In article nzo9d.22131$a41.19778@pd7tw2no,
Elaine Jackson wrote:
Far out!! Thanks very much for telling me about this! I would like to find out
what's known and/or speculated about an evolutionary explanation for the fact
that most organisms have a finite lifespan even (presumably) under ideal
conditions. Can you point me toward any references?

A multicellular organism is a complicated system, and a lot can go
wrong. The longer it lives, the more likely something is to go wrong.
The repair mechanisms aren't perfect, and they too can fail. A sick or
deteriorating organism can't compete with healthy ones for resources,
and will produce fewer offspring.

Evolutionarily, a handwave is to say that the old models have to give
way to the new ones, or what's sexual reproduction for? More
carefully, we can say that there's a tradeoff between investing
resources in self-maintenance and in reproduction, so there's a point
at which improved self-maintenance is unfavorable. Other than top
predators, few animals live long enough to deteriorate with age in
nature, so there's not much selection for traits that deal with conditions
that arise long after almost every individual has been eaten by a
predator, long after an individual has produced most of its offspring.

When we say an organism is "immortal", or potentially so, it's usually
an organism that propagates clonally. That 80,000 year old aspen clone
is genetically the same individual, but it's not the same tree.
Similarly, single celled organisms that reproduce by fission could be
said to be immortal, but each individual can die before it fissions.

Note that parthenogenesis has arisen numerous times in several families
of lizards. Some of these all-female "species" are extremely
successful - some geckos, for example, are now pan-tropical. There may
be many millions of individuals in each "immortal" clone, but each
lizard meets its doom eventually.

You might ask, so what good is sex? (Or even, who needs men? ;-))
Genetic diversity is the raw material of selection, and conditions
eventually change. Asexual propagation is a win in the short term --
it lets you really multiply your genotype -- but when conditions
change, you could lose all in the Darwinian sweepstakes. Those poplar
clones may form the entire poplar population of a substantial piece of
ground, those geckos may be the main gecko fauna in their area, but
when some new disease or competitor comes around all members of the
clone are equally vulnerable. In the long term it's a better bet to go
for the diversity possible with sexual reproduction, but selection has
no way of considering the long term, so we have these unusual cases,
"immortality" through natural cloning.