L03 — The Peppered Moth

Module: M07 — The Arms Race Periods: 1 NGSS: HS-LS4-2, HS-LS4-4 Lesson Type: Explore / Explain

Learning Objective

By the end of this lesson, students will be able to analyze historical data on peppered moth populations during the Industrial Revolution to explain how environmental change drives natural selection, using the four conditions framework from L01.

Phenomenon / Hook

“In 1800s England, almost every peppered moth was light-colored with dark speckles. By 1900, in industrial cities, almost every moth was solid black. Same species. Same forests. What changed — the moths or the world around them?”

Show two images side by side: a light moth on a lichen-covered tree trunk, a dark moth on a soot-blackened tree trunk. “Can you spot the moth? Predators can’t either — if the moth matches the bark.”

Materials

Peppered moth images (light and dark morphs on clean vs. polluted bark)
Historical data set: [[handouts/L03-peppered-moth-data]] (population percentages 1848–2001)
Graphing paper or Chromebooks with spreadsheet access
Projector for phenomenon images
Colored pencils for graphing

Agenda

Opening (7 min)

Show “spot the moth” images — light moth on clean bark, dark moth on sooty bark (2 min)
Quick poll: “Hands up if you can see the light moth on the clean tree. Hands up if you can see the dark moth on the sooty tree.” Both should be easy.
Flip: show light moth on sooty bark. “Can you see it now?” Much harder.
Think: “If you’re a bird hunting moths, which moth would you eat more of in each environment?” (2 min)
Bridge to L01: “This is selection pressure in action. Let’s see the data.”

Explore (18 min)

Peppered Moth Data Analysis (pairs)
- Students receive a data table with moth population percentages from 1848–2001, alongside major environmental events:
  - 1848: 99% light, 1% dark (pre-industrialization)
  - 1895: 5% light, 95% dark (peak industrial pollution in Manchester)
  - 1950s: Clean Air Acts begin
  - 1990s–2001: Light moths recovering, dark moths declining
- Tasks:
  1. Graph the data: Line graph with two lines (light % and dark %) over time
  2. Identify the pattern: When did dark moths increase? What was happening in England at that time?
  3. Apply the four conditions:
    - Variation: (light and dark color morphs)
    - Inheritance: (color is genetic)
    - Selection pressure: (predation — birds eat what they can see)
    - Differential reproduction: (camouflaged moths survive to reproduce more)
  4. Predict: If pollution levels increase again, what would happen to moth populations?
  5. Critical thinking: Did individual moths change color, or did the population change? How do you know?

Explain (10 min)

Teacher-led synthesis:
- Historical timeline: Industrial Revolution → soot kills lichens → trees go dark → dark moths hidden → dark moths survive → population shifts
- Clean Air Acts → lichens recover → trees go light → light moths hidden → population shifts back
- Key insight: The moths didn’t change. The environment changed which color was advantageous. Natural selection acted on pre-existing variation.
- Show Kettlewell’s original experiment (mark-recapture method): released marked moths, recaptured more of the camouflaged type
- Address a common criticism: “But moths don’t rest on tree trunks!” — Majerus’s massive 6-year experiment (4864 moths, published posthumously by Cook et al., 2012, in Biology Letters) confirmed they do rest on trunks and predation rates match Kettlewell’s predictions
Students add annotations to their graphs: mark where selection pressure shifted

Elaborate (8 min)

Mystery Moth Scenario: Students receive a new scenario:
- “A new species of moth lives in a forest. The forest has both light-barked and dark-barked trees in equal amounts. There are two moth colors: brown and green. Which moth color would you expect to be more common? Design a simple experiment to test your prediction.”
- Students write a 3-sentence experimental design: what they’d do, what they’d measure, what they’d expect
- Share with a partner — peer feedback: “Does this experiment actually test natural selection?”

Closing (5 min)

Exit Ticket: [[exit-tickets/L03-exit-ticket]]
1. Explain how the Industrial Revolution changed the selection pressure on peppered moths. Use the terms “variation” and “selection pressure” in your answer.
2. After the Clean Air Acts, light moths became more common again. Does this mean evolution “went backward”? Explain.

Differentiation

Support	Extension
Provide a pre-labeled graph template with axes and key dates marked — students plot points and draw lines	Ask: Kettlewell’s experiment was criticized for using dead moths pinned to trees. Design a better experiment that addresses this criticism.
Give a word bank: camouflage, predation, Industrial Revolution, pollution, allele frequency	Research: Are there other examples of rapid color change in populations due to environmental change? (e.g., rock pocket mice in lava fields)

Assessment

Formative: Check graphs during data analysis — are students correctly correlating environmental changes with population shifts?
Exit Ticket: Mechanism explanation + “backward evolution” misconception check

Teacher Notes

Graphing is the activity. Don’t shortcut this. Students who can graph this data and explain the trend understand natural selection deeply.
The “did evolution go backward?” question catches the most common misconception in this lesson. Students who say “yes” think evolution has a direction. Use the discussion to clarify: evolution responds to current conditions, it doesn’t have a goal.
This is a data-heavy lesson. If students struggle with graphing, pair them with a stronger partner or provide the pre-labeled template.
The Mystery Moth scenario connects to the concept of balanced polymorphism — both colors can persist if the environment has mixed backgrounds. Advanced extension if time allows.
Bridge preview: “We’ve seen selection act on color. But what about body parts — like beaks? That’s Darwin’s big idea, and it’s next.”