PrepTest 152, Section 1, Question 8
Monarch butterflies must contend with single-celled parasites that(51 more words not shown)
The reasoning in the argument(10 more words not shown)
- Amonarch butterflies are(9 more words not shown)
- Blong migrations are(9 more words not shown)
- Cpopulations of monarch(11 more words not shown)
- Dmonarch butterflies infected(7 more words not shown)
- Epopulations of monarch(11 more words not shown)
Reading the argument
What if the parasites keep the butterflies from migrating?
What we’re looking for
We're already told that the parasites can deform butterflies in ways that interfere with flight. That gives us a reason to suspect that the heavily infected butterflies are the ones unable to make the trip. The migrating population would then have fewer infected members because infection makes migration difficult, not necessarily because migration protects against infection. For the flaw, I'd look for that alternative explanation.
Working the answers
- A
Migration could help butterflies avoid parasites without the butterflies consciously detecting parasite-free areas. They might move for some other reason and benefit anyway. Their ability to recognize safe areas doesn't address the more basic possibility that infected butterflies cannot make the journey.
- B
The evidence compares migrating populations with populations that haven't migrated. It doesn't compare long migrations with short ones. Both lengths could offer equal protection while migration still offered more protection than staying put. B doesn't undermine the author's comparison.
- C
The argument already compares percentages, not just the number of infected butterflies in each group. A larger population could contain more infected butterflies simply because it is larger, but that doesn't explain why its infection rate is so much higher. Group size alone doesn't address the claimed cause of that rate difference.
- DCorrect
This gives us the alternative explanation. If infected butterflies are typically unable to migrate, the migrating group will tend to contain the butterflies that weren't infected enough to prevent the trip. We can then observe relatively few infected migrants without establishing that migration protected them. D doesn't prove migration has no protective effect; it shows why this comparison alone doesn't prove that it does.
- E
Stable food could explain why a population stays in one place. But E doesn't connect food stability to parasite infection. Without that additional connection, it doesn't explain why the groups have such different infection rates or expose the author's mistaken direction of explanation.
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