Question 114·Hard·Command of Evidence
Migratory songbirds are thought to sense the Earth’s magnetic field using cryptochrome proteins in their retinas. When these proteins absorb blue light, they form pairs of molecules whose electron spins remain correlated (a radical pair) for several microseconds; the orientation of the spins is subtly altered by geomagnetic forces, creating a faint visual pattern that points toward magnetic north. Critics argue that the radical pair’s brief lifetime is too short to provide a stable cue during long nocturnal flights. In a recent investigation, researchers discovered that European robins produce a variant of the protein, Cry4b, whose radical pairs last nearly ten times longer than those of the common form, Cry4a. The team proposes that the prolonged lifetime of Cry4b’s radical pairs allows robins to maintain a reliable magnetic compass throughout migration.
Which finding, if true, would most strongly support the researchers’ proposal?
For SAT questions asking which finding would best support a hypothesis, first restate the hypothesis in simple terms and identify its key cause-and-effect claim. Then predict what ideal evidence would look like—usually an experiment or observation that directly links the proposed cause to the predicted effect while ruling out alternative explanations (for example, by removing other cues). Finally, test each answer against this prediction, eliminating options that are irrelevant, only loosely related, or that actually contradict or weaken the hypothesis, and choose the one that most directly and strongly confirms the proposed relationship.
Hints
Clarify the proposal first
Before looking at the choices, underline or note what the researchers are actually proposing about Cry4b and migration. What do they think Cry4b does for robins?
Think about cause and effect
Ask yourself: If the researchers are right, what change in Cry4b (especially where it is located) should cause what change in the birds’ ability to orient or navigate?
Look for an experiment that isolates magnetic cues
Pay attention to any answer choice where the animals must rely mainly on the magnetic field, not on visual landmarks or other cues. Which option shows behavior that depends on magnetic information?
Beware of details that distract from navigation
Some choices mention Cry4b but focus on things like light sensitivity or organs other than the eye. Make sure the evidence connects Cry4b in the retina to magnetic orientation, not just any effect.
Step-by-step Explanation
Restate what the question is asking
You are asked which finding would most strongly support the researchers’ proposal. This is a strengthen-the-argument question: pick the option that, if true, makes the researchers’ explanation more believable.
Identify the key parts of the researchers’ proposal
Summarize the proposal in your own words:
- Songbirds sense Earth’s magnetic field with cryptochrome proteins in their retinas.
- A special variant, Cry4b, forms radical pairs that last about ten times longer than those from Cry4a.
- The researchers propose that these longer-lasting radical pairs allow robins to maintain a reliable magnetic compass throughout migration.
So strong supporting evidence should connect Cry4b in the retina to better magnetic orientation/navigation.
Predict what strong supporting evidence would look like
Ask: If their idea is right, what experiment result would we expect?
We’d expect something like:
- Animals with more Cry4b in their retinas navigate better using magnetic cues.
- Or, changing Cry4b levels changes how well they orient in a magnetic field, especially when visual landmarks are removed, so they must rely on magnetoreception.
Keep this prediction in mind as you test each choice.
Evaluate each answer choice against the proposal
Now match each option to the proposal:
- A) Mice with Cry4b show more blue-light sensitivity but no change in navigation. This suggests Cry4b does not help navigation, which undermines the proposal.
- B) Cry4b mainly in the liver, not the retina, contradicts the idea that retinal Cry4b supports magnetoreception, so it weakens the proposal.
- C) Talks about Cry4a (the common form) and how its radical pairs last longer in darkness than dim blue light. This doesn’t show anything about Cry4b helping navigation.
- D) Robins with elevated Cry4b in their retinas can orient accurately in an artificial magnetic field even when all visual landmarks are removed. This directly links more Cry4b in the correct location (retina) to successful navigation using only magnetic information.
The choice that most clearly supports the researchers’ proposal is D) Robins with elevated concentrations of Cry4b in their retinas orient accurately in an artificial magnetic field even when all visual landmarks are removed.