Logical reasoning PrepTest 102 · Section 2 · Question 12

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Traditional belief - Lizards evolved from As, but there are no A fossils older than 300m years

New discovery - A lizard fossil 340m years old
Conclusion - Lizards didn't evolve from As

Answer Anticipation:
This question stem is interesting. First, it just mentions an assumption, not a necessary or sufficient one. When that happens, the question is a Strengthen with Necessary Premise question—the correct answer is frequently also sufficient, but it's always necessary. Second, the way it's phrased tells us a bit about our approach. We're looking for an answer that brings up something that doesn't exist—in other words, it's going to give us a possibility that would hurt the argument. To see why, think about it this way—the question asks us to find something that the argument assumes there are none of. So if there are some of that thing, it would kill the argument.

Now, to the argument itself. This argument uses new evidence to argue against a traditional belief. That traditional belief is that lizards evolved from As. But there's no evidence that As existed before 300m years ago, and the recent discovery shows that lizards did exist at least 340m years ago. If lizards were around before As, then lizards couldn't have evolved from As, which would justify the conclusion.

Since we know we're looking for a possible thing that would kill this argument, we should approach our analysis from that lens. What, if it existed, would suggest that lizards did evolve from As? Well, there was a recent discovery of a lizard fossil that made them look older than believed—what if there are A fossils out there that are even older than that? The argument assumes that they don't exist in stating that lizards were around before As, so let's find an answer stating that.

And let's be careful about the negative in the question stem—we're looking for an answer that builds off of that stem, so we're looking for something that says "there are no . . . A fossils that are older than 340m years old."

Answer Explanation:
This answer perfectly matches our anticipation. If there are A fossils older than 340m years, then the As could have existed before the lizards, and it's possible that the lizards evolved from them.

Key Takeaway:
When there's a negative in the question stem, be very careful—it can be easy to forget about it while reading through the answer choices! Make sure your anticipation takes that negation into account.

Incorrect. The author concludes that lizards didn't evolve from As because lizards are at least 340m years old, but As are only at least 300m years old. If lizards are even older than that, that doesn't change the calculus, so this answer doesn't represent a necessary premise.

Incorrect. There's no indication that the As ceased to exist once lizards started. A species can evolve from another while that original species still exists!

Incorrect. This answer is the reverse of (C) and is thus wrong for the same reason.

Incorrect. First, fossils of an unknown age don't really provide information that would call anything into question. Second, lizard fossils of an unknown age are even less relevant here, since we already have a lizard fossil that predates all A fossils.