"Doesn't allopatry also mean not having gene flow, a reduced gene pool and therefore eventually the development of certain characters (due to environmental factors)? If so, any allopatric population would become a subspecies and later perhaps a species over time, and could definitly be considered as sibling species prior to subspecific or specific level?



Where is my error in thinking then?"



Any population, sympatric or allopatric, *can* become differentiated from other populations and become a new subspecies or species. The question is simply whether or not it will. If there is selection for the characters used to diagnose a species, an isolated segregate experiencing the same selection pressures can remain similar to the main body of the species indefinitely, and never warrant recognition as a separate taxon.



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.



Patrick Alexander

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