PrepTest 155, Section 2, Question 3
Biologist: DNA analysis shows that Acacia heterophylla ,(70 more words not shown)
Which one of the following,(10 more words not shown)
- AA. koa branches(10 more words not shown)
- BThere are mountain-nesting(7 more words not shown)
- CA. koa is(14 more words not shown)
- DOcean currents have(8 more words not shown)
- EMany seabird species(12 more words not shown)
Reading the argument
The trees are related. How did their seeds get from Hawaii to Réunion?
Floating across the ocean is one proposed answer, but the biologist gives us reasons to doubt it. Soaking the seeds in seawater prevents them from germinating, and both trees grow in the mountains rather than along the shore. The seeds would need a way to reach the water and survive the trip.
Birds sound like another possibility. We can imagine a bird eating something containing a seed, carrying it away, and depositing it elsewhere.
But imagining a route isn't evidence that this particular journey happened. Would these seeds survive a bird's digestive system? Could they have traveled some other way, perhaps in a storm? Those are questions to investigate, not facts we're adding to the passage.
What we’re looking for
The question asks us to strengthen the bird explanation. I'd like evidence connecting birds with these particular trees and islands. Evidence of birds carrying the seeds would be especially helpful. We may have to accept something less conclusive, but it should make this proposed route more plausible.
Working the answers
- A
This helps the floating explanation, not the bird explanation. A floating branch could give the seeds a different way to travel by water from simply soaking alone in the sea. Whether that would fully solve the survival problem is another question. Either way, A supplies no new support for birds as the carriers.
- BCorrect
These birds nest in the mountains, where the trees grow, and species of these birds occur on both islands. That connects possible seed carriers with the places the seeds would need to leave and reach. It doesn't show that a particular bird flew between the islands carrying a viable seed. Still, it makes the proposed bird route more plausible, which is all a strengthener needs to do.
- C
The Hawaiian tree's earlier ancestry is a different journey. We want to know how seeds from Hawaii reached Réunion, not how the Hawaiian tree's ancestors might have arrived from Australia. C gives us no evidence for birds causing the dispersal at issue.
- D
This is another fact supporting travel by water. Ocean currents can carry floating objects between the islands, but the biologist concludes that birds probably carried these seeds. Showing that objects can make the ocean journey doesn't strengthen that bird hypothesis.
- E
Returning to land infrequently doesn't establish a connection between these trees and birds traveling between these islands. The birds might still carry seeds during their visits; we shouldn't assume they never eat merely because they rarely come ashore. But E gives us no specific reason to favor the bird explanation. B does.
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