Charles: Excellent posts. Welcome to evcforum!
Both the allopatric speciation and the more general discussion on the other thread are right on the money. This is precisely what is actually observed. The point that many creationists seem to miss is that it isn't a species that diverges magically into another. Rather, two isolated populations of a single species gradually accumulate differences. If the populations are in contact, there will be an intermediate area on the border of their ranges where genetic mixing can occur. Eventually, groups or populations far from each other will have developed either pre- or post-zygotic barriers that permit scientists to identify them as separate species. As you noted, this is merely a convenience. In reality, there is likely a gradation (at least if the populations are in contact) of allele frequency between the two groups (c.f., ring species, etc).
Hope you enjoy the discussions!
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