Taxonomy is the branch of biological science that is concerned with the classification of organisms. To use birds as an example, all birds are classified into the Class Aves within the Subphylum Vertebrata (which includes all animals with backbones) and the Phylum Chordata (which includes sea squirts as well as vertebrates).

The Class Aves is divided into a number of orders. For example, the Order Gaviiformes includes the loons while the Passeriformes includes all the perching birds. In turn, each order is broken into a number of families. The Corvidae (jays and crows) and the Vireonidae (the vireos) are but two of the many families in the Order Passeriformes. Families are broken down into genera (the singular is genus). Finally, each genus contains one or more species. Following the convention developed by the Swedish taxonomist, Carolus Linnaeus, every species is referred to by its genus and species name. The scientific name of the Black-billed Cuckoo is Coccyzus erythropthalmus. It’s a good thing we have standardized common names for our birds!

Ideally, the taxonomic system should reflect the relatedness of the species within a group of organisms. So, all species that belong to a particular genus are more closely related to each other than they are to other members of their family that are in different genera. Let’s consider the sparrow family, the Emberizidae. Within this family, the White-throated Sparrow and White-crowned Sparrow both belong to the genus Zonotrichia. Our classification suggests these two species are more closely related to each other than either is to the Chipping Sparrow, classified into the genus Spizella. But all the sparrows, classified into many different genera, are all considered more closely related to each other than to any member of another family like the finches (Fringillidae) or tanagers (Thraupidae).

Of all the different levels of a taxonomic system, the species is the only one that is defined by nature. We have techniques that allow us to determine if a population of organisms constitutes a species. All of the other levels are defined by humans. There is no formal definition of what constitutes a genus or a family or an order.

Taxonomy operates on a system of priority. The first classification of a group of organisms is the one that is usually followed. Revisions to taxonomy can be published to reflect better understanding and new discoveries but such revisions have to be reviewed and approved by other taxonomists before an older taxonomy can be changed. As an example, barnacles used to be classified in the Phylum Mollusca, the group that contains snails, clams and squids. Like most mollusks, barnacles have a calcareous shell around the body. Careful examination of the internal structures of barnacles showed them to be crustaceans, more closely related to lobsters, shrimp and crabs. Barnacles were re-classified into the Phylum Arthropoda. The rapid increase in our knowledge of DNA sequences of many organisms has caused us to revise many taxonomic classifications.

Different groups of organisms were originally classified by different taxonomists. Each taxonomist had a subjective notion of how similar species should be to allow them to be classified into the same genus. There are 500 species of marine tropical snails called cone shells. All 500 of these species are classified into the genus Conus. The frog genus Pristimantis contains over 400 species of frogs native to South America and northern Central America. Different taxonomists might have defined each genus more narrowly and might have established ten or more genera for the same groups of species. At the other extreme, some species are considered so distinctive that they are put into their own genus. The Gray Hawk is the only species in the genus Asturina. The distinctive Shoebill from Africa is not only the sole species in its genus but the only member of the family Balaenicipitidae. The Black Skimmer is one of only three species in the family Ryncopidae. Birds are split into smaller genera than many other groups of organisms.

In the end, taxonomy is a way of organizing earth’s biodiversity. A taxonomic system is really a collection of pigeonholes, nested within larger pigeonholes. It is up to each taxonomist to decide how large a genus pigeonhole or a family pigeonhole should be. In other words, all taxonomic units above the level of the species are artificial groupings created by taxonomists.

We recognize the species level as the base of all taxonomy. Species are not artificial units created by scientists but groups determined by the biology of the organisms. However, recognizing a species is tricky and often controversial. We’ll jump into that fray in the next column.

[Originally published on November 15, 2009]