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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.

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.

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?

Where is my error in thinking then?

Cheers,
Wulf
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http://www.leiopython.de - the white-lipped python site -
http://www.herpers-digest.com - herp related eBooks search -

>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.

>>>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.

Molecular data are also misused for identifying cryptic species. Herps are small animals with limited dispersal powers. Geographical barrier to gene flow can produce distinctive molecular races or so-called cryptic species. There is a trend to name these molecular races as full blown species without any demonstration of reproductive isolation.

>>
>>>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.

That is of course one school of taxonomic thought. If two populations are geographically isolated and they are "diagnosable" than they are on different "phylogenetic trajectories," which means they are different species. That sort of philosophy is misguided, and it has resulted in a return to typological taxonomy that was practiced in the 19th century.

>>
>>>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.

Lots of people still use the subspecies rank. As Ernst Mayr pointed out, the subspecies category is useful and taxonomists will continue to use it. Those who refuse to use the subspecies category simply have an ideological aversion to the subspecies category. To them, the smallest identifiable taxonomic unit is the species. Therefore there is no room in their ideology for subspecies. Since this sort of ideology may come and go in ways that are similar to fashion, the subspecies category will likely outlast those ideologies that do not favor its use.

"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?

Where is my error in thinking then?"

Any population, sympatric or allopatric, *can* become differentiated from other populations and become a new subspecies or species. The question is simply whether or not it will. If there is selection for the characters used to diagnose a species, an isolated segregate experiencing the same selection pressures can remain similar to the main body of the species indefinitely, and never warrant recognition as a separate taxon.

The "evolutionary species concept" in one form sometimes employed in the herp world often involves making the kinds of assumptions you mention, and calling something a separate taxon because the researcher thinks that it probably will become differentiated in the future; my feeling is simply that systematists must deal with current reality rather than trying to read the future. An allopatric population may become differentiated, may remain isolated without differentiation, or may rejoing the main body of the species--we just don't know.

Patrick Alexander

>>The "evolutionary species concept" in one form sometimes employed in the herp world often involves making the kinds of assumptions you mention, and calling something a separate taxon because the researcher thinks that it probably will become differentiated in the future; my feeling is simply that systematists must deal with current reality rather than trying to read the future. An allopatric population may become differentiated, may remain isolated without differentiation, or may rejoing the main body of the species--we just don't know.

Depends a bit on where you look for differentiation. Whereas allopatric populations may not beocme differentiated morphologically if selection pressures stay the same, they will become differentiated at the molecular level, given enough time. The question is whether we should only acknowledge this once differentiation affects morphology, or whether consistent molecular differences (e.g., separate mtDNA haplotype clades) are enough.

The reverse is of course also true - what do we do with allopatric populations that have undergone rapid morphological differentiation within a very short timespan, leaving them largely undifferentiated at the molecular level.

Very often, what a taxonomist does will simply depend on the marker he happens to use or favour, or on what evidence is available. Hence the diversity of opinions among taxonomists.

Cheers,

Wolfgang
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WW Home

"Depends a bit on where you look for differentiation. Whereas allopatric populations may not beocme differentiated morphologically if selection pressures stay the same, they will become differentiated at the molecular level, given enough time. The question is whether we should only acknowledge this once differentiation affects morphology, or whether consistent molecular differences (e.g., separate mtDNA haplotype clades) are enough."

Well, I figure any species concept has to be evolutionary (ironically, this is my main objection to the "evolutionary species concept" ) and therefore only differences that can reasonably be assumed to be evolutionarily important should be considered in species boundaries. This would mean, essentially, differential adaptation to environmental factors or sufficient differentiation in neutral genetic characters to preclude interbreeding.

That you have two mtDNA clades tells you that the two sets of organisms have been isolated in some form. It doesn't tell you anything about differential adaptation, and only at fairly high levels would it imply reproductive isolation. Basically, mtDNA clades tell you something about the past, but rarely say anything about the present or future. Burbrink's split of Pantherophis obsoletus provides a good example; he has nice mtDNA clades, but no evidence for differential adaptation and no evidence for current or future isolation of the clades.

"The reverse is of course also true - what do we do with allopatric populations that have undergone rapid morphological differentiation within a very short timespan, leaving them largely undifferentiated at the molecular level."

You call them separate species. Or subspecies if you're uncertain.

"Very often, what a taxonomist does will simply depend on the marker he happens to use or favour, or on what evidence is available. Hence the diversity of opinions among taxonomists."

True enough.

My feeling at this point, though, is that molecular data are excellent for phylogenies, but cannot, of themselves, tell us much of anything about taxon boundaries.

Patrick Alexander

>>
>>"Depends a bit on where you look for differentiation. Whereas allopatric populations may not beocme differentiated morphologically if selection pressures stay the same, they will become differentiated at the molecular level, given enough time. The question is whether we should only acknowledge this once differentiation affects morphology, or whether consistent molecular differences (e.g., separate mtDNA haplotype clades) are enough."
>>
>>Well, I figure any species concept has to be evolutionary (ironically, this is my main objection to the "evolutionary species concept" ) and therefore only differences that can reasonably be assumed to be evolutionarily important should be considered in species boundaries.

I think "evolutionarily important" is going to be pretty tricky to define, and even trickier to implement in the real world.

>> This would mean, essentially, differential adaptation to environmental factors or sufficient differentiation in neutral genetic characters to preclude interbreeding.
>>
>>That you have two mtDNA clades tells you that the two sets of organisms have been isolated in some form. It doesn't tell you anything about differential adaptation, and only at fairly high levels would it imply reproductive isolation. Basically, mtDNA clades tell you something about the past, but rarely say anything about the present or future. Burbrink's split of Pantherophis obsoletus provides a good example; he has nice mtDNA clades, but no evidence for differential adaptation and no evidence for current or future isolation of the clades.

True, but here we are getting away from allopatric populations, which started this out, and opening a whole different can of worms - the big difficulty with Burbrink's interpretation of his data is that although he has nice mtDNA clades (but with plenty of "fuzzy edges" where they meet and where his sampling allows you to assess to what extent they are geographically separate), there is no information on nuclear gene exchange - for all we know, the bearers of these mtDNA haplotype clades may have rampant male-mediated gene flow between them, there is no way of telling. On the other hand, where have allopatric populations characterised by different mtDNA haplotype clades, that would not be a tenable hypothesis, so that particular problem with that study would disappear.
>>
>>"The reverse is of course also true - what do we do with allopatric populations that have undergone rapid morphological differentiation within a very short timespan, leaving them largely undifferentiated at the molecular level."

>>You call them separate species. Or subspecies if you're uncertain.

Which takes us back to an extreme typological species concept that would elevate every mildly differentiated lizard population on a different rock outcrop in a sea somewhere as a different species - not always very convincing, and not really what many people are doing.

>>My feeling at this point, though, is that molecular data are excellent for phylogenies, but cannot, of themselves, tell us much of anything about taxon boundaries.

MtDNA can't for contiguously distributed taxa, agreed - but if the forms are allopatric, it can certainly provide evidence of extended periods of separation between lineages... at which point we can go back to arguing about whetehr such differences are "evolutionarily important"

If one subscribes to the fundamental notion that species are best defined as independently evolving lineages, then I would have thought that evidence of differentiation at the molecular level would be just as relevant as morphological differences (with the caveat that mtDNA cannot test for paternal gene flow between contiguously distributed populations).

Just bouncing some ideas around...

Cheers,

Wolfgang
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WW Home

"I think "evolutionarily important" is going to be pretty tricky to define, and even trickier to implement in the real world."

True enough, but a general rule of thumb might be easy:
a population or set of populations is a species if it is disjunct from other populations both genetically and in morphology or physiology.

"True, but here we are getting away from allopatric populations, which started this out, and opening a whole different can of worms - the big difficulty with Burbrink's interpretation of his data is that although he has nice mtDNA clades (but with plenty of "fuzzy edges" where they meet and where his sampling allows you to assess to what extent they are geographically separate), there is no information on nuclear gene exchange - for all we know, the bearers of these mtDNA haplotype clades may have rampant male-mediated gene flow between them, there is no way of telling. On the other hand, where have allopatric populations characterised by different mtDNA haplotype clades, that would not be a tenable hypothesis, so that particular problem with that study would disappear."

But the other problem, which I think is just as severe, is that Burbrink has no evidence of any morphological, etc., differentiation between his "species", and this is perfectly applicable to allopatric species. Having two clades of indistinguishable organisms is a distinction without a difference...

It's also worth mentioning that mtDNA lineages coalesce much more quickly than nuclear DNA lineages, so will give higher estimates of isolation and genetic divergence even if males don't contribute significantly to gene flow.

"Which takes us back to an extreme typological species concept that would elevate every mildly differentiated lizard population on a different rock outcrop in a sea somewhere as a different species - not always very convincing, and not really what many people are doing."

Well, yeah, there's a fair amount of fuzziness when it comes to how differentiated is differentiated enough. I figure if there isn't good evidence for genetic differentiation the morphological differentiation ought to be very strong, and vice versa.

"MtDNA can't for contiguously distributed taxa, agreed - but if the forms are allopatric, it can certainly provide evidence of extended periods of separation between lineages... at which point we can go back to arguing about whetehr such differences are "evolutionarily important"."

But extended separation does not demonstrate lack of future reintegration, nor does it necessarily imply differentiation.

To go back to Burbrink for a moment... his mtDNA clades imply that there was some reasonably extended separation, yet those previously separated sets of populations don't look to have any boundaries between them now and are presumably in the process of re-integrating. If correct, his suggestion that the mtDNA clades are the result of isolation in separate glacial refugia (which seems perfectly reasonable to me) would make this an excellent example of how changing climatic or other environmental conditions can induce genetic differentiation even though long-term the isolated populations do not represent independent lineages...

"If one subscribes to the fundamental notion that species are best defined as independently evolving lineages, then I would have thought that evidence of differentiation at the molecular level would be just as relevant as morphological differences (with the caveat that mtDNA cannot test for paternal gene flow between contiguously distributed populations)."

Basically, I'd just change the tense. Species are independently evolved lineages. Once you start calling things species when you think they're evolving independently but they haven't really diverged yet, you put yourself in the position of guessing the future.

Patrick Alexander

>>"I think "evolutionarily important" is going to be pretty tricky to define, and even trickier to implement in the real world."
>>
>>True enough, but a general rule of thumb might be easy:
>>a population or set of populations is a species if it is disjunct from other populations both genetically and in morphology or physiology.

Previously, you seemed to regard the genetic part as an optional extra...

>>
>>"True, but here we are getting away from allopatric populations, which started this out, and opening a whole different can of worms - the big difficulty with Burbrink's interpretation of his data is that although he has nice mtDNA clades (but with plenty of "fuzzy edges" where they meet and where his sampling allows you to assess to what extent they are geographically separate), there is no information on nuclear gene exchange - for all we know, the bearers of these mtDNA haplotype clades may have rampant male-mediated gene flow between them, there is no way of telling. On the other hand, where have allopatric populations characterised by different mtDNA haplotype clades, that would not be a tenable hypothesis, so that particular problem with that study would disappear."
>>
>>But the other problem, which I think is just as severe, is that Burbrink has no evidence of any morphological, etc., differentiation between his "species",

That's not quite true. In his Herpetol. Monogr. paper, he did show that the pattern of variation in overall morphology correspodned more closely to the mtDNA haplotype clades than to the conventional subspecies. That in itself does not demonstrate species status, but does suggest that the historical lineages do have relevance beyond just the mtDNA molecule.

>> and this is perfectly applicable to allopatric species. Having two clades of indistinguishable organisms is a distinction without a difference...

I would disagree - it simply says that two long-standing independent lineages have not evolved differences in those characters we have looked at. Since there are molecular differences (and if they have been independent for a long time, then other molecular markers will also be affected), I would not regard that as a distinction "without a difference"

>>It's also worth mentioning that mtDNA lineages coalesce much more quickly than nuclear DNA lineages, so will give higher estimates of isolation and genetic divergence even if males don't contribute significantly to gene flow.

True, but one could also regard that as a strength - I recall hearing (sorry, don't have a ref for that) that some alleles have not reached coalescence between humans and chimps... the view that we are the same species would pobably be a minority opinion...

>>
>>"Which takes us back to an extreme typological species concept that would elevate every mildly differentiated lizard population on a different rock outcrop in a sea somewhere as a different species - not always very convincing, and not really what many people are doing."
>>
>>Well, yeah, there's a fair amount of fuzziness when it comes to how differentiated is differentiated enough. I figure if there isn't good evidence for genetic differentiation the morphological differentiation ought to be very strong, and vice versa.

That is the sort of compromise I can live with, and, I suspect, that many people follow as a gut instinct. As a herp example, I fully agree with Scott Keogh's sinking of the various Notechis scutatus/ater subspecies, which differ mostly in body size but are minimally differentiated genetically, just as, on the other hand, I haveno problem with recognising Bothrops insularis as a species, despite the fact that, in terms of mtDNA, it is rooted within Bothrops jararaca - it is just too different and not the same snake.

>>"MtDNA can't for contiguously distributed taxa, agreed - but if the forms are allopatric, it can certainly provide evidence of extended periods of separation between lineages... at which point we can go back to arguing about whetehr such differences are "evolutionarily important"."
>>
>>But extended separation does not demonstrate lack of future reintegration,

And yet, in a previous post, you wrote:
" my feeling is simply that systematists must deal with current reality rather than trying to read the future"

>> nor does it necessarily imply differentiation.
>>
>>To go back to Burbrink for a moment... his mtDNA clades imply that there was some reasonably extended separation, yet those previously separated sets of populations don't look to have any boundaries between them now and are presumably in the process of re-integrating. If correct, his suggestion that the mtDNA clades are the result of isolation in separate glacial refugia (which seems perfectly reasonable to me) would make this an excellent example of how changing climatic or other environmental conditions can induce genetic differentiation even though long-term the isolated populations do not represent independent lineages...

The jury is still out on that one - he would need to either reanalyse his morphological data to test whether there is any evidence of hiatuses between phylogroups, or, better, use a genotyping approach (microsats, AFLP) to determine how much intergradation there is between the mtDNA lineages. The evidence published so far does not allow us to judge one way or the other.

>>"If one subscribes to the fundamental notion that species are best defined as independently evolving lineages, then I would have thought that evidence of differentiation at the molecular level would be just as relevant as morphological differences (with the caveat that mtDNA cannot test for paternal gene flow between contiguously distributed populations)."
>>
>>Basically, I'd just change the tense. Species are independently evolved lineages. Once you start calling things species when you think they're evolving independently but they haven't really diverged yet, you put yourself in the position of guessing the future.

I guess we just differ as to what markers we accept as indicators of differentiation - I'd go with profound genetic divergences alone, you would not.

I'd like to carry on this discussion, which has been interesting, but since I am away for the next month, I will have to take a rain check on that. Anyhow, it's been fun.

Cheers,

WW
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WW Home

"Previously, you seemed to regard the genetic part as an optional extra..."

I was being facetious to some extent. Although, if you have fixed morphological differences that aren't just the result of plasticity, you necessarily have genetic differentiation as well.

"That's not quite true. In his Herpetol. Monogr. paper, he did show that the pattern of variation in overall morphology correspodned more closely to the mtDNA haplotype clades than to the conventional subspecies. That in itself does not demonstrate species status, but does suggest that the historical lineages do have relevance beyond just the mtDNA molecule."

Yes, his mtDNA clades correspond better with scale measurements than do the traditional subspecies, but he does not demonstrate that the variation is anything other than clinal. He could apply the same techniques to arbitrary sections of a cline and get the same results.

"I would disagree - it simply says that two long-standing independent lineages have not evolved differences in those characters we have looked at. Since there are molecular differences (and if they have been independent for a long time, then other molecular markers will also be affected), I would not regard that as a distinction "without a difference"."

Suppose you have two clades with divergent sequences in a gene that still codes for exactly the same protein with exactly the same functionality. What's the difference? It's like calling "behavior" and "behaviour" different words because they're spelled differently. The letters are important only in so far as they convey information, and the same is true for nucleotides.

"True, but one could also regard that as a strength - I recall hearing (sorry, don't have a ref for that) that some alleles have not reached coalescence between humans and chimps... the view that we are the same species would pobably be a minority opinion..."

Then again, the main reason why we would be very reluctant to view humans and chimps as conspecific is that there is such abundant morphological, developmental, etc., differentiation.

>>"Which takes us back to an extreme typological species concept that would elevate every mildly differentiated lizard population on a different rock outcrop in a sea somewhere as a different species - not always very convincing, and not really what many people are doing."

"That is the sort of compromise I can live with, and, I suspect, that many people follow as a gut instinct. As a herp example, I fully agree with Scott Keogh's sinking of the various Notechis scutatus/ater subspecies, which differ mostly in body size but are minimally differentiated genetically, just as, on the other hand, I haveno problem with recognising Bothrops insularis as a species, despite the fact that, in terms of mtDNA, it is rooted within Bothrops jararaca - it is just too different and not the same snake."

Agreed.

>>But extended separation does not demonstrate lack of future reintegration,

"And yet, in a previous post, you wrote:
"my feeling is simply that systematists must deal with current reality rather than trying to read the future"."

Yes. If you've got mtDNA clades that might reintegrate or might not, you've got to guess the future in order to come to the conclusion that they do, in fact, represent diverging lineages. OTOH, if you've got evidence that they have already undergone divergent evolution then you don't have to make that kind of guess.

>>"If one subscribes to the fundamental notion that species are best defined as independently evolving lineages, then I would have thought that evidence of differentiation at the molecular level would be just as relevant as morphological differences (with the caveat that mtDNA cannot test for paternal gene flow between contiguously distributed populations)."

"I guess we just differ as to what markers we accept as indicators of differentiation - I'd go with profound genetic divergences alone, you would not."

Yup.

If the differences are really, really strong they alone might be enough. But the genetic sequences most useful for phylogenetic reconstruction are those either not under selection or under constant, stable selection for conserved protein function. They're most useful when they provide the least information about evolutionary divergence, when they either have no impact on phenotype or have the same impact on phenotype throughout the study taxa.

I guess my basic feeling is that the organism is the phenotype. The genotype is important in so far as it causes the phenotype, but has no inherent importance to me; without the phenotype, it's just chemistry rather than biology...

"I'd like to carry on this discussion, which has been interesting, but since I am away for the next month, I will have to take a rain check on that. Anyhow, it's been fun."

I've been enjoying it, too. Perhaps we'll continue later.

Patrick Alexander

>>>>The "evolutionary species concept" in one form sometimes employed in the herp world often involves making the kinds of assumptions you mention, and calling something a separate taxon because the researcher thinks that it probably will become differentiated in the future; my feeling is simply that systematists must deal with current reality rather than trying to read the future. An allopatric population may become differentiated, may remain isolated without differentiation, or may rejoing the main body of the species--we just don't know.
>>
>>Depends a bit on where you look for differentiation. Whereas allopatric populations may not beocme differentiated morphologically if selection pressures stay the same, they will become differentiated at the molecular level, given enough time. The question is whether we should only acknowledge this once differentiation affects morphology, or whether consistent molecular differences (e.g., separate mtDNA haplotype clades) are enough.

As Ernst Mayr and Peter Ashlock pointed out, what taxonomists want to classify are meaningful evolutionary changes, not meaningless neutral genetic drift. When their book was written, there was no way to isolate neutral genetic drift from evolutionary (adaptive) changes in DNA. In the future, as genomic information become more detailed, perhaps biologists can pinpoint the genetic changes that are associated a particular phenotypic change. Even before then though, most biologists would likely agree that mtDNA changes are mostly neutral genetic drift. As geneticists know, most mtDNA lineages within a population have the tendency to become extinct so that in any given population, only one or a few mtDNA lineages will predominate due to chance alone. Occasionally, however, a particular mtDNA haplotype may by chance be associated with a particular phenotype that is favored by natural selection. Since there is no method to distinguish these two types, it is best to refrain from recognizing mtDNA lineages as species.

>>The reverse is of course also true - what do we do with allopatric populations that have undergone rapid morphological differentiation within a very short timespan, leaving them largely undifferentiated at the molecular level.

Are you talking about humans and chimps? Lots of morphological and behavioral changes but very little genetic differentiation? Of course, in such cases, reproductive isolation will result. After all, reproductive isolation functions to preserve well functioning, harmonious genotypes that we call species, as Ernst Mayr pointed out.

>>Very often, what a taxonomist does will simply depend on the marker he happens to use or favour, or on what evidence is available. Hence the diversity of opinions among taxonomists.
>>
>>Cheers,
>>
>>Wolfgang
>>-----
>> WW Home

No matter what marker(s) a taxonomist may use, he/she wants to answer the same questions. And the questions are whether and how much evolutionary changes have occurred between two or more lineages and whether two or more lineages are close relatives or whether they are merely convergently similar. Before these questions have been answered, there is not going to be a satisfactory classification. Unless the classification codifies the amount of evolutonary changes, it is not going to be a useful classification.

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.