Question 111·Hard·Inferences
During a decade-long study, ornithologist Mateo Rios observed the nesting habits of cliff swallows along two highways: Highway A, which had been fitted with broad concrete overhangs above the lanes, and Highway B, which lacked such structures. Over the ten-year period after the installation of the overhangs, the average wingspan of swallows nesting near Highway A decreased by 4 millimeters, while no significant change was recorded in the population near Highway B. Rios theorized that the architectural modification had altered the selective pressures on the birds, leading to the smaller wingspan, suggesting that ____.
Which choice most logically completes the text?
For SAT "logical completion" questions, first identify the function of the blank: here, it must explain how the described cause leads to the observed effect. Lock onto key phrases like "altered the selective pressures" and connect them to the data given (smaller wingspan at Highway A, no change at Highway B). Then reject any option that introduces a new, unsupported cause, contradicts the stated results, or ignores the specific comparison between the two highways. Finally, choose the option that clearly and directly connects the described change (overhangs) to a mechanism (differences in survival and reproduction) that logically produces the outcome in the passage.
Hints
Use the sentence right before the blank
Reread the part that says the overhangs "altered the selective pressures on the birds, leading to the smaller wingspan." Ask yourself: what kind of explanation must follow this claim?
Connect cause and effect for Highway A vs. Highway B
Only the swallows near Highway A changed in average wingspan. Which answer gives a cause that is specifically linked to Highway A’s new overhangs and explains why the wingspans there, and not at Highway B, would get smaller?
Think about natural selection
For a physical trait to change in a population over time, what has to happen to individuals with that trait in terms of survival and reproduction? Look for the choice that clearly describes that process in relation to wingspan and the concrete overhangs.
Step-by-step Explanation
Understand what the blank must explain
The sentence tells you that after the concrete overhangs were added on Highway A, the swallows there developed a smaller average wingspan, while those at Highway B did not change. Rios theorized that the man-made structures changed the selective pressures on the birds. The blank must give a logical explanation of how the overhangs could favor certain birds and lead to a smaller average wingspan over time.
Focus on key phrases: "architectural modification" and "selective pressures"
"Architectural modification" refers to the new concrete overhangs. "Selective pressures" is a biology term connected to natural selection: traits that help organisms survive and reproduce become more common over generations. So the missing idea should:
- Tie directly to the overhangs (not some unrelated change), and
- Explain how a particular trait (wingspan size) would help certain birds survive and reproduce better, changing the population average.
Eliminate choices that don’t match natural selection or add unsupported causes
Check each option against what the passage actually says and what "selective pressures" implies:
- Some options introduce new causes (like predators or diet changes) that the passage never mentions and that aren’t clearly tied to the overhangs as the driver.
- Another option explains the change as birds choosing to nest near the overhangs (a relocation/preference idea) rather than as a survival-and-reproduction advantage, which does not match the claim about selective pressures. Options that rely on unmentioned causes or replace natural selection with a different mechanism should be rejected.
Choose the explanation that fits natural selection and the observed pattern
The only remaining option directly links the architectural change (concrete overhangs) to a survival advantage specifically for birds with smaller wingspans. Because these smaller-winged swallows could maneuver better under the overhangs, they were more likely to survive and reproduce, causing the average wingspan in that population to decrease over the decade. This matches the idea of altered selective pressures leading to a trait becoming more common, so Choice D is the most logical completion.