>Unless I missed something, you agree with Gary and you're both
>agreeing that micro and macro are inseparable. One is but the other in
>microcosm (both physical and time).
You missed something. I think that microevolution and macroevolution are
different processes. In some cases they will cause the same pattern; in
other cases they will not. My guess would be that almost all allopatric
speciation is caused by microevolution, but that some sympatric speciation
is caused by macroevolution. (Allopatric = different homeland; sympatric =
same homeland.)
In allopatric speciation, geographically separated populations diverge
until they are different enough that taxonomists call them different
species. In sympatric speciation, genetically different individuals within
a species stop interbreeding despite living in mixed populations. Thus, in
allopatry populations diverge until they speciate. In sympatry, they
speciate and then they diverge. (Maybe this yields the difference between
gradualism and punctuated equilibrium?)
I would guess that allopatric speciation doesn't usually result in genuine
novelty. Genetic differences might be caused by founder effect, genetic
drift, recombination, occasional point mutations, but you end up with
something that isn't much different from the parent species, and probably
lives in a similar habitat and so experiences similar selective pressures.
In sympatric speciation, the change must usually be more profound (although
it might still be caused by a point mutation). Maybe a mutation of some
sort causes some individuals to switch from diurnal to nocturnal or to
switch to a different food source. Such changes will mean that they are
unlikely to have opportunity to mate with individuals that don't have the
mutation. But these changes also mean that they are living in different
environmental conditions and therefore experience different selective
pressures which will cause further divergence.
Sympatric speciation might also result from a mutation that caused mate
choice, say orange individuals would mate only with orange, not with yellow
or brown. But orange would still be living in the same habitat as yellow
and brown and competing with it, which would mean that either that would
evolve to use different resources (character displacement) and therefore
experience different selective pressures, or one would go extinct (since
ecological theory says two species can't occupy exactly the same niche).
Yet another mechanism for sympatric speciation is polyploidy, a doubling of
the number of chromosomes that leaves the individual unable to mate with
its parents. Polyploids have an extra set of genes around, which
immediately make them different, because of dosage effects--twice as much
of some proteins, depending how production of a given protein is regulated.
It also gives them much greater potential for mutational change. A mutation
that is normally fatal might not be, because the other chromosome has a
copy of the original gene. That leaves the second copy free to evolve a new
function. Macroevolution by gene, chromosome or genome duplication is thus
is much more likely to be a creative force than microevolution. About 70%
of plant species are now thought to have polyploidy somewhere in their
ancestry, and evidence is growing for its importance in vertebrates.
Another difference between allopatric and sympatric speciation is that the
populations that diverge allopatrically are generally small. It's less
likely that the really rare non-lethal macromutations will occur in them.
The parent species probably didn't change much because of its size (rate of
change is inversely related to population size), but is its size makes it
much more likely to have the rare mutations that results in sympatric
speciation.
A problem with allopatric speciation is that the action generally happens
in the small population, which is usally offstage where you're not
looking--hence the observability problem in the fossil record. Sympatric
speciation suggests that in some cases we should be able to see divergence
directly in the fossil record.
Hey, Ross, Doug and Brian, I'm having more trouble fending off Marlo and
Paul than you guys. Am I playing into your hands? I'd still like to hear
what kind of evidence you'd accept.
Gary
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Gary Rosenberg, Ph.D. [log in to unmask]
Malacology & Invertebrate Paleontology gopher://erato.acnatsci.org
Academy of Natural Sciences http://www.acnatsci.org
1900 Benjamin Franklin Parkway Phone 215-299-1033
Philadelphia, PA 19103-1195 USA Fax 215-299-1170
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