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

A taxonomy is a hierarchical system for classifying and identifying organisms. This organizational system was developed by Swedish scientist Carl Linnaeus in the 18th century. In addition to being a valuable tool 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. The first is the name of the organism's genus, and the second is the name of the organism's species. 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 name is sapiens. These terms are unique, and no other organism has the same name.

Classification Categories

The second feature of Linnaeus's taxonomy system that simplifies organism classification is the ordering of species into broad categories. The broadest of the these categories is kingdom. Linnaeus divided the world's living organisms into two kingdoms, the animal kingdom and the plant kingdom (he placed minerals in their own third kingdom). Linnaeus further divided organisms into classes, orders, genera, and species.

These categories were later revised to include: kingdom, phylum, class, order, family, genus, and species.

Due to further scientific advancements and discoveries, the 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: This domain includes prokaryotic organisms (those that lack a nucleus) 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.
  • Bacteria: This domain includes prokaryotic organisms with unique cell wall compositions and RNA types. As part of the human microbiota, bacteria are vital to life. However, some bacteria are pathogenic and cause disease.
  • Eukarya: This 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. They include Archaebacteria (ancient bacteria), Eubacteria (true bacteria), Protista, Fungi, Plantae, and Animalia.

Intermediate Categories

Taxonomic categories can be further divided into intermediate categories such as subphyla, suborders, superfamilies, and superclasses. An example of this taxonomy scheme appears 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 
Domain   
Kingdom SubkingdomSuperkingdom (Domain)
PhylumSubphylumSuperphylum
ClassSubclassSuperclass
OrderSuborderSuperorder
FamilySubfamilySuperfamily
GenusSubgenus 
SpeciesSubspeciesSuperspecies

The table below includes a list of organisms and their classification within 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

Tarantula

DomainEukaryaEukaryaEukaryaEukaryaEukaryaEukarya
KingdomAnimaliaAnimaliaAnimaliaAnimaliaAnimaliaAnimalia
PhylumChordataChordataChordataChordataChordataArthropoda
ClassMammaliaMammaliaMammaliaMammaliaMammaliaArachnida
OrderCarnivoraCarnivoraCarnivoraCetaceaCarnivoraAraneae
FamilyUrsidaeFelidaeCanidaeDelphinidaeCanidaeTheraphosidae
GenusUrsusFelisCanisOrcinusCanisTheraphosa
SpeciesUrsus arctosFelis catusCanis familiarisOrcinus orcaCanis lupusTheraphosa blondi