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RE: Elaphe/Pantherophis/Pituophis?

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Posted by: CKing at Fri Apr 18 01:22:48 2008  [ Report Abuse ] [ Email Message ] [ Show All Posts by CKing ]  

>>I normally get a lot of questions regarding native U.S. identification, and I like to be as thorough as possible. But this whole mess has got me at a stand still. Which genus is most commonly accepted now for North American Ratsnakes?
>>Has the DNA really proven the Pituophis placement?

The North American species of ratsnakes have traditionally been classified in the genus Elaphe, the same genus as the ratsnakes of Eurasia. There have been a recent attempt to split this genus by a group of researchers who, on the basis of mtDNA evidence and hemipenis morphology, resurrected a large number of genera from the synonymy of Elaphe. One of the problems with this study is its reliability, if the placement of Senticolis triaspis (the green ratsnake) is any indication. Another problem is that these authors fail to provide any definition for the genera they recognize, so it is impossible to know what the differences exist between one genus and another, if any. The main reason for dividing the genus Elaphe appear to be philosophical. There is no evidence that the genus Elaphe is an unnatural polyphyletic assemblage of species, but there is evidence (in fact it has been well known for decades) that the genus Elaphe is paraphyletic. The proposal to split the genus Elaphe is apparently a philosophical intolerance of paraphyletic taxa.

The distinction between paraphyletic and polyphyletic is important. A polyphyletic taxon is unnatural because it consists of species that are descended from two or more recent common ancestors. No taxomist will knowingly recognize polyphyletic taxa, although it has been pointed out that cladists are more likely to recognize polyphyletic taxa because of their uncritical choice of taxonomic characters and the likelihood that the characters they use are convergent similarities, which are not evidence of common ancestry but which can lead to the recognition of polyphyletic taxa. A paraphyletic taxon, on the other hand, is descended from a single recent common ancestor. For centuries, paraphyletic taxa (e.g. Reptilia) have been recognized by taxnomists as valid. However, about a half a century ago, the German scientist Willi Hennig proposed that paraphyletic taxa should not be recognized. His proposal was largely ignored for decades. Recently, a group of taxonomists known as the cladists adopted Hennig's classification and began dismantling paraphyletic taxa.

Traditionally, taxonomists recognize only monophyletic taxa. To them, paraphyletic taxa is a term that did not exist, since paraphyletic taxa are considered monophyletic. Hennig, however, redefined monophyletic as a group that consists of all of the descendants of a common ancestor, and excluded paraphyletic groups (which consists of the descendants of a common ancestor but not all of the descendants) from the definition of monophyletic. The traditional Reptilia is a paraphyletic taxon, since the birds and mammals are descended from the same ancestor that gave rise to the reptiles, but the birds and mammals are excluded from Reptilia. To the cladists, the taxon Reptilia is paraphyletic and therefore it is "non-monophyletic". As such it is not valid. To the traditionalists, the taxon Reptila is in fact monophyletc. This hair-splitting difference between the philosophies of two schools of taxonomists is the source of a great deal of taxonomic controversy which exists today.

The group of researchers who propose to split Elaphe do so because Elaphe is paraphyletic or "non-monophyletic." The traditionalists would disagree with this distinction, because paraphyletic taxa are indeed monophyletic. It is a well known fact for decades that Elaphe is paraphyletic by the definition of Hennig, and yet nobody bothered to split Elaphe on that basis. The proposal to split Elaphe is therefore unnecessary and it will only lead to taxanomic chaos.


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<< Previous Message:  Elaphe/Pantherophis/Pituophis? - MikeFuture, Wed Aug 15 21:37:03 2007