On 4/18/2013 11:09 AM, John Clark wrote:
On Thu, Apr 18, 2013 at 1:29 PM, meekerdb <[email protected]
<mailto:[email protected]>> wrote:
> It's been proposed that the susceptibility to mutation is itself a
characteristic subject to natural selection.
>> If a animal is undergoing stress (too hot, too cold, too thirsty, too
hungry
whatever) that means there is something about it that is not well adapted
to its
environment; I can imagine a gene that in times of stress would switch on
and
produce a chemical that increases the rate of random mutation in the genes
of the
offspring of that stressed animal. Most of the offspring would have mutated
in the
wrong direction and die but they would have probably died anyway because
they would
have been as poorly adapted as there parent was, but if the mutational
effect was
not too strong (even if it's in the right direction you can change things
too far)
it could increase the likelihood that at least one of its children would be
better
adapted than its parent. However I maintain that such a stress induced
mutation
producing gene has had no significant effect on the history of life, at
least not
in animals that reproduce sexually.
> That's a kind of Lamarckian adjustment of mutability. What I was
referring to is
simple Darwinian adjustment of mutability. There are error correcting
mechanisms
for DNA reproduction. Suppose they worked perfectly: then there would
never be any
genetic variation and when the evironment changed the species would go
extinct. But
if they had a slight error rate then there would develop a range of genetic
diversity that might, under environmental change, result in survivors or
even new
species. So on strictly Darwinian theory the DNA error correction may be
selected
to be less than perfect.
No because in a sexually reproducing animal the genes that make the error correcting
machinery are inherited independently of the very genes that they have corrected, and
the vast majority of mutations are detrimental not helpful. So in any generation the
offspring of a animal with good error correcting machinery will almost always do better
than offspring from a animal with poor correcting machinery.
I think this overlooks the fact that there can be many mutations that are neutral relative
to a given environment, but under a change or in combination with another may be
advantageous. But I'll look up Dawkins argument.
Brent
And even in the very rare cases where the mutation caused a improvement in a gene the
animal will do better if it has the gene for the better error correcting machinery,
because otherwise that good gene is likely to mutate again and this time the mutation
will almost certainly be bad. As Richard Dawkins said in his wonderful book "Climbing
Mount Improbable":
"The predaliction to mutate is always bad, even though individual mutations occasionally
turn out to be good. It is best, if more than a little paradoxical, to think of natural
selection as favoring a mutation rate of zero. Fortunately for us, and for the
continuance of evolution, this genetic nirvana is never quite attained."
John K Clark
Brent
Such a stress-mutation gene has never been found in a sexual animal and
it's easy
to see why. In sex all the genes are not inherited in one big package but
are
shuffled around with the genes of the other parent, so a animal that was
lucky
enough to inherit the good genes produced by the hypothetical
stress-mutation gene
but not the stress-mutation gene itself would do just as well or better
than a
animal that got both the good genes and the stress-mutation gene that is no
longer
active because the animal is no longer under stress. So even if such a
stress-mutation gene did occur in one individual in a population it would
vanish in
just a few generations from the gene pool. Natural Selection doesn't figure
"I
better keep that stress-mutation gene because even though there is no
stress now
that could change and such a gene might come in handy in the future".
Evolution
has no foresight and can't think and all that matters to it is what's
happening
right here right now.
John K Clark
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