You can't always get what you want But if you try sometimes,
well you just might find You get what you need
M. Jagger/K. Richards
Does a population whose survival is threatened by some
environmental stress "evolve faster" than one which is not?
Well sure. If there is a low frequency gene that usefully
responds to the stress, Fisher's fundamental theorem and
common sense agree that the frequency of this gene will
increase faster when the stressor is actually present in the
environment than when it is not.
But some researchers go farther and claim that mutations
leading to alleles that usefully respond to the stress are
more likely when the stress is present. This idea is widely
considered to be heresy. It apparently conflicts with one of
the core axioms of neo-Darwinism - that mutation is
"random". Nevertheless, apparently solid empirical results
in microorganism systems repeatedly back up this claim of
environmentally directed mutation.
In a typical experiment, a strain of bacteria that is
sensitive to some toxin is divided into two groups. One is
exposed to moderate levels of the toxin, the other is not.
Periodically, a sample is taken from both populations and
cultured under high toxin levels. A check is made to see if
there are any survivors. If so, then a beneficial mutation
must have occurred. Such experiments repeatedly find that
the beneficial mutations happen more frequently in the
stressed population. And statistical analysis of the results
suggests that it is not just a case of the stressed
population having a higher mutation rate. There is a much
lower tendency for a stress by one toxin to induce an
immunity to a second, non-present, toxin.
Results like these definitely challenge the neo-Darwinnian
axiom, IMO. One frequently sees the label "Lamarckian"
attached to these results - I suppose there could be debate
over whether that label is appropriate. In any case, results
like these have always puzzled me. What could be the
mechanism?
Well, I finally ran across a paper that suggests a
mechanism.
Wright BE. A biochemical mechanism for nonrandom mutations
and evolution. J Bacteriol. 2000 Jun;182(11):2993-3001
jb.asm.org2993Open ↗
Without endorsing what she says about evolution, I think
that I can agree with what she says about mechanism.
Basically, it is that DNA segments that are being actively
transcribed are more at risk of mutation than segments that
remain continually in double-stranded form.
Suppose that we have a species of bacteria that have an
inducible defense against a toxin. Two genes are involved
here - one to sense the toxin and one to deal with it. Now,
isolate a mutant strain that has a defect in the deal-with-
it gene. Use this strain in your anti-neo-Darwinist
experiment. Your control group never transcribes the
defective deal-with-it gene, but your stressed group does -
ineffectively, of course. But a back mutation to an
effective deal-with-it gene is more likely in the stressed
group than the control group.
OK, it works for back mutation, but can this mechanism
explain how immunity can develop in an environmentally
directed way to a novel toxin. Yes it can, if the novel
toxin is chemically similar to a known toxin that the
microorganism can already deal with. If the gene to sense
the known toxin is non-discriminating, it will respond to
the novel stress by promoting the transcription of the deal-with-the-known-
stress gene. And that gene can probably mutate to one that
deals with the novel stress more readily that can some
randomly chosen gene from the genome.
It can be debated whether these ideas really challenge neo-
Darwinism and whether they support neo-Lamarckism. But I
like them because they tend to explain the origin of the
signalling mechanism involved in inducible enzymes. A new
inducible enzyme pair arises by the duplication and
evolution of an old pair. The two genes involved in the
communication do not have to originate separately and then
later come to an agreement on the "signalling protocol".
organisms. Recombination is also more likely in DNA segments
that are actively being transcribed!