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Taxonomy and Systematics

What is a Boa? What is a Python?

by Mitch Hiers

For those who may be unfamiliar with the term "taxonomy" or who may, from time to time, find themselves a bit bewildered as to its meaning, welcome to the club! While the purpose of this article is not to serve as a primer on the subject of taxonomy, I will attempt to present you with as clear an explanation as I can as to how the process works and why recent changes in the taxonomic classification of boas and pythons have taken place. My personal definition of taxonomy is that the term refers to the process by which biologists attempt to classify the diverse life forms on our planet and categorize them according to assumed relationships based on apparent similarities between those life forms. These classifications are made through the application of various theories and methodologies, and should take into consideration a variety of factors discovered through careful observation and scientific analysis.

Systematics is the science of investigating the relationships between the taxa, and recent years have seen many changes in theories as well as advances in analytical methods, such as molecular systematics. Extensive field research is conducted along with exhaustive analysis in the laboratory, after which hypothesized relationships and proposed changes to current classifications are debated at length by recognized authorities before they can be accepted or rejected by the scientific community at large. If a proposed taxonomic change is ultimately accepted, then the current scientific nomenclature for that life form is to be changed in a manner consistent with the Latin-based conventions of the Linnean system.

The taxonomists who debate the relative merits of a proposed change in classification can themselves be classified into two basic groups! These are the "lumpers" and the "splitters". At one extreme, the quintessential "lumper" might be the systematist who is such a purist from his school of thought that he would assert there are no such things as "subspecies", citing molecular systematic studies and DNA analysis as his or her reasoning. At the other extreme end of the scale you might find the ultimate example from the "splitter" school of thought, someone who would consider naming new subspecies based on the slightest phenotypic variations. In my opinion, both approaches would be equally ludicrous. I have always thought that in order to come up with a sensible approach to taxonomy, one must take everything into account -- molecular systematics, physiology, morphology, overall phenotypic similarities, common evolutionary trajectories, populations that have apparently been evolving in isolated habitats, etc. I think the subspecies concept most definitely has merit, even if only to classify life forms that appear to have begun taking the first few steps of an evolutionary detour from their conspecifics -- life forms that appear to have recently entered a kind of transitory stage, so to speak.

A relatively recent taxonomic concept that appears to be gaining widespread acceptance is that classification should be based on phylogenetic relationships. Think of this as a modern variation of the Linnean system in which overall similarity was the basis for classification. The modern guideline is that a taxon at any level of classification should include all of the descendants of the common ancestor of that group. Thus, in theory at least, one should be able to assume that all of the species in the same genus include all of the descendants of the common ancestor for that genus. Sound familiar? This is what I meant by "common evolutionary trajectories." Therefore, it should also follow that the species in that genus would be more closely related to each other than to other genera in that same subfamily. This type of group is called a monophyletic group, or "clade." Using "cladistic" analysis, herpetologists have in recent years begun questioning previously assumed relationships between herp taxa, and this is the reason for many of the recent changes in the classification of the snakes we have heretofore known as boas and pythons -- those currently (and some now formerly) included in the family Boidae. In Dr. Arnold Kluge's 1993 study of the subfamily Pythoninae, he compared no less than 121 behavioral and morphological characteristics of 24 species of pythons and generated hypothesized relationships based on cladistic analysis. These relationships are illustrated in the Figure 2 diagram accompanying this article. The results of his study point to Australia as the geographic origin of the pythons. Likewise, similar studies conducted by Kluge (1991) and others using cladistic analysis have supported revisions in the taxonomic classification of the subfamilies Boinae (the "true" boas) and Eryclnae (the sand boas, rosy boas, rubber boas, and the oviparous Calabar burrowing "python"). These relationships are illustrated in the Figure 1 and Figure 3 diagrams. Such studies have led to some genera previously considered members of the Boidae to be moved out of the family altogether. The Mexican burrowing "python" (Loxocemus bicolor) was considered to be more closely related to the Xenopeltidae (the sunbeam snakes) and was subsequently moved out of the Pythoninae subfamily of the Boidae. Two previously recognized Boidae subfamilies, the Tropidophiinae (the dwarf boas, or wood snakes, of the Caribbean islands) and the Bolyeriinae (the extremely rare, oviparous Round Island boas), were moved out of the family Boidae and elevated to full family status as the Tropidophidae and Bolyeridae, respectively. The phylogenetic relationships of these two families are illustrated in the Figure 4 and Figure 5 diagrams.

So... exactly what is a boa? And what is a python? Well, I suppose technically speaking, only those snakes classified in the family Boidae. However, there will still be herpers who cannot or will not shake their sentimental attachments and will persist in making references to Mexican burrowing "pythons" or Caribbean dwarf "boas", despite the fact that those animals are no longer "officially" considered boids. Even staying within the family Boidae, there are herpers who still insist on referring to their green tree pythons as Chondropython, or Chondros for short, even though they are now classified Moreila viridis. Even I am having a hard time getting accustomed to using the Boa genus instead of the old Acrantophis for my Dumeril's boas. It is now Boa dumerili for the Dumeril's boa, Boa madagascadensis for the Madagascar ground boa, and Boa mandrita (instead of Sanzinia madagascadensis) for the Madagascar tree boa, and I'm constantly having to remind myself! But you know what? We all might as well just get used to it. Time marches on, and evolution along with it. Every year new methods of scientific analysis are developed and "new" discoveries are made. Advancing technology provides the keys that open doors to more in-depth knowledge of the natural world around us. The prospects are exciting. By being observant and keeping an open mind, amateur herpers can contribute greatly to the growing knowledge base. As for me, regardless of how it may be officially classified, if it is a boa or a python and it is obtainable, the challenge will be to get it to breed successfully in my facility. Each new clutch is like a Christmas present!