Q22 - After a hepadnavirus inserts itself

[mshinners]

Question Type:
Strengthen

Stimulus Breakdown:
Let's call it virus H.

The argument concludes that H is at least 25 millions years old. This is based on premises about the finch and the junco, which each have signs of the virus in the same place of their DNA and split from each out about 25 millions years ago.

Answer Anticipation:
These heavy science argument's are always confusing, so it's important to just get the important pieces down. We're using the fact that the virus is in the same place for both of these species to show that they were infected at the same time. I'm not sure how the answer is going to do it, but it will have to prove that the infection happening at the same time is more likely than these two species individually getting infected after they split.

Correct Answer:
(C)

Answer Choice Analysis:
(A) Out of scope. The argument doesn't care about why these species diverged, or the impact of the virus on that divergence. It uses the divergence point because of the heritability of the virus to show that the virus was "inherited" before that divergence.

(B) Out of scope. A lack of other virus fragments doesn't make anything about this virus fragment any more or less likely. It'd be like saying humans lacking wings making it more/less likely that we have tentacles.

(C) Bingo. This probably wouldn't be an automatic pick, but I'd leave it because it talks about the virus being in a different location, and, in arguments that are comparing two things (here, finches and juncos), similarities and differences are relevant.

In this case, we know the virus is in the same location for both birds. If viruses enter the DNA randomly, it'd be unlikely for the DNA to randomly insert itself into the exact same place in two different bird species. This makes it more likely the virus entered the bird DNA before they split, which means it's more likely it happened at least 25 million years ago.

(D) Out of scope. Not only are other bird species not relevant, but this answer doesn't tell us where the fragments are located, which the argument relies on to drive home its point.

(E) Out of scope. Survival doesn't matter to this argument, and the H virus is never related to survival, and we're looking at birds that presumably have survived.

Takeaway/Pattern: Whenever an argument deals with a comparison, and you're in a strengthen/weaken question, leave any answer choice that compares/contrasts the elements of the comparison in play during your first pass.

#officialexplanation

[SJK493]

Bingo. This probably wouldn't be an automatic pick, but I'd leave it because it talks about the virus being in a different location, and, in arguments that are comparing two things (here, finches and juncos), similarities and differences are relevant.

In this case, we know the virus is in the same location for both birds. If viruses enter the DNA randomly, it'd be unlikely for the DNA to randomly insert itself into the exact same place in two different bird species. This makes it more likely the virus entered the bird DNA before they split, which means it's more likely it happened at least 25 million years ago.

How does this relate to being precisely in the same location? Can I ask for an extended explanation as I am unable to follow?

Would very much appreciate it! Thanks!

[BrianM353]

Bingo. This probably wouldn't be an automatic pick, but I'd leave it because it talks about the virus being in a different location, and, in arguments that are comparing two things (here, finches and juncos), similarities and differences are relevant.

In this case, we know the virus is in the same location for both birds. If viruses enter the DNA randomly, it'd be unlikely for the DNA to randomly insert itself into the exact same place in two different bird species. This makes it more likely the virus entered the bird DNA before they split, which means it's more likely it happened at least 25 million years ago.

How does this relate to being precisely in the same location? Can I ask for an extended explanation as I am unable to follow?

Would very much appreciate it! Thanks!

Full disclosure, I missed this question but I can see where I went wrong.

We need to strengthen the fact that it's 25 million years old.
A current hole in the argument is nothing is disproving that the finch and junco didn't develop it side-by-side at some later point.
So, if we take the answer to be true, then it's very, very unlikely that it would be in the same chromosome position if they were to develop it side-by-side. Thus it means they got it from a common ancestor, and their common ancestor was 25 million years ago.

My takeaway is that I didn't focus on the conclusion and how the argument was assembled .

[ohthatpatrick]

I tend to filter all causal assumptions/arguments through the same schema.

What was the CURIOUS FACT the author made a causal assumption about?
why is it that the finch and junco have the hepadnavirus in precisely the same location of their chromosome?

What is the AUTHOR'S CAUSAL STORY for that Curious Fact?
a common ancestor of the finch and junco got the hepadnavirus, more than 25 million years ago, and since this virus gets passed genetically to all descendants, when the finch and junco species branched off from this common ancestor, they each still had the hepadnavirus in the same part of their genetic code.

Answers either deal with
- OTHER STORIES to explain the curious fact
- the plausibility of the AUTHOR'S STORY

On Strengthen, we'd pick an answer because it either
- UNDERMINES some other story for why the finch and junco have the h-virus in the same location
or
- BOLSTERS the plausibility that the common ancestor of the finch and junco had the h-virus

One other way we might explain the curious fact is to say that it's just a coincidence that the finch and junco have the h-virus at the same location in their chromosome. Maybe they each separately got the h-virus, some time in the last 25 million years, and it just happens to be at the same spot.

(C) diminishes the plausibility of that storyline by letting us know that it would be a crazy coincidence for two different insertions to appear at the same spot (remember, it's "precisely" the same location).

If there are 100 different possible spots on the chromosome, there's only a 1% chance they'd be at the same spot.

If there are 10,000 different possible spots on the chromosome, there's only a 0.01% chance they'd randomly be at the same spot.

So (C) makes it seem 99% or better that "it ISN'T through coincidental matching of two separate insertions" that the finch and junco have the h-virus at the same spot.

Hope this helps.