15.10 New Systems of Classification

A free GCSE Biology lesson for AQA (8461) from OBA School: woese’s three-domain system and how to read evolutionary trees. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why classification systems have changed since Linnaeus
• Name the three domains and describe what is in each
• Explain what evidence is used to build evolutionary trees
• Interpret evolutionary trees to find common ancestors and how closely species are related

Key words

• Domain: The highest level of classification, above kingdom. There are three domains.
• Archaea: Single-celled prokaryotes, once thought to be bacteria, that are chemically very different. Many live in extreme conditions.
• Bacteria: The domain of true bacteria: single-celled prokaryotes.
• Eukaryota: The domain of organisms whose cells have a nucleus: protists, fungi, plants and animals.
• Evolutionary tree: A diagram showing how scientists think organisms are related, through common ancestors.
• Common ancestor: An ancestral species from which two or more later species evolved.

Why classification has changed

Linnaeus could only use features he could see. Since then, scientists have gained:

• better microscopes, which show the internal structures of cells;
• a better understanding of biochemical processes inside organisms;
• the ability to compare DNA and RNA sequences between organisms.

For much of the 20th century living things were split into five kingdoms: animals, plants, fungi, protists and prokaryotes. New evidence showed this was not the whole story.

The three-domain system

In 1977 the American scientist Carl Woese used chemical analysis of the RNA in cells. He found that some single-celled organisms thought to be bacteria were very different chemically. In 1990 he proposed dividing all living things into three domains:

Key fact: Archaea and Bacteria are both prokaryotes, but their chemistry (e.g. cell membranes and RNA) is so different that they are in separate domains. Archaea are in some ways closer to eukaryotes.

Evolutionary trees

An evolutionary tree shows how scientists think groups of organisms are related. It is built using current classification data (including DNA) for living organisms and fossil data for extinct organisms.

• The tips (right-hand ends) are species living today.
• Each branching point is a common ancestor. The further back in time it is, the longer ago the species separated.
• Two species are more closely related if their most recent common ancestor is more recent.

Worked example: reading the tree

Which ape is most closely related to humans? Humans and chimpanzees share the most recent common ancestor, about 6 million years ago.
How long ago did the common ancestor of the gorilla and the orangutan live? Follow both branches back until they meet: about 14 million years ago.

Answer: chimpanzee; about 14 million years ago

Common mistakes

• Saying humans evolved from chimpanzees. They share a common ancestor; both evolved from it.
• Thinking species that are next to each other at the tips are always closest relatives. Follow the branches back to the nearest branching point.
• Putting Archaea in the Bacteria domain because both are prokaryotes. Chemical analysis shows they are very different.
• Forgetting that evolutionary trees use fossil data as well as data from living organisms.

Revision questions

Try these, then check your answers below each one.

Question 1. Which are the three domains in Woese’s system? (Choose from: Archaea, Bacteria, Eukaryota, Animals, Plants, Fungi, Prokaryotes, Protists, Eukaryotes, Bacteria, Viruses, Eukaryota.)

Answer: Archaea, Bacteria, Eukaryota

Working: Archaea, Bacteria and Eukaryota.

Question 2. Who proposed the three-domain system? (Choose from: Carl Woese, Carl Linnaeus, Charles Darwin, Alfred Russel Wallace.)

Answer: Carl Woese

Working: Carl Woese, using chemical analysis of RNA.

Question 3. True or false? Fungi belong to the domain Eukaryota.

Answer: True

Working: True. Fungal cells have a nucleus, so fungi are eukaryotes.

Related lessons

• Previous lesson: 15.9 Classification
• Next lesson: 16.1 The Importance of Communities
• In this chapter: 15.1 The History of Genetics
• In this chapter: 15.2 Theories of Evolution
• In this chapter: 15.3 Accepting Darwin's Ideas

Continue in the full course

This lesson is section 15.10 (AGCBH1510) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.