#22 - After a hepadnavirus inserts itself into a chromosome

[Adam Tyson]

The stimulus doesn't indicate that, LSAT2018, but that's what is needed to strengthen it. Why would the author think the hepadnavirus must be that old? Why does the time frame of the two birds diverging in their evolution matter? What does it matter that the hepadnavirus shows up in the exact same spot in both birds?

The divergence date would matter if the two birds inherited the virus from their common ancestors, rather than each of them getting it independently later. So to strengthen the argument, we want evidence that they got it way back then, before diverging, rather than more recently. All we know is that they have it in the same location. Connect those ideas by prephrasing "if you have it in the same location, you probably got it from a common ancestor," or perhaps "if you got it independently, it wouldn't be in the same location."

Answer C matches both these prephrases by reducing the odds of the virus showing up in the exact same spot on two different occasions. If the placement is random, yet they have it in the same place, it makes more sense for them to have inherited it from a common ancestor than for them to have gotten it independently after they diverged in their evolution.

[Lily123]

Would it be accurate to say there's causal reasoning in this stimulus? The virus being in the same location is caused by the infection of a common ancestor

[Adam Tyson]

I'd say there is some implied causation here, Lily, yes - the author is implying that the cause of the virus showing up in both birds at the same location is that a common ancestor had the virus and passed it on. C can be viewed as a "no cause, no effect" answer. It's not directly causal, and I think this could be solved without looking at it directly through a causal lens, but doing so in this case is, imo, very helpful. Good eye!

[lsatnoobie]

I'd say there is some implied causation here, Lily, yes - the author is implying that the cause of the virus showing up in both birds at the same location is that a common ancestor had the virus and passed it on. C can be viewed as a "no cause, no effect" answer. It's not directly causal, and I think this could be solved without looking at it directly through a causal lens, but doing so in this case is, imo, very helpful. Good eye!

I'm having a really hard time seeing how "C can be viewed as a "no cause, no effect" answer. Could you further elaborate?

[Brook Miscoski]

lsatnoobie,

The stimulus argues that since the virus is at the same place on each species' chromosome, the virus existed 25 million years ago, at the time the species diverged.

The question is whether there is some other reason why the virus would be at the same place on each species' chromosome, and our goal is to defend (strengthen) the argument from that problem.

(C) does this by saying that the place the virus inserts at is random--meaning that if the insertion happened at different times, you should expect different places of insertion. This strengthens the argument that the insertion happened at the same time, because the virus is in the same place in each species--thus, at least 25 million years ago for the single insertion.

You can call this what you like. You're showing that the cause isn't that the virus always inserts at the same place. You're showing that the cause of the equal placement was an insertion 25 million years ago. Take your pick, different people will phrase it different ways.

[PresidentLSAT]

Can the powerscore team kindly break down this stimulus for me? English isn't my first language so I'm having a hard time understanding what is going go.

Strengthening this parrticular argument means increasing the plausibility that the virus is at least 25million years old.

We're told that when this virus enters an animal, its fragments are passed on to descendants; I'm assuming children, grandchildren etc.

This virus is in the same location of the zebra finch and dark-eyed junco who have been diverged from each other for 25million years. How does the location of it impact the age? I'm lost and the explanations exacerbated my confusion.

My reason for eliminating A is that if viruses affect the evolution of organisms and influence their diverges, how does this strengthen the conclusion that it's at least 25 years old? Then again I probably don't know what this answer is saying.

B. makes sense as a wrong answer. The could contain 12 other viruses. I don't care.

c. The insertion of the virus occurs at a random spot. The stimulus says they're found at the same location. How is this increasing the plausibility of 25 years of existence? How is it harmed if they're are NOT found at a random spot?

D. the elimination is fair. I'm not concerned about other species.

E. It can affect an animal's survival and still bolster the conclusion about a 25million year run

[PresidentLSAT]

I think after writing down my own thoughts in the forum, I understand and want to give this a shot.

My job here is to support the author's conclusion that the fact that this virus is located in the same spot of these two species (I'm assuming one descended from the other) proves that the virus is at least 25million years old. Why? because that's how long they've been diverged.

C. Strengthens it by saying that virus contacts that are not a result of descendant inheritance occur at random spots. This further tightens the author's reasoning, that because this isn't random, it rules out a doubt that they may have contracted the virus elsewhere. Because if they did, it wouldn't be the same spot.

The takeaway pattern for strengthening which I've seen on more recent LSATs is that you make a line of reasoning more plausible by ruling out an alternative factor or weakening any doubt that could be attributed to the line of reasoning. I've spent like an hour on this question. I hope I'm not just rambling lol

A. I don't see what other students are seeing. If anything, it's focused on the likeliness of diversion.

[KelseyWoods]

Hi PresidentLSAT!

Seems like you've figured this one out! If virus insertion occurs at a random spot, that suggests that, if these two species of bird have it in the exact same spot, it's likely that the insertion occurred before they diverged into two separate species. If, instead, insertion always occurred at the same spot on a chromosome, that would make the similarity between these two species less rare which would weaken the idea that the virus had to be inserted 25 million years ago, before they diverged.

And yes, a very common scenario that happens in Strengthen questions is that you strengthen the author's explanation by eliminating another explanation. Here, the author observes that these two bird species have the virus in the same spot and concludes that the explanation for this similarity is that the virus was inserted 25 million years ago, before the species diverged. Answer choice (C) eliminates an alternate explanation for having the virus in the same spot--that it just always shows up in the same location.

Good job!

Best,
Kelsey

[andrewb22]

I got this one right, but my prephrase did not show up in the answers, though it did help me select (C). My prephrase was "The hep virus died out before the two birds separated." My thinking was: if the virus was dead before the two birds separated (>= 25 million years ago), then that gives a lot of strength to the idea that the virus was passed on genetically. Obviously this didn't show up as a choice, but it definitely helped me connect it to (C) because was similar enough in function to what (C) did, bridging the genetic transmission gap.

When strengthen questions come up that are not causal, I struggle to come up with a very accurate prephrase because it seems like the options are numerous. What is a good pre-phrase for this questions and what is the best strategy for prephrasing similar strengthen questions.

[Poonam Agrawal]

Hi Andrew!

As a general strategy, I pre-phrase answers to strengthen questions by looking at the evidence given for the argument's conclusion and trying to make this evidence even more compelling. In this stimulus, it seemed like the author was using the fact that the virus fragment was in precisely the same location in both birds to prove that one of their common ancestors received the virus before the two species diverged. However, we were kind of just taking for granted that the virus being in the same location was indicative of this conclusion - we were never really told that this was rare or unlikely to happen in two different animal species.

So, a good pre-phrase to this question might include a statement that tells us that the virus is usually inserted in different chromosomes for different animals. With that piece of information, we would then know that because the two birds had the virus in the same location, they must have received it when they were part of one species, which was 25 million years ago. Answer choice (C) drives this point pretty closely, which is why it is the correct answer here.

Let us know if you have any other questions!