Constructing keys

Part 1 of 6Choosing good questions

• A question is suitable for a key only if two different people looking at the same organism always give the same answer.
• Good questions use features that can be counted or seen clearly.
  • "Do the flowers have five or more petals?" can be counted.
  • "Are the leaves darker on the upper surface than the lower surface?" is a direct comparison on the same plant.
• Poor questions need an opinion or have no fixed measure.
  • "Is the plant tall?" has no fixed measure to compare with.
  • "Does the plant have dark green leaves?" depends on the person's opinion.
• Avoid words such as tall, big, dark and long on their own.
• Avoid features that change with season, weather, age or the observer's opinion.
• Every step must have exactly two possible answers.

Part 2 of 6Building a key

1. List all the organisms and observe their features.
2. Find one feature that splits the whole group into two, for example male and female learners.
3. Write it as a question or a pair of statements.
4. Take one of the two groups and find a feature that splits it into two.
5. Repeat step 4 for every group that still has more than one organism.
6. Stop when each organism is alone at the end of a branch with its name.
7. Test the key by working through it for each organism.

• A key for n organisms needs n − 1 splits, so a key for four organisms needs three pairs of statements.

Part 3 of 6Rules for the two statements

• 01The two statements must be opposites that cover every organism.
• 02Each route must lead on to another number or to a name, and no route may be a dead end.
• 03Each step must add new information, so the same feature must not be repeated.
• 04End with names, with one organism at each ending.
• 05A statement must not name the answer, so ask about a feature and not "Is it a snail?".

Part 4 of 6Two styles of key

• 01A branching key has questions with Yes and No branches.
• 02A numbered key has pairs of statements, 1a and 1b, 2a and 2b, each ending in "go to" a number or in a name.
• 03Either style is correct if the key works.

Part 5 of 6Testing and improving a key

1. Exchange keys with a partner.
2. Try their key on the real organisms and not just by reading it.
3. Note exactly where it went wrong.
4. Rewrite the failing step with a feature that can be counted or seen clearly.
5. Test again.

• A good key:
  • Is made of pairs of statements to choose between.
  • Makes it easy to choose between the statements each time.
  • Has no more pairs of statements than needed.
  • Works, so someone else can identify all the organisms with it.

Part 6 of 6Why classification decisions matter

• 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, species living only there may become extinct.
  • Whether the forest holds 200 species or only 100 changes the strength of the case for protecting it.
• This is why the definition of a species, a group that can reproduce together to produce fertile offspring, must be applied carefully.