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 scheme for classifying and identifying organisms. It 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. The two main features of this taxonomy system, binomial nomenclature and categorical classification, make it convenient and effective.

Binomial Nomenclature

The first feature of Linnaeus's taxonomy, which makes naming organisms uncomplicated, is the use of binomial nomenclature. This naming system devises a scientific name for an organism based on two terms: The name of the organism's genus and the name of its species. Both of these terms are italicized and the genus name is capitalized when writing.

Example: The bionomical nomenclature for humans is Homo sapiens. The genus name is Homo and the species name is sapiens. These terms are unique and ensure that no two organisms have the same scientific name.

The foolproof method of naming organisms ensures consistency and clarity across the field of biology and makes Linnaeus's system simple.

Classification Categories

The second feature of Linnaeus's taxonomy, which simplifies organism ordering, is categorical classification. This means narrowing organism types into categories but this approach has undergone significant changes since its inception. The broadest of these categories within Linnaeus's original system is known as kingdom and he divided all of the world's living organisms into only an animal kingdom and plant kingdom.

Linnaeus further divided organisms by shared physical characteristics into classes, orders, genera, and species. These categories were revised to include kingdom, phylum, class, order, family, genus, and species over time. As more scientific advancements and discoveries were made, domain was added to the taxonomic hierarchy and is now the broadest category. The kingdom system of classification was all but replaced by the current domain system of classification.

Domain System

Organisms are now grouped primarily according to differences in ribosomal ​RNA structures, not physical properties. The domain system of classification was developed by Carl Woese and places organisms under the following three domains: 

  • Archaea: This domain includes prokaryotic organisms (which 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 grouped into six kingdoms which include Archaebacteria (ancient bacteria), Eubacteria (true bacteria), Protista, Fungi, Plantae, and Animalia. The process of classifying organisms by categories was conceived by Linnaeus and has been adapted since.

Taxonomy Example

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

Taxonomic Hierarchy Example
  Brown Bear House Cat Dog Killer Whale Wolf

Tarantula

Domain Eukarya Eukarya Eukarya Eukarya Eukarya Eukarya
Kingdom Animalia Animalia Animalia Animalia Animalia Animalia
Phylum Chordata Chordata Chordata Chordata Chordata Arthropoda
Class Mammalia Mammalia Mammalia Mammalia Mammalia Arachnida
Order Carnivora Carnivora Carnivora Cetacea Carnivora Araneae
Family Ursidae Felidae Canidae Delphinidae Canidae Theraphosidae
Genus Ursus Felis Canis Orcinus Canis Theraphosa
Species Ursus arctos Felis catus Canis familiaris Orcinus orca Canis lupus Theraphosa blondi
Taxonomic Classification Example

Intermediate Categories

Taxonomic categories can be even more precisely divided into intermediate categories such as subphyla, suborders, superfamilies, and superclasses. A table of this taxonomy scheme appears below. Each main category of classification has its own subcategory and supercategory.

Taxonomic Hierarchy With Subcategory and Supercategory
Category Subcategory Supercategory
Domain    
Kingdom Subkingdom Superkingdom (Domain)
Phylum Subphylum Superphylum
Class Subclass Superclass
Order Suborder Superorder
Family Subfamily Superfamily
Genus Subgenus  
Species Subspecies Superspecies