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RE: another try: Sibling species...

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Posted by: WW at Thu Sep 1 13:25:01 2005  [ Report Abuse ] [ Email Message ] [ Show All Posts by WW ]  
   

>>
>>"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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