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8.5Unit 4: Living organisms and classification

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

5 min read 9 parts Lesson 5 of 5 1 quiz

📚 Cambridge Checkpoint Biology 0893 Hub / Topical Revision
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Lesson summary

8.5 Constructing keys — CHECKPOINT BIOLOGY 0893 notes: Construct branching and numbered keys from observable features, test and improve them, and explain why classification decisions matter.List all the organisms and their observable features.Choose a feature that splits the grou…

  • Branching key
  • Dichotomous key
  • Choose a feature that splits the group into two
  • Write it as a yes/no question or a pair of statements
  • Repeat for each smaller group using a new feature
  • Stop when each organism is alone on a branch

Section 1 of 9Making a key

1
List all the organisms and their observable features.
2
Choose a feature that splits the group into two.
3
Write it as a yes/no question or a pair of statements.
4
Repeat for each smaller group using a new feature.
5
Stop when each organism is alone on a branch.
6
Test the key on each organism to check it works.
  • For n organisms a dichotomous key needs exactly n − 1 questions.

Section 2 of 9Good and poor questions

1
A good question can be answered the same way by two different people.
2
Good questions use observable features with a clear yes/no answer.
  • Does it have legs?
  • Are the leaves divided?
  • Does it have more than six legs?
  • Do the flowers have five or more petals?
3
Poor questions are opinions or have no fixed measure.
  • "Is the plant tall?" has nothing to compare with.
  • "Is it pretty?" is an opinion.
  • "Is it big?" needs a measurement, such as "Is the body longer than 5 cm?".
4
Avoid features that vary with age or season, such as flowers.

Section 3 of 9Rules for the statements

1
Each question splits the group into two smaller groups, ideally about equal in size.
2
The two statements must be opposites that cover every organism.
3
Every route must end in a number or a name.
4
A statement must not name the organism, so ask "Does it have eight legs?" and not "Is it a spider?".

Section 4 of 9Two styles

Branching key
  • Branching key questions with Yes and No branches.
Numbered key
  • Numbered key paired statements, such as 1a and 1b, with "go to 2".
Either style is correct as long as the key works.

Section 5 of 9Testing and improving a key

  • Exchange keys with a partner and try them on the real organisms.
  • Note exactly where the key went wrong.
  • Replace an unclear feature with one that can be counted or seen clearly.
  • Test again.

Section 6 of 9Uses of keys

Uses of keys
Keys are used by scientists and ecologists to identify organisms in the field quickly and consistently, even by people who are not experts.
Keys link to classification, for example insects have 6 legs and arachnids have 8.

Section 7 of 9Why classification matters

Why classification matters
Biologists sometimes disagree about whether two kinds of organism are the same species or different species.
The decision can affect efforts to save an organism or protect a habitat.
  • If a forest is cut down, some species may become extinct.
  • Whether the forest holds 200 species or only 100 changes the case for protecting it.

Section 8 of 9Key terms

TermMeaning
Dichotomous keyA key that gives 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.
ClassificationSorting organisms into groups based on shared features.

Section 9 of 9Exam technique

1
Choosing "Is it big?" — use a measurement or a fixed feature.
2
Writing questions with three answers — only two choices are allowed.
3
Naming the organism in the question, such as "Is it a spider?" — ask about a feature.
4
Leaving a route with no ending — every route must lead to a name or a number.
5
Not testing the key on every organism.

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