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8.4Unit 4: Living organisms and classification

Using keys — Cambridge Checkpoint Biology 0893 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 5 1 quiz

📚 Cambridge Checkpoint Biology 0893 Hub / Topical Revision
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Lesson summary

8.4 Using keys — CHECKPOINT BIOLOGY 0893 notes: Use a dichotomous key, in branching or numbered style, to identify an organism.A key is a tool used to identify an unknown organism by answering a series of questions about its features.Biologists use keys because looking at picture…

  • A dichotomous key gives two choices at each step (di
  • Branching key
  • Numbered key
  • Observable feature
  • Biologists use keys because looking at pictures does not always give a name
  • Keys are used by scientists in fieldwork to identify organisms quickly without expert knowledge

Part 1 of 7Keys

Keys
A key is a tool used to identify an unknown organism by answering a series of questions about its features.
Biologists use keys because looking at pictures does not always give a name.
Keys are used by scientists in fieldwork to identify organisms quickly without expert knowledge.

Part 2 of 7Dichotomous keys

1A dichotomous key gives two choices at each step (di
A dichotomous key gives two choices at each step (di two).
2
Each answer leads either to another question or to the name of the organism.
3
Questions must use observable features with a clear yes or no answer.
  • Does it have wings?
  • Does it have more than six legs?

Part 3 of 7Styles of key

Styles of key
A branching (tree) key is drawn as a diagram, with yes and no arrows from each question.
A numbered key is written as paired statements such as 1a and 1b, each ending in "go to 2" or in a name.
The same key can be written in either style.

Part 4 of 7Using a key

1
Choose one organism to identify.
2
Start at the first question.
3
Look closely at the organism and choose the answer that matches.
4
Follow the instruction to the next question.
5
Repeat until a name is reached.
  • Do not skip steps, and imagine the whole animal rather than only a picture.

Part 5 of 7An example key

StepStatementGo to / answer
1aIt has legs.go to 2
1bIt does not have legs.go to 3
2aIt has exactly six legs.go to 4
2bIt has more than six legs.crab
3aIts body is made up of rings.earthworm
3bIts body is not made up of rings.slug
4aIt has four wings.dragonfly
4bIt has two wings.housefly

Part 6 of 7Choosing features

Choosing features
Good features can be counted or seen clearly and do not change.
Poor features are:
  • Opinions, such as "Is it pretty?".
  • Features that change with age or season, such as size or having flowers.
  • Colour, which can vary between individuals.
  • Comparisons without a measurement, unless a measurement such as "longer than 5 cm" is given.

Part 7 of 7Making a key

Making a key
List all the organisms and pick a feature that splits them into two groups.
Keep splitting each group until each organism is on its own.
For n organisms, a dichotomous key needs exactly n − 1 questions.

Key terms

TermMeaning
KeyA set of questions or statements used to identify an organism.
Dichotomous keyA key with two choices at each step.
Branching keyA key drawn as a diagram of questions.
Numbered keyA key written as numbered pairs of statements.
Observable featureA feature that can be seen or counted clearly.

Avoid these mistakes

1
Trying to identify two organisms at once — use the key for one organism at a time.
2
Writing questions with three possible answers — use only two choices.
3
Using size or colour as a feature — use a fixed structural feature.
4
Writing "It is a beetle because it looks like one" — state the features that lead the key to beetle.
5
Skipping steps — follow the path from question 1.

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