Taxonomy is the science of defining and naming groups of biological organisms based on shared characteristics. It uses a hierarchy of increasingly inclusive categories with Latin names. The smallest units of taxonomy, species and genus, are used to assign a formal, taxonomic name to each species in a system. This classification system, referred to as binomial nomenclature, was formalized by Carolus Linnaeus in the 18th century.

Hierarchy of Taxonomy

The hierarchy that Carolus Linnaeus first proposed is still used today, although it has been expanded upon. The order of ranking—from the highest or largest group to the smallest or most specific—is as follows: domain, kingdom, phylum, class, order, family, genus, and species.

Binomial Nomenclature

Beginning from the smallest unit of taxonomy, similar species are grouped into the same genus. For example, the arctic hare and the black-tailed jackrabbit both belong to the genus Lepus; however, they belong to different species—arcticus and californicus, respectively. Within an organism’s taxonomic name, both the genus and species are italicized, and the first letter of the genus is capitalized. This two-part format for naming and categorizing specific organisms is referred to as binomial nomenclature.

Higher Level Taxonomy

Members of the same genus belong to the same family. For example, hares and rabbits belong to the Leporidae family. They also share the same order, Lagomorpha, with some other rodent species like pikas (Ochotonidae family), which resemble hares and rabbits but have smaller, rounder bodies and no visible tail. All the lagomorphs belong to the class Mammalia, which also includes monotremes like the platypus, marsupials like kangaroos, and placental mammals like goats, mice, and humans.

All vertebrate animals belong to the phylum Chordata and the kingdom Animalia. Finally, at the top of the hierarchy are the three domains. Eukarya includes all animals, plants, fungi, and protists. Archaea and Bacteria are prokaryotes—single-celled organisms without nuclei and other organelles.