A dichotomous key identifies an organism by a series of yes-or-no questions about features you can observe. Following each choice carefully leads to the right name; one wrong choice leads to the wrong one.
A field guide doesn't show you every animal in Australia and ask you to find the match. It asks you a question about the one in front of you, then another, then another.
Last lesson: kingdoms. This lesson: a tool for naming organisms one careful choice at a time.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: how does a key work?
ExploreTry it on the board and compare what happens with your prediction.
Explore: identify the specimens
Choose a specimen and read its description carefully. Answer each question about that specimen. The name appears only when you get it right.
Try it yourselfYour edits are saved in this browser.
Use the key to identify at least 5 specimens.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: how keys work
In your book: write the path you took to identify the platypus, naming the feature you checked at each step.
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: a different key
Try it yourselfYour edits are saved in this browser.
Identify 3 specimens with this second key.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: a broken key
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
A dichotomous key identifies an organism with a series of two-way choices about observable features.
Answer each question about the specimen in front of you; each answer leads to the next step, until you reach a name.
One wrong answer leads to the wrong name: check each feature carefully.
Good questions have a clear yes or no and use features that don't vary within a kind (legs, covering, backbone), not colour or size.
For teachers
Students use a dichotomous key to identify organisms, name the feature at each decision, and explain how a wrong choice leads to a wrong name (AC9S7U01, node 3, a hinge node). About 25 minutes.
I can use a dichotomous key to identify at least five organisms.
I can name the observable feature used at each step.
I can explain why a wrong choice leads to a wrong name.
Before class
A concept tutorial for introducing or revising the idea; it does not replace keys with real specimens or a field guide outside. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk is procedural key-following: students reach a name without being able to say what each step checked, and then build broken keys in node 4. Keep asking "what feature did that step check?".
Board notes: specimens are anonymous descriptions; names appear only when the key is followed correctly. A wrong turn names the first step where the student's path left the right one.
Running the lesson
Engage: if anyone chose 'guess the name, then check', point out that the board hides names on purpose.
Explore: insist on reading the description before each answer; ask a student to narrate one path aloud.
Explain: the crocodile question is the wrong-turn reasoning; the good-question item previews node 4.
In your own words: the platypus path is the evidence.
The hard part (on purpose)
Reading each step against the organism, not jumping to a likely name. When a student reaches the wrong name, ask where their path left the right one.
When students get stuck
A student keeps reaching the wrong name. Ask: Read the step the board points to. What does the description say about that feature? Fix: Answer from the description, not from what you expect the animal to be.
A student says the key is wrong. Ask: Where did your path leave the right one? (The board tells you.) Fix: Check that step's answer against the description.
A student can't say what a step checked. Ask: What were you looking at when you answered? Fix: Name the feature: backbone, feathers, number of legs.
Stretch and support
Stretch: Add a cassowary to the first key: where would it go, and does the key still work?
Support: Print the whole key (open 'The whole key' on the board) and let students tick each step as they go.
Where it leads
Next: constructing keys (node 4), the same logic in reverse: choosing features that split a group cleanly in two.
In class: a key to the leaves on the school grounds, or to a tray of mixed hardware (screws, nails, bolts) as a warm-up.
What each step is for
Engage: how does a key work?: Students who think a key is a list of names to match will choose that option; working through it one feature at a time is the logic the node needs.
Explore: identify the specimens: Using a real key, with feedback on wrong turns, makes the binary decision logic concrete.
Explain: how keys work: Naming the feature at each decision and explaining wrong turns is the understanding students need to build keys in node 4.
Mix it up: A key is a classification tool for a purpose, and the purpose it serves is identification.
Elaborate: a different key: Two keys for the same animals reach the same names by different questions, showing that the features, not the order, carry the identification.
Elaborate: a broken key: Spotting a flaw in a key (a question that doesn't split cleanly) prepares students to build keys of their own.
Now you can
Use a dichotomous key to identify at least five organisms.