Question 136·200 Super-Hard SAT Reading Questions·Information and Ideas
Urban ecologists studying white clover (Trifolium repens) proposed that prolonged exposure to heavy-metal contamination in cities has driven the evolution of lead tolerance in urban populations of the plant. To test this hypothesis, they germinated seeds collected from urban and rural clover populations and measured the dry biomass of the resulting seedlings after four weeks in soils containing either no added lead or 300 parts per million (ppm) of lead nitrate. Urban seedlings produced 30 percent more biomass than rural seedlings in the 300-ppm treatment, but the two groups did not differ in the no-lead treatment. On the basis of these results, the researchers concluded that urban clover populations have evolved genetically based tolerance to lead.
Which choice describes an additional finding that, if true, would most directly strengthen the researchers’ conclusion?
For SAT Reading & Writing questions that ask which finding would "most directly strengthen" a scientific conclusion, restate the conclusion and identify the key assumption it depends on (here: the trait is genetic, not environmental). Then choose the option that most directly tests that assumption—typically by controlling conditions and/or showing the effect persists across generations—while eliminating choices that only describe background conditions or possible mechanisms without establishing heredity.
Hints
Focus on the exact wording of the conclusion
Look closely at the phrase "have evolved genetically based tolerance to lead." What kind of evidence would specifically show a genetic difference rather than just a difference in current conditions?
Think about ruling out environmental explanations
Imagine someone argued that urban plants only do better because their parents grew in polluted soil or because their home soil has different nutrients. What sort of experiment or finding would rule out those kinds of short-term environmental effects?
Test each option for heredity, not just correlation
For each answer choice, ask: Does this show that the tolerance difference persists when urban and rural plants are raised in the same environment for at least one generation, or does it only describe current conditions or a possible mechanism?
Step-by-step Explanation
Pinpoint the researchers’ exact conclusion
The key line is that the researchers "concluded that urban clover populations have evolved genetically based tolerance to lead." This is stronger than just saying urban plants currently tolerate lead better; it says the difference is due to genetic evolution in urban populations.
Decide what kind of evidence would best support that conclusion
To support a claim of genetically based tolerance, we need evidence that the difference between urban and rural plants:
- Is heritable (passed down across generations), and
- Is not just caused by recent environmental conditions, such as polluted soil, parental exposure, or better current growing conditions.
A very strong design is to raise both lineages for one or more generations in identical, controlled, lead-free conditions and then compare their performance when exposed to lead. If the urban plants still do better, that strongly indicates a genetic difference.
Check how each choice relates to genetics vs environment
Now think about what each option actually tells you:
- One option talks about how contaminated urban vs rural soils are. That describes environmental exposure, not inherited traits.
- One option talks about how much lead ends up in the leaves. That describes a physiological difference, but doesn’t tell you if it’s genetic or temporary.
- One option says urban plants have higher photosynthesis across all treatments. That suggests general vigor rather than specific lead tolerance, and it doesn’t directly test whether any difference is inherited.
Only one option clearly shows that the tolerance difference remains after environmental conditions have been equalized for multiple generations.
Identify the choice that directly shows an inherited tolerance difference
The finding that after being grown for two generations in identical, lead-free greenhouse conditions, urban descendants still produced more biomass than rural descendants when exposed to 300 ppm lead means the advantage of urban plants persists even when their recent environment has been made the same. Because the difference remains after two generations in identical, lead-free conditions, the most plausible explanation is a genetic difference that has evolved in urban populations. Therefore, this choice most directly strengthens the researchers’ conclusion.