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