Natural Selection & Adaptation: How Populations Change Over Time

Estimated Time: 50-65 minutes Materials: Computer or tablet with internet access, calculator.

Part 1: Engage — The Case of the Changing Moths

In 19th-century England, the peppered moth (Biston betularia) existed in two color forms: light (speckled white) and dark (nearly black). Before the Industrial Revolution, the light form was common. As factories covered trees with dark soot, the dark form became dominant. Why?

  1. Initial Observations:
    • What environmental factor changed and how might this have affected the moths?
    • Write down two “need to know” questions about how populations change over time.

Part 2: Explore — Simulating Natural Selection

Open the Natural Selection & Adaptation simulation. You’ll observe a population of organisms with variable traits under environmental pressure.

  1. Experimental Setup:
    • Observe the starting population. Note the range of trait variation (e.g., fur color in rabbits, beak size in finches).
    • Identify the selection pressure (e.g., predators, food availability, climate).
  2. Data Collection — Baseline Generation (No Selection Pressure): Run the simulation for 5 generations without any selection pressure.

Table 1: Control — No Selection Pressure | Generation | Avg Trait Value | Population Size | Trait Range (min-max) | |:—|:—|:—|:—| | 0 | | | | | 1 | | | | | 2 | | | | | 3 | | | | | 4 | | | | | 5 | | | |

  1. Data Collection — With Selection Pressure: Now introduce a selection pressure (e.g., predator that preferentially catches organisms with certain trait values).

Table 2: Experimental — With Selection Pressure | Generation | Avg Trait Value | Population Size | Trait Range (min-max) | |:—|:—|:—|:—| | 0 | | | | | 1 | | | | | 2 | | | | | 3 | | | | | 4 | | | | | 5 | | | | | 10 | | | |

Part 3: Explain — Making Sense of Natural Selection

  1. Analyzing the Results:
    • Compare the control and experimental groups. How did the average trait value change under selection pressure?
    • Did the population go extinct or adapt? What evidence supports your conclusion?
    • Was there any trait variation still present after 10 generations of selection? Why might this matter?
  2. The Mechanism of Natural Selection: Darwin’s theory of natural selection requires three conditions:
    • Variation: Individuals in a population differ in their traits.
    • Differential Survival/Reproduction: Some variants survive and reproduce more than others.
    • Heritability: Traits are passed from parents to offspring.

    For each condition, provide specific evidence from your simulation data.

Part 4: Elaborate — Testing Your Understanding

  1. Predict and Test:
    • If you reversed the selection pressure (e.g., changed which trait value was favored), predict what would happen to the population. Then test your prediction in the simulation.
    • Were your results consistent with your prediction? Explain any discrepancies.
  2. Real-World Connection: Antibiotic resistance is a modern example of natural selection. Bacteria with genetic mutations that confer resistance survive antibiotic treatment and reproduce. Explain how this is the same mechanism you observed in the simulation.

Part 5: Evaluate — Constructing an Evidence-Based Argument

  1. Write a Claim-Evidence-Reasoning (CER) argument:
    • Claim: Populations change over generations through the process of natural selection.
    • Evidence: Cite at least three specific data points from your simulation (trait shifts, population changes, generational patterns).
    • Reasoning: Explain how variation, differential survival, and heritability work together to drive adaptation — connecting your evidence back to the three conditions for natural selection.

Teacher Notes & NGSS Alignment

Performance Expectation: HS-LS4-4. Construct an explanation based on evidence for how natural selection leads to adaptation of populations.

Alignment to Dimensions:

Evidence Statement Mapping: