top of page

B14.6 New Systems of Classification

GCSE Combined Science · AQA · Chapter B14: Genetics and Evolution

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: why classification has changed, Woese’s three-domain system, and 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
  • Compare the Linnaean system with the three-domain system
  • Explain what evidence is used to build evolutionary trees
  • Interpret evolutionary trees to find common ancestors and how closely organisms 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 earlier 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 between organisms (genome sequencing).

This new evidence sometimes shows that organisms which look alike are not closely related, so they are moved to new groups. Scientists must be willing to change a classification when new evidence appears.

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. He proposed that all living things should be divided into three domains:

  • Evidence used; Linnaean system structure and characteristics you can see; Three-domain system chemical analysis (RNA), plus structure
  • Largest group; Linnaean system kingdom; Three-domain system domain (above kingdom)
  • Prokaryotes; Linnaean system all in one group; Three-domain system split into Archaea and Bacteria
  • Naming; Linnaean system binomial names; Three-domain system still uses binomial names

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

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 groups separated.
  • Two species are more closely related if their most recent common ancestor lived more recently.

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 gorillas and orangutans live? Follow both branches back until they meet: about 15 million years ago.

Answer: chimpanzee; about 15 million years ago

Common mistakes

  • Saying humans evolved from chimpanzees. They share a common ancestor; both evolved from it.
  • Thinking species next to each other at the tips are always closest relatives. Follow the branches back to where they meet.
  • 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 of classification? (Choose from: Archaea, Bacteria and Eukaryota, Animals, Plants and Fungi, Prokaryotes, Protists and Eukaryotes, Bacteria, Viruses and Eukaryota.)

Answer: Archaea, Bacteria and Eukaryota

Working: The three domains are Archaea (primitive bacteria), Bacteria (true bacteria) and Eukaryota.

Question 2. Which scientist proposed the three-domain system? (Choose from: Carl Woese, Carl Linnaeus, Charles Darwin, Gregor Mendel.)

Answer: Carl Woese

Working: Carl Woese, using evidence from chemical analysis of RNA in cells.

Question 3. Use the evolutionary tree. Which animal is most closely related to the grey wolf? (Choose from: Coyote, Red fox, Grey seal, Lion.)

Answer: Coyote

Working: The wolf and coyote share the most recent common ancestor (about 3 million years ago), so they are the most closely related.

Related lessons

Continue in the full course

This lesson is section B14.6 (AGCSC1406) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

bottom of page