Posted by:
curteck
at Wed Mar 31 10:53:34 2004 [ Email Message ] [ Show All Posts by curteck ]
Your observation on the western and eastern hognoses being placed in a genus by convention is well placed.
Indeed, it appears by all lines of evidence (genetic, immunological, morphological) that the eastern and western hognose snakes have been separated for a fairly lengthy period. However, when you include the hognose snake species in with the other Xenodontines (the closest known relatives), the hognose species always come out together.
In actuality most of the confusion has come from where H. simus belongs. It is pretty clear to me now that H. simus is very closely related to H. nasicus based on all lines of evidence. Pinou, in her dissertation, made the suggestion that H. simus was closely related to H. platyrhinos but there are many reasons why the immunological data she was examining would not apply to relatively recent divergences.
Anyway, taxonomic classifications are indeed somewhat arbitrary on some level. We use very strict guidelines to name and classify organisms but the most important variable to be as acurate as possible is the one thing we don't have... the timing of events. As a result, Heterodon, as a genus is a much older genus than many other snake genera. Does this mean that they aren't part of a genus? As a matter of convention it is perfectly acceptable in my opinion. The fossils that indicate that H. nasicus and H. platyrhinos do not have a common ancestor still show all of the hognose traits and so still share traits through a common ancestor though not an immediate one.
Hognose snakes are very unusual not only for their morphology or behavior. They seem to be the last of a line of snakes going back many millions of years. This is the very reason why the treatment of the classification of the Xenodontines as a group has received many treatments and all are at odds with one another in some way or another. The next closest relative to Heterodon is either Diadophis, Farancia or Contia and support for the different associations is found in different places. (Disclaimer: I do not agree with the Contia connection named by Pinou. Someday I'll actually finish sequencing my contia samples and show it.)
So it is an interesting discussion from a taxonomic standpoint because of its relevance to how we classify organisms. Until we have a better way of estimating the timing of events we will always have discrepencies in taxonomic levels. For hognose, I don't think the genus classification will change anytime soon. There are only three species and so there is little to go on in trying to find those defining evolutionary events in the group.
Curtis Eckerman
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RE: Evolutionary History of Heterodon - curteck, Wed Mar 31 10:53:34 2004