Taxonomy and Organism Classification

Carolus Linnaeus
circa 1760: Swedish physician and botanist Carl von Linnaeus (1707-1778), founder of the modern system of binomial nomenclature for plants. Original Publication: From a copy by Pasch of an original painting. Hulton Archive / Stringer/ Hulton Archive/ Getty Images

Taxonomy is a hierarchical system for classifying and identifying organisms. This system was developed by Swedish scientist Carolus Linnaeus in the 18th century. In addition to being a valuable system for biological classification, Linnaeus's system is also useful for scientific naming.

Binomial Nomenclature

Linnaeus's taxonomy system has two main features that contribute to its ease of use in naming and grouping organisms.

The first is the use of binomial nomenclature. This means that an organism's scientific name is comprised of a combination of two terms. These terms are the genus name and the species or epithet. Both of these terms are italicized and the genus name is also capitalized.

For example, the scientific name for humans is Homo sapiens. The genus name is Homo and the species is sapiens. These terms are unique and no other species can have this same name.

Classification Categories

The second feature of Linnaeus's taxonomy system that simplifies organism classification is the ordering of species into broad categories. Linnaeus classified organisms under the broadest category of Kingdom. He identified these Kingdoms as animals, plants, and minerals. He further divided organisms into classes, orders, genera, and species. These major categories were later revised to include: Kingdom, Phylum, Class, Order, Family, Genus, and Species.

Due to further scientific advancements and discoveries, this classification system has been updated to include Domain in the taxonomic hierarchy. Domain is now the broadest category and organisms are grouped primarily according to differences in ribosomal RNA structure. The domain system of classification was developed by Carl Woese and places organisms under three domains: Archaea, Bacteria, and Eukarya.

  • The Archaea domain includes prokaryotic (no true nucleus) organisms that differ from bacteria in membrane composition and RNA. They are extremophiles capable of living in some of the most inhospitable conditions on earth, such as hydrothermal vents.
  • The Bacteria domain includes prokaryotic organisms with unique cell wall composition and RNA type. As part of the human microbiota, bacteria are vital to life. However, some bacteria are pathogenic and cause disease.
  • The Eukarya domain includes eukaryotes, or organisms with a true nucleus. Eukaryotic organisms include plants, animals, protists, and fungi.

Under the domain system, organisms are further grouped into six Kingdoms. The Kingdoms include: Archaebacteria (ancient bacteria), Eubacteria (true bacteria), ProtistaFungiPlantae, and Animalia.

A helpful aid for remembering the taxonomic categories of Domain​, Kingdom, Phylum, Class, Order, Family, Genus, and Species is the mnemonic device: Do Keep Plates Clean Or Family Gets Sick.

Intermediate Categories

Taxonomic categories can be grouped into intermediate categories such as subphyla, suborders, superfamilies, and superclasses. An example of this taxonomy scheme is below. It includes the eight main categories along with subcategories and supercategories.

The superkingdom rank is the same as the Domain rank.

Taxonomic Hierarchy
 Category  Subcategory  Supercategory 
Kingdom SubkingdomSuperkingdom (Domain)


The table below includes a list of organisms and their classification in this taxonomy system using the major categories. Notice how closely dogs and wolves are related. They are similar in every aspect except species name.

Taxonomic Classification
  Brown BearHouse CatDogKiller WhaleWolf


SpeciesUrsus arctosFelis catusCanis familiarisOrcinus orcaCanis lupusTheraphosa blondi