Taxonomy is the practice 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. These are: 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 into multiple classes such as mammals, reptiles, and amphibians.
The broadest category splits all organisms into three groups called ‘Domains’. The three Domains of life are Bacteria, Archaea and Eukaryota.
Kingdom
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 to 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.
Phylum
A phylum (plural phyla) is still a very broad classification but it splits kingdoms into multiple 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.
Class
A class is the next level down. As mentioned earlier some classes from the phylum Chordata include mammals, reptiles, and amphibians. Arthropod classes include the likes of insects and arachnids (spiders, mites, and scorpions).
Order and family
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.
Genus and species
The final two categories are genus and species. The genus and species that an organism belongs to are how an organism receives its 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 remember the order of the taxonomic hierarchy from domain to species, people often use mnemonics to make it easier. The phrase that 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’s original classification had three kingdoms – animals, plants, and minerals (natural, non-living elements). 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 that we use to uniquely name every different organism 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 within 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: 30 August 2020
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