>>>>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.
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>>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.

As Ernst Mayr and Peter Ashlock pointed out, what taxonomists want to classify are meaningful evolutionary changes, not meaningless neutral genetic drift. When their book was written, there was no way to isolate neutral genetic drift from evolutionary (adaptive) changes in DNA. In the future, as genomic information become more detailed, perhaps biologists can pinpoint the genetic changes that are associated a particular phenotypic change. Even before then though, most biologists would likely agree that mtDNA changes are mostly neutral genetic drift. As geneticists know, most mtDNA lineages within a population have the tendency to become extinct so that in any given population, only one or a few mtDNA lineages will predominate due to chance alone. Occasionally, however, a particular mtDNA haplotype may by chance be associated with a particular phenotype that is favored by natural selection. Since there is no method to distinguish these two types, it is best to refrain from recognizing mtDNA lineages as species.

>>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.

Are you talking about humans and chimps? Lots of morphological and behavioral changes but very little genetic differentiation? Of course, in such cases, reproductive isolation will result. After all, reproductive isolation functions to preserve well functioning, harmonious genotypes that we call species, as Ernst Mayr pointed out.

>>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.
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>>Cheers,
>>
>>Wolfgang
>>-----
>> WW Home

No matter what marker(s) a taxonomist may use, he/she wants to answer the same questions. And the questions are whether and how much evolutionary changes have occurred between two or more lineages and whether two or more lineages are close relatives or whether they are merely convergently similar. Before these questions have been answered, there is not going to be a satisfactory classification. Unless the classification codifies the amount of evolutonary changes, it is not going to be a useful classification.

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