LR Question 22 Explanation
QUESTION TEXT: After a hepadnavirus inserts itself into a chromosome…
QUESTION TYPE: Strengthen
CONCLUSION: The hepadnavirus is at least 25 million years old.
REASONING: When an animal’s chromosome has a hepadnavirus inserted into it, bits of that virus are passed on to the animals descendants. Both zebra finches and dark-eyed juncos have a hepadnovirus in the same location. These bird species split around 25 million years ago.
ANALYSIS: This argument makes an assumption: the hepadnovirus won’t go to the same place in different species. If every hepadnovirus infection of a new species always goes to the same spot in the chromosome, then we have no evidence. The two species could have gotten the virus at different times.
The divergence is only significant on the assumption that the species had the virus before the divergence. To prove that, we have to show that the virus would normally go in a random spot. (And so the fact that both species have it in the same spot proves that their precursor species had it, in that same spot.)
___________
- This suggests that the hepadnavirus caused the diverging. But the word “can” is very weak. We have to take answers at their least useful, so this could mean that there is 0.0001% chance that’s what actually occurred.
- Who cares? Maybe most animals have dozens of viruses in their chromosomes. We’re only talking about one specific virus.
- CORRECT. If this is true, then the two species could only have the virus in the same location because their ancestor had it. It would be too much chance for the insertion to have randomly occurred in the same spot in two distinct species.
- So? The author never said only those species have the virus. The author mentioned them because their divergence 25 million years ago may tell us something about the hepadnavirus’ age.
- This doesn’t matter. The two species obviously did survive, whether or not the virus helped. We only care what these two species’ divergence can tell us about the hepadnavirus’ age.
More Resources for Strengthen Questions
- Intro Course lesson: This intro course lesson covers Strengthen questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers strengthen questions.
This is a strengthen question. Your answer just seems to blow off “A” too easily. If the virus can thereby influence the divergence of the species, you could argue that this answer makes room for the possibility of it existing 25 million years ago (it could have been the virus that caused them to diverge).
I can see why C is a strengthener too, but perhaps the case is worth making as to why its STRONGER than A.
Thanks.
I think you’re right that I should reword A, but fundamentally it is a very weak answer. “Can” is an incredibly weak word, it might mean this happens 0.0000000001% of the time. You have to interpret Strengthen answers at their least useful.
Note: This is an old comment but I wanted to address the point.