molecular drive encore

DF_1 at MB1.BIO.CAM.AC.UK DF_1 at MB1.BIO.CAM.AC.UK
Wed Dec 4 07:57:33 EST 1991


 Date:	Nov 28, 1991
 Subject:	Molecular Drive
 From:	Garbiel A. Dover

	In his reply to my posting, Joe Felsenstein conceded that concerted
evolution cannot be used as a name for both the observed phenomenon and the
process underlying it. That's good: hurdle number one has been crossed. Hurdle
number two involves the realisation that the genomic mechanisms (biased and
unbiased conversion; unequal crossing over; slippage, transposition;
retroposition) feed into one single process that I called molecular drive. The
mechanisms were already known and under active investigation by many labs,
including ours. There's hardly any need to tell me that. However, as I
explained in my last posting, it was not generally recognised that the
operation of any given mechanism WITHIN and BETWEEN chromosomes in a sexual
population would not only homogenise a variant repeat through an array in a
single chromosome lineage (in a vacuum so-to-speak, as previously modelled by
others) but would also spread, concomitantly, the variant repeat through all
arrays (lineages) in a population. It is this extra POPULATION DIMENSION which
was previously missing which is, of course, the CRITICAL requirement for ALL
processes of evolutionary change (e.g., as embodied in the previous two of
selection and drift). As I wrote, Ohta & Kimura, as good population
geneticists, realised the need for this extra dimension, but their "double
diffusion" process was unsatisfactory for the reasons I gave. I would have
thought that Joe Felsenstein would have had no difficulty in understanding this
requirement and welcome the proposal of ONE umbrella term of molecular drive
(embracing all 5 internal mechanisms) of a dual process (homogenisation coupled
to fixation) to explain the concerted evolution patterns. As I repeatedly
state, this process is OPERATIONALLY different from selection and drift but is
expected to interact with them. Hence, the evolution of any given trait is a
mix (to be experimentally dissected) of three evolutionary processes. We needed
a term to encompass all the genomic mechanisms, whether biased or unbiased. The
term is not wholly satisfactory in that "drive" can be seen to imply biased
(directional) mechanisms only; but I gave a definition of molecular drive in
the first paragraph of my 1982 NATURE paper which embraced both modes (biased
and stochastic). People who read into the details have no difficulty with this,
and, secondly, for many given concerted evolution patterns we don't know how
much bias versus unbias contributions there are - bias can switch in direction
or be lost altogether in several examined systems. So, ONE term is required
which doesn't prejudice the input, and which is distinct from selection and
drift. We all now accept the term natural selection (derived from artificial
selection) even though we know there is no actual external force doing the
selection.
	Finally, Joe writes that unbiased mechanisms do not increase the net rate
of evolution. But nobody said that they did. I've heard this criticism before,
but I can't understand where molecular drive (in the unbiased mode) has ever
been described as increasing evolutionary rates. Ah, well - attention to detail
problem.
	In conclusion, I'm well aware of the history of work in genomic dynamics
and I'm more than familiar with the need (back in 1982) to provide a MISSING
term for an evolutionary process effected by internal genomic mechanisms that
is clearly, operationally, distinct from selection and drift, as these are
traditionally understood. Once again, sorry about this, but that's what's going
on in the real, complex world out there.



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