Teacher sheet · Ages 15–16 · about 45–50 minutes
Evolution: evidence and numbers
Lesson page: echoesof.science/school/evolution/ages-15-16/ · Pupil worksheet: echoesof.science/school/evolution/ages-15-16/print/
Learning aims
- Explain why heritable variation is essential for natural selection.
- Calculate changes in the frequency of a variant after selection.
- Explain antibiotic resistance as natural selection.
- Describe five lines of evidence for evolution and correct common misconceptions.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: 'Evolution is just a theory.' Discuss what theory means in science. |
| 10 min | Read the text; worked example of the moth calculation. |
| 20 min | Paper-moth selection model over four generations. |
| 5 min | Play Which line of evidence? |
| 5 min | Quiz and discussion. |
Materials for the experiment
- Two large sheets: one plain pale paper, one dark or newsprint paper
- 60 small paper discs: 30 pale and 30 dark (hole-punch circles work well)
- A stopwatch
- Graph paper or a spreadsheet
Safety: Keep the hole-punch away from fingers. Pick up discs with fingers only, gently; no grabbing between groups. Tidy away the discs so nobody slips.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Bacteria become resistant because they 'get used to' antibiotics. | Resistant variants already present survive and multiply when susceptible ones are killed. |
| Evolution always makes organisms more complex or better. | Evolution is not goal-directed; populations track their environment. |
| Individuals evolve during their lives. | Populations evolve: the proportions of heritable variants change between generations. |
Worksheet answers
- Answers vary; on the dark sheet % dark should rise and on the pale sheet it should fall; uncertainties include different predators, uneven spreading and small numbers (chance).
- Green: 100 × 0.6 = 60 survive; brown: 300 × 0.9 = 270 survive; 60 ÷ 330 × 100% ≈ 18%.
- Variation: some bacteria happen to be resistant; selection: the antibiotic kills susceptible bacteria, so resistant ones survive; inheritance: they multiply and pass on resistance, so the population becomes resistant.
- Three limbs with matching colours for the upper arm bone, the two forearm bones, and the wrist and finger bones.
Quiz answers
- By 1898, dark peppered moths outnumbered light ones 99 to 1. What percentage were dark? 99%. 99 ÷ (99 + 1) × 100% = 99%.[3]
- A population has 160 light and 40 dark moths. Birds eat 50% of the light ones and 10% of the dark ones. What percentage of the survivors are dark? About 31%. Survivors: 80 light and 36 dark; 36 ÷ 116 ≈ 31%.[3]
- Why must variation have a genetic basis for natural selection to change a population? Otherwise the trait is not passed to the next generation. Selection only changes the next generation if the trait is inherited.[2]
- Which is an example of a vestigial structure? Wings on a flightless bird. Vestigial structures are reduced remnants of ancestral traits.[1]
- How does antibiotic use speed up antimicrobial resistance? It kills susceptible bacteria, so resistant ones survive and multiply. This is selection: resistant bacteria are left to multiply.[4]
- What does 'theory' mean in 'the theory of evolution'? A well-tested, well-supported explanation. Scientific theories are supported by large amounts of evidence.[1]
Going further
Model the peppered moth over 10 generations in a spreadsheet: start at 1% dark and assume birds remove 30% of light and 10% of dark moths each generation. How many generations until most moths are dark? Discuss what the model leaves out.
Sources used in this lesson
- [1] Biology 2e, 18.1 Understanding Evolution (OpenStax (Rice University), open textbook): reliability High
- [2] Concepts of Biology, 11.1 Discovering How Populations Change (OpenStax (Rice University), open textbook): reliability High
- [3] Peppered moth (Encyclopaedia Britannica): reliability High
- [4] About Antimicrobial Resistance (US Centers for Disease Control and Prevention (CDC)): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/evolution/sources/
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