PrepTest 147, Section 4, Question 22
After a hepadnavirus inserts itself into a chromosome(70 more words not shown)
Which one of the following,(6 more words not shown)
- AViruses can affect(17 more words not shown)
- BThe chromosomes of(16 more words not shown)
- CWhen a virus(13 more words not shown)
- DMany bird species(14 more words not shown)
- EThe presence of(15 more words not shown)
Reading the argument
What if both birds got the virus after their species split? Suppose the virus entered each species 10 million years ago and happened to land in the same place in their chromosomes. Then the matching fragments wouldn't show that the virus already existed 25 million years ago.
What we’re looking for
That would be an odd coincidence, but the argument hasn't yet told us how odd. Inheritance from a shared ancestor is one explanation for the match; separate later insertions are another. I'd strengthen the argument by making that second explanation much less likely. If the odds of two separate insertions matching were one in a trillion, for example, that would make the coincidence objection far less persuasive. Those are hypothetical odds, not a fact we have been given.
Working the answers
- A
A virus could influence how a species evolves without telling us when this virus entered these particular birds' chromosomes. The argument needs evidence that the matching fragments were present before the species split. A general ability to affect evolution doesn't establish that timing.
- B
Finding no other viruses' fragments doesn't tell us when the hepadnavirus fragments arrived. The two birds could still have acquired this one virus separately after their species split. Other viruses being absent doesn't resolve the competing explanations for the matching locations.
- CCorrect
Random insertion helps explain why two independent infections ending up at precisely the same spot would be a coincidence worth doubting. The virus isn't simply directed to that one spot every time. That makes inheritance of the matching fragment from a shared ancestor more persuasive, supporting a date before the species diverged. C doesn't give us the exact odds I imagined, and it doesn't make coincidence impossible. A strengthener only needs to make the argument better supported.
- D
Other bird species also carrying fragments shows the virus isn't limited to these two species. But without information about when or how those other birds acquired it, we still can't date the matching fragments in the finch and junco. More infected species doesn't settle the before-or-after question.
- E
Whether the virus affects a species' survival doesn't tell us when it entered the chromosomes. The fragments could be harmless and either inherited from an ancient common ancestor or acquired later in separate infections. We need help choosing between those histories, not information about their effects on survival.
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