Question 174·200 Super-Hard SAT Reading Questions·Information and Ideas
Seeds of a coastal grass collected from several populations differ in how long they remain dormant. The populations differ genetically and also experience rainfall at different times, so field observations alone cannot identify the source of the variation. Researchers propose that seed dormancy is determined by the timing of rainfall experienced by a parent plant while its seeds are developing, rather than by inherited differences among the populations.
Which finding, if true, would most directly support the researchers' proposal?
For evidence questions involving competing causal explanations, identify the proposed causal variable and the main alternative. Prefer a finding that manipulates the proposed cause, controls the alternative explanation and later testing conditions, and reports an outcome specifically predicted by the proposal.
Hints
Separate the explanations
Consider what evidence would distinguish an effect of the parent plant's environment from an inherited difference among populations.
Check both stages
Look for a finding that controls genetic background, changes the parent plants' watering conditions, and prevents the offspring seeds' testing conditions from creating the observed difference.
Step-by-step Explanation
Identify the competing explanations
The researchers propose that parental rainfall timing determines seed dormancy. The competing explanation is that inherited differences among populations account for the observed variation.
Determine the necessary comparison
A strong test should assign genetically identical parent plants within each population to different watering schedules while seeds develop. It should then test all offspring seeds under identical conditions so that the seeds' own environments cannot account for differences in dormancy.
Evaluate the evidence
If dormancy differs according to the parental watering schedule but not according to population of origin, the result directly links dormancy to the proposed environmental cause while weighing against the inherited-difference explanation.