"r norman"  wrote in message
news:ci0foo$2p7i$1{at}darwin.ediacara.org...
> On Sat, 11 Sep 2004 20:34:10 +0000 (UTC), Brett Aubrey
>  wrote:
> >Merriam-Webster Online has the following definitions:
> >
>
> There are several things at work.  You may be missing something and
> the definitions are definitely somewhat flawed.
>
> Evolution is generally defined as something like "a change in the
> genetic composition of the population from generation to generation."
> There are several aspects to the "genetic composition" that can
> change.  Ordinarily there is some variability in the specific DNA
> sequence from individual to individual.  That is, each gene can exist
> in several different forms or alleles.  The "genetic composition"
> really refers to the relative abundance or frequency of each allele
> for each gene in the population.
>
> In a small population, it is quite possible (even very likely) that
> the abundance of a particular allele might change just be chance over
> generations.  For example, those individuals with big noses just
> happened to have more babies with big noses than expected.  It is sort
> of like flipping a coin twice. You expect one head, one tail, but
> sometimes you end up with two heads. That is the definition of genetic
> drift: a change in the genetic composition of the population (in the
> allele frequencies).  If the change is drastic enough, it can in fact
> lead to the elimination or the fixation of an allele: so few people
> with small noses happen to have small-nosed babies that the small-nose
> allele simply disappears and the big-nose allele is then "fixed" in
> the population.  The flaw in the definition is that drift is any
> random change in gene frequency whether or not it leads to
> "preservation or extinction" of a gene.

Thanks muchly.  This is now making sense.  Is there any quanification (or
guidelines) of what constitutes a "small population" with regards
to genetic
drift?

> As a completely separate matter, an "error" might occur in cell
> division that causes a change in the sequence of DNA or in the
> structure and arrangement of the genes along a chromosome (or even in
> a rearrangement of the chromosomes, themselves).  Once such a change
> occurred, a mutation, it would then be faithfully copied (in the
> absence of another mutation) and so is a "permanent" change.  The
> second flaw in the definition is that inappropriate emphasis on the
> permanence of the mutation.  The important thing is that the change is
> the introduction of a completely new variation into the population,
> not the change in frequency of existing alleles.
>
> For better definitions, consult Wikipedia, usually a good reference
> for many things. Googling "genetic drift mutation" will give you many
> academic sites with instructional guides to these topics, too.

Regards, Brett Aubrey.   (Thanks also for "Wikipedia".)
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