Michael,
 
The process you describe is probably how most speciation has occurred as well as how
99% of species became extinct (something that happens to most species).  An
individual with a mutational characteristic which is a benefit to survival or use of
the habitat does not just share the habitat but is able to better compete for
resources and the original species is driven out, out-competed and maybe on the
slick slope of extinction.  A bit brutal but very Darwinistic.
 
Whether this happens is small stages or large leaps or goes forward and backwards -
I believe it does happen.  Thus the black moth survives on the coal blackened trees
and the original white moth dies out.  Of course when coal laden soot no longer
blackens the trees, the black moth is in trouble and a new lighter colored moth will
be far more likely to survive.  The book by Jonathan Weiner, "The Beak of the
Finch," describes just such a process among the finches of the Galapagos Islands.
Now the magic question - can the newly dominate group interbreed with the original
group.  Well it really doesn't matter for if the new guys are truly able to better
survive, they will do just that and not be able to breed with the original gene pool
as the original guys are now ancestors.  A new species?  Really?  Maybe, maybe not
-- even probably not.  But carry this process through millions of years and the
changes become large enough for even a skeptic to see a new species.  Thus my
original example of x-a, x-b, x-c, and x-d has not only a physical distance
parameter but a temporal one as well.  Today the callused cypraea and the uncallused
cypraea are arguably the same species - but what about in a million years/  Or
five?  Or 100?  Today the cichlid fish x-a is able to breed with x-d, but in time?
 
Again, we are attempting to put a large sloppy process into neat little cubicles
with nicely typed data slips.  Our nicely typed data slips will undoubtedly need
changing as little in nature seems as exact as we would like to make it.  We keep
learning more and eventually we may truly have the key.  Now, hopefully we do have a
hazy outline of the whole.
 
Fun stuff,
 
Tom Eichhorst in New Mexico
 
 
 
Michael Reagin wrote:
 
>      My feeling towards the mutational change would be that it would not be
>      cause for a new species.  If the change occurs within an individual in
>      a thriving population, then the characteristic would eventually show
>      up only occasionally due to dilution in the gene pool.  However if the
>      mutation gave it an ecological advantage, then this trait would
>      eventually predominate within the colony (much like the text book
>      example of the melanic variety of the English moth during the
>      industrial revolution). Of couse if found in the fossil record and
>      noticably different from the parent, I guess it would be considered a
>      different species.  Or, if an individual with this genotypic
>      characteristic was geographically isolated and interbreeded then maybe
>      it should be classified as a different species.
>
>