Q18 - The flagellum, which bacteria use

[ManhattanPrepLSAT1]

The argument claims that the use of a flagellum to swim requires many parts. Therefore, if something did not have those parts, it would gain no survival advantage from a flagellum. There’s a serious gap here!

Evidence: ~MP → ~UFS

Conclusion: ~MP → ~SA

Gap: ~UFS → ~SA
(key: MP = many parts, UFS = use a flagellum to swim, SA = survival advantage)

Back into English, taking the contrapositive of the gap, "In order to gain a survival advantage from using a flagellum, something would have to use it to swim." This is best expressed in answer choice (B).

(A) does not bridge a gap in the argument but rather seems to be merely a restatement of the information within the conclusion.
(B) fills the gap and is the assumption of the argument.
(C) is not necessary to the argument. Even if it were the case that some parts of the flagellum were not vital, the conclusion could still be true.
(D) misses the topic at hand. It’s not about whether these ancestors had the parts needed to use the flagellum, it’s about a hypothetical situation dealing strictly with those ancestors that did not have all of the parts needed to use a flagellum to swim.
(E) isn’t needed for the argument to stand. Even if they could swim by some other means, the parts of the flagellum might still not offer any survival advantages.

[farhadshekib]

The argument claims that the use of a flagellum to swim requires many parts. Therefore, if something did not have those parts, it would gain no survival advantage from a flagellum. There’s a serious gap here!

Evidence: ~MP → ~UFS

Conclusion: ~MP → ~SA

Gap: ~UFS → ~SA
(key: MP = many parts, UFS = use a flagellum to swim, SA = survival advantage)

Back into English, taking the contrapositive of the gap, "In order to gain a survival advantage from using a flagellum, something would have to use it to swim." This is best expressed in answer choice (B).

(A) does not bridge a gap in the argument but rather seems to be merely a restatement of the information within the conclusion.
(B) fills the gap and is the assumption of the argument.
(C) is not necessary to the argument. Even if it were the case that some parts of the flagellum were not vital, the conclusion could still be true.
(D) misses the topic at hand. It’s not about whether these ancestors had the parts needed to use the flagellum, it’s about a hypothetical situation dealing strictly with those ancestors that did not have all of the parts needed to use a flagellum to swim.
(E) isn’t needed for the argument to stand. Even if they could swim by some other means, the parts of the flagellum might still not offer any survival advantages.

Hey Matt,

Can you please evaluate my thought process?

P1: Flagellum requires some parts before it allows bacteria to swim.

Conclusion: evolutionary ancestors of bacteria, which only had a few of these parts, would gain no survival advantage from them.

The conclusion presents a new, or rogue, element (i.e. survival advantage). The correct answer must connect the premise with this element for the conclusion to work.

Thus, the correct answer must connect "survival advantage" to the Flagellum/Swimming.

This brings us to (B), which connects the survival advantage to swimming.

A - does not mention "survival advantage".
C - does not mention "survival advantage".
D - does not mention "survival advantage".
E - does not mention "survival advantage".

Also, what is the logical negation of (B)? Thanks!

[tzyc]

Hi, let me double check the first sentence...
It's A require B form, and A is sufficient conditionor, B is necessary conditioner, right?
So it's the flagellum (use to swim)→many parts [after "before~" is not involved in the condition]
and take contrapositive,
~many parts→~flagellum,
correct??
Also, I eliminated (E) just because it does not address the gap...only about premise.
As the previous poster says, does not say about servival advantage...correct? Sorry but I'm a little confused to read your explanation about (E) so would appreciate if you could explain it in different words...


Thank you

[ttunden]

I spotted the gap during the test but the answer choices got me. B does mention the gap(new term survival advantage) but it rephrases it and I didn't pick up on it.

It all makes sense now.


I picked E, but it is way 2 strong and doesn't have to be necessarily assumed to derive our conclusion. "any" is too extreme.

A is too extreme as well, the author isn't comparing or contrasting ancestors with few parts and organisms with no parts.

[roflcoptersoisoi]

Premise: The flagellum which bacteria use to swim requires many parts before it can propel a bacterium at all.

Conclusion: Therefore an evolutionary ancestor of bacteria that had only a few of the many parts that bacteria need to swim would gain no survival advantage from them.

Assumption: Bacteria need all the parts of the flagellum that enable it to swim in order to gain a survival advantage.


(A) We don't know what type of disadvantage they're talking about in this answer choice. Even if we assumed that they were talking about a survival disadvantage which is a bit of a stretch, we need not assume that this relative disadvantage necessitates the preclusion of any survival advantage. Further, even if the evolutionary ancestors in question were not at a survival disadvantage relative to other organism, it could still be conceivably true that they did not have any survival disadvantage. Eliminate.
(B) This matches the gap we identified, looks good keep for now.
(C) First of all, as far as this argument is concerned, we only care about the flagellum's ability to help bacteria swim and whether that is necessary to confer survival advantages. It's other functions are irrelevant and out of scope of the argument so this doesn't need to be true for the argument to be true. If some of it's parts were not vital to each of it's functions, the argument could still be true.
(D) This just contradicts a portion of the conclusion, eliminate.
(E) Irrelevant. The bacteria in question didn't lack a flagellum just some of the parts needed to allow it to swim.