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Wulf wrote:

"Hello folks,

as far as I have learned, sibling species are (morphologically) indistinguishable from (a certain) other species. But in fact, "sibling species" are no valid taxonomic entity, but I have read some papers regarding such as separate sub- or full species."

Me:
Sibling species are simply just that, siblings. That means they share a more recent common ancestor than they do with other species. An example of 2 sibling species would be the common chimp and the pygmy chimp. Sibling species are not necessarily morphologically indistinguishable from one another, although they tend to be morphologically similar in many cases.

"I understand that even allopatric populations are not always considered a subspecies or at specific rank, at least if morphologically indistinguishable from the 'nominate species'. Although this was commonly done in the past.
So, what is to do with allopatric species that are indistinguishable from a certain other species."

Me:
Allopatry is a poor determinant of species distinctness. A much better and universally accepted criterion is reproductive isolation.

Wulf:
"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)?"

Me:
True. Allopatry means that two populations that are geographically isolated from each other will each develop their own unique alleles. This is a process known as genetic drift. Some biologists suggest that the presence of such unique alleles is evidence that the two populations are different species. As Ernst Mayr pointed out, however, systematists want to classify organisms on the basis of evolutionary changes, not neutral genetic drift.

Wulf:
"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?"

Me:
That is the rationale that some would use to suggest that two allopatric populations, if they are going to drift apart, might as well be recognized as distinct species NOW. That too is absurd because no one can predict the future. These two populations may become extinct before either or both of them have evolved into a different species. Or they may meet again and then interbreed freely sometime in the future. Many of the island populations off the coast of California, for example, may come in contact with mainland populations during the next ice age. Many of them have become isolated from mainland populations only since the end of the last ice age, about 12,000 years ago.
Therefore it would be wise not to regard allopatric populations as distinct species unless reproductive isolation can be demonstrated (premating isolation) or unless it can be shown that they are so different ecologically that a hybrid would be unlikely to survive in the wild (postmating reproductive isolation) because of reduced fitness.

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