Tree of animal life

What is Taxonomy?

Taxonomy is the method we use to identify and group organisms based on their similar morphological (physical) characteristics. It is founded on the concept that morphological similarities descend from a common evolutionary ancestor.

At a very broad level, all living organisms are grouped into one of the 7 Kingdoms of Life each of which are split further into Phylum > Class > Order > Family > Genus > Species. E.g. European green crab:

A top down view of a crab on a white background, on the left side of the image is a list of its classification scheme. Kingdom: Animalia. Phyla: Arthopoda. Class: Malacostraca. Order: Decapoda. Family: Carcinidae. Genus: Carcinus. Species: maenas.Carcinus maenas. © Shutterstock, 2019.

History of taxonomy

This history of how we have identified and classified organisms begins with Aristotle.

A marble bust of Aristotle on a black background.Aristotle (384-322 BC). © Shutterstock, 2019.

Early attempts to classify organisms: The Greek philosopher, Aristotle (384-322 BC) was the first to formally classify living organisms. In his ‘History of Animals’, he grouped animals based on similarities such as those with blood, or those without blood; those that live on land or those that live in water. Although organisms were categorised, it had no methodical foundation and animals were not grouped based on evolutionary similarities. Aristotle also introduced the concept of binomial (two-name) definition, something which has since been refined and is still in use today. By this system an organism is defined by the two names of its ‘genus and difference’. Using this he placed similar animals in a genus and then split them based on a defining characteristic which distinguished them from others placed in the same genus.

A hand drawn black and white portrait of LinnaeusLinnaeus (1707-1778). ©Shutterstock, 2019.

Carolus Linnaeus: Linnaeus (1707-1778) was the first person to combine binomial definition (nomenclature) with a hierarchical structure of classification. His system organised both plants and animals from the level of kingdom, right down to species. He used this system consistently to identify every species of plant and animal he came across and this is the basis of the system we use today.

Process of species identification

Traditional taxonomy: To identify species, organisms are divided into smaller and smaller groups using the characteristics defined in a ‘dichotomous (2-way) key’. This is made up of a series of 2-option questions which split organisms into two groups. Questions start with the broadest, most fundamental differences, and as the key progresses each group is split further and further based on increasingly unique characteristics until the lowest classification is reached. An example of this can be seen below.

Let’s take these 5 animals: A composite image of a goldfish, a blue butterfly, a chicken, a mouse and a brown and white stripey lizard on a white background© Shutterstock, 2019.

Without knowing anything about them other than where they were collected from and what they look like, we can create a dichotomous key to split them into groups and identify them based on their features.

dichotomous key example ©Shutterstock, 2019


Molecular Phylogenetics: is a branch of taxonomy dedicated to understanding the evolutionary links between organisms. Instead of using morphological (physical) characteristics, this method uses molecular techniques, such as DNA and protein sequences, to determine evolutionary similarities within and amongst individuals, populations and species.

phylogenetic tree ©Shutterstock, 2019


(Optional) Further information

If you wish for more information on the history of taxonomy and Carl Linneaus, watch this short video ‘A crash course in taxonomy.’ (12 mins).

This is an additional video, hosted on YouTube.


Look again at the picture above featuring 5 animals. We split them into groups based on certain similar features. Can you think of other characteristics that could be used to create a key for these animals?

Share your thoughts in the comments area.

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