PrepTest 132, Section 4, Question 7
Researcher: Over the course of three decades, we(47 more words not shown)
Which one of the following,(9 more words not shown)
- AThe small-beaked wild(12 more words not shown)
- BThe large-beaked wild(12 more words not shown)
- CChanges in the(17 more words not shown)
- DThe average body(12 more words not shown)
- EThe researcher measured(13 more words not shown)
Reading the argument
Something changed for the wild birds. What was it?
The same species lives in both settings, but the pressures on the birds can differ. Captive birds receive food from their keepers; wild birds must survive on what's available. If their food supply changed in a way that favored smaller beaks, smaller-beaked birds could become more common in the wild while the captive population stayed the same.
What we’re looking for
That's my prediction for explaining the findings: a change affecting survival in the wild. We need to explain a decrease over thirty years, not just why wild and captive birds might have different average beak sizes at one moment.
Working the answers
- A
This might make smaller-beaked birds easier to include in the measurements. But why would it make the measured average decrease over time? If small-beaked birds were easier to capture throughout the study, that could affect the measurements thirty years ago as well as the measurements today. A gives us no change that explains the downward trend.
- B
This has the same timing problem as A, with large beaks replacing small ones. An unchanged tendency to capture large-beaked birds doesn't explain a declining average. We'd need to know that the measurement process or the population changed in a relevant way.
- CCorrect
The food supply changed during the study, and the change favored survival of small-beaked wild birds. As those birds became more likely to survive, the wild population's average beak size could fall.
That change concerns the wild food supply. It need not affect the food provided to captive birds, so it can explain why the two populations followed different paths. It addresses both the decrease and the difference between settings.
- D
We're explaining beak size, not body size. The fact that captive birds' bodies stayed the same size doesn't tell us why the wild birds' beaks became smaller. No connection is supplied that would make this fact explain the finding.
- E
Measuring a bird more than once isn't automatically a problem; a study of change might do that deliberately. More importantly, this doesn't say that repeated measurements favored smaller beaks increasingly over time or that the birds' beaks changed. It supplies no reason for the particular decrease we're trying to explain.
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