thing as "immortality", if you eliminated all causes of death
people would still have an average age at death of 600 years.  A few lucky
individuals might live to be 1-2000 yrs old before the grim reaper show up.

>
>Of course, the other question that looms large in my head is:
>"What is the difference between a living cell and a cell the instant after it 
>'dies'?"

There is probably no "instant" a cell dies.  It is a process of
damage accumulation (to nucleic acids, lipds and proteins) until a
point is reached where the maintenance and repair processes cannot
maintain homeostasis.  There may be an exception to this in "aooptosis"
for at the instant the signals for "programmed cell death" are recognized
and the genes which cause the process are turned on the cell is doomed.

Robert Bradbury
Newsgroups: sci.bio