Hello Conch-L,
I must disagree with the posted conclusions concerning the a-b-c-d
fish populations.  The situation described would actually constitute
proof that these populations ARE all the same species, even though
the extremes are geographically, and therefore genetically, isolated
from each other.  The fact that gradual intergrades exist from one
form to another is evidence that the extreme forms do not represent
separate species, even if long-time geographic/genetic isolation were
to render the extreme forms incapable of producing offspring together.
If pop-a and pop-b are forms or subspecies of the same species, then
the same must be said of pop-b and pop-c; and likewise of pop-c and
pop-d, and so on down the line to pop-y and pop-z.  So it kind of
boils down to a basic theorem of logic, namely if A=B and B=C then
A=C.  This example involves two geographically isolated populations
which are "connected" by intermediate populations.  What about
geographically/genetically distinct populations which are NOT
connected in this way?  Nowhere are more distinct examples of this
found than in the Mollusca.  There are species of Cymatium and Bursa
for example that are found virtually worldwide.  Cymatium
parthenopeum can be found on both sides of the Atlantic and both
sides of the Pacific.  These populations are separated from one
another either by an ocean, or by a continent.  Do they thereby
automatically qualify as distinct species?  I think not.  The most
that can be said is that they are in a position to diverge
genetically, perhaps to the point of speciation, given sufficient
time.  In the meantime, a Bursa thomae or Cypraecassis testiculus or
Architectonica nobilis from west Africa, and a specimen of the same
species from Florida, are still one and the same species.  In  some
cases such populations are assigned subspecific names (Cypraecassis
testiculus senegalica).  I don't believe such names are warranted by
the mere fact of genetic isolation.  Such names should be attached
only if genetic isolation has already resulted in some observable
morphologic difference between the two populations.  In other words,
if you can tell by looking at a specimen whether it came from Africa
or Florida, then a new taxon might be reasonable.  Otherwise,
specimens transplanted to a new locality (such as several species of
Japanese origin now found in western Canada) would automatically
become a new entity, which is no more logical for mollusks than it
would be for any other organisms.
Paul Monfils
Rhode Island, USA