For a command-of-evidence question about a causal interpretation, separate the observation from the proposed cause. Then identify an alternative explanation that the original study did not eliminate and choose the finding that tests that alternative most directly. Evidence showing that a pattern disappears when the alternative influence is removed is generally more decisive than evidence that merely confirms the pattern or explains why it could be advantageous.
Hints
- Hint 1
Focus on the causal claim
The researchers are not merely claiming that the populations differ; they are claiming that genetic inheritance primarily causes the difference.
- Hint 2
Examine the experimental control
Raising hatchlings in identical conditions controls their environments after hatching, but consider what influences they may already have received from their mothers.
- Hint 3
Look for a disappearing pattern
Evidence would challenge the claim most directly if the difference faded when a parental carryover effect was removed.
Step-by-step
- Step 1
Identify the researchers’ inference
The researchers infer that the difference among hatchlings is genetic because the hatchlings themselves were raised in identical environments.
- Step 2
Recognize what the experiment did not control
Although the hatchlings shared an environment, their mothers did not. Conditions experienced by the mothers could have affected the nutrients or other materials placed in the egg capsules, creating a first-generation difference without genetic divergence.
- Step 3
Select the evidence that tests the alternative
A difference that disappears after another generation in the same environment—and that is linked to nutrients supplied by the original mothers—is best explained by parental environmental effects. Therefore, the correct answer is After two generations bred separately by origin in the same cue-free conditions, the shell-thickness difference disappears, while first-generation thickness tracks nutrient levels in the original egg capsules.