I believe that macroevolution is generally taken to mean evolution
above the species level (that leads to the production of higher
taxonomic groups) and microevolution to that occurring at the level
of species or populations.  The question is, do the same processes
account for both macro- and microevolution?  The answer to this
question probably depends on what one calls a significant
difference.  If macroevolutionary events depend upon the rather
sudden introduction of radically new and divergent characters and a
subsequent period of accelerated evolutionary change which leads
to a major new taxonomc category, is this process different or is it
just an extreme example of ordinary microevolutionary events?

Macroevolutionary events must, surely, originate within populations
and, therefore, they originate at the microevolutionary level.  So we
could ask what sort of populations would most likely encourage the
production of "novel" types and the more or less rapid evolution of
new morhphological, ecological, physiological and/or behavioral life
styles which ultimately go on to become new higher taxonomic
groups.  Where would such "evolutionary revolutions" occur?

Here, I would have to disagree with Gary.  I think that the most
likely scenario is that such evolutionary events would most often
occur within small, isolated and usually peripheral populations.
Such allopatric speciation provides a mechanism by which a
population can rapidly diverge from and become reproductively
incompatible with its parent group. The reasons for this are
extensively discussed by Ernst Mayr in his various books.  Before
isolation, such populations may exist at the environmental
extremes of a species' range and may thus also represent an
unusual genetic subset within the parent species.  The selective
pressures experienced by the recently isolated population may also
be quite different than those experienced by the parent group and,
especially if the isolated population has become established in
some sort of "virgin territory," permit the survival of mutations which
could be quickly eliminated or "swamped" in the parent species.
Favorable mutations will also spread more rapidly within a small
population than a large one and so the rate of genetic change will
be accelerated within the small isolated group.

While it is possible that sympatric speciation may sometimes
occur, it does not seem likely to me that it is the major source of
speciation events simply because it requires the sudden production
of individuals unable, or at least unlikely, to interbreed with their
fellows.  This would, of course, be a significant selection pressure
against such mutants who would be forced to find similar mutants
as mates within a population overwhelmingly dominated by
nonmutant individuals.

Polyploidy in plants may perhaps be a good example of a mode of
sympatric speciation, but it is less likely to be a source of
sympatric speciation in animal groups.  Plants have the advantage,
in many cases, of self fertilization or of vegetative reproduction
giving rise to individuals which can then later interbreed.  Isolated
polyploid mutants may thus initially persist without finding a mate.
Some animal groups have similar reproductive possibilities but most
do not.

kate