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Biologist: Species with broad geographic ranges probably tend to endur

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AlexTheTrainer

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gmatt1476

GMAT CR only asks for an assumption that is necessary, meaning the negation test will always be applicable.

While LSAT LR (progenitor of GMAT CR) features two types of Assumption questions (Necessary Assumption and Sufficient Assumption, the latter asking for an answer that proves/guarantees that the conclusion must be true) and while this question stem is phrased as an LSAT Sufficient Assumption question, the correct answer is both necessary and sufficient.

On the LSAT, not all assumptions are both necessary and sufficient, but they certainly can be. In other words, a Necessary Assumption and Sufficient Assumption are not necessarily mutually exclusive ideas.

While an assumption that is both necessary and sufficient need not be phrased in if/then form, such a form does indicate an answer that goes both ways.

Negating “if x then y” results in “(even) if x then not (necessarily) y”.

So negating D: “If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to NOT gradually increase with time.”

This negation will forever create an invalid argument. That is, the negation above, when added to the argument from the stimulus, means that the argument will always be invalid, no matter what additional evidence might be added.

For the record, I’ve been teaching both the GMAT and LSAT since the 20th century. The reply above is consistent with the one-off 2007 LSAT publication known as The Official LSAT SUPER PREP, the only LSAT publication with an actual guide to Logical Reasoning. It specifically discusses the issue of how some assumptions might be both Necessary and Sufficient.

Negating a sufficient assumption that is not also necessary does not permanently invalidate the argument, it merely leaves the original invalid argument as-is. Only when a necessary assumption is negated will a forever invalid argument result. This is why some folks (like me) recommend against using LSAT LR as a way to study. Causes too much confusion.

Too much?

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Biologist: Species with broad geographic ranges probably tend to endure longer than species with narrow ranges. The broader a species’ range, the more likely that species is to survive the extinction of populations in a few areas. Therefore, it is likely that the proportion of species with broad ranges tends to gradually increase with time.

P : Species with broad geographic ranges tend to endure longer
P : broader range -> more likely to survive extinction.
A : more likely to survive and endure longer means getting gradually more proportion
C : Proportion of species with broad range will increase.

The biologist’s conclusion follows logically from the above if which of the following is assumed?

A. There are now more species with broad geographic ranges than with narrow geographic ranges.
-> Current situation cannot be a determinant in determining what will happen in the future.

B. Most species can survive extinctions of populations in a few areas as long as the species’ geographic range is not very narrow.
-> Concept 'not very narrow' is very vague and cannot be determined with ease.

C. If a population of a species in a particular area dies out, that species generally does not repopulate that area.
-> If cetain species does not repopulate in a particular area where it has died out, then the range of that species will become smaller and smaller, making it to have narrow geographic range gradually as time flows. Thus option C cannot be the assumption to make the conclusion persuasive.

D. If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time.
-> Correct. Option D provides linkage between two concepts, enduring longer and procuring more proportion.

E. Any characteristic that makes a species tend to endure longer will make it easier for that species to survive the extinction of populations in a few areas.
-> Characteristic helping a certain species to survive the extinction in a few areas does not guarantee that certain type of species will gradually increase its proportion among others.

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Biologist: Species with broad geographic ranges probably tend to endure longer than species with narrow ranges. The broader a species' range, the more likely that species is to survive the extinction of populations in a few areas. Therefore, it is likely that the proportion of species with broad ranges tends to gradually increase with time.

The biologist's conclusion follows logically from the above if which of the following is assumed?

The question asks which of the assumptions fills in the gap between the premises and the conclusion.

A. There are now more species with broad geographic ranges than with narrow geographic ranges.
B. Most species can survive extinctions of populations in a few areas as long as the species' geographic range is not very narrow.
C. If a population of a species in a particular area dies out, that species generally does not repopulate that area.

(C) even if species generally doesn't repopulate the area after dying, our conclusion holds: proportion of species with broad ranges can still gradually increase with time.

D. If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time.

If we negate this choice, then conclusion falls apart. If a characteristic helps species endure longer, then the proportion of species will NOT tend to gradually increase with time. BAM! Our conclusion is broken. This is our answer.

E. Any characteristic that makes a species tend to endure longer will make it easier for that species to survive the extinction of populations in a few areas.

Is extinction really what our conclusion is supported by? It says characteristics of broad range leads to 1) longer endurance, 2) higher extinction survival rate, and 3) an increase in proportion of population. This answer choice states: broad - > #2, but never gets us to #3, which is our conclusion. However, (D) gets there: broad -> #3.

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gmatt1476

Hi,

If we negate Option C "Even if a population of a species in a particular area dies out, that species generally repopulates that area." , then shouldn't this break the conclusion that the proportion of broad range species will increase over time as the narrow range species will repopulate those areas.

Thanks

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Biologist: Species with broad geographic ranges probably tend to endure longer than species with narrow ranges. The broader a species' range, the more likely that species is to survive the extinction of populations in a few areas. Therefore, it is likely that the proportion of species with broad ranges tends to gradually increase with time.

The biologist's conclusion follows logically from the above if which of the following is assumed?

Arrived at the answer by POE. Here is my thought process:

A. There are now more species with broad geographic ranges than with narrow geographic ranges.
This looks like a result that the conclusion supports. Out

B. Most species can survive extinctions of populations in a few areas as long as the species' geographic range is not very narrow.
What does the author mean by "not very narrow"? Even if this is assumed, it still does not explain why the proportion of species with broader geographic range will increase over time. Out

C. If a population of a species in a particular area dies out, that species generally does not repopulate that area.
Okay, but what if they repopulate in another area? Further, this answer choice does not talk about the broad and narrow geographic range. Out

D. If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time.
Connect the premise and the conclusion perfectly!

E. Any characteristic that makes a species tend to endure longer will make it easier for that species to survive the extinction of populations in a few areas..
Yes, but this doesn't support the conclusion or explain the gap

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KittyDoodles

If we negate Option C "Even if a population of a species in a particular area dies out, that species generally repopulates that area." , then shouldn't this break the conclusion that the proportion of broad range species will increase over time as the narrow range species will repopulate those areas.

The negation of an assumption that takes the form of a IF-THEN conditional is tricky. And this is a tricky question anyway, because it requires us to identify an assumption that guarantees that the conclusion is true. We know this from the way the question is phrased: "The biologist's conclusion follows logically from the above if which of the following is assumed?"

This is the argument:
Premise: The broader a species' range, the more likely that species is to survive the extinction of populations in a few areas.
Intermediate Conclusion: Species with broad geographic ranges probably tend to endure longer than species with narrow ranges.
Main Conclusion: Therefore, the proportion of species with broad ranges tends to gradually increase with time.

Option C is 'If a population of a species in a particular area dies out, that species generally does not repopulate that area.'

If C is true, does it guarantee that the proportion of species with broad ranges will increase? No, it does not.

Why not? Because it depends on the condition 'if a population of a species in a particular area dies out'. If no population dies out in any area, then the general rule in Option C is irrelevant. There will be no reason for the proportion of broad range species to increase. So the conclusion is not necessarily true.

Option D, on the other hand, does guarantee that the conclusion is true.

Option D says that 'If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time.'

We know from the argument that 'Species with broad geographic ranges probably tend to endure longer than species with narrow ranges.' Then, as per Option D, we arrive at the conclusion that the proportion of species with broad ranges will increase over time -- and that is the conclusion of the argument.

Hope this helped!

Digression on Necessary vs Sufficient Assumptions
Most GMAT questions require us to identify an assumption 'on which the assumption depends'. This assumption is necessary for the conclusion to be true, but it may not guarantee that the conclusion is true. The reason is that the argument could depend on multiple assumptions, and ALL the assumptions must be true for the conclusion to be true. One assumption being true does not guarantee the truth of the conclusion.
A sufficient assumption guarantees that the conclusion is true.

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In general there are three types of question which involve analyzing "Must Be True" components in an argument:

1. Inference Questions - in which we need to find a Conclusion that MUST be true, given the premises;

2. Necessary Assumption Questions - in which we need to find an Assumption that MUST be true, given the premises and the conclusion;

3. Sufficient Assumption Questions - in which we need to find an Assumption, which will render the conclusion to be 'MUST BE TRUE' given the premises and the conclusion.

Note that the first and second type of questions above are quite common on the GMAT, but the third type - an example of which is this question - is quite rare.

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The biologist's conclusion follows logically from the above if the following assumption is made:

D. If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time.

This assumption establishes a general principle that species with characteristics that promote their long-term survival will gradually increase in proportion over time. The biologist's argument is based on the premise that species with broad geographic ranges are more likely to survive population extinctions in specific areas. If we assume that any characteristic promoting longer endurance leads to an increase in the proportion of species with that characteristic over time, it supports the biologist's conclusion that the proportion of species with broad ranges tends to gradually increase.

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shridhar786

conclusion of the argument-- it is likely that the proportion of species with broad ranges tends to gradually increase with time.

the author reached the conclusion by saying that
-Species with broad geographic ranges probably tend to endure longer than species with narrow ranges.
-The broader a species' range, the more likely that species is to survive the extinction of populations in a few areas

the assumption is the unstated premise that needs to be true for the conclusion to be true
--negating this destroys the conclusion

A. There are now more species with broad geographic ranges than with narrow geographic ranges.
--negating this does not shatter our conclusion
--so, this cant be the assumption of the argument (incorrect)

B. Most species can survive extinctions of populations in a few areas as long as the species' geographic range is not very narrow.
---this answer choice says most species can survive as long as the range is not very narrow but this answer choice does not help our conclusion in any way
---this answer choice does not bridge the gap between our premise and conclusion
---negating this does not shatter our conclusion hence, this is not the assumption (incorrect)

C. If a population of a species in a particular area dies out, that species generally does not repopulate that area.

-- the assumption is something that needs to be true for the conclusion to hold but this answer choice does not reinforce our conclusion in any way
--negating this does not shatter our conclusion--this answer choice is irrelevant (incorrect)

D. If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time.

--this is the correct answer choice it bridges the gap between premise and conclusion
--negating this---If a characteristic tends to help species endure longer, then the proportion of that species does not increase with time---then we cant say that the proportion of species with broad ranges tends to gradually increase with time and our conclusion falls apart (correct)

E. Any characteristic that makes a species tend to endure longer will make it easier for that species to survive the extinction of populations in a few areas.

--this answer choice is too extreme
--this cant be the assumption
--Any characteristic that makes a species tend to endure longer will make it easier for that species to survive the extinction (incorrect)

Thanks for the explanation and request you to clarify a doubt with regards to the explanation for choice C. If we negate choice C, it says that once a species in a certain area is extinct, the species generally repopulates that area.
This means that if a poplulation with narrow range goes extinct in an area then it repopulates that area and also that if a population with a broad range goes extinct then it repopulates that area. Therefore, the proportion of species with broad range would not grow over time.

I was thinking of this and had hence selected option C. Would appreciate your comments and also request expert inputs.

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This is a tricky one.

Conc: Therefore, it is likely that the proportion of species with broad ranges tends to gradually increase with time.
The question, we'd like to answer is "Would the proportion of species with broad range increase with time?"

A. There are now more species with broad geographic ranges than with narrow geographic ranges - Are there more species with broad range than with narrow range? This seems to answer a different conclusion. Drop

B. Most species can survive extinctions of populations in a few areas as long as the species' geographic range is not very narrow. The option can be paraphrased as follows: Most species with non-narrow range can survive local extinctions. Alright what's the logical implication here? Well, let's say most (70%) of the non-narrow species can survive local extinctions. That's good to hear. However, what does that tell us about the survival characteristics of narrow range species? Nothing. Even if most of the non-narrow species does/doesn't survive local extinctions, as long as the number of such species surviving extinctions is greater than those with narrow range, the conclusion would be true. Therefore, what this option says is not necessary for the conclusion to be true. One good way to drop this option, is to identify that this talks/provides a threshold, i.e., most (could be any number) for survival for a particular group of species. However, the conclusion focuses on the proportion of one type of species vs another. Drop

C. If a population of a species in a particular area dies out, that species generally does not repopulate that area. Can the same species that went locally extinct, re-populate the area (let's say if re-introduced naturally or through human intervention)? Whether the species can or can't repopulate that area or repopulates a similar but geographically separate location, doesn't tell us the required information, i.e., the proportion of broad vs narrow range species. Drop

D. If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time. Sounds like trap doesn't it. Given the reasoning required to eliminate B. This probably is the right answer. But let's check. Negate - "If a characteristic tends to help species endure longer, then the proportion of species with that characteristic doesn't increase gradually with time". This hypothesis seems to be in the opposite direction of the conclusion stated, i.e., weakens the option. Keep

E. Any characteristic that makes a species tend to endure longer will make it easier for that species to survive the extinction of populations in a few areas. Sounds true, based on the passage. However, we want to substantiate whether species with broad range tend to endure longer than narrow range. Does this assumption about "any characteristic" transfer over to "broad range"? Not necessarily, there can be some characteristics, which might help a species endure longer, i.e., reduced dependence on food or water or ability withstand extreme temperatures, but not allow it to survive a local extinctions which need not be related to any of these, i.e., loss of habitat, increased predator presence, etc. Drop

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STEP #1 -> IDENTIFY THE QUESTION TYPE

Let us read the question stem to identify the question type.

Quote:

The biologist's conclusion follows logically from the above if which of the following is assumed?

The stem indicates an assumption question.

STEP #2 -> X-RAY THE ARGUMENT

In an assumption question, it is a must to x-ray the argument and deconstruct it. Let us therefore read the argument first and deconstruct it by figuring out the conclusion and the premise(s) soon after.

Quote:

CONCLUSION: It is likely that the proportion of species with broad ranges tends to gradually increase with time.
PREMISES: -> Species with broad geographic ranges (species that live in many different places around the world) probably tend to endure longer than species with narrow ranges (species that are found in limited areas).
-> The broader a species' range, the more likely that species is to survive the extinction of populations in a few areas.

STEP #3 -> FRAME A SHADOW ANSWER

To frame a shadow answer, we need to know what the correct answer should do. In an assumption question, the correct answer must bridge the gap between the conclusion and the premise. In other words, we should be able to figure out what is there in the conclusion that is missing from the premise. In this argument, the conclusion speaks of the proportion of species with broad ranges tending to gradually increase with time; however, the first premise makes a comparison between species with broad ranges and those with narrow ranges in terms of tending to endure, with the author stating that the former probably tend to endure longer than the latter. The second premise indicates that the broader a species' range, the more likely the species is to survive the extinction of populations in a few areas. We can thus simplify the argument in the format below.

Species with broad range -> Tend to endure longer -> More likely to survive extinction in a few areas -> Proportion tends to gradually increase with time

SHADOW ANSWER: An option that links the phrases 'tend to endure longer' and 'proportion tends to gradually increase with time', thereby bridging the gap between the conclusion and the premise.

Let us also keep in mind that for the conclusion to be true, the assumption must be true.

STEP #4 -> ELIMINATE INCORRECT OPTIONS

Options that do not match the shadow answer can be eliminated.

A. There are now more species with broad geographic ranges than with narrow geographic ranges. | NOT A MATCH | First, this option does not link the phrases we are looking to link. Second, even if there are not more species now with broad geographic ranges than with narrow geographic ranges (negating the option), the conclusion that it is likely that the proportion of species with broad ranges tends to gradually increase with time remains on solid ground. Since what is stated in this option does not need to be true for the conclusion to be true, we can safely eliminate this option. | ELIMINATE

B. Most species can survive extinctions of populations in a few areas as long as the species' geographic range is not very narrow. | NOT A MATCH | The phrases 'most species' and 'not very narrow' make this option out of scope, meaning even if what is stated in this option does not happen to be an assumption, the argument will not be impacted in any way. In other words, even if most species cannot survive extinctions of populations in a few areas despite the species' geographic range not being very narrow (negating the option), we have a reason to logically believe that there could be some species with a very narrow range in those areas that may still survive the extinctions of populations. Or there could be many species with a narrow range that might survive the extinctions of populations. The conclusion, nonetheless, discusses species with broad ranges, and more importantly, this option does not link the phrases we are looking to link. | ELIMINATE

C. If a population of a species in a particular area dies out, that species generally does not repopulate that area. | NOT A MATCH | The argument does not concern itself with anything concerning repopulating. | ELIMINATE

D. If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time. | MATCHES THE SHADOW ANSWER | Based on the shadow answer we have, this is our best bet. Also, if a characteristic (in this case having a broad geographic range) actually tends to help species endure longer but if the proportion of species with that characteristic does not tend to gradually increase with time, the argument will fall flat. Clearly, what is stated in this option must be true for the conclusion to be true. | KEEP

E. Any characteristic that makes a species tend to endure longer will make it easier for that species to survive the extinction of populations in a few areas. | NOT A MATCH | The argument does not concern itself with whether having a broad geographic range makes it easier or difficult to survive the extinction of populations. | ELIMINATE

Hence, (D) is the correct answer.

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Biologist: Species with broad geographic ranges probably tend to endure longer than species with narrow ranges. The broader a species' range, the more likely that species is to survive the extinction of populations in a few areas. Therefore, it is likely that the proportion of species with broad ranges tends to gradually increase with time.

The biologist's conclusion follows logically from the above if which of the following is assumed?

Please check if my reasoning for the above question is correct?
The conclusion says that proportion of species with broad ranges tends to gradually increase with time - this can happen in two ways - either the broad ranges are reproducing exponentially, and the narrow ranges are not able to contribute much since they are not able to endure longer. Or the total of broad ranges is greater than narrow ranges, hence the proportion of broad ranges is greater.
Ex Initially - Broad- 100 Narrow - 100 (50% each)
Then, broad ranges increase to 200 (61%) and Narrow increases to 125- this proves that broad ranges proportion increases with time OR broad ranges remain same - 100 (57%) and Narrow reduces to 75 from 100.

A. There are now more species with broad geographic ranges than with narrow geographic ranges.
B. Most species can survive extinctions of populations in a few areas as long as the species' geographic range is not very narrow.
C. If a population of a species in a particular area dies out, that species generally does not repopulate that area.
They can repopulate the area and still the proportion of species of broad ranges be higher. Hence, this doesnt look like a necessary assumption

D. If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time.

This looks correct and is according with the examples used above.

E. Any characteristic that makes a species tend to endure longer will make it easier for that species to survive the extinction of populations in a few areas.

GMATNinja - Is the understanding above correct? karishma

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Hi GMATNinja IanStewart KarishmaB

Can we simply reject option C on the basis of reasoning that we don't know whether population of species with narrow range is going to die out or not, hence, conditional statement won't be fulfilled, in this case, we cannot logically comment that the proportion of species with broad ranges tend to increase with time because we don't have enough info with us even after learning option C.

Please let me know if this is also the correct way to think and reject C.

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Hi, for C) C. If a population of a species in a particular area dies out, that species generally does not repopulate that area.

I am not sure I follow your reasoning. It is not that we don't know, but that we don't care. This statement has no impact on the argument. All you need to say in this case is that C) is irrelevant. You can just eliminate it on the basis of being not helpful. It does not matter at all if it populates or does not populate when it comes to the argument and assumption we are looking for.

I recommend identifying the assumption after reading the passage (before you read the answer choices) and then choosing the one that matches your answer rather than pecking through multiple answer choices and trying to reason how to eliminate them. It is usually harder and less accurate doing it through the process of elimination.

E.g. my self-identified assumption after reading the passage was: "If species can survive, they will multiply over time". Now, it was was not exactly D but D is just the more elaborate version and it fit the description a lot better than others, though I had to think about it for a sec. However, it took me less than 1 min to get the correct answer. You just can't get this fast with evaluation of every single answer choice. (The evaluation method makes reasonable sense to use but only as the second choice in your arsenal. If you are looking for your keys in your house, would you search every room and every shelf at first or would you just check your coat pockets and your desk first?)

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Kindly explain why option E wrong

GMATNinja

Argp

AjiteshArun GMATNinja nightblade354

Can you explain why option C is incorrect?

Negating C - If a population of a species in a particular area dies out, that species generally do repopulate that area. This would mean that the species that go extinct in small areas are repopulated again i.e. they are back to their original numbers. Then, the proportion of broad range species will not increase eventually. So, conclusion will not hold true, and hence C is an assumption.

As we discussed in our previous post, the question is not asking for an assumption that is made by the argument. Rather, it’s asking for an assumption that, if made, ensures the conclusion follows logically. In other words, it’s not asking for a necessary assumption — something that must be true in order for the conclusion to be properly drawn. Instead, it’s asking for a sufficient assumption — something that, if true, means the conclusion MUST be true.

The negation technique only applies to necessary assumptions. But even there, we generally do not recommend it because it has its limitations as we discuss in this post.

With all that in mind, here’s (C):

Quote:

C. If a population of a species in a particular area dies out, that species generally does not repopulate that area.

(C) tells us that when a population in a particular area dies out, the species generally does not repopulate that area.

Does that mean that it MUST be true that “it is likely that the proportion of species with broad ranges tends to gradually increase with time”? Not necessarily.

First, (C) only says that species generally don’t repopulate the area. Also, it’s possible that while (C) is true, the range occupied by a particular species is only a minor determinant in the ability of that species to survive over time. For both of those reasons, while (C) may strengthen the argument, it is not a SUFFICIENT assumption. Eliminate (C).

I hope that helps!