LSAT 79, Logical Reasoning II, Q22, LSATHacks

QUESTION TEXT: After a hepadnavirus inserts itself into a…

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.)

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1. This doesn’t matter. Species can obviously diverge without a virus as well.
2. Who cares? Maybe most animals have dozens of viruses in their chromosomes. We’re only talking about one specific virus.
3. 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.
4. 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.
5. 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.

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