Logical reasoning PrepTest 147 · Section 4 · Question 22

Incorrect. If there was evidence that the hepadnavirus inserted itself into the DNA shortly before the divergence, then this answer may be relevant. However, since those two events aren't connected, this answer doesn't necessarily say anything that affects the argument and can be eliminated.

Incorrect. Their lack of other virus DNA doesn't tell us anything about when this virus DNA entered them, and whether it was in a parent species. In fact, there being other virus DNA might help to figure out when and if they diverged!

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen

Stimulus Summary:
Phenomenon - Hepadnavirus DNA is in the exact same spot of the chromosomes of two birds
Background - Those two birds diverged from the same species 25m years ago
Implication - The hepadnavirus is at least that old

Answer Anticipation:
This argument is interesting in that it doesn't bring up an explanation for a given phenomenon—instead, it brings up an implication of it.

That phenomenon is that a certain virus's DNA is found in the same spot in 2 birds that are known to be descended from the same species, splitting 25m years ago. From this, the argument concludes that the virus is at least 25m years ago.

What's the assumption there? The argument is assuming that DNA in exactly the same spot of a chromosome wouldn't happen at two different times—if it's in the same place, then it was put there when the birds were still the same species. Anything that makes that more likely to be true will strengthen this argument. For instance, if the virus always inserts the DNA in the same place, then that evidence no longer supports the conclusion, so a statement that it rarely inserts the DNA in the same place would strengthen it.

Answer Explanation:
The evidence here is that the virus fragment was found in the same place in the genomes of two species. If the virus always inserts itself into the same location, then that similarity isn't evidence the birds diverged from the same species and thus that point of divergence couldn't be used to date the virus. Since it's random, it's very unlikely that the virus would randomly insert itself in the same place in a genome twice, and thus finding it in the same place in two species suggests those species diverged from the same one, the virus inserted itself before that happened, and thus that 25m years old date is likely correct.

Key Takeaway:
While there are common argument structures on the LSAT—e.g., Phenomenon/Explanation—not all arguments that feature an element of one of these forms feature all the elements in it. Here, trying to frame it as an argument about an explanation could work, but it doesn't do much to help answer the question. Don't force the tools to work if they don't fit! They're there to help, not be the only path you have to solve a question.

Incorrect. If it told us something about the location of the DNA in their chromosomes, we might be able to see the effect this has on the argument. Short of that, it doesn't add anything.

Incorrect. The birds have the hepadnavirus DNA and they've survived, so even if this were true, it didn't affect these species.