Question 90·Hard·Command of Evidence
Marine ecologist Dr. Mei-Ling Leung compared two adjacent coral reefs in the South China Sea. Reef A is dominated by Acropora corals that currently host a strain of heat-tolerant algal symbionts, whereas Reef B’s Acropora corals still host the more common, heat-sensitive strain. Leung notes that both reefs have experienced several short heat waves during the past decade and proposes the following: the corals at Reef A survived those episodes by expelling the sensitive algae and acquiring the tolerant strain; subsequently, the corals began passing the tolerant symbionts directly to their offspring through their eggs, causing the tolerant strain to predominate in the population.
Which finding, if true, would most directly support Leung’s proposal?
For “Which finding would most directly support the scientist’s proposal?” questions, first restate the proposal in your own words and identify its key causal claim (what the scientist says is happening and why). Then imagine what ideal evidence would look like—usually a controlled comparison that isolates that cause. Finally, test each answer: eliminate choices that only give background, describe unrelated variables, or are merely consistent with the idea, and pick the one that directly shows the claimed cause-and-effect (such as offspring inheriting a trait when raised in identical conditions).
Hints
Clarify the proposal’s main claim
Reread the sentence starting with “Leung notes that both reefs have experienced…” and focus on what she says happened after the corals survived the heat waves. What does she say about how the tolerant algae ended up predominating?
Think about the kind of evidence needed
To back up the idea that corals pass tolerant algae through their eggs, would it be more helpful to compare water conditions, coral genetics, temperatures, or what is found inside the offspring themselves?
Look for a controlled comparison
Which answer choice describes a situation where offspring from Reef A and Reef B are raised under the same conditions, so that any difference between them most likely comes from their parents rather than from the environment?
Step-by-step Explanation
Pinpoint what the scientist is proposing
Leung’s proposal has two key parts:
- During heat waves, Reef A corals swapped out their heat‑sensitive algae and took up a heat‑tolerant strain.
- Afterward, these Reef A corals started passing the heat‑tolerant algae directly to their offspring through their eggs, so over time the tolerant strain became common in the population.
The question asks which finding would most directly support this proposal, so we want evidence that clearly shows this parent‑to‑offspring transfer of tolerant algae (and, secondarily, that the change is not just due to local water conditions).
Imagine ideal supporting evidence
Ask: If Leung were right, what evidence would we expect to see?
We would expect that:
- Offspring (larvae) from Reef A parents already contain the heat‑tolerant algae because of their parents, not because of the surrounding water.
- Offspring from Reef B parents do not contain the tolerant algae if their parents don’t have it.
- A strong test would raise larvae from both reefs in the same controlled conditions, so any difference must come from the parents, not the environment.
Check each answer against that ideal evidence
Now, match each option to what we’re looking for:
- One option talks about DNA differences (or lack of them) in the coral hosts.
- One option is about nutrient levels in the water near the reefs.
- One option compares larvae from Reef A and Reef B parents raised in identical lab conditions, and what algae they contain.
- One option compares the peak temperatures the two reefs experienced.
Only the option that directly examines what algae the larvae have when raised in the same environment tells us whether the tolerant strain is being passed from parents to offspring, which is the central claim of the proposal.
Select the choice that shows inherited tolerant algae
The finding that larvae reared in the laboratory from parent corals collected at Reef A contain the heat‑tolerant algal strain, whereas larvae from Reef B parents do not, even though all larvae are raised in identical water conditions most directly supports Leung’s proposal. It shows that the difference in algae between Reef A and Reef B larvae comes from the parents (inheritance) rather than from differences in the local environment, backing up the idea that Reef A corals now pass the tolerant symbionts directly to their offspring.