PT51.S3.Q18 - the flagellum, which bacteria use to swim

Sarah889Sarah889 Alum Member
edited February 2017 in Logical Reasoning 877 karma

Hi All,

Super confused by this question, mostly because I probably don't understand what the correct answer choice is actually saying.

The argument is as follows:

P1: The flagellum is what bacteria uses to swim.
P2: The flagellum needs many parts before it can even propel the bacterium at all.
C: Therefore, any evolutionary ancestor of bacteria who only had a few of these parts would gain no survival advantage from these parts.

The question is asking for a sufficient assumption. I cannot understand what (B) is saying or why it is correct.

This is what I think (B) is saying: "At this point in time, all of the parts used for survival would have had to help the bacteria swim." So say there was Part A, Part B, and Part C. Part A helped the bacteria to eat, Part B to procreate, and Part C to sleep. (B) is essentially saying that, in addition to these functions, Part A, Part B, and Part C also help the bacteria to swim. Again, we are talking about bacteria NOW (not the evolutionary ancestors that the conclusion brings up).

So present day bacteria have Part A, Part B and Part C (as well as others, since there are "many"), each of which help them survive but also are utilized for their swimming abilities.

The conclusion talks about evolutionary ancestors that only had a few of these parts. So lets say that an ancestor only had Part A and Part C (pretend they morphed from being asexual to sexual creatures-- clearly not a scientist, but go with it). From my perspective, we need more information that (B) does not give us. For example, why would the functions of Parts A and C have no "survival advantage"? Parts A and C still allow the bacterium to eat and sleep. Is swimming necessary for survival? We are told that they need many parts to swim, but if they CANNOT swim, why would that render their other parts useless when it comes to "survival advantage"?

We know that there needs to be "many parts" in order for the bacteria to swim. We also know that the ancestors in question had "few parts," so we can infer that they could not swim. But the conclusion is very strong in that it is saying there are NO survival advantage to ANY of its existing parts. I just don't see how (B) gives us the information we need to arrive at this conclusion.

I believe I must just not be understanding what (B) is actually saying.

I chose (C). My thought was that, if all the parts of flagellum are vital to each of its functions, then it makes sense that, if an ancestor didn't have certain parts, they wouldn't be able to do anything. For example, Part A, Part B, and Part C are all necessary for ANY of them to function; therefore, if an older ancestor didn't have Part B, then Part A and Part C would provide no survival advantage because they cannot perform without that third part. So unless all of the parts are present, then none of the parts would provide a "survival advantage". (C) provides a very broad and strong claim, but I am seeing it fill a gap that (B) isn't.

Thanks in advance.
https://7sage.com/lsat_explanations/lsat-51-section-3-question-18/

Comments

  • DumbHollywoodActorDumbHollywoodActor Alum Inactive ⭐
    edited February 2017 7468 karma

    The question is asking for a sufficient assumption.

    Right here is your problem. This is not a sufficient assumption. It's a necessary assumption ("assumption on which the argument depends"). Your analysis of the stimulus is spot on, though, so I'll just go over the answers.

    (C) is a classic necessary assumption wrong answer. It's too strong (which is probably why you thought it was a good sufficient assumption), or rather it includes more parts of a flagellum than are necessary to support this argument. First, let's think about the negation of (C). "Some parts of the flagellum are not vital to each of the flagellum's functions." Even if this were true, it's entirely possible that an evolutionary ancestor of bacteria that had only a few of the parts a bacteria's flagellum would still gain no survival advantage from those few parts. The reason? We have no idea WHICH of the parts of the flagellum the argument is talking about.

  • DumbHollywoodActorDumbHollywoodActor Alum Inactive ⭐
    edited February 2017 7468 karma

    Now, realize (B) is a conditional statement; its contrapositive is very telling: "If the bacteria did not have the parts incorporated into the flagellum to help that bacteria swim, then those parts now incorporated into the flagellum would not have aided the organism's survival." The trick is to recognize that the evolutionary ancestor of bacteria that had only a few of the parts from the conclusion is the logical equivalent of the bacteria not having the parts incorporated into the flagellum to help the bacteria swim in the sufficient condition of the contrapositive of my translation. Hopefully, you can see that that statement essentially BRIDGES the premises to the conclusion, and hence, is necessary to the argument.

  • Sarah889Sarah889 Alum Member
    877 karma

    @DumbHollywoodActor said:

    Right here is your problem. This is not a sufficient assumption. It's a necessary assumption ("assumption on which the argument depends").

    YESSS. Thank you! That is why I'm confused. Whenever I see the word "depends," it often trips me up because I conceptualize the argument depending on one specific assumption for the conclusion. Classic mistake. Thank you for pointing that out. This question makes so much sense now.

  • DumbHollywoodActorDumbHollywoodActor Alum Inactive ⭐
    7468 karma

    Any time. :)

  • paniz.gh91paniz.gh91 Free Trial Member
    82 karma

    Hi,
    I just checked the question and it is asking for a necessaey assumption and not a sufficient assumption. The question asks for "which one of the following is an assumption the argument depends?".
    As for the correct answer choice "B" is the assumption and the gap between the premise and the conclusion. Also if you negate "B" the argument will be destroyed.
    C" is wrong because we don't have to assume that all parts of the flagellum are vital. We just know that it requires many parts to swim.

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