Logical Reasoning

PT147 · S4 · Q22 After a hepadnavirus inserts itself

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After a hepadnavirus inserts itself into a chromosome of an animal, fragments of the virus are passed on to all of that animal's descendants.

Conclusion

The hepadnavirus is at least 25 million years old.

Evidence

Once a hep virus inserts itself into a chromosome, fragments of the virus get passed to all its descendants.

A hep virus fragment is found in precisely the same location in a certain chromosome of the junco and the zebra finch, and these two species diverged about 25 million years ago.

Evaluate

This argument is hard to understand, let alone critique.

The author is assuming that the junco and the finch had a common ancestor (more than 25 million years ago, before the two species diverged), that a hep virus inserted itself into a certain spot of a certain chromosome in this common ancestor, and that this is reason why we now see the junco and the finch having a hep virus fragment in the same spot of the same chromosome.

We could use a Curious Fact / Explanation mindset to get through this problem.

Curious Fact Why does the junco and finch both have a hep virus fragment in the exact same spot of the same chromosome?

Author's Explanation The junco and the finch had a common ancestor; the hep virus inserted itself into the common ancestor and since the hep virus gets passed down to all of an animal's descendants, now the junco and finch both have it.

We affect these arguments by either - discussing alternative explanations for the curious fact - affecting the plausibility of the author's explanation

What could be a different explanation for why the junco and the finch have a hep virus fragment in the same spot?

Couldn't it be that it didn't come from some common ancestor, but rather it came from two different ancestors, each of which had the hep virus insert itself into the same spot of the same chromosome?

Goal

This answer could come in 3 different flavors: - rule out an alternate explanation for how the junco / finch have hep virus in the same spot

- increase the plausibility that they have it in the same spot because of a common ancestor

or

- make it more likely that the hep virus is at least 25 million years old (maybe we find it in the fossils of animals that went extinct 30 million years ago)

22.

Which one of the following, if true, most strengthens the argument?

  1. Viruses can affect the evolution

    Weak / No Impact

    The strength of wording like "some / sometimes / can / not all" means that one example is enough to make it true. This sort of weak wording is almost always wrong on Strengthen, Weaken, and Paradox. This answer seems to be trying to strengthen a connection between getting the hep virus and diverging into two species (so if there were a common ancestor to the junco/finch that got the hep virus, the hep virus may have influenced that species diverging 25 million years ago). But the weak strength of the answer makes it unappealing. We have no idea if the hep virus is one of the viruses that can influence the likelihood, and even if it were one of them, it's not a great answer. We don't need to be convinced that the junco/finch's common ancestor diverged into two species. We already know that that happened 25 million years ago. The fact that they diverged certainly doesn't prove that the species from which they came had a virus.

    27% picked this

  2. The chromosomes of the zebra

    No Impact

    Whether the chromosomes do or don't have other virus fragments doesn't change this conversation at all. We know they both have the hep virus fragment; we know that the hep virus fragment gets sent down the lineage to all of an animal's descendants; and the author can still make the same argument whether or not either animal has other fragments lying around.

    15% picked this

  3. Correct

    When a virus inserts itself

    Why this is right

    This is a brutal correct answer, but it's helping to rule out the alternative explanation that the junco and the finch each got their hep virus from a different ancestor. If the hep virus inserts itself randomly, then it would be a 1 in a million chance that two organisms would magically get the hep virus inserted into precisely the same location of the same chromosome. Since the junco/finch have the fragment at the same exact spot, it would be almost unthinkable that they got it separately. It makes it much more likely that they got it from a common ancestor who passes it along to all its descendants in that same exact spot. Consider how the flipside of this answer would weaken -- if we heard that "when a virus inserts into a chromosome, it always inserts into the gap between chromosome 19 and 20", then that would make it very possible that the junco and the finch both got their virus fragment from different sources.

    Skill tested: Strengthen · how this choice captures the argument's function is the move to repeat next time.

    47% picked this

  4. Many bird species other than

    Weak Impact

    This might seem to somewhat strengthen, if we think that "if all these birds are getting the hep virus, then maybe they all got it from some common bird ancestor more than 25 million years ago". But the author's case is that the junco and finch must have had a common ancestor, because they have the fragment at precisely the same location of the same chromosome, not because they're both birds. This answer choice doesn't do anything for us unless we know that these other bird species also have the fragment in the same spot, as well as whether these other bird species also diverged from the junco/finch at least 25 million years ago.

    2% picked this

  5. The presence of a hepadnavirus

    Irrelevant: species' survival

    Nothing in this argument would be changed by whether the hep virus helps a species, hurts it, or is neutral to its survival likelihood.

    9% picked this

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