PrepTest 121

Incorrect. Since the conclusion is limited to discussing just the species studied (one, really), it's not a problem that the sample was so limited. Only if she had concluded something about rodents generally would this be an issue.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
Common belief: Species in the same order share a common ancestor

Researcher: A certain rodent (guinea pigs) is more genetically similar to non-rodents than other rodents, so it must have a separate ancestor from the other rodents.

Answer Anticipation:
This answer is about two things: genetics, and comparisons. This is the LSAT, so the comparisons are more important! That's a good thing, because very few people taking this test will have a genetics degree.

The argument rebuts a common belief by examining a few examples. Before we start to think that the problem here might be that it generalizes too much from rats, mice, and guinea pigs, we can see that the conclusion limits itself to discussing guinea pigs, so that's not the flaw we can exploit to weaken this argument.

Instead, it uses the fact that guinea pigs are more genetically similar to non-rodents than to mice (a rodent). From this, it concludes that guinea pigs must have a different ancestor.

We have no idea how factual that is. What we do know is that Weaken questions have flaws in them, and it's not established that different genetic patterns indicate different ancestors. Any answer that suggests two species can have relatively different genetic patterns while still being of the same species and descending from the same ancestor will serve to weaken this argument. It's a blunt and direct attack on the argument here, but we expect that to be what is given in the answer since the science is likely here just to distract.

Answer Explanation:
This answer essentially just says the proof used by the Researcher doesn't prove the point she's trying to make. If species that lack a common ancestor can be more genetically similar than species that have one, then the comparative genetic differences between guinea pigs and other animals raised in the stimulus wouldn't lead to the conclusion discussed.

Key Takeaway:
Don't get wrapped up and lost in the science! It doesn't matter on the LSAT. Focus on the elements of logic in the stimulus and work from those, not the genetics.

Incorrect. This answer is a little science-heavy, which is always suspect. These questions generally have one answer like this to make you think it sounds science-y and thus must be right. However, it also deals with comparisons. Viewing it through that lens, we can see that other nonrodent mammals that are different from those in the study wouldn't weaken the argument. Why not? Because the argument uses the guinea pig's similarity with certain nonrodent mammals to highlight a difference between them and other rodents. As such, other nonrodent mammals that aren't established as being similar to guinea pigs are out of scope.

Incorrect. It's unclear how the recency of the common ancestor would play into the argument here, or if the orders discussed here include the ones in the stimulus, so this answer has no effect on the argument.

Incorrect. If anything, similarities between guinea pigs and other rodents suggest a common ancestor.