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RE: Bull snake taxonomy

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Posted by: CKing at Fri Apr 18 23:34:36 2008  [ Report Abuse ] [ Email Message ] [ Show All Posts by CKing ]  
   

>>Since this is a "hot topic", maybe I should say something.
>>
>>First, figuring out what the "real" scientific name is is a bit problematic. Systematists disagree much of the time and there is no governing body decreeing which is right (although CNAH & SSAR both produce "standardized" names lists, these have no more authority than we choose to give them), so all we can do is either look to what the majority seems to be doing or come to our own decisions on what we think is right.

A third alternative is to seek the opinions of those who we respect, especially in cases where we are unfamiliar with the taxa in question. In the case of the genus Pituophis, we very fortunately have a published paper based on mtDNA data, detailing the relationships within the genus. This paper is fortunately available online:

http://faculty.unlv.edu/jrodriguez/21.pdf

>>In this case, Pituophis catenifer sayi is the most commonly used name, with the others being synonyms. I don't trust CNAH too much, but they use it. I think SSAR does the same, but I can't figure out how to get to their list at the moment.
>>My personal guess is that P. c. sayi is more closely related to P. melanoleucas than to other P. catenifer but has moved west since its origin, so that it is now in contact with other P. catenifer rather than to the more closely related P. melanoleucas. If this is right, grouping P. c. sayi based on relatedness or morphological similarity would place it with P. melanoleucas, but grouping it based on interbreeding would place it with P. catenifer. How we ought to classify taxa in this kind of situation isn't entirely clear, but the present generally accepted split between P. catenifer and P. melanoleucas, with P. c. sayi as a member of the former, doesn't strike me as unreasonable.

Looking at the mtDNA trees of Rodriguez-Robles and De Jesus-Escobar, the genus Pituophis originated in the east, with the mugitus-lodingi-melanoleucus complex as basal to the other taxa in this genus. When the range of this ancestral complex expanded westward, there is apparently an adaptive radiation (shown as an unresolved polytomy in one of the mtDNA trees), which has resulted in the taxa currently recognized as P. catenifer, P. deppei, and P. lineaticolis. From the mtDNA data it appears that sayi is descended from a population of affinis, and that ruthveni in turn is derived from a population of sayi, which is of course counterintuitive. The Louisiana pine snake (ruthveni) is morphologically and geographically closer to the ancestral melanoleucus stock than to affinis and sayi. ruthveni is to me almost certainly basal to the other Pituophis lineages west of the Mississippi River given its morphology and its geographic location. It is most likely that ruthveni begets sayi, which begets affinis and other taxa within Pituophis to the west of sayi. Currently ruthveni is considered a distinct species primarily because it is geographically isolated from other taxa within Pituophis. H. Dowling has shown that the catenifer-melanoleucus complex is probably a single species since sayi appears to bridge the taxa to the west with those to the east both morphologically and geographically. Therefore to me, P. melanoleucus, "P. ruthveni", and "P. catenifer" form a single wide ranging species, instead of 3 different species. The single species arrangement is not contradicted by the mtDNA data since such a grouping is not polyphyletic.


   

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