Damien wrote:
> I go off like an alarm clock every time I see this. I agree that
> antagonistic pleiotropy is the principle answer (animals that foresake
> later benefits for earlier ones survive better and procreate more reliably,
> if only because predators and accidents get you sooner or later even if
> you're in tip-top well-maintained shape).
>
> But the `declining force of natural selection'? What *is* this:
[snip]
Damien, I can't tell if how much of your following discussion was
intended to be humorous or discussion material [I think my humor
detection meter is dead today...] -- so I'll simply explain the
reasoning.
You are getting to a large degree into whether species move
towards K-strategy or R-strategy (which I'm too lazy to
go lookup now to define { I must be catching the Spike Virus }).
Here is how it works though.
For longevity:
(a) "Big" is better;
(b) "special defenses" is better;
(c) "intelligence" is better
E.g.:
(a) Elephants, whales, sturgeon; giant clams; parrots;
(b) tortises, giant clams, lobsters, all birds & bats (mobility);
(c) Elephants, chimpanzees (& other great apes), parrots, man.
Why are these better? Because they provide you with ways to
"beat" the hazard function. What determines the 'hazard function'?
If you are "small" you are generally prey and die from predation.
(your gene strategy becomes -- live fast, die young and leave
lots of children -- which I think may be the R-strategy.)
If you've managed to get yourself into one of the 'special'
categories listed above, evolution (generally) moves you in
the other direction (K-strategy?) where you have fewer young
over a much longer reproductive period (which is what I think
you suggested in your note).
*But* the bigger you are the greater your nutritional requirements
are and the greater risk you have of starving during environmental
disruptions or simply "overpopulation" of the local carrying capacity.
(Remember that starvation weakens the immune system giving
microorganisms a chance to develop into overwhelming numbers in
you and potentially migrate to other members of your species).
You may have "defenses" but your extended longevity gives microorganisms
all that much more time to figure out their weaknesses. If your
defenses reduce your mobility, then you fall victum to "local" hazards.
Tortises don't "relocate" from the Galapagos to Hawaii when the
local food resources go sour due to an El Nino.
Intelligence works fairly well. Human propagation shows it beats
all other hands. But even that trait runs up against limits of endless
reproduction running head first into the carrying capacity of the
environment (e.g. global warming).
My take on "the declining force of natural selection" is that
evolution hasn't had enough substrate on which to evolve a
"perfect" program. [If you think it can roll the perfect program
out of the starting gate then you need a much better understanding
of biochemistry and physiology -- start with Jay Olshansky's recent
article in Scientific American about how humans "should" be designed
if were were engineered for longevity.]
A simple example would be the "SOS"-program built into bacteria.
It gets activated when nothing else is working and bacteria are dropping
like flies and the only way out of a dead-end in your "local" environment
is to produce an entirely new genetic program. Its evolution's way of
saying you have to kill any other passengers to save your children.
Unfortunately only bacteria have a "real" "SOS"-program.
You "can" produce progams that survive the extinction events but
only at a cost of becoming something completely different.
If you look at the multi-parameter optimization vector, you can
see how difficult the problem is --
(a) get big -- but don't exceed the carrying capacity of the environment
(b) get great defenses (e.g. armour) -- but remain mobile
(c) get mobile -- but sacrifice the armoured defenses
(d) get intelligent -- again this requires a large fraction of
resources (15-20% of overall body energy consumption) to
support advanced intelligences. (Such resources will be
in short supply during famine periods). Also have children
that require lengthy gestation and education periods (during
which they are incapable of protecting themselves).
Nature does "push" on these vectors. But as the dinosaurs show,
you can get very big and when your food supply goes belly-up
so do you.
The 'declining force of natural selection' is just a way of saying
that as one grows older there is very little left in the gene pool
(due to predation and accidents) in terms of "longevity genes" which
Nature can select from.
It gets worse due to the "reproduction runaway" problem. Individuals
who have won the genetic lottery and who devote their resources more
to maintenance and repair rather than reproduction get competed out
of the gene pool! Why? Well if you have a generation time of 20
years (# offspring = replacement levels) and your rabbit-copying
cousins have a generation time of 10 years (you have to devote your
resources to maintenance and repair while they devote them to mating,
producing and feeding offspring); then after 100 years your "long-lived"
clad has 2^5 people while the short-livers have 2^10 people. You will
note that 2^10 people are much more able to harvest the available
resources than 2^5 people. Furthermore your clad has a problem finding
mates who can compete against the short-livers clad for resources
and has a problem of in-breeding producing non-productive lethals
amongst the long-livers. [You will note I am sure that Heinlein
"stacked" the deck by providing an un-natural reward for people
to marry within the long-lived clad.]
Now, there is a way out of this problem -- you have to become
more fecund with age! [i.e. as you get older you have to
increase your reproductive output so to maintain the fraction
of longevity determinant genes in the overall gene pool].
This is precisely the strategy followed by many trees (more
seeds as the get larger and older), sturgeon and perhaps
lobsters and giant clams. However note the problem -- to
produce more offspring to maintain your fraction of the gene
pool you have to get larger (or at least harvest and recycle
an increasingly larger fraction of your local resources) --
once again you are up against the "getting big" solution to
longevity -- eventually you hit the carrying capacity of
the environment.
In other words even if you are big, with great defenses,
and are very intelligent and win the genetic longevity
lottery you will lose the genetic reproductive lottery
because sooner or later your environment will not support
the fecundity you need to retain your "share" in the
species gene pool.
The above hopefully provides clear explanations as to
why organisms age as you will probably hear from anyone
alive today. { If it isn't clear hit me and I'll try to
explain the details further. }
Robert
This archive was generated by hypermail 2b30 : Mon May 28 2001 - 09:59:41 MDT