Hey VC,
Sorry to miss this one and take a few days to reply...
It's a tough explanation, because absolutely most of the time you'd be correct even with the thinking that missed this particular question. The possessive should require a "those of" or "that of" response from the second item in the comparison. It's just that, in this case, you don't really "have" a species in that possessive form...
So I guess another parallel example would be:
Some anthropologists theorize that racism is a function of the natural human tendency to extend favoritism to members of their own families over others.
Here the comparison isn't between "members of their own families" and "members of other families"; it's "members of their own families" and "other people". So the above is correct even without the "those of" - in fact, adding "members of their own families over those of others" is wrong. It misses the point of the comparison, which is really:
Members of one's family vs. Everyone else
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Now, let me further this discussion by talking about why the GMAT does that. Last night in a math class I was teaching, we went through an equilateral triangle problem in which the key to solving it was to note that finding the height of an equilateral triangle (for use in finding the area) also involves breaking the big triangle into two identical 30-60-90 triangles. So then the side of the equilateral triangle becomes the 2x / 90-degree side of the 30-60-90 and you can find the height from there.
Well, I had a student complain after I had demonstrated those steps that she knew the area-of-equilateral-triangle formula (s^3 * sqrt 3 / 4) and that using that formula would be faster. So why did we do it the long way?
I had to reply with a different question. "If a triangle has two 60-degree angles and an area of 9sqrt3, what is the shortest distance between any side and its opposite angle?" (I think that's how they could 'officially' ask you about the height). That question asks you to use that exact same relationship between the area of an equilateral triangle and the fact that it's a pair of identical, side-by-side 30-60-90s, but it doesn't let you get away with just plugging in the formula.
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And that's how the GMAT gets "harder". There are dimensions of intellectual ability. The ability to know/remember is important, but it's at a lower level than the ability to synthesize information, draw parallels, and logically solve complex problems. Educators talk a lot about "higher-order thinking" as the goal of higher levels of education - recall is important, but it's lower-level. So the GMAT has a responsibility to test you in ways that you can't just get from a flashcard. Now, there will be some questions that are more or less content based, but as you get to higher scoring ranges on the test they have to continue to break away from pure knowledge and instead test more strategy and problem solving skills. The average MBA student should be able to remember information , so to some extent they need to test recall to weed out those who simply struggle to retain what they read or hear. But that alone doesn't make a terrific manager; it's the higher-order thinking, the ability to find synergy or common ground where others might not see that; the ability to solve problems efficiently, maximize the value of assets, look at problems from different angles to find elegant solutions, etc. The GMAT has to test that because that's the thought process that's in short supply in the world, and that's what business schools want to accept, hone, and turn out into the work force.
So that's what's going on in the question further up in this thread. It's - and this is just a number for the sake of a number - a 550-level skill to recognize that a possessive should typically be compared to a possessive, and schools probably don't want students who can't learn and apply at least some of those concepts. But it's a much more useful skill - maybe 720 level, again just for sake of demonstration - to recognize that this isn't your typical comparison. You're not comparing "the species of one animal" to "the species of others"; you're actually comparing "this animal's species" to "other species".
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Now, as far as strategy goes, I don't know exactly how to advise you. The general rule you're applying is one that will work on average-difficulty questions and probably above-average questions. So I don't want you to overthink every question you see and doubt everything you already know, and therefore miss some of the 600 level (again, numbers just for the sake of proving a point...just guessing on actual difficulty) questions that you'd ordinarily get right.
But I would definitely advise that you think about meaning in sentences in practice, especially when you get them wrong and wonder why. The harder SC questions tend to hinge more on meaning issues than on "grammar rules" issues. I've talked to three people in the last week about this - they've scored 760, 780, and 790 within the past 6 months - and they all said that the hardest questions they saw on the test were "tricky parallelism Sentence Correction" related. And when I've brought up that they were probably of this type - that the grammar rule you wanted to see had an illogical meaning - they all agreed that it was probably it. I actually used this question in leading a Veritas instructor development session the other night, and a couple instructors mentioned having seen something very similar on recent tests.
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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