Q26 - Researcher: It is commonly believed

[deanmx]

I had some trouble with this question. Particularly in deciding between A), B) and C).

For B), I liked the answer but I ultimately rejected it because it said "some". I thought it was a weak answer and wasn't enough to ensure that the conclusion couldn't be the case.

For C), I thought if the researcher selected non-rodents with specific genes, then maybe the comparison that the researcher made was non-representative and thus would weaken the conclusion.

Could someone please explain this question and specifically why my reasoning for B) and C) were incorrect? Thanks

[ManhattanPrepLSAT2]

We can think of the argument core as follows:

Species belonging to same biological order descended from same ancestor +
Genetic differences between mice and guinea pigs as great as between mice and some non-rodents.

-- >

Guinea pigs from separate ancestor.

The gaps in the core are pretty significant -- the fact that the genetic differences between mice and guinea pigs is as great as between mice and some non-rodents is not enough to prove that guinea pigs and mice don't share a common ancestor. Perhaps mice share an ancestor with certain non-rodents. Perhaps the genes that do align between mice and guinea pigs are far more significant than the genes that align between mice and some non-rodents.

Your job is not to identify these or other potential explanations. However, you want to be able to focus on on the fact that this genetic evidence provided does a very weak job of proving the conclusion. We want an answer choice that exploits this issue.

Let's evaluate the incorrect answers first:

(A) it doesn't matter that they only examined three rodents -- only three rodents are in question.

(C) The implied significance of this answer is that mice might be more genetically similar to certain non-rodent species than to others. We already know this -- and since the premise in the argument is simply that mice are genetically similar to some species, this information neither weakens nor strengthens the argument.

(D) It's unclear what impact this has on the conclusion.

(E) can be tempting, because it hints at the idea that other factors besides genes can be used to determine commonality, but (E) is not directly related to the reasoning in the argument.

That leaves us with (B).

(B) directly exposes a gap we saw in the argument initially. If (B) is true, the genetic evidence presented does not justify the conclusion.

[gotomedschool]

I had this between A and B and had a tough time choosing between the two...


My thing with A) is that, I think it does matter that only 3 rodent species were analyzed...


Same biological order(rodents) ---> common ancestor.

guinee pigs, rats, and mice are all rodents so they theoretically should all have a common ancestor.

however, in the comparison of genetic pattern, rats and mice have roughly the same genetics; whereas the differences between guinea pigs and mice are as great as mice-->non rodent mammals.

therefore, guinea pigs stem from a separate ancestor?


this is where i was like, wait wtf?


what if guinea pigs are genetically very similar to gerbils or chimpunks or another rodent species? wouldn't that discredit the argument's hasty conclusion that guinea pigs do not stem from the rodent common ancestor?

[ManhattanPrepLSAT1]

So there's a gap in the reasoning here. The evidence is about the genetic similarity of two rodents and the conclusion is about whether they shared a common ancestor. To undermine the argument we can suggest that such genetic dissimilarity is does not preclude sharing a common ancestor - answer choice (B) accomplishes this.

I see your point illmalak, but I think the issue is that you might be misreading the conclusion. It suggests that the guinea pig does not share a common ancestor with mice. It is not suggesting that the guinea pigs do not share a common ancestor with another rodent, but that it's not true that all rodents shared at some point a common ancestor.

The argument is set up to challenge a commonly held belief. Does that answer your question?

[zainrizvi]

The reason I think C is wrong is because it does not address the key flaw.

Even if we assume that the researcher chose a species that is really really close to mice on purpose, implying the difference between guinea pigs and mice is not THAT large - the only way we know this means anything at all is because genetics should somehow be related to a common ancestor. So C sorta superficially weakens the argument but not really.

[timsportschuetz]

(E) is wrong since the argument specifically addresses this point: the researcher explicitly states that, "despite their similar physical form, guinea pigs stem from a separate ancestor" as mice. Thus, stating that the above two animals have "peculiar" biological similarities does absolutely nothing to the argument since it already stated this fact prior to drawing its' conclusion.