>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.

Sounds like a cryptic species to me. This would be something that was indistinguishable from others, but has now been demonstrated to be different based on new data. Molecular data are often used for this.

>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.

Well if they are diagnosable, one could consider them different species. But if they are indistinguishable then they are not really a different species.

>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?

Currenly most taxonomists (at least the ones dealing with herps in North America) use the species rank for diagnosable entities (populations or groups of populations).
Few people use subspecies or sibling species. Some people consider sibling species to be the sister taxon at the species level, which seems to make the most sense to me. What you described above sounds like it could be considered a population.

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