Natural Selection: Following the Shift in Traits

Estimated Time: 45-60 minutes Materials: Computer or tablet with internet access, calculator or spreadsheet.

Part 1: Engage (Anchoring Phenomenon)

Before the Industrial Revolution in England, most peppered moths were light-colored with dark spots, which camouflaged them on lichen-covered trees. After factories began covering trees with soot and darkening the bark, the moth population shifted to mostly dark-colored. Why did the population change?

1. Observations and Questions:

Why did the peppered moth population shift from mostly light to mostly dark during the Industrial Revolution?
If the environment (trees) changed dramatically, what determines which individuals survive and reproduce?
Generate at least two “need to know” questions about how traits change in a population over time.

Part 2: Explore (Simulation Investigation)

Open the Natural Selection simulation. The simulation includes a background color slider, mutation rate slider, predation level slider, play/pause/next gen controls, a population average color graph, and a data table.

2. Data Collection:

Part A: Baseline — Light Environment

Set Background Color to 10% (very light/mostly white)
Set Mutation Rate to 0% (no new mutations)
Set Predation Level to 70% (high predation)
Click Play. Let the simulation run for 20 generations
Record the Average Color of the population every 5 generations
Observe the Population Average Color graph

Part B: Dark Environment

Reset the simulation
Set Background Color to 90% (very dark/mostly black)
Keep Mutation Rate at 0% and Predation Level at 70%
Run for 20 generations. Record Average Color every 5 generations

Part C: Mutation Effect

Reset. Set Background Color to 50% (medium gray)
Set Mutation Rate to 5%
Set Predation Level to 50%
Run for 30 generations. Record Average Color every 5 generations
Watch the data table. Note any new colors that appear in the population.

Part D: Predation Level Effect

Reset. Set Background Color to 75% (dark)
Set Mutation Rate to 1%
Set Predation Level to 90% (high)
Run for 20 generations. Record final Average Color
Repeat with Predation Level = 10% (low). Compare results.

Data Tables:

Data Table 1: Average Color Over Time (Light vs. Dark Background) | Generation | Avg Color (Light Bkg) | Avg Color (Dark Bkg) | |:—|—:|—:| | 0 | | | | 5 | | | | 10 | | | | 15 | | | | 20 | | |

Data Table 2: Mutation and Background Color Effects | Generation | Avg Color (50% Bkg, 5% Mutation) | |:—|—:| | 0 | | | 5 | | | 10 | | | 15 | | | 20 | | | 25 | | | 30 | |

Part 3: Explain (Sensemaking)

3. Analyzing Trait Shift Patterns:

Compare your light-background and dark-background data. How did the average color change over 20 generations in each environment? What pattern do you observe in the rate of change?
Look at your predation level data. How did changing the predation level affect how quickly the population adapted to the dark background? Why do you think this happens?
Based on your mutation experiment, what role does mutation play in natural selection? Can a population adapt without new mutations?

4. Connecting to the Peppered Moth Phenomenon:

The peppered moth example is a classic case of natural selection. Based on your simulation data, explain step by step how the moth population shifted from light to dark:
- Before the Industrial Revolution: Why were light moths more common?
- During the Industrial Revolution: Why did the proportion of dark moths increase?
- What would happen if the environment returned to light-colored trees?
Use your data to support each step of your explanation.

Part 4: Elaborate / Evaluate (Argumentation & Modeling)

5. CER: Explaining Trait Shift Through Natural Selection

Construct a complete CER (Claim, Evidence, Reasoning) explanation answering the driving question: Why did the peppered moth population in England change from mostly light to mostly dark during the Industrial Revolution?

Your response must include:

Claim: A clear statement about what caused the population shift in peppered moths.
Evidence: Specific data from at least two of your simulation experiments. Include:
- Quantitative data (average color values at specific generations)
- Comparisons between different environmental conditions (light vs. dark background, high vs. low predation)
- Data table references and calculated trends
Reasoning: Explain how the evidence supports your claim by describing:
- Organizing Data: How your data tables and graphs show the distribution of traits over time
- Identifying Relationships: How statistical patterns (averages, rates of change) demonstrate the relationship between environmental conditions and trait frequencies
- Interpreting Data: How your analysis provides evidence that natural selection (differential survival and reproduction based on trait-environment fit) caused the changes in the population

Teacher Notes & NGSS Alignment

Performance Expectation: HS-LS4-3. Apply concepts of statistics and probability to support explanations that organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait.

Alignment to Dimensions:

SEP: Analyzing and Interpreting Data — Students organize trait distribution data into tables, identify patterns of change in average color over generations, and interpret their analyses as evidence for natural selection.
DCI: LS4.B (Natural Selection) — The traits that positively affect survival are more likely to be passed on to the next generation. In the peppered moth simulation, dark moths survive better on dark backgrounds, leading to an increase in dark-colored individuals in the population.
DCI: LS4.C (Adaptation) — Natural selection leads to adaptation — the population becomes better suited to its environment over time as individuals with advantageous traits contribute more offspring.
CCC: Patterns — Students identify patterns in trait frequency data across time and environmental conditions, using these patterns to infer the underlying mechanism of natural selection.

Evidence Statement Mapping:

1 (Organizing data): Students organize data on trait distribution over time using tables and graphs. Demonstrated in Part 2 when students record average color values at regular generation intervals under different conditions.
2 (Identifying relationships): Students use statistical analyses (averages, comparison of rates) to determine patterns of change across environmental conditions. Demonstrated in Part 3 when students compare light vs. dark background data and analyze the effect of mutation and predation rates.
3 (Interpreting data): Students use their analyses as evidence for positive/negative effects on survival, interpreting natural selection as the cause of observed changes. Demonstrated in Part 4 when students construct a CER argument connecting simulation data to the peppered moth phenomenon.