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My apologies for what is probably very much a beginner's question. I am curious when (what year) new world water snakes (Nerodia) split from the old world equivalent (Natrix). Further, which trait, genetic or otherwise, was found to differ enough to warrant the distinction, or was it mostly geographic? Thanks for any insight...

The split was made in 1977, in this paper:
ROSSMAN-D-A; EBERLE-W-G. `PARTITION OF THE GENUS NATRIX WITH PRELIMINARY OBSERVATIONS ON EVOLUTIONARY TRENDS IN NATRICINE SNAKES.' Herpetologica-. 1977; 33 (1): 34-43.

The abstract of the paper is:

`The generic distinctiveness of each of the geographically isolated species clusters currently placed in the genus Natrix is demonstrated by investigations of blood proteins, chromosomes, scutellation, cranial osteology and hemipenes. The European species retain the name Natrix. Nerodia is revived for the North American species, and the names SINONATRIX and AFRONATRIX are proposed for the Asian and African genera, respectively. Nerodia and the 8 other New World natricine genera [Adelophis, Clonophis, Regina, Seminatrix, Storeria, Thamnophis, Tropidoclonion and Virginia] are placed in a new tribe, the THAMNOPHIINI, to emphasize the closeness of their relationship. Possible evolutionary trends are discussed for all characters except blood proteins.'

Having not read the paper, I don't know what features in particular of the traits mentioned were important, but that at least gives the general idea...

Patrick Alexander

Thanks a ton. Very interesting!

Rossman and Eberle cite biochemical data that suggests New World "Natrix" are more closely related to each other and to New World natricine genera than they are to Old World Natrix. Nonetheless, such results are possible if Old World Natrix is paraphyletic with respect to New World watersnakes. Looking at the data of Dowling et al., this scenario appears to be strongly supported. Old world watersnakes are basal to the New World watersnakes in Dowling et al.'s data. Comparing the definitions of the genus Natrix with that of Nerodia in Rossman and Eberle, they are identical with the exception of the number and types of chromosomes and hemipenial structure.

The differences in karyotypes can be the result of an evolutionary change in the derived lineage of the New World natricines (an autapomorphy in this lineage), therefore it does not refute a close relationship between New World and Old World watersnakes. The hemipenial structure of Nerodia is indeed disparate from Old World Natrix. However, Regina, which was once included as part of New World "Natrix", has a hemipenial morphology that is very similar to that of Old World Natrix (compare figs. 4D and 4 H of Rossman and Eberle), suggesting that the hemipenial structure of Nerodia is a derived condition. Regina is basal to Nerodia and Old World watersnakes are in turn basal to Regina. Therefore the similarities between the hemipenis of Regina and Natrix appear to be a shared ancestral character which has since been modified in the derived lineage Nerodia.

Old World Natrix and New World "Natrix" (which includes Regina, Colonophis and Seminatrix) appear to be part of a monophyletic group. The extensive morphological similarities between Old World and New World watersnakes therefore appear to be due to common ancestry rather than convergent evolution.

The species in Regina, Nerodia, Storeria, Colonophis and Seminatrix could have been retained in a paraphyletic Natrix with Old World species of Natrix. Without including Regina and Colonphis, which are basal to Nerodia, Nerodia plus Natrix would be polyphyletic. Therefore, unless there is a major revision of the watersnakes, for the sake of taxonomic stability, it is best to recognize Natrix, Regina, Nerodia and Clonophis as valid genera.

The evolutionary history of this group is pretty clear from the morphological and biochemical data:

Water snakes originated in the Old World. One species of Old World watersnake migrated to the New World (most probably from Europe across a land bridge connecting the eastern US and Europe) and this species is ancestral to Storeria, Regina, Nerodia, Clonophis and Seminatrix. Storeria, Regina, Clonophis and Seminatrix are basal among the New World Natricines and they resemble Old World Natrix morphologically. Nerodia is a more derived group and differ more from Old World Natrix. These species also are restricted to the eastern US in distribution. It is from Nerodia that the species of Thamnophis evolved. Thamnophis appears to be more suited to the drier conditions of the western US and the members of this genus are the only Natricines that have spread westward. Thamnophis sauritus is probably not a member of the genus Thamnophis, since it is basal to the clade that includes Nerodia, Thamnophis elegans and Thamnophis sirtalis. T. sauritus may only be convergently similar to the other species of Thamnophis.

After looking up paraphyletic and monophyletic I was actually able to follow that. Thanks a million. My interest in this stems from the fact that I grew up in Europe and have been living in the U.S. for quite a while, where my childhood interest in herpetology seems to have awoken from a 20 year hibernation period. I was always fascinated with the Natrix genus, specifically the "grass snake" Natrix natrix. In keeping with my interest in the local fauna, I have now begun to educate myself on New World natricines, specifically Thamnophis and Nerodia.