On Wed, 22 Mar 2000, Aaron Davidson wrote:
> M. E. Smith Wrote:
>
> > ... and ONLY undergoing said mutation in that
> >situation (that is, a control colony of the same
> >bacteria does not undergo the same mutation in an
> >environment where it has food it can absorb).
>
> Speculating, It could be the case that past evolution has engineered
> the bacterium's genome to actually increase the chance of that
> specific mutation when in such environments. So the apparent
> "accelerated beneficial mutations" may actually have been evolved
> over a much longer time scale. It only appears to be evolving too
> fast.
There is no need to speculate on these topics. The machinery for
and mutation rates in bacteria and to a lesser extent in higher
organisms are well studied.
Good books on these topics would be DNA Replication by Kornberg
or DNA Repair and Mutagenesis by Friedberg, Walker and Siede.
In a normal "good" environment, most mutations are deleterious.
This is largely because the genomes of lower organisms are very
dense and the highly competitive nature of their environments has
tuned to some local optima for survival. When a situation occurs
that clearly poses a threat to survival, a "stress response" will be
activated that increases the mutation rate. In rare cases it might
appear that this allows for directed mutation, but I suspect
this might be due to some lingering genetic memory for
how to survive in a different environment. If you can
figure out how to cheaply flip a gene between two forms
that are optimal in different environments, that would
be better than carrying around two copies of that gene
and the regulatory machinery for flipping the expression
from one to another.
However, most of the time, the hypermutation state simply
changes bits at random (similar to computer ALIFE programs).
Some bacteria get lucky and invent something new and survive.
Most don't and die. The strategy doesn't work for an individual
but works for the species as a whole. The mechanism presumably
evolved because it is what allows species to survive what would
otherwise be extinction events. Species that live in very
stable environments and are under selective pressure to
minimize their genomes (to be physically small or replicate
faster) will probably lose the stress response machinery.
So the cost of being able to survive extinction events is
limiting the environmental niches in which you can compete.
Complex animals, having invented sex are already doing a lot
of mixing and matching and have the advantage of being
able to carry two gene variants which allows the production
of gg, Gg and GG individuals (where "g" or "G" is any gene) that
have selective advantages in different environments. That
allows a wide variety of mutations to accumulate in the
gene pool and extinction events will select those with
the lucky combination(s). An example would be the few
percent of people who have a mutated receptor that allows them
to be much less susceptible to HIV infection.
Robert
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