1.5 - Dichotomous Keys

Using and constructing dichotomous keys

• A dichotomous key (also called a paired-statement key) is a written tool for working out the name of an unknown organism from its observable features.
• It's made of a series of numbered steps. Each step offers two contrasting statements - for example "wings present" or "wings absent".
• The two statements are opposites, so for any one organism exactly one of them is true.
• "Dichotomous" means dividing into two - which is what happens at every step: the key repeatedly splits the group of possible organisms into two smaller groups.

Using a key to identify an organism

1. Start at the first numbered step and look at your organism to decide which of the two statements is true for it.
2. Beside each statement is an instruction: either a number telling you which step to go to next, or the name of an organism.
3. Follow the true statement to the step it points to, look at the next pair of statements, and again pick the one that matches.
4. Each choice rules out one group and narrows down what your organism could be.
5. Keep going until a statement leads not to another step but to a single name - that's the identity of your organism.

Constructing a key of your own

1. Start with a sample containing every organism the key will identify.
2. Choose one observable feature that divides the sample cleanly into two groups (for example, "has six legs" against "has more than six legs"), and write a statement describing each group.
3. Take each of those two groups and split it again using a different feature, writing a statement for each new group.
4. Carry on splitting in the same way until every organism has been separated into a group on its own.
5. Finally, link the statements together in the right order, numbering the steps so that each statement points to the next step or to a named organism.

Good features to split on are ones that are easy to observe and clearly contrasting, so that each step puts every organism unambiguously into one of the two groups. A key is often drawn as a branching diagram, like the one below, with each step written as a question and its two answers.