Richard Klim (post #25) evidently was a bit faster than I with that last post, and he dealt with the same issue I mentioned in approx. the central section of post #26.
... I don't see how you can write that "numerous subspecies come into contact, and if there was little or no intergradation, they would be species rather than subspecies". Ultimately, of course that is the case, but are we not talking about the now, not the remote future here?
I certainly mean
now, not in the future. Take any Australian species of bird where the map shows a continuous population (except migratory species, of course), yet there are more than one subspecies in Australia. These subspecies intergrade where they come into contact. If not, they are species, not subspecies. This is the very definition of the Biological Species Concept, and that is the concept followed by the major Australian authorities (e.g. Christidis & Boles, 2008, and before that Schodde & Mason, 1999). If the Dutch and South African authorities have been the authorities most willing to adopt possible PSC species and novel taxonomic ideas, the Australian authorities have generally been in the opposite end of that spectrum. Just to give a few examples that match what I described above (subspecies come into contact and where they do, they intergrade), here are the Australian species of honeyeaters where this is mentioned in the recently published Handbook of the Birds of the World vol. 13 (chapter written by an all-Australiam team of top authorities; Higgins, Christidis & Ford): Yellow-spotted Honeyeater, Singing Honeyeater, Yellow Honeyeater, White-eared Honeyeater, Yellow-tufted Honeyeater, Grey-fronted Honeyeater, Fuscous Honeyeater, White-plumed Honeyeater, Yellow-throated Miner, Red Wattlebird, Brown Honeyeater, Black-chinned Honeyeater, Brown-headed Honeyeater, White-throated Honeyeater, Blue-faced Honeyeater, & Little Friarbird (my mistake if a missed one or two, but it was only a fast check through the species accounts). This is just the species where they specifically mention it in the text – as it is the default option when dealing with BSC subspecies that come into contact, there generally is no reason to mention it.
...The general practice in my part of the world (and I believe in yours as well) over the last 20 or so years has been to gradually whittle away the inconsistencies in the way we have assigned species and subspecies rank to populations between which substantial gene flow exists.
The included "substantial" is of course open for interpretation (as it is in the last section further down), but as long as the variation versus distribution do not act entirely or largely as a "smooth" cline, above is incorrect for subspecies. I doubt Australian biologists following BSC view this any differently than biologists in European or the US. See also previous section with references & examples.
... The sitellas are a case in point: they used to be regarded as individual species but are no longer. The present classification of them as subspecies endures for the time being because we simply don't know enough about them to discard our current (clearly unsatisfactory) classifications and replace them with new ones. It appears clear that the Varied Sittella (current sense) is paraphyletic, but it isn't clear whether the best response is to re-split, or to lump them with the related New Guinea taxa. So the current unsatisfactory classification remains.
I am not aware of any evidence that suggests the Varied Sittella
sensu lato is paraphyletic. Compared to what? The very different Black (so different that it has been placed in a different genus)? I have not seen any data for the Black, but that sounds unlikely, and I would be very surprised if the Varied Sittella
sensu lato was anything but monophyletic. That, of course, does not necessarily mean it is a single species. The Australian situation is quite well known, and the zones of intraspecific hybridization have been extensively documented (indeed, a region where all five Australian ssp's intergrade has been identified in central Qld). As long as staying with the standard BSC, the only real issue that remains is Australian Varied versus New Guinea Varied (and/or possible variations within New Guinea). Unless, of course, someone can provide evidence of lowered fitness, but at present I am not aware of anything pointing in that direction. If anyone is, I would be interested in a reference.
... Nevertheless, many former bird and mammal subspecies have been merged into a single taxon in recent years on the basis of evidence of substantial interbreeding.
Assuming you use "taxon" in the above as a substitute for subspecies rather than species (as taxon can be used for all levels), I cannot agree, as long as disregarding clear clines. If you know of examples of this under the BSC, I would be interested in knowing about them. However, numerous ssp's have disappeared from usage when it was realized that they represented nothing but a hybrid population between two other ssp's. If that was what you meant, my mistake.