Evolutionary necessity of senescence?

Andy Peters adpeters at sunflower.bio.indiana.edu
Thu Feb 18 19:20:21 EST 1993


In article <45258 at sdcc12.ucsd.edu> wsun at jeeves.ucsd.edu (Fiberman) writes:
>In article <1ludla$2c6 at agate.berkeley.edu> tyersome at toxic (Randall Tyers) writes:
>> [stuff deleted]
>
>>production of toxins within our bodies.  This seems to reinforce the
>>general idea that we die because it is more advantageous to put energy
>>into reproduction than a (futile) attempt to reverse 'ageing' processes.
>>
>>Randall Tyers
>
>Why is it more advantages to put energy into reproduction?  It
>seems to me that theres less energy requirement to keep an
>organism at homeostasis.  Growth requires more energy than
>homeostasis.  Reversing aging may be futile, but I think
>stopping aging is definitely possible, and more economical than
>reproduction.

While stopping aging (through somatic repair) is certainly possible,
stopping _death_ isn't.  Even if there's no senescence (i.e. decrease
in fitness components of an individual with increasing age), there's
still a given probability of death from stochastic environmental
factors each season.  So, as age increases, the strength of selection
decreases, because there's a lower probability that the individual has
survived to that season.  Hence, one (or both) of two things can
occur:
  (1) Deleterious mutations which act later in life can build up
(since selection against them is weak, they reach mutation-selection
balance at a higher frequency than those acting early in life).
  (2) Phenotypes involving a tradeoff between early and late survival
or reproduction can be selected for.  So energy use that leads to
later free-radical buildup, the use of certain proteins which are
limited in quantity, etc., can increase early fitness, and the
decrease in late fitness they cause isn't selected against strongly
enough to counteract the early benefit.

In a nutshell, the individual organism is going to die one way or the
other.  The only way to achieve immortality is through reproduction.

>-fm

-- 
--Andy
     "God is a real estate developer / with offices around the nation
          They say one day he'll liquidate / his holdings on High
             I say it's all speculation." -- Michelle Shocked




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