--- title: "Why did the beetles change?" subtitle: "Year 10 Biology · Evolution · Lesson 5" --- # Why did the beetles change? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** After a bushfire, the ground is black. A few years later, a beetle population on it is darker than before. No beetle changed colour. :::teacher-note **In one sentence:** Students model natural selection as predators, explain it in steps, reject the idea that individuals change because they need to, and connect it to common ancestry and the tree of life (AC9S10U02, node 2, a hinge node). **Students should be able to say:** I can explain natural selection in steps. I can explain that populations change, not individuals. I can explain why "fittest" depends on the environment. I can explain that all living things are related in a branching tree, and why features alone can mislead about relatedness. **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 **Birds eat the beetles they can see most easily. Over many generations the population becomes better camouflaged. How?** ☐ Beetles that blend in survive more often and have more offspring, so the population's colours shift ☐ Beetles choose a ground that matches ☐ The birds teach the beetles ☐ Each beetle changes colour to hide, because it needs to My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Beetles that blend in survive and breed more, so their offspring (with similar colours) make up more of the next generation. Individuals never change; the population does. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: natural selection :::keep-together ### The steps **Put the steps of natural selection in order.** Groups: 1st, 2nd, 3rd, 4th, 5th - Beetles in the population vary in colour: {{field-15}} - Birds eat the beetles that stand out most: {{field-15}} - Beetles that blend in are more likely to survive: {{field-15}} - Survivors breed, passing on the alleles for their colour: {{field-15}} - Over generations, the population's average colour shifts: {{field-15}} ::: :::teacher-note **Answer:** Beetles in the population vary in colour → 1st; Birds eat the beetles that stand out most → 2nd; Beetles that blend in are more likely to survive → 3rd; Survivors breed, passing on the alleles for their colour → 4th; Over generations, the population's average colour shifts → 5th ::: :::keep-together ### Who changed **Did any single beetle on the board change its colour?** ☐ Yes, the survivors got paler ☐ No: each beetle kept its colour; the population changed because of who survived and bred ☐ Yes, the eaten ones got darker ☐ Only the babies changed ::: :::teacher-note **Answer:** No: each beetle kept its colour; the population changed because of who survived and bred ::: :::keep-together ### No variation **What would happen if every beetle started exactly the same colour?** ☐ Selection couldn't change the population: there would be no variation to select from ☐ They would all be eaten ☐ They would turn the ground's colour ☐ They would still evolve the same way ::: :::teacher-note **Answer:** Selection couldn't change the population: there would be no variation to select from ::: :::keep-together ### A branching tree **Over millions of years, populations split apart and natural selection changes each part differently. What does that mean for living things today?** ☐ Each species began separately, so they aren't related ☐ They are all related, in a branching family tree: close relatives share a recent common ancestor ☐ They form a ladder, with humans at the top as the most evolved ::: :::teacher-note **Answer:** They are all related, in a branching family tree: close relatives share a recent common ancestor Splitting and selection, repeated over millions of years, give a branching tree of life. Every living thing is related; the question is how closely. ::: :::keep-together ### Closest DNA **If relatives share ancestors, which pair would you expect to have the most similar DNA?** ☐ A human and a mushroom ☐ A human and a mouse ☐ A human and a chimpanzee ::: :::teacher-note **Answer:** A human and a chimpanzee The more recently two species shared an ancestor, the more similar their DNA. Comparing DNA is how biologists now draw the tree of life. ::: :::keep-together ### Back to the kingdoms **In Year 7, kingdoms put mushrooms in their own group, and many people think of them as plant-like. DNA shows fungi share a more recent ancestor with animals than with plants. Why don't features alone show this?** ☐ Similar features can evolve in unrelated groups, and close relatives can come to look very different ☐ Fungi evolved from plants ☐ DNA tests are unreliable ::: :::teacher-note **Answer:** Similar features can evolve in unrelated groups, and close relatives can come to look very different Natural selection shapes features to suit each environment, so features can mislead about family. That is why classification moved from kingdoms by features to groups by shared ancestry, and changes as evidence improves. ::: :::typical ## Elaborate: after the bushfire :::keep-together ### Fittest **Before the fire, paler beetles survived best. After it, darker ones did. What does that tell you about 'survival of the fittest'?** ☐ Fittest means strongest ☐ Fittest means best suited to the current environment, which can change ☐ The darker beetles were always fitter ☐ Fitness doesn't depend on the environment ::: :::teacher-note **Answer:** Fittest means best suited to the current environment, which can change ::: ::: :::wider ## Elaborate: antibiotic resistance :::keep-together **Why does using antibiotics more often make resistant bacteria more common?** ☐ Each use kills the bacteria that aren't resistant; any that happen to be resistant survive and multiply ☐ Bacteria learn to resist ☐ Antibiotics make bacteria resistant ☐ It doesn't make a difference ::: :::teacher-note **Answer:** Each use kills the bacteria that aren't resistant; any that happen to be resistant survive and multiply ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Natural selection:** individuals **vary**; more are born than survive; those **best suited** to the environment are more likely to **survive and reproduce**; they pass on their **alleles**; over **generations**, the population changes. - **Individuals don't change** to suit the environment; the population does. - "Fittest" means best suited to the **current** environment. Selection needs **variation**. - Over millions of years, populations split and diverge: all living things are related in a **branching tree of life**, not a ladder. The more recent the **common ancestor**, the more similar the DNA. - Features can mislead about relatedness (fungi are closer to animals than to plants), so modern classification uses shared ancestry, and changes as evidence improves. ## Exit ticket :::keep-together **Explain why the beetle population got paler on pale ground.** {{lines-3}} ::: :::teacher-note **Answer:** Beetles varied in colour; birds ate the darker, more visible ones; paler beetles survived and bred, passing on their colour; over generations, the average colour got paler. :::