Principles of Taxonomy and Binomial Nomenclature

Taxonomy is the science of naming, describing and classifying organisms, and binomial nomenclature is the system in which every species has a two-part Latin name consisting of its genus and species, such as Homo sapiens. In brief: organisms are arranged in a hierarchy of ranks from the broadest to the narrowest: domain, kingdom, phylum, class, order, family, genus, species. A common mnemonic is “Dear King Philip Came Over For Good Soup”. The genus name is capitalized, the species name is lowercase, and both are written in italics.

Classification gives scientists a universal language, because common names vary and can refer to different species. This guide explains the purpose of classification, the taxonomic ranks, the rules of binomial nomenclature, shows the classification of a few organisms in a table, works through an example of classifying a lion and corrects common mistakes.

Why do we classify living things?

About two million species have been named, and there are probably many millions more. Classification puts them in order, which makes it possible to identify an organism, to find information about it, to predict its characteristics from its group and to show evolutionary relationships. Taxonomy covers three tasks: identification (determining what an organism is), naming (nomenclature) and classification (placing it in a group). The related science of systematics studies evolutionary relationships among organisms.

Common names are not reliable. “Robin” refers to different birds in Europe and North America, and “gopher” is used for several animals. A scientific name is the same worldwide and in all languages.

What are the taxonomic ranks?

The Linnaean system arranges groups in a hierarchy of ranks (taxa, singular: taxon). Each level contains one or more groups of the level below, so groups are nested like boxes inside boxes.

1. Domain: the highest rank: Bacteria, Archaea or Eukarya.
2. Kingdom: for example Animalia, Plantae, Fungi.
3. Phylum (plural: phyla): for example Chordata (animals with a notochord).
4. Class: for example Mammalia.
5. Order: for example Carnivora.
6. Family: for example Felidae (cats).
7. Genus (plural: genera): for example Panthera.
8. Species: for example Panthera leo.

The higher the rank, the more organisms it contains, and the fewer features they share. Organisms in the same genus are very similar, and those in the same kingdom share only basic features. Plants and fungi use “division” in place of “phylum” in some traditional systems. The memory aid for the order of ranks has many versions: “Dear King Philip Came Over For Good Soup” is among the most common.

How does binomial nomenclature work?

Carl Linnaeus introduced the system in the 1750s. The scientific name of a species has two parts: the genus name and the species name (also called the specific epithet).

• The genus name starts with a capital letter, and the species name begins with a lowercase letter: Homo sapiens, not Homo Sapiens.
• Both are written in italics in print. In handwriting, they are underlined separately.
• After the full name has been given once, the genus can be abbreviated to its first letter: H. sapiens, E. coli.
• Names are in Latin or Latinized, so that they can be used internationally. They often describe a feature, a place or honor a person. Tyrannosaurus rex means “tyrant lizard king”.
• The name is unique. No two animals can have the same genus and species name, and codes govern naming: the International Code of Zoological Nomenclature and the International Code of Nomenclature for algae, fungi, and plants.

A species name alone, such as sapiens, is not enough, because the same epithet can be used with different genera (for example vulgaris or alba).

Classification of selected organisms

Some ranks differ between classification schemes. For the oak, modern systems often use clades such as the eudicots, so the table follows the traditional school scheme.

Worked example: classifying the lion

To place a lion in its taxonomic position, ask a series of questions from the broadest to the narrowest.

1. Domain: its cells have a nucleus, so it is in Eukarya.
2. Kingdom: it is multicellular, has no cell walls and eats other organisms, so it is an animal (Animalia).
3. Phylum: it has a backbone, so it is a chordate (Chordata).
4. Class: it has hair and feeds its young with milk, so it is a mammal (Mammalia).
5. Order: it has the teeth and claws of a meat-eater (carnassial teeth), so it is in the order Carnivora.
6. Family: it has retractable claws and belongs to the cat family, Felidae.
7. Genus and species: it belongs to the genus Panthera (roaring cats), species leo.

Its full scientific name is Panthera leo. The tiger is Panthera tigris, in the same genus, which shows that they are closely related. The domestic cat, Felis catus, is in the same family (Felidae), but a different genus. The wolf, Canis lupus, is in the same order (Carnivora), but a different family (Canidae). The number of ranks that two organisms share indicates how closely related they are.

What is a species in taxonomy?

The species is the basic unit of classification. As explained in the article on speciation, a common definition is a group that can interbreed and produce fertile offspring. In practice, taxonomists often use physical characteristics and DNA sequences. Some species are divided into subspecies, and a third name is added, as in Panthera tigris altaica (Siberian tiger). In bacteria, species are defined by DNA similarity, since they reproduce asexually.

How did taxonomy develop?

Aristotle classified animals in the fourth century BC. John Ray defined the species concept in the late seventeenth century. Carl Linnaeus published Systema Naturae (tenth edition, 1758) and Species Plantarum (1753), which established binomial names. After Darwin, taxonomy shifted to reflect evolutionary relationships. In the twentieth century, Robert Whittaker proposed a five-kingdom system (1969), and Carl Woese introduced the three-domain system in 1990. DNA sequencing has led to many revisions in the classification of groups.

Which mistakes are common?

Mistake 1: Writing the species name with a capital letter

Only the genus is capitalized: Homo sapiens.

Mistake 2: Forgetting italics or underlining

Scientific names of genus and species are italicized in print. Higher ranks such as Mammalia are not italicized.

Mistake 3: Using the species name alone

The full binomial is needed, because the species epithet alone is not unique.

Mistake 4: Thinking that classification is fixed

As new evidence on evolutionary relationships appears, groups are moved or renamed.