The ABC's of Animal Taxonomy

by Donald L. Blanchard

Taxonomy:   The science of classifying plants and animals into species and logical groups of species.

This article was originally published in The Cold Blooded News, the newsletter of the Colorado Herpetological Society, Vol 26, #1, January, 1999.

There are two distinct taxonomic systems currently in vogue among professional zoologists today. The traditional, or Linnaean, taxonomy is still largely in favor among field workers, conservationists, and husbandry people. The alternative, Cladistic taxonomy, is overwhelmingly supported by evolutionary biologists.

The undisputed father of taxonomy was the Swedish botanist Karl von Linné (1707-1778). Because virtually all science in the 18th century was written in Latin, von Linné is better known by his Latinized name, Carolus Linnaeus. Linnaeus created the system of scientific nomenclature still in use today, wherein every species is given two Latin names, a genus, or group name, and the species name. In formal Latin usage, the genus name is a noun and is capitalized, while the species name is an adjective, which should agree in gender with the genus, and is written in lower case.

Over time, biologist added additional, larger and higher level group names, called taxons (plural: taxa), from Family up to Kingdom, arranged in a hierarchical order, until a standardized 7-level hierarchy was established, as follows:
Kingdom   Animalia
  Phylum   Chordata
    Class   Reptilia
      Order   Squamata
        Family   Colubridae
          Genus   Pituophis
            species   catenifer
              (Bullsnake, Gopher Snake)

To further facilitate grouping similar or closely related groups, these taxa may optionally be divided with from one to three named intermediate-level taxa, as required. For example:
Class   Major division (required)
  Subclass   1st optional subdivision
    Infraclass   3rd optional subdivision
      Superorder   2nd optional subdivision
        Order   Major division (required)
The taxa Superkingdom and Infraspecies are generally not used, leaving a maximum of 26 possible taxonomic categories, although all are rarely required for any given species (or subspecies).

One sub-taxon that is most commonly used is the subspecies, as in Pituophis catenifer sayi, the bullsnake, as shown in the example above. Others that should be familiar to herpers are Suborder Lacertilia (lizards), and Suborder Serpentes (snakes), two divisions of the Order Squamata. (The third division of Squamata is Suborder Amphisbaenia, the worm lizards.)

The original purpose of taxonomy was the recognition, categorization, and identification of organisms. Therefore, species were grouped into higher level taxa based primarily by apparent resemblance or by the possession of shared traits. With the widespread acceptance of the theory of evolution, more of an attempt was made to group species in accordance with their evolutionary history. This endeavor is what lead to the creation of the sub-taxon groups, in order that taxonomists could more accurately depict evolutionary relationships between species or other taxa. As our understanding of evolution increased, and our tools for determining evolutionary history and genetic relationships became more sophisticated, the task of ordering species into only 26 or 28 taxons became impossible. That is when cladistics was invented.

A clade is just a fancy word for group, but in cladistic taxonomy, it has a very precise definition. Any clade can be defined as two member species (or any other taxonomic group) and all the descendants of their nearest common ancestor. Depicted graphically as a cladogram, relationships are shown on a binary tree, where every fork has exactly two branches, each clade represented by a fork. No matter what the relationship is between the groups depicted in a cladogram, there will always be one less clade than the number of groups. Thus, any cladogram depicting eight taxonomic groups will always contain seven clades, regardless of their arrangement, as shown:

Cladogram 1 Cladogram 2.
Cladogram 1.
Groups G and H are most closely related, while group A is the most distant. Clades are represented by numbers 1 through 7.
Cladogram 2.
Groups A and B are more closely related to each other than to any other group, as are C and D, E and F, and G and H. Clade 4 is more closely related to 5, and clade 6 is more closely related to 7.

Due to the sheer number of clades (all of which are generally assigned names), cladistic taxonomy becomes impractical when trying to simultaneously describe all of the world's 3,000 some odd reptile species, let alone the two to ten million species of plants, animals, and micro-organisms that are known or presumed to exist. It does, however, provide a very powerful tool for analyzing relationships within smaller assemblages of organisms.