PrepTest 148, Section 4, Question 25
Scientist: A small group of islands near Australia(56 more words not shown)
Which one of the following,(7 more words not shown)
- AA number of(12 more words not shown)
- BGenetic analysis indicates(18 more words not shown)
- CDocumented cases of(9 more words not shown)
- DFossils of iguana(14 more words not shown)
- EThe lineages of(23 more words not shown)
Reading the argument
Couldn't the iguanas have come from Australia?
The islands are near Australia, and Australia used to be joined to South America. Imagine a population living before that split. Some descendants could have ended up in South America and others in Australia, with the Australian population later reaching the nearby islands. That's a possible history, not something the passage has established. But it shows why I'd question the claim that the island ancestors must have crossed the Pacific directly from the Americas.
The statement that related species exist nowhere else is about where they exist now. It doesn't tell us where related iguanas lived in the past. Their absence from Australia today wouldn't settle whether Australia once supplied the island population.
What we’re looking for
For a weakener, I'd look for evidence supporting another route. Fossils of related iguanas in Australia would be useful. Evidence of people transporting them could also challenge the rafting story. We don't have to show a long ocean crossing was impossible; we need to weaken the claim that it was the necessary explanation.
Working the answers
- A
Those other species could have entirely different histories. Perhaps some came from Australia, while the iguanas came from the Americas. Finding other island animals absent from the Americas doesn't give us a competing source for these iguanas. The argument concerns their ancestors, not the origin of every island animal.
- B
Closely related species don't have to be genetically identical. Descendants that became separate species could differ in several respects while still sharing ancestors from the Americas. B adds detail about their differences without showing that those differences conflict with the proposed ancestry or identify a different route.
- C
Uncommon documented crossings still allow a rare crossing that established a population. We also shouldn't expect a complete record of ancient rafting events. This answer doesn't offer the concrete alternative source that D does. An effective weakener needn't prove rafting impossible, but merely saying few such journeys have been documented leaves the proposed event quite possible.
- DCorrect
Related iguanas once lived in Australia, supplying a possible nearby source for the island population. That evidence supports the alternative we considered: perhaps the ancestors reached the islands from Australia rather than crossing the Pacific directly from the Americas. The fact that related species aren't alive in Australia now doesn't erase the fossil evidence that they were there before.
Could those Australian iguanas themselves have arrived from the Americas? Yes. D doesn't rule that out or prove our whole proposed history of a population splitting with the continents. But the scientist hasn't established that history either. The fossils provide a real alternative starting point for the trip to these nearby islands, undermining the claim that the island ancestors must have made the direct trans-Pacific journey described.
- E
Ancient lineages of other plants and animals don't establish where these iguanas lived. Even if many species predate the breakup, we still need evidence connecting the island iguanas to a possible Australian source. D supplies closely related fossils; E's broad statement about numerous other lineages doesn't.
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