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RE: another try: Sibling species...

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Posted by: aspidoscelis at Thu Sep 1 23:47:56 2005  [ Report Abuse ] [ Email Message ] [ Show All Posts by aspidoscelis ]  
   

"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


   

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