PrepTest 143, Section 3, Question 6
If newly hatched tobacco hornworms in nature first(80 more words not shown)
Which one of the following,(9 more words not shown)
- ATobacco hornworms that(18 more words not shown)
- BIf taste receptors(19 more words not shown)
- CTobacco hornworm eggs(7 more words not shown)
- DIndioside D is(10 more words not shown)
- EThe taste receptors(12 more words not shown)
Reading the argument
The first meal seems to matter. The proposed explanation for why it matters still needs support.
Hornworms that start with nightshade plants stick to that family afterward. Those that start with other plants will eat other sorts of plants. The scientists think the difference comes from taste receptors becoming accustomed to indioside D, a chemical found only in nightshades. Once that happens, other plants no longer taste good.
Maybe. But what if another chemical in nightshades creates the preference? We know the behavior occurs; we haven't established that this particular chemical acting through taste receptors explains it.
What we’re looking for
For a strengthener, one possibility would be evidence ruling out competing chemicals. Another would test whether taste is actually responsible: if a hornworm lost its ability to taste, would its refusal to eat other leaves disappear too? That would help connect the proposed mechanism to the behavior.
Working the answers
- A
The hornworms' lack of preference among nightshade varieties doesn't tell us why they reject plants outside that family. All the varieties might contain indioside D, but they could also share another feature responsible for the behavior. A doesn't isolate the proposed explanation.
- BCorrect
Remove the taste receptors, and hornworms that previously ate only nightshades will eat other leaves. That is the change we'd expect if those receptors were responsible for the restricted diet. Disabling the proposed mechanism removes the behavior it is supposed to explain. B doesn't prove that indioside D is the particular chemical involved, but it directly supports the taste-receptor part of the hypothesis.
- C
The location of the eggs could explain why a newly hatched hornworm first encounters nightshade leaves. It doesn't explain why that first meal leads to a lasting refusal to eat other plants. The hypothesis concerns that continuing preference, not merely where the meal happened.
- D
Another chemical unique to nightshades gives us an alternative to investigate. Perhaps that chemical, rather than indioside D, produces the preference. D doesn't support singling out indioside D as the cause.
- E
Taste receptors reacting to several chemicals is a broad fact about how they work. We need evidence linking this particular lasting preference to the proposed process. E doesn't show that a first meal changes the receptors or that indioside D drives the later food choices.
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