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Why are there subspecies?

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Old Friday 1st May 2009, 15:20   #26
Rasmus Boegh
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Quote:
Originally Posted by Argon55 View Post
Turning the question around somewhat, it is interesting to speculate why some of the members of genus Pan should be reclassifed as Homo.
Of note that there are no fixed rules for genera, and as such the claim of how much a genus should/shouldn't include is to some extend arbitrary.

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Originally Posted by Tannin View Post
Determination of "species" and "subspecies" has nothing to do with the size or number of observable differences between populations. Repeat, nothing. Species and subspecies classifications are only assigned where there is a lack of gene flow between the two populations.
It is a bit more complex. First, the vast majority of subspecies that are recognized are based on observable differences between populations. Indeed, while there are cases where genetic differences have been confirmed between different populations, but there are no other known differences (morphology, voice, etc.) between them, the general standard has been to maintain them in a single taxon. When using the BSC, a clade doesn't necessarily equal a taxon and vice versa (though some have argued for modified BSC where these are more equal). Strictly speaking this is only the case if using e.g. PSC. Secondly, lack of gene flow: In species, yes. In subspecies, no. Otherwise, for example the Australian population of the Varied Sitella would fall into a single subspecies, as there are large zones of intergradation where the currently recognized subspecies come into contact, i.e. there is geneflow. This is why the idea of considering them as separate BSC species was abandoned in the first place. Numerous subspecies come into contact, and if there was little or no intergradation, they would be species rather than subspecies (per BSC), though one can discuss how much gene flow is needed for two taxa to be separate spp. rather than ssp's (as in e.g. Corvus cornix/corone). So, if limiting it to allopatric speciation (which of course also applies for ssp's), then the isolation only has to have been there at some historical point, but when including parapatric and - especially - sympatric speciation, it becomes far more complex (as briefly noted in earlier posts). Finally, specifically regarding the case discussed here, the current absolute free movements between human populations is very recent. Depending on how you view it, it goes back perhaps a few hundred years, and that figure is so small that it is of little - if any - importance from an evolutionary point of view in a species with a generation length as humans. What genetics have confirmed is that 1) The spread of humans is relatively recent (this brings in possible issues of punctuated equilibrium, but that's for another thread), and the figure is generally too low for subspeciation to have had the chance of taking place. 2) While it would have been entirely sensible to believe that certain human populations have been very isolated (e.g. Americas versus remaining populations), geneflow appears to largely have persisted. Anyhow, the above issues aside, the result is the same; no human subspecies.
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Old Friday 1st May 2009, 17:11   #27
Tannin
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Rasmus, I'd make three points here. First, I was not writing for clarity, not detail. So yes, the minutiae does become a bit more complex!

Secondly, 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?

Finally, variations in the actual practical application of a set of rules do not make those rules invalid. As you say, once you go beyond the species level, it all becomes rather arbitrary. (And I for one don't see how this could not be so.) Also, as you say, to assign subspecies (or species) rank to a population there must be an observable difference - but difference alone does not define a species! And whatever the practice commonly is, the theory which (mostly) guides that practice is quite clear. (I'm assuming that we are talking BSC here - once we start veering off into alternative species concepts the water becomes so muddy that you could drink it with a fork.)

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 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. (We may well discover that the sitellas are yet another of those troubling examples where the observed facts of nature refuse to fit in neatly with our (inevitably simplified) artifical classification schemes.)

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. Little by little, our "map" of the biological world starts to better resemble the territory.
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Old Friday 1st May 2009, 19:53   #28
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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.

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Originally Posted by Tannin View Post
... 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.

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Originally Posted by Tannin View Post
...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.

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Originally Posted by Tannin View Post
... 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.

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Originally Posted by Tannin View Post
... 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.

Last edited by Rasmus Boegh : Friday 1st May 2009 at 22:33. Reason: typo
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