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CAT Critical Reasoning Practice question with Solution 14

Only 1,000 to 2,000 species of fruit flies exist worldwide. Nowhere in the world are fruit flies more taxonomically diverse than in the Hawaiian Islands, which host some 500 species. A subset of fruit flies called the picture-winged drosophilids is represented in Hawaii by 106 species. All of the fruit fly species now present in the Hawaiian archipelago are thought to be the descendants of the same one or two ancestral females.

Which one of the following can be inferred from the passage?

[A]. All of the picture-winged drosophilids in Hawaii are believed to be the descendants of the same one or two ancestral female fruit flies.
[B]. Picture-winged drosophilids are found only in the Hawaiian Islands.
[C]. All of the 1,000 to 2,000 species of fruit flies worldwide are believed to be the descendants of one or two females.
[D]. If 500 new species of fruit flies were discovered, then Hawaiian fruit flies would no longer be the most taxonomically diverse population.
[E]. Some fruit flies originated in Hawaii and spread from there to other parts of the world.
Answer: A

Here we have lots of formal logic statements hidden in what appears to be a casual argument. Working backwards from the last sentence: ALL Hawaiian fruit flies are thought to be descendent from the same one or two ancestral females. Now look at the sentence right before this: The picture-winged drosophilid is a subset of Hawaiian fruit flies. That means one and only one thing: All Hawaiian picture-winged drosophilids must therefore have the same characteristic of the entire group of Hawaiian fruit flies—they must all be believed to be descendent from the same one or two ancestral females, choice (A).

(B) We have no way to infer that picture-winged drosophilids are unique to Hawaii; just because this species is found there doesn’t prevent it from appearing elsewhere.

(C) distorts the argument by attempting to forge a connection between the first and last sentences. However, no such connection is indicated, because the scope of the first sentence is “worldwide” while the last sentence strictly concerns Hawaii. Specifically, it could very well be that NOT all, or even very few of the 1000 - 2000 worldwide species of fruit flies share the Hawaiian fruit flies’ characteristic descent from one or two ancestral females.

(D) takes its thrust from the idea in the second sentence, that Hawaii has the most diverse population of fruit flies in the world. Even if 500 new fruit fly species were discovered, how would we know whether Hawaii would lose its fruit fly diversity crown? There’s no way to tell other than by pure conjecture, and we’re in no position for that. (D) is not inferable.

(E) Again, like in (C), we get a connection between Hawaii and the world that is in no way supported by the stimulus. While (E) certainly could be true, there’s simply no way to logically infer it from the limited and focused information in the stimulus.

Get past the wordy and cumbersome terminology to see the real relationship among the groups in the stimulus. Here, this situation boils down to nothing more complicated than this: ALL X (picture-winged drosophilids) are Y (Hawaiian fruit flies). ALL Y (Hawaiian fruit flies) are Z (thought to be descendent from the same one or two ancestral females). Therefore, ALL X (picture-winged drosophilids) are Z (thought to be descendent from the same one or two ancestral females).

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