Developments in classification

Inheritance, variation and evolution · Classification of living organisms · note 2 of 2

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Developments in classificationSpec 4.6.4

In short

The three-domain system divides organisms into Archaea, Bacteria and Eukaryota, based on evidence from chemical analysis. Archaea are primitive bacteria usually living in extreme environments, Bacteria are true bacteria, and Eukaryota include protists, fungi, plants and animals. New models of classification were proposed as better microscopes revealed internal structures and the understanding of biochemical processes progressed.

As evidence of internal structures became more developed due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed.

Due to evidence available from chemical analysis, there is now a three-domain system developed by Carl Woese. In this system, organisms are divided into:

The three-domain system
Domain	Description
Archaea	Primitive bacteria, usually living in extreme environments
Bacteria	True bacteria
Eukaryota	Includes protists, fungi, plants and animals

Evolutionary trees

Evolutionary trees are a method used by scientists to show how they believe organisms are related. They use current classification data for living organisms and fossil data for extinct organisms. The more recently two branches split from a common ancestor, the more closely related the organisms are.

Evolutionary tree with time running left to right from a common ancestor of all six species to five living species A to E at the present day and one extinct species F, with the most recent common ancestor of D and E marked. — D and E share the most recent common ancestor, so they are the most closely related species on this tree.

Exam tip:

To read a tree, find the point where two branches split. The nearer that point is to the present, the more closely related the two species are.

Quick check

List the Linnaean groups from largest to smallest.
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Kingdom, phylum, class, order, family, genus, species.
Which two groups make up a binomial name?
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Genus and species.
Name the three domains in Woese's system.
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Archaea, bacteria and eukaryota.
What data are used to build evolutionary trees?
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Current classification data for living organisms and fossil data for extinct organisms.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

What is the binomial system of naming organisms?

The binomial system names each organism by two parts: its genus and its species. For humans, the genus is Homo and the species is sapiens, giving Homo sapiens. The genus is written first with a capital letter and the species second with a lower-case letter, and the whole name is written in italics or underlined.

What are the seven levels of classification?

The seven levels of classification, from largest to smallest, are kingdom, phylum, class, order, family, genus and species. Living things were traditionally classified into these groups depending on their structure and characteristics. The last two groups, genus and species, are used to give each organism its two-part binomial name, such as Homo sapiens.

What is the three-domain system?

The three-domain system is a model of classification that divides organisms into Archaea, Bacteria and Eukaryota. It was proposed because of evidence from chemical analysis. Archaea are primitive bacteria usually living in extreme environments, Bacteria are true bacteria, and Eukaryota include protists, fungi, plants and animals.

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