Taxonomy is the practise of identifying different organisms, classifying them into categories and naming them. All organisms, both living and extinct, are classified into distinct groups with other similar organisms and given a scientific name.

The classification of organisms has various hierarchical categories. Categories gradually shift from being very broad and including many different organisms to very specific and identifying single species.


Taxonomic categories

There are eight distinct taxonomic categories: Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species. With each step down in classification, organisms are split into more and more specific groups.

For example, all of the animals in the Kingdom Animalia are split into multiple phyla (plural of phylum). All of the animals in the phylum Chordata are split in multiple classes such as mammals, reptiles and amphibians.

The broadest category splits all organisms in three groups called Domains. The three Domains of life are Bacteria, Archaea and Eukaryota.

For a long time all life was separated into five or six kingdoms. These included kingdoms such as animals, plants, fungi, protists, archaea and bacteria.

With new genetic data we now know that some protists are more closely related with animals, plants and fungi than they are to other protists. This suggests that the protist kingdom could be separated into multiple kingdoms. Thoughts are similar for the bacteria and archaea kingdoms.

A phylum, plural phyla, is still a very broad classification but it splits kingdoms into several groups. An example of phyla from the animal kingdom is Arthropoda which includes all insects, spiders, crustaceans and more. All vertebrate animals belong to one phylum called Chordata. Invertebrates are separated into many different phyla.

A class is the next level down. As mentioned earlier some classes from the phylum Chordata include mammals, reptiles and amphibians. Arthropod classes includes the like of insects and arachnids (spiders, mites and scorpions)

From class organisms are placed into an Order and then a Family. Using grasses as an example from the plant kingdom, they belong to the order Poales and the family Poaceae.

The final two categories are genus and species. The genus and species that an organism belongs to is how an organism receives it scientific name. This naming system is called binomial nomenclature and was invented by a brilliant biologist named Carl Linnaeus.

An identified species is placed into a specific group in each of these categories. For example, the taxonomic classification of humans is:

Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Family: Hominidae
Genus: Homo
Species: Homo sapiens

To make it easier to remember the order of the taxonomic hierarchy people often use mnemonics to make it easier. The phrase I was taught and still use to help me remember is ‘King Phillip Came Over From Germany Swimming’. There are many different phrases people have come up with. If you’re not keen on the sentence I use and want another one, take a look at these taxonomy mnemonics


Carl Linnaeus

Carl Linnaeus was a Swedish naturalist from the 18th century and is considered the father of taxonomy. It was Linnaeus who first began to separate organisms into hierarchical categories. He also developed the system that we use to name new species called binomial nomenclature. Linnaeus is credited with identifying over 10,000 different plant and animal species in his lifetime, more than any other biologist.

Systema Naturae

When Linnaeus developed his system of hierarchical categories he called it ‘Systema Naturae’. It contained three kingdoms, classes, orders, genera and species. We have since added two more categories – domains and phyla.

Linnaeus included natural non-living elements in a kingdom called minerals. We now only use this system for classifying organisms and we have since separated all of life into more than two kingdoms.

Binomial nomenclature

Binomial nomenclature is the method we use to uniquely name every different organisms on Earth, living or extinct. All organisms have a scientific name that includes two Latin words.

The two words are made from the names of the genus the species belongs to and a second word to separate each of the species from the same genus. The second word is known as the specific epithet. Hence, the scientific names of all organisms are made from the name of their genus and a specific epithet.

For example, the scientific name given to humans includes their genus Homo and the specific epithet sapiens. The overall name is Homo sapiens.

Scientific names are also written in either italics or underlined.



Taxonomy is not a perfect science and as you will find out there is a lot of disagreement and uncertainty about the structure of taxonomic classifications. In general however, taxonomy is a great way to quickly learn about how an organism slots into the tree of life.

Last edited: 3 November 2017


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