>>The "evolutionary species concept" in one form sometimes employed in the herp world often involves making the kinds of assumptions you mention, and calling something a separate taxon because the researcher thinks that it probably will become differentiated in the future; my feeling is simply that systematists must deal with current reality rather than trying to read the future. An allopatric population may become differentiated, may remain isolated without differentiation, or may rejoing the main body of the species--we just don't know.



Depends a bit on where you look for differentiation. Whereas allopatric populations may not beocme differentiated morphologically if selection pressures stay the same, they will become differentiated at the molecular level, given enough time. The question is whether we should only acknowledge this once differentiation affects morphology, or whether consistent molecular differences (e.g., separate mtDNA haplotype clades) are enough.



The reverse is of course also true - what do we do with allopatric populations that have undergone rapid morphological differentiation within a very short timespan, leaving them largely undifferentiated at the molecular level.



Very often, what a taxonomist does will simply depend on the marker he happens to use or favour, or on what evidence is available. Hence the diversity of opinions among taxonomists.



Cheers,



Wolfgang
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