A summary of the Linnaean system of taxonomy, including a description of basic taxonomic categories and where the definition of genus and species fit in.

Systematics and Taxonomy

To understand the binomial nomenclature of scientific names, it helps to have at least a general understanding of how scientists categorize living things. The science of taxonomy and systematics involves the classification of organisms according to evolutionary relationships. Historically, organisms were classified based on physical similarities and differences.

Taxonomic Categories

Through this system, organisms are hierarchically classified into increasingly specific groupings. The seven basic taxonomic categories are: Kingdom, Phylum, Class, Order, Family, Genus and Species; kingdom being the broadest category, and species being the most specific.

Beginning students of biology often use mnemonic devices to remember the identity and order of these taxonomic categories. One commonly used mnemonic device being ‘King Philip came over for green soup.’

Carolus Linnaeus

Linnaeus (1707-1778), a Swedish physician and botanist, was the founder of taxonomy. He originated the system of binomial nomenclature used for naming plants and animals and grouping similar organisms into increasingly general categories. Today, biologists still use this basic system of classification, but advances in the fields of genetics and evolutionary theory has resulted in some of Linnaeus’ original categories being changed to better reflect the relationships among organisms.

Modern Systematics

Modern systematics combines data from many sources, including: the fossil record, comparative homologies (similarity of structures due to shared ancestry), and comparative sequencing of nucleic acids (DNA and RNA) among organisms.

Binominal Nomenclature

Also called binary nomenclature, this formal system of naming organisms consists of two Latinized names, the genus and the species. All living things, and even some viruses, have a scientific name.

The binomial aspect of this system means that each organism is given two names, a ‘generic name,’ which is called the genus (pl. genera) and a ‘specific name,’ the species. Having a universal system of binomial nomenclature allows scientists to, in essence, speak the same language when referring to living things, and avoids the confusion of multiple common names that may differ based on region, culture or native language.

When written, a scientific name is always either italicized, or if hand-written, underlined. The genus is capitalized and the species name is lower case. For example, the proper format for the scientific name of humans is Homo sapiens.

What Is a Genus?

In biology, ‘genus’ is the taxonomic classification lower than ‘family’ and higher than ‘species’. In other words, genus is a more general taxonomic category than is species. For example, the Grey Wolf, or Timber Wolf (Canis lupus) belongs to the same genus as the domestic dog (Canis domesticus). Although in the same biological family (Canidae) as wolves and dogs, foxes belong to a different genus (Vulpes). This reflects a closer evolutionary relationship between the wolf and the domestic dog than between either and the fox.

What is a Species?

The species name, also called specific epithet, is the second part of a scientific name, and refers to one species within a genus. A species is a group of organisms that typically have similar anatomical characteristics and that can successfully interbreed to produce viable offspring. A mule, for example, is not a distinct species. It is an infertile hybrid of a male donkey (Equus asinus) and a female horse (Equus caballus).

Natural History Resources

There are many interesting on-line and in-print resources on taxonomy, systematics and natural history, including Science Prof Online and the American Museum of Natural History. Also see other interesting systematics articles on Suite101.com, including Invertebrate Classification.

Additional Binomial Nomenclature Sources

Campbell, N. and Reece, J. (2005) Biology, Seventh Edition. Pearson, Benjamin Cummings.