A finding question about competing causes tests whether evidence backs one cause over another. Read the proposal and split it into two parts: what the researchers say causes the result and what they rule out. To test a cause, a study changes it while keeping other possible causes alike. Predict evidence for both parts, because ruling out one cause doesn't show which other cause produced the result. Findings that let several things change together can't show which one caused the result.
Hints
- Hint 1
Read the last sentence of the passage. Separate the proposed cause from the cause the researchers reject.
- Hint 2
The opening says genes and rainfall timing differ together. What would let researchers separate those possible causes?
- Hint 3
For any dormancy difference a finding reports, ask what could have caused it. Could more than one difference between the groups explain the result?
Step-by-step
Test the cause and rule out the rival
Step 1Put the proposal in two parts
Genes and rainfall timing differ together, so field observations alone cannot identify the source of the variation. What must the finding support? The proposal points to rainfall timing while the parent's seeds develop and rejects inherited differences among the populations. Evidence against inheritance alone won't show that rainfall timing is the cause.
- Step 2
Separate the parent's water from the seeds' water
How could we test the effect of the rain experienced by a parent plant while its seeds are developing? Comparing parents with the same genes but different watering schedules separates rainfall from inheritance. The offspring must then be tested alike, because their own watering could change dormancy. Different watering for both generations leaves two possible causes, so a dormancy gap wouldn't tell us which generation's watering mattered.
- Step 3
Find the gaps in the weaker findings
Which findings leave a gap? Several measured genetic markers still uses the original populations, whose genes and rainfall timing vary together. Schedules matching those of their parents allows the offspring's own watering to explain the gap. With grown and watered alike, population differences disappear, but the parents' rainfall timing never varies, so that finding doesn't test the proposed cause.
- Step 4
Check both parts in one finding
The finding with dormancy differs by parental watering schedule compares parents with the same genes given different watering. All offspring are tested alike, so the gap points to the parent's watering. Does the result also argue against inheritance? Dormancy doesn't differ by population of origin, which argues against inherited differences among the populations. Choice C.