The Three Domains

Traditionally, all living things have been classified into groups depending on their structure and characteristics
Organisms were first classified in this way by a Swedish naturalist called Linnaeus
This system allows the subdivision of living organisms into smaller and more specialised groups
- The species in these groups have more and more features in common the smaller and more subdivided the groups get
The sequence of classification is: Kingdom, Phylum, Class, Order, Family, Genus, Species

Linnaeus’ system of classification

For a long time, biologists have regarded there to be five kingdoms at the top of the classification hierarchy
These five kingdoms include:
- Prokaryotes (all single-celled organisms without a nucleus e.g. bacteria)
- Protists (eukaryotic single-celled organisms e.g. algae)
- Fungi (mushrooms, toadstools, yeasts etc.)
- Plants (grasses, trees etc.)
- Animals (fish, mammals, reptiles, amphibians, birds etc.)

Originally, organisms were classified using morphology (the overall form and shape of the organism e.g. whether it had wings or legs) and anatomy (the detailed body structure as determined by dissection)
As evidence of internal structures became more developed due to improvements in microscopes, and the understanding of biochemical processes and genetics progressed, new models of classification were proposed
As technology advanced, genetic analyses (such as DNA sequencing) allowed us to classify organisms using a more scientific approach
Studies of DNA or RNA sequences of different species show that the more similar the base sequences in the DNA or RNA of two species, the more closely related those two species are (and the more recent in time their common ancestor is)
- For example, the base sequences in a mammal’s DNA are more closely related to all other mammals than to any other vertebrate group, such as birds or fish

Due to evidence available from genetic analysis, there is now a ‘three-domain system’ of classification
This was developed by Carl Woese in 1990
In this system, organisms are divided into three large groups called domains
These domains are:
- Archaea (primitive bacteria usually living in extreme environments such as hot springs and salt lakes)
- Bacteria (true bacteria such as E. coli and Staphylococcus)
- Eukaryota (which includes protists, fungi, plants and animals)
These domains are then subdivided into the smaller groups previously used (i.e. kingdom, phylum, class, order, family, genus, species)

Today, biologists regard the ‘domain’ (rather than ‘kingdom’) to be the highest level of classification for organisms