PrepTest 141, Section 2, Question 22
Because the native salmon in Lake Clearwater had(58 more words not shown)
Which of the following, if(7 more words not shown)
- ANeither of the(10 more words not shown)
- BWhen the native(15 more words not shown)
- CMost types of(17 more words not shown)
- DOne of the(14 more words not shown)
- EThe total number(21 more words not shown)
Reading the argument
Different habitats could put different pressures on the fish. Perhaps one environment favors a particular body size or a way of hiding from predators. Those are possibilities to investigate, not facts we're given about these salmon. But I can see why researchers would suspect that the deep-water and shallow-water populations adapted differently.
Is adaptation the only way they could have become genetically different?
The native salmon nearly disappeared. They didn't disappear completely. What if one sockeye population interbred with some of the remaining native salmon? Imagine native fish surviving in the deep part of the lake and breeding with the sockeyes there. That could produce a genetic difference without the researchers' habitat-adaptation account explaining it.
What we’re looking for
To strengthen the hypothesis, we could look for evidence tying a genetic change to conditions in each habitat. Or we could eliminate another explanation, such as this possible interbreeding. Either route would help; we don't need to prove the entire hypothesis beyond doubt.
Working the answers
- ACorrect
Neither sockeye population interbred with the native salmon. That removes the alternative we raised: genetic mixing with the remaining native fish cannot explain the difference between the two sockeye populations. It doesn't establish every detail of adaptation, but removing a competing explanation makes the researchers' account more convincing.
- B
Two distinct native populations might resemble the later sockeye situation, but we aren't told what genetically distinguished those native populations or how they came to differ. Their past separation alone doesn't establish that the sockeyes' current genetic differences arose through adaptation to different habitats.
- C
Most salmon types spending time at different depths doesn't tell us what caused the genetic difference between these two populations. The passage specifically describes one in the deep areas and one in the shallow areas. A general fact about where other salmon spend time doesn't provide the missing explanation here.
- D
Being virtually identical to the original population gives little affirmative evidence that this group adapted genetically. “Virtually” could allow a small change, so this doesn't necessarily rule adaptation out. But it doesn't strongly support the claim that each population adapted to its distinct habitat, which is what we need.
- E
The total number of fish doesn't explain why the two populations are genetically different. A smaller population than the lake once supported could coexist with adaptation or with another cause of genetic change. Without a connection to that cause, the head count doesn't support the hypothesis.
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