Reading Trait-Frequency Data: Natural Selection Across Generations

Objective

Use trait-frequency data to explain how natural selection changes a population over generations.

Materials & Prep

Project or draw the beetle-survival scenario and data table below; students need notebooks or a half-sheet exit ticket.
Prepare pairs to share one CER explanation. No special materials beyond usual classroom setup.

Opening

0–7 min

Display“On dark lava, the proportion of dark beetles rose from 30% to 78% in four generations. Did beetles turn dark because they needed camouflage? Predict yes or no, then write one reason.”

Students commit silently, then compare with a partner. Ask: “What do you remember about inherited traits: can an individual beetle choose or practice a new inherited color?” Elicit that offspring inherit traits from parents. Tell students the class will use data to decide what changed and what did not.

Direct instruction

7–16 min

Present the phenomenon: A beetle population contains inherited light and dark color variation. Birds more easily spot light beetles on dark lava. Emphasize observation versus inference: “78% dark beetles in Generation 4” is an observation from data; “dark beetles were less likely to be eaten” is an inference supported by the pattern and scenario.

Model a short data set

Generation: 1 2 3 4 Dark beetles (%): 30 45 63 78 Light beetles (%):70 55 37 22

Think aloud“First, I compare the same trait over time: dark rises by 48 percentage points. Next, I ask which individuals are more likely to survive and reproduce in this environment: dark beetles, because they are harder for birds to see. Finally, I explain population change: more surviving dark beetles leave offspring, so dark-color alleles become more common in later generations. The critical point is that the population changes; an individual beetle does not turn dark during its lifetime.”

Use one simple comparison: “It is like a playlist: one song does not transform into another song, but songs played more often become a larger share of the playlist.”

Ask the targeted check for understanding: “Point to the data that proves a single light beetle changed color.” Students should answer that there is no such data; the table gives population percentages, not individual transformations. If students claim beetles changed on purpose, return to inherited variation already present in Generation 1.

Guided practice

16–26 min

Pairs analyze a new case, displayed on the board

On pale sand, birds more easily spot dark lizards. Generation: 1 2 3 4 Striped lizards (%):25 38 51 64 Dark lizards (%): 75 62 49 36

Partners complete these prompts in notebooks

Observation: Which trait frequency changed, and by how much?
Inference: Which lizards were more likely to survive and reproduce? Use the habitat information.
Explanation: Why did the population change over generations without any lizard changing its trait on purpose?

Give the sentence frame to students who need it: “The frequency of changed from % to %. In this environment, lizards were more likely to survive because . Because they reproduced more, became more common in the population over generations.” Circulate and press for evidence: “Which numbers support that?” Briefly cold-call two pairs, including one that initially predicted purposeful change, and revise wording together if needed.

Independent work

26–38 min

Students independently write a 5–7 sentence CER for the lizard case.

ClaimState how natural selection changed the population.

EvidenceCite at least two frequencies from the table and the pale-sand/bird observation.

ReasoningConnect inherited variation, differential survival/reproduction, and trait frequency over generations. Include the phrase “the population changed, not individual lizards on purpose.”

Students ready for extension respond to: “If the habitat became dark rock before Generation 5, predict the likely long-term trend and explain why the data would not guarantee an immediate change.” This pushes them to distinguish a likely selective pressure from certainty and to note that variation and reproduction are required.

Closing

38–45 min

Exit ticket, collected individually

“An insect population has 20% pesticide-resistant insects before spraying and 70% resistant insects after several generations of spraying. Write a two- to three-sentence explanation of the change using the terms inherited, survive/reproduce, population, and generations. Also state one observation and one inference.”

Sort quicklystudents meeting the objective describe preexisting inherited resistance and differential reproduction; students needing follow-up say insects became resistant because they needed to. Use those responses to begin the next lesson with a contrasting case and a re-teach of population versus individual change.