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

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• c.janson35 Free Trial Inactive Sage Inactive ⭐

  August 2015 2398 karma

  This is a taxing question, so don't feel bad about not really understanding it.
  
  The conclusion of the argument is basically that only having a few of the parts of the flagellum confers no survival advantage. Why? Because the flagellum needs many parts before it can enable an organism to swim.
  
  There's a gap here, though, and seeing it really helps you with the right answer. Why would having only a few parts confer no advantage? What if it's the case that having a few parts allows the organism to do other things that are helpful to its existence, even though they aren't helping it swim? If this hypothetical were true, it would destroy the argument--so it is necessary that, if the parts helped the survival, then it would do so by allowing the organism to swim.
  
  This is essentially what answer choice B says:
  
  If a part aided an organism's survival, it would have had to help it swim.
  
  The negation of this is: even if a part aided an organisms survival, it could do so without having helped it swim.
  
  This pretty much matches our hypothetical case from above, because if it's true that a part helped survival without having helped it swim, then the conclusion that "only having a few of the parts would confer no survival advantage" would fall apart.
  
  Hope this helps!
• c.janson35 Free Trial Inactive Sage Inactive ⭐

  August 2015 2398 karma

  As for the wrong answers,
  
  A: talks about being at a disadvantage relative to other organisms, which is completely out of scope.
  
  C: we know that the flagellum is used to swim, but we don't know about any of its other functions, so we don't care if all of its parts are vital to EACH of its functions. We only care about one function: swimming. So it is not necessary that all of its parts are vital to each.
  
  so what? If an evolutionary ancestor did have only a few parts of the flagellum, we don't know that the bacteria could swim, and we don't know that these parts conferred any kind of advantage on it. Eliminate.
  
  E: we don't care about lacking a flagellum, we care about the parts of the flagellum, and if the parts by themselves could have aided the bacteria's survival. Eliminate.
• nye8870 Alum ⭐

  August 2015 1749 karma

  Thanks too. It all seems so simple now. Our task is to solidify the connection between swimming and a survival advantage. Because, if swimming is not necessary for survival advantage then the argument (vague as it is) will not make sense. These parts could have helped the organism in other survival advantage ways.