r norman  wrote in message
news:...
> On Wed, 19 Feb 2003 19:06:27 +0000 (UTC), Steve Schaffner
>  wrote:
> 
> >r norman  writes:
> >
> >> To say that genetic drift is a cause of speciation is, indeed,
> >> incorrect.  But to say that genetic drift is a cause of evolution is
> >> another matter indeed.  Genetic drift is definitely one of the
> >> important mechanisms of evolution, a change in the genetic composition
> >> of a population over time.  Given that populations do change by means
> >> of a variety of mechanisms (including drift, selection, mutation, etc)
> >> then additional factors such as the development of a reproductive
> >> isolating mechanism as one of the changes will indeed produce a new
> >> species.
> >
> >I don't get it.  Why is it incorrect to say that genetic drift is a 
> >cause of speciation?  Drift may need to be combined with another 
> >process, like geographic separation, in order to cause speciation, 
> >but under the right circumstances it certainly can do so.
> 
> Evolution is defined as a change in the genetic composition of a
> population.  

This is how SOME people (neoDarwinists) define it.  

Genetic drift is simply a change in allele frequencies
> (the genetic composition) due to random deviations from the expected
> Hardy-Weinberg equilibrium.

The Hardy-Weinberg equilibrium is an artificial construct. 
Consequently the existence of drift is dependent upon the belief in
the validity of a artificial construct.

> 
> Speciation is more than simply a change in the genetic composition.
> There must be enough change to result in a reproductive isolating
> mechanism developing to keep populations from interbreeding.  It
> really is quite unlikely that drift alone could produce such a change.
> Simply changing the frequency of existing alleles is unlikely to cause
> interbreeding to fail.  More likely, a mutation combined with
> selection, or a chromosomal mutation that renders the genomes
> incompatible is responsible.  Similarly, geographical separation, with
> or without drift, also does not "cause" speciation.  Separation will
> prevent hybridization and therefore allow changes to accumulate, but
> it does not "cause" anything.  Speciation involves not only change,
> but change of sufficient magnitude of the right type.
> 
> Note:  This is all being described in terms of "simple minded"
> introductory biology level evolution.

Well stated.  

  If your question really
> involves some subtle aspect of more advanced evolutionary theory, then
> you may well be correct that an unusual confluence of genetics and
> circumstances can possibly allow drift alone to produce a new species.
> But I'll bet that mutation plus selection are far more important
> factors.
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