PrepTest 133, Section 2, Question 18
The flagellum, which bacteria use to swim, requires(31 more words not shown)
Which one of the following(8 more words not shown)
- AAny of bacteria's(28 more words not shown)
- BFor parts now(18 more words not shown)
- CAll parts of(9 more words not shown)
- DNo evolutionary ancestor(12 more words not shown)
- EAny of bacteria's(12 more words not shown)
Reading the argument
Not enough parts to swim. Does that mean not enough parts to help at all?
The flagellum needs many parts working together to propel a bacterium. We can accept that without assuming each part is useless on its own. Imagine a simplified system needing parts A through Z to move. An ancestor with only A through D couldn't use that system to swim, but perhaps one of those parts could help protect it from predators. That would be a survival advantage even without propulsion.
Those letters are just a way to track the reasoning; we don't need to know the actual biology. The author's evidence concerns one function, swimming. The conclusion rules out every survival benefit from the parts. What if they serve another purpose?
What we’re looking for
The assumption needs to close that gap. I'd expect the claim that these parts could aid survival only by helping the organism swim. If swimming is the only possible route to a survival benefit, then the inability to swim with a few parts becomes relevant to the much broader conclusion.
Working the answers
- A
No advantage isn't the same as a disadvantage. The parts could make no difference to survival without actively harming the organism relative to one that lacked them. The argument concludes that they don't help; it doesn't require them to hurt.
- BCorrect
To aid survival, these parts would have had to help the organism swim. Now the missing swimming ability rules out a survival benefit from the parts. The predator-protection possibility is excluded: under B, a part can't provide an independent survival advantage that doesn't involve swimming. This is the link the argument takes for granted.
- C
Even if every part were needed for each function of the complete flagellum, a few parts might still help the organism in some other way. We need to rule out a separate benefit of those parts, not just describe what the fully assembled structure requires. Also, 'many parts' in the passage doesn't itself tell us that every single part is vital to every function.
- D
The argument concerns what an ancestor with only a few parts would gain from them. It doesn't depend on proving that such an ancestor never existed. Whether the situation actually occurred is different from whether those parts could have helped an organism in that situation.
- E
This rules out other ways for an ancestor without a flagellum to swim. But our objection doesn't require another way to swim. The few parts could provide a completely different survival benefit, such as protection. E leaves that possibility open, whereas B excludes survival benefits unrelated to swimming.
More Necessary Assumption questions
Learn the LSAT our way
Our platform drills Necessary Assumption questions at your level and adjusts as you improve.