--- title: "Who is it? Using a key" subtitle: "Year 7 Biology · Classification · Lesson 3" --- # Who is it? Using a key **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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. :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **You have a dichotomous key and an animal you don't know. How do you think you use the key?** ☐ Look for the picture that matches ☐ Guess the name, then check it in the key ☐ Count its legs and look up the number ☐ Answer two-way questions about the animal's features, one at a time, until the key names it My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Answer two-way questions about features you can observe, one at a time. Each answer sends you to the next question, until you reach a name. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: how keys work :::keep-together ### Two ways **Why is it called a dichotomous key?** ☐ Each step splits into exactly two choices ☐ It identifies two organisms ☐ It uses two kinds of feature ::: :::teacher-note **Answer:** Each step splits into exactly two choices Dichotomous means dividing into two. ::: :::keep-together ### Wrong turns **A student identified the crocodile as a green tree frog. Where did they most likely go wrong?** ☐ At "Does it have a backbone?" ☐ At "Does it have dry, scaly skin?": they answered no ☐ The key is wrong ::: :::teacher-note **Answer:** At "Does it have dry, scaly skin?": they answered no A wrong answer at one step sends you down the wrong branch, and every later answer can't fix it. ::: :::keep-together ### Good questions **Which is a good question for a key?** ☐ Does it have six legs? ☐ Is it big? ☐ Is it scary? ☐ Is it a kangaroo? ::: :::teacher-note **Answer:** Does it have six legs? A good question is about an observable feature with a clear yes or no. ::: ## Mix it up :::keep-together **Looking back at why we classify: what job is a dichotomous key for?** ☐ Giving a new organism its name ☐ Identifying an organism you do not know ☐ Showing exactly how organisms are related ☐ Deciding which organism is better ::: :::teacher-note **Answer:** Identifying an organism you do not know A key identifies an organism by asking about features you can observe. Different classifications suit different purposes. ::: :::typical ## Elaborate: a different key :::keep-together ### Same names **The two keys ask different questions in a different order, but give the same names. Why?** ☐ Each animal has the same features whichever order you check them in ☐ One of the keys must be wrong ☐ Keys always ask the same questions ::: :::teacher-note **Answer:** Each animal has the same features whichever order you check them in A key is one way of checking features. Different keys can sort the same organisms in different orders. ::: ::: :::wider ## Elaborate: a broken key :::keep-together **A key's first question is "Is it brown?" The specimens include a brown kangaroo, a grey kangaroo and a brown spider. What is wrong with this question?** ☐ Colour varies within a species and isn't reliable: the two kangaroos go different ways ☐ It should ask about size instead ☐ Nothing: colour is observable ::: :::teacher-note **Answer:** Colour varies within a species and isn't reliable: the two kangaroos go different ways Good keys use features that don't vary within a kind of organism: legs, body covering, backbone. ::: ::: ## 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. ## Exit ticket :::keep-together **Why must each question in a dichotomous key be about an observable feature with a clear yes or no?** {{lines-3}} ::: :::teacher-note **Answer:** So that anyone checking the specimen gets the same answer and reaches the same name. :::