Materials & Prep
3 min- Project the trait-frequency table below and prepare one copy per pair.
- Provide graph paper or a half-sheet with axes labeled Generation (0–3) and Percent of population.
- Set up a board space with: Claim / Evidence from data / Reasoning.
Opening
3–10 minDisplay“A population of beetles lives on dark soil. By generation 3, 88% are dark. Did the beetles turn dark because they needed camouflage?” Students silently commit to Yes, No, or Not enough information with a hand signal, then write one reason.
Ask“What does it mean for a trait to be more frequent in a population?” Have students recall that a frequency is the percent of individuals with a trait, not a description of one individual changing.
ConfrontCollect two or three explanations, including any “they changed because they needed to” idea, without correcting it yet. Tell students the data will test that explanation.
Direct instruction
10–20 minExplainNatural selection occurs when individuals in a population already vary, some inherited traits help individuals survive and reproduce in a particular environment, and those traits become more frequent across generations. The environment does not give organisms traits because they need them.
DisplayUse this data set from a beetle population on dark soil.
| Generation | Dark beetles | Light beetles | Dark-trait frequency |
|---|---|---|---|
| 0 | 50 | 50 | 50% |
| After predation in generation 0 | 35 survive | 10 survive | 78% of survivors are dark |
| 1 | 70 | 30 | 70% |
| 2 | 78 | 22 | 78% |
| 3 | 88 | 12 | 88% |
Think aloud“First, I observe that dark beetles rise from 50% to 88%. That is an observation because the numbers show it. Next, I infer that dark beetles were harder for predators to see on dark soil. The survival row supports that inference: 35 dark beetles survived, but only 10 light beetles survived. My claim is not that a light beetle changed color. Both colors were present at generation 0, and the data show different survival and reproduction.”
EmphasizeThe common error is treating the population change as an individual beetle changing. Point to the generation 0 row: selection acts on existing variation in individuals; the population’s trait frequency changes over generations.
Guided practice
20–32 minStudentsIn pairs, graph the dark-trait frequency for generations 0–3: 50%, 70%, 78%, 88%. Then annotate the graph with an arrow from the survival data to generation 1.
PromptPartners complete this spoken or written frame: “My claim is that natural selection made the trait more frequent. My evidence is . This supports my claim because individuals were more likely to survive and reproduce, so .”
Check for understandingCold-call pairs to distinguish one observation and one inference. Expected: Observation: “Dark beetles increased from 50% to 88%.” Inference: “Dark color provided camouflage on dark soil.” Ask, “Which statement is supported directly by the table, and which explains the table?”
ScaffoldProvide the graph with the 50% point plotted and the CER frame above. Students may point to the table while orally rehearsing their explanation before writing.
ExtensionAsk ready students to explain why the dark-trait frequency after predation in generation 0 (78%) is not identical to generation 1 (70%). They should infer that offspring frequencies depend on reproduction and inherited traits, not a simple copy of one survival count.
Independent work
32–45 minDisplayA moth population lives on light-colored tree bark. The table shows trait frequencies after several generations.
| Generation | Speckled moth frequency | Plain light moth frequency |
|---|---|---|
| 0 | 25% | 75% |
| 1 | 40% | 60% |
| 2 | 58% | 42% |
| 3 | 72% | 28% |
WriteStudents produce a four-sentence CER response: (1) claim about which trait became more frequent, (2) numerical evidence from at least two generations, (3) reasoning that connects better camouflage to survival and reproduction, and (4) a correction to this statement: “The moths developed speckles because they wanted to blend in.”
CirculateLook for use of population and generations. Prompt students who write “moths changed” to revise with: “Moths with speckles were already present. Over generations, they….”
ChallengeHave students predict what would likely happen to the frequencies if pollution darkened the bark for many generations, then justify the prediction without claiming any individual moth chooses or develops a needed trait.
Closing
45–50 minExit ticket“In a lizard population, the long-leg trait rises from 30% in generation 0 to 68% in generation 4 after a drought leaves widely spaced food sources. Write a claim, cite both percentages as evidence, and explain how natural selection could cause this change. Include the phrase ‘over generations.’”
Listen forStudents must state that long-legged lizards likely survived and reproduced more successfully in that environment, causing the inherited long-leg trait to become more frequent. Sort responses for next lesson: accurate population-level explanation, data-only claim, or purposeful-change misconception.