Taxonomy

Main Points

  • Taxonomy is the classification of life and helps us communicate with other people.
  • A taxon (plural is taxa) is an organism.
  • Linnaean taxonomy helps provide a structure for the classification of life but life does not fall simply into the seven described groups.
  • Phylogenetic nomenclature uses the natural evolutionary groups (clades) that are supported by specific features as groups.

Taxonomy is the classification of life or how we organize organisms into groups. Often the term systematics is used interchangeably with taxonomy. By classification we mean the ordering of organisms into groups by a taxonomist (person who studies taxonomy). The whole reasoning for having a classification scheme is to be able to communicate findings with other scientists, friends, family, and everyone! Read more about taxonomy on the Digital Encyclopedia of Ancient Life here.

A taxon (plural taxa) is an organism. Sometimes when we are talking about major groups we don’t use their proper names but more of a nickname that is often easier to say. Here is an example: Clams are part of the group Bivalvia but in conversation we discuss them as ‘bivalves’ or ‘clams’ rather than ‘Bivalvia’. This is true for Foraminifera (=foram(s)), Blastoidea (blastoid(s)), Mammalia (mammal(s)), and even Hominidae (hominid(s)).

You may be already be familiar with the term taxidermy. Both “taxidermy” and “taxonomy” are derived from Ancient Greek. Taxidermy means “moving/arranging skins” (taxis=arranging, derma=skin), whereas taxonomy means “the rules of arranging” (nomia=rules/laws). Taxidermists and taxonomists have very different jobs, but taxidermy was very important to the early taxonomists.

Linnaean Taxonomy

Linnaean taxonomy was established by Carl Linnaeus, a Swedish scientist, and marks the onset of consistent use of binomial nomenclature (genus + species; e.g., Homo sapiens). His volumes of Systema Naturae were what later became known as Linnaean taxonomy. This is the traditional type of taxonomy that places life into a classic hierarchical system. It is also commonly taught in grade school and consists of seven levels of classification: Kingdom, Phylum, Class, Order, Family, Genus, Species. This system was based on morphological (shape or anatomical features) and developmental similarities that could be visually identified on fossil forms. Organisms with the most similarities were grouped within a species and each successive group outward would have less and less similarity (Boggs, 2012).

Linnaean Classification scheme in black with additional groupings added in in orange.

There are many issues with this system. The first is that it is incredibly simple, does life really fit into seven bins? No, it really doesn’t! This means that extra subdivisions have to be added, resulting in Superphyla, Subphyla, Infraorders, etc. The rank terminology does not provide additional meaning to the group of organisms, it is simply to organize. If I say Class Insecta, what do you think of? What if I just said ‘Insecta’? Same thing! The term class doesn’t change the meaning of the group. If I say Tetrapoda (four legs) you can think of everything that has four legs as being included but snakes are actually tetrapods as well. They lost their legs secondarily, which is something we can test in an evolutionary framework. Although these ranks are arbitrary it does not take away from the impact of this first attempt to organize life was invaluable to the biological sciences. The organization within Linnaean taxonomy is not based on evolutionary relationships but on the similarities that we can observe on organisms.

Phylogenetic nomenclature

Phylogenetic nomenclature uses clades (natural taxa bounded by origination and extinction) to describe life. Rather than using a diagnosis (written description of an organism or group), this system defines groups based on the tree of life. Branching points on a tree, called nodes, split based on the changes in anatomy or morphology of the organisms. In this system, names go at the branching points on the tree and are supported by the changes in anatomy.

Branching points (nodes) can be designated with a group name and is supported by specific morphological features that separate the organisms. These names now carry increased meaning with the support of specific features rather than using the Linnaean terms, which were arbitrary.

This system ties the names directly to the clades (=natural evolutionary groups) whereas traditional taxonomy indirectly does this through the hierarchical ranks. This allows the name to be tied to the clade rather than the rank. This system of nomenclature provides a system that more closely follows evolutionary concepts (de Queiroz and Cantino, 2001; Cantino and de Queiroz, 2003). It is not necessary for us to go into extreme detail but the system of phylogenetic nomenclature removes the confusing the rank-based groups and allows for us to name natural evolutionary groupings (monophyly).

As paleontologists, we often only have morphological data to assess these patterns but biologists have access to molecular data, such as DNA. This makes things slightly more complicated but there are a series of tests we can use to assess specific skeletal elements. If you are interested in reading more on that – head to Homology. If you are interested in learning how to properly read branching diagrams – head to Reading the Tree of Life.

Proceed to ‘Reading the Tree of Life’