#2 - Only 1,000 to 2,000 species of fruit flies exist

[Administrator]

Complete Question Explanation

Must Be True. The correct answer choice is (A)

This stimulus consists entirely of information. However, since the information is delivered in very absolute terms, you might be able to make an inference. You should pay particular attention to statements that are related to each other. The other statements might be important as well; however, when a stimulus that contains only information nevertheless has statements that force a conclusion, the answer to a Must Be True question will very often be that conclusion.

Since we know that fruit flies called drosophilids inhabit Hawaii, and all fruit fly species in the Hawaiian archipelago are thought to be descended from "the same one or two ancestral females," we can infer that all the drosphilids that inhabit Hawaii are thought to be descended from the same one or two ancestral females.

Answer choice (A): This is the correct answer choice, and is the exact inference we can make from the latter statements in the stimulus.

Answer choice (B): This inference is unjustified. The stimulus states that drosophilids are "represented in the Hawaiian archipelago by 106 species," and if anything the implication of "represented" is that drosophilids are found elsewhere as well. In any case, the stimulus contains no information that would exclude drosophilids from areas other than the Hawaiian archipelago, and you should eliminate this choice.

Answer choice (C): Choosing this answer choice would involve misreading the stimulus to associate the Hawaiian fruit flies as the ancestors of all fruit flies, or mistakenly assuming that the case in Hawaii should be reflected everywhere. Also, you should not assume that the diversity in Hawaii has anything to do with the general origin of fruit flies. The correct answer should follow from the passage, not possibly valid scientific assumptions, so this choice is wrong.

Answer choice (D): If 500 new species of fruit flies were discovered all in some comparably sized region, we might be able to claim that the Hawaiian selection was no longer the most diverse. However, this answer choice is not even that specific, and it would actually be possible for those new species to be discovered in Hawaii, making the choice false. You should eliminate this choice.

Answer choice (E): It very likely could be true that some fruit flies originated in Hawaii and spread to other parts of the world. In fact, given shipping it might be absurd to disbelieve that answer choice (E) is true in reality. However, you are supposed to pick answers that proceed from the passage alone, and this choice is incorrect.

[hanvan]

Hi, in question 20, powerscoretest preparation, page 1-80 , the right answer is A. It says :" all of the picture-winged drosophilids in Hawaii are believed to the descendants..." it mentioned about the " drosophilids " kind, not archipelago kind, but in the last sentence of the stimulus stated " all of the fruit fly species now present in the Hawaiian archipelago..." I am confused this answer choice. Please help! Thanks!

[Nikki Siclunov]

The terms "Hawaiian archipelago," "Hawaiian islands," and "Hawaii" are used interchangeably in the stimulus. Since we know that fruit flies called drosophilids inhabit Hawaii, and all fruit fly species in the Hawaiian archipelago are thought to be descended from “the same one or two ancestral females,” we can infer that all the drosphilids that inhabit Hawaii are thought to be descended from the same one or two ancestral females.

[rameday]

I had a few LR questions.

For question 20 on page 1-80 what would be an ideal pre phrase? Mine was that the one or two ancestral female butterflies in haiwaii mated with a wide variety of different species male butterflies. That pre phrase was ultimately rather useless.


A

[KelseyWoods]

Hi A,

Your prephrase for this questions goes a bit beyond the scope of the information we have in the stimulus. We know that all of the fruit fly species in Hawaii are thought to be descended from one or two female fruit flies, but that doesn't mean we can make any inference about how many or what kind of male fruit flies they mated with. In fact, we really don't know much about fruit fly reproduction in general based on this stimulus so we can't make any inferences about it. Careful not to infer things you can't actually prove.

Remember that a prephrase doesn't always have to be super specific. Not all questions or question types lend themselves to specific prephrases. The process of prephrasing is just thinking about what you are looking for in the correct answer choice. Sometimes, based on the information you have and the question, you know very specifically what you are looking for. Sometimes, you really don't know specifically what the answer choice will say so you need to think more broadly about what you're looking for.

Prephrasing is always important. But it doesn't always have to be an exact guess as to what the answer choice will say.

My process for prephrasing this question is to study each of the facts closely making sure I understand each of them and am keeping track of the difference between all fruit flies in the world, all fruit flies in Hawaii, and this one subset of fruit flies in Hawaii (picture-winged). Then, I read the question stem and classify it as a MBT. From that, I know that I will be using the information in the stimulus to prove an answer choice. So I can't bring in outside information and I am going to make sure my correct answer passes the Fact Test.

That's my prephrase, and that's as specific as it gets. Because the truth is, there are multiple things that must be true based on the information I have above. If I try to guess which of the multiple things that must be true is going to appear in the answer choices, I risk getting so enamored of my specific prephrase, that I miss the correct answer.

This holds true for many MBT questions: there are usually many things that must be true and you can't prephrase too specifically because you may then overlook the correct answer. The MBT questions that lend themselves best to specific prephrases are ones involving conditional reasoning. Even then though, you have to be careful because there are still usually multiple thinks that must be true. Always make sure that you make sure you can prove your answer based on the facts in the stimulus.

When I go through the answer choices in this question, most of them are outside the scope of the things I definitely know based on the facts in my stimulus. I can prove answer choice (A) with the information above, though. If ALL of the fruit flies in Hawaii are believed to be descended from the same one or two females (last sentence) and the picture-winged drosopholids are a subset of fruit flies found in Hawaii (3rd sentence), then it must be true that ALL of the picture-winged drosophilids found in Hawaii are believed to be descendants of the same one or two females. It sounds too simple, which is why it's probably not something most people would prephrase. But it passes the Fact Test, so it's the correct answer.

Finally, I'm going to link you to a page with an excellent discussion of prephrasing written by Jon for further reading.

Hope this helps!

Best,
Kelsey

[rameday]

Thanks for the response kelsey. I already was aware of that drill. I have done the exercise that Jon recommended and thus far it has been helpful. Still need to keep repeating it.

[rrurrec]

I understand why A is the correct answer, however, how can we be sure that it is not just an implication of a belief? In which case it cant be inferred that the implication is also thought to be true?

[katehos]

Hi rrurrec,

When we are faced with Must Be True questions, we accept the information in the stimulus to be true and know that we will be using that information to prove the correct answer choice. So, we don't have to worry about whether or not the author of the stimulus simply believes statements in the stimulus to be true or if they were rigorously scientifically proven -- we accept the stimulus to be true and figure out what can be logically inferred!*

*In an instance, however, when an opinion or viewpoint is expressly stated in a stimulus, then it is important to approach that particular portion of the stimulus with caution because you cannot logically infer a fact from an opinion.

I hope this helps
Kate

[rrurrec]

I think I'm confused because Im comparing this question to PT 69, S4, Q20 which is also a must be true. In that question, a distinction is made saying "many scientists believe" indicating a difference between opinion and fact and the answer explanation says to make sure to be aware of this difference and not conflate it. In this question, #2 says "it is THOUGHT that," which I read to also be an opinion. Can you please explain why the first question is treated differently than the second?

[katehos]

Hi rrurrec, great follow up question!

To put it simply: they're not being treated differently! In both instances, we're not fighting the information or trying to disprove the stimulus, rather, we accept the stimulus to be true (so, if the stimulus states something as a fact we accept it as a fact and if the stimulus states something as an opinion we accept that as an opinion). With this acceptance, we still distinguish between fact and opinion, as this distinction can limit what can be properly inferred from the stimulus itself (as the explanation for PT 69 Q20 discusses, we want to avoid conflating fact and opinion).

My apologies if I misunderstood your previous question; what I was trying to say was even if there is an opinion in the stimulus, we don't need to fight its existence (similar to how if something is stated as a fact, we don't need to rigorously prove that fact before we accept it). We do, however, still need to avoid conflating fact and opinion in order to find what Must Be True. In the case of this question, answer choice (A) can be inferred in part from the last sentence of the stimulus that uses the phrase "are thought to be". This is not an issue, however, since answer choice (A) matches that language by using the phrase "are believed to be". If answer choice (A) in this question made the same mistake as answer choice (A) on PT69 Q20 by asserting a factual claim from an opinion, then it would certainly be an issue!

I hope this helps clear things up!
Kate