PrepTest 142, Section 4, Question 22
Fish with teeth specialized for scraping algae occur(65 more words not shown)
The reasoning in the argument(5 more words not shown)
- Ainfers a cause(4 more words not shown)
- Binfers that just(16 more words not shown)
- Ctakes a sufficient(5 more words not shown)
- Dinfers merely because(9 more words not shown)
- Eappeals to the(17 more words not shown)
Reading the argument
Not closely related doesn't tell us how many times the teeth evolved.
The argument gives us a rule: if these species are closely related, the algae-scraping specialization evolved only once. Let's accept that rule. It tells us what follows when close relatedness is present; it doesn't tell us what follows when close relatedness is absent.
That's the situation the genetic tests give us. The species aren't closely related, although they may be related more distantly. Could the specialization still have evolved once? The stated rule leaves that possibility open. It never says close relationship is the only situation compatible with a single origin.
What we’re looking for
For the flaw question, look for that change from one way to establish a single origin to a requirement for a single origin. The conclusion needs a rule about species that aren't close relatives, and we weren't given one.
Working the answers
- A
The inference doesn't go from two things occurring together to one causing the other. It uses a conditional rule and a genetic finding to conclude how many times a feature evolved. The mistake concerns what that rule establishes when its stated condition is absent.
- B
The argument isn't merely waiting for confirmation of close relatedness and declaring it false. The genetic tests affirmatively show the species aren't closely related. Accepting that finding still leaves the question of how often the specialization evolved unresolved.
- CCorrect
Close relatedness is sufficient under the stated rule: it would establish that the specialization evolved once. The author treats it as necessary too, as though a single origin couldn't occur without close relatedness.
But no such restriction was given. The species could fail to be close relatives while the specialization still had a single origin. By ruling that out without support, the author makes the leap C describes.
- D
The biologists initially express an expectation of close relatedness, but the argument doesn't conclude that something happened merely because it was likely. It uses genetic evidence against that expectation and then misapplies the rule about a single evolutionary origin. D targets a different move.
- E
The author doesn't simply accept the biologists' view because of their authority. Their expectation of close relatedness is challenged by genetic tests. Whether they represent every other expert's opinion doesn't fix the final move from 'not closely related' to 'evolved more than once.'
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