Emilio,
I think you are right on.  The entire concept of species is clear cut only in old
texts.  As we learn more, the definition of species becomes a bit wider and very,
very cloudy.  One way species are separated and defined is whether or not they
mate with each other in their natural habitat.  This is not a separation of
species because their offspring would be infertile but a separation because they
are separate.  Thus if they have differentiating physical traits and are
geographically separate OR separated by closely related subspecies, then they can
be considered separate species that may indeed have viable offspring if they were
brought together in another environment.
 
To use your cichlid example, four subspecies of fish-X are found in Lake Malawi;
labeled pop-a, pop-b, pop-c, and pop-d.  Pop-a is on the North end of the lake,
pop-b is adjacent to pop-a to the South, pop-c is next, and finally pop-d is the
most Southern population (fried chicken, accent with a deep drawl and all).
Sorry got carried away.  Anyway, in this example pop-a can interbreed with pop-b,
and pop-b with pop-c, and pop-c with pop-d.  However, pop-a cannot interbreed
with pop-d, it is too far and there are all of these darn fish in between.  These
two (a and d) will probably be considered species and the two populations between
(b and c) will be intergrades or subspecies.  And no one knows or really cares if
a and d can successfully interbreed!!!  It is enough that they do not.
 
One more example and I'll quit, I promise.  Where I live in New Mexico, USA, it
is against the law to keep and raise exotic mammals unless you have a specific
license, over 3,000 acres or are an established zoo.  This is unlike our eastern
neighbors in Texas where any ranch can have African antelope, giraffes, or
whatever - no matter how small the ranch.  Not here.  However, if you go to my
youngest brother's place just down the road, you will see a beautiful Ibex goat.
This is certainly an exotic and yet my brother lives on only 10 acres.  He can
legally do this because the goat is only 15/16 Ibex and 1/16 domestic goat.
These two separate species have been interbred for years and all of the offspring
are viable.  Both goats are from common old world stock that speciated many, many
years ago.  For countless years they did not mate and reproduce across species
lines, not because they couldn't but because they didn't.  Now, if we want a goat
with wild horns we can cross Ibex with domestics, get our wild horns but still
have a legally domestic goat.  A bizarre law but not nearly as bizarre as some in
nature.
 
The proceeding sermon was brought to you by a history major so don't get too
worked up if I bent a biological point or two.
 
And speaking of shells, I recently purchased a deep water limpet, Amathena
tricarinata (Linnaeus, 1758) from Tom Rice's place but I can't seem to find its
proper family.  Anyone know where this guy belongs?  Thanks,
 
Tom Eichhorst in New Mexico, USA