Biologist: Species with broad geographic ranges probably tend to endur

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)

Official Explanation

Argument Construction

This question asks us to find an assumption that allows the biologist's conclusion to be logically drawn if made in conjunction with the premises of the biologist's argument.

The biologist claims that species with broader ranges are more likely to survive the extinction of populations in certain areas than are species with narrow ranges. The biologist concludes that over time the proportion of species with broader ranges will probably increase.

Note that the argument as it stands is not logically valid: it is possible that if species that now have broad ranges survive the extinction of populations with narrow ranges, the proportion of species with broad ranges could still decline. That is, decimation of populations in certain areas may in fact cause the ranges of species that now have broad ranges to shrink in size, thereby becoming narrow ranges.

Were this to happen at a faster pace than the extinction of species that currently have narrow ranges, the proportion of species with broad ranges would decline rather than increase. The correct answer to this question must rule out this possibility.

A. This choice does not rule out the possibility that the proportion of species that have broad ranges would decline.

B. This choice does not rule out the possibility that the proportion of species that have broad ranges would decline.

C. This choice does not rule out the possibility that the proportion of species that have broad ranges would decline. In fact, this assumption helps to support the claim that as certain populations of a species that once had a broad range die out, that species' range could narrow.

D. Correct. This assumption rules out the possibility described above; furthermore, it rules out any other possibility that allows the biologist's conclusion to be false even if the premises were true.

E. This choice does not rule out the possibility that the proportion of species that have broad ranges would decline.

The correct answer is D.

GMATNinja specifically - How is D correct? It's a rehash of the exact reasoning of the conclusion, not an assumption to arrive at the conclusion. The way to arrive at their conclusion is to assume that only species with broad ranges or characteristics that can help them endure will still exist after extinctions in a few populations whereas species of narrow ranges cannot do such, which is what B says. That is the only way that the ratio of broad range to narrow range species can increase, because those species with narrow ranges, naturally, have not survived as many 'extinctions.'

Broader range -> more endurance & chances to survive ext.
Assumption: % of species w/ broad ranges will increase


Not necessarily. Out.

‘Most’ – extreme. Out.

Irrelevant. Out

So this one is a bit more ‘safer’ since it specifically focuses on species with that characteristic, described in the premise. Seems ok.

‘Any’ – extreme. Out.

A causes B
B causes C
Therefore A causes D.

Choice D fills the missing link between A and D.
So, Choice D is the winner.

Thanks VeritasKarishma for teaching us this technique.
God bless you.

Regards
Basim

Posted from my mobile device

We need the conclusion to follow logically. The conclusion must follow if we include one of the options.

Premises:
Species with broad 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.

Conclusion: it is likely that the proportion of species with broad ranges tends to gradually increase with time.

The premises tell us that broad range leads to longer survival. There is no mention of what proportion of all species do they make. If I want my conclusion to follow, I must look for something that talks about how the proportion of species changes over time.

A. There are now more species with broad geographic ranges than with narrow geographic ranges.

This tells us that currently, more than 50% species are those with broad geographic ranges. But this does not help us establish that the proportion of broad ranges species increases with time. What if some time back 90% species were with broad ranges but now only 60% are there?

B. Most species can survive extinctions of populations in a few areas as long as the species' geographic range is not very narrow.

Doesn't talk about changing proportion.

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

Doesn't talk about changing proportion.

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. If a characteristic helps in longer endurance, then more species acquire that characteristic. It tells us that the proportion of species with the longer endurance characteristic increases with time. Then we can say that over time a higher proportion of the species will have broader ranges (they will be able to endure longer)

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.

Doesn't talk about changing proportion.

Answer (D)

My understanding of the passage was that,
Since the smaller range species become extinct more likely and the larger range ones endure such extinctions more likely then over a period of time there will be more Longer range species and that is pointed out in the conclusion.
So, I thought we may need to counter any other possibilities, and for me Option C seemed to rule out a possible option.
Also, I thought Option D is re-iterating what is already there in the argument, and I thought assumptions generally don't do that. Here I may have thought Option D is already implied by the premises and hence ruled it out as an assumption. But literally that is what such questions test us. What is that one assumption we take for granted.

Now, after reading all the explanations by experts I understand why D is correct. But I need some help in redefining my thought process in such questions.
Experts, do you think me inferring the smaller range species become extinct more likely and the larger range ones endure such extinctions more likely, then over a period of time there will be more Longer range species and that is pointed out in the conclusion. is a bit out of scope and I am bringing in more understanding and deviating out of scope of the argument ?
GMATNinja VeritasKarishma

We are not looking for just one assumption. We are looking for a "sufficient" assumption. This means that the conclusion should logically follow if that assumption is also included with the premises.

e.g.
Premises:
A is B.
B is C.

Conclusion: A is D.

What assumption will make the conclusion follow logically? How about 'C is D'?

Premises:
A is B.
B is C.
C is D.

Conclusion: A is D.

Makes sense now.

Now for what confused you - Broad range species endure longer. Narrow range species do not endure as long.

Conclusion: Over time, proportion of broader range species will increase.

What can we say to conclude that proportion of broad range species will increase over time?

Option (D) tells us that if a specie has characteristics that lead to longer endurance, the proportion of that specie increases over time. This means proportion of species that have broad range will increase over time. So our conclusion logically follows now.

Premise:
Species with broad geographic ranges tend to endure longer.
Conclusion:
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?

This is actually a justify question in disguise.
For a conclusion to follow logically, it must be justified by the given information.
Question stem, rephrased:
Which of the following justifies the biologist's conclusion?

D: If a characteristic tends to help species endure longer, then the proportion of species with that characteristic tends to gradually increase with time.
Since it is given as a PREMISE that a broad geographical range is a characteristic that tends to help species endure longer, option D justifies the biologist's conclusion: that the proportion of species with this characteristic -- a broad geographical range -- tends to gradually increase with time.

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):

(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!

This seems to me like a supporter vs defender type assumption. We are generally accustomed to supporter assumptions because they fill in the logical gaps / loophole in the argument. But in this case, the defender type assumption i.e. D allows the conclusion to be drawn from the given premises. Am i thinking this right? GMATNinja VeritasKarishma ?

That said, had C omitted the word "generally" it would have been a case of supporter type assumption?

That's irrelevant and I do not subscribe to this terminology for GMAT. Students waste too much time trying to distinguish the type of assumption though GMAT couldn't care less about it. All you need to do is ensure that it is not just necessary but sufficient too to arrive at the conclusion.

dear
GMATNinja, GMATNinjaTwo, VeritasKarishma, AndrewN,GMATRockstar

I realized that GMATninja says if stuck into the passage as evidence, makes it so the conclusion MUST be true., and also VeritasKarishma says "a sufficient assumption ", this hits me harder, because at first I regard it as general assumption during my GMAT study, which means I need to find an assumption that the author must believe the conclusion. not the conclusion must be true.

furthermore, I found the approach is different the assumption question I met during my study.

how can I disguise these two, what the difference to handle when I encounter these two different kind of assumption questions.

thanks in advance.

The best thing to do is to read each question individually, and answer the exact question on the page. If you just react to the word "assumption," (or "strengthen," or "support," or many other examples), then you run the risk of missing important details in the particular question in front of you.

For example, "Which of the following, if true, provides the most additional support to the argument?" is very, very different than "The statements above, if true, provide the most support for which of the following?"

Similarly, the key to understanding what kind of assumption you're looking for is always going to be present in the question on the page.

If the question asks: "Which of the following is an assumption on which the argument depends?", then you're looking for something that MUST be true in order for the author to reach his/her conclusion. In other words, you're looking for an assumption that's necessary to the author's argument -- after all, the argument depends on this assumption! The vast majority of GMAT assumption questions are phrased like this.

If, on the other hand, the question asks: "The biologist's conclusion follows logically from the above if which of the following is assumed?", then you're looking for something much different. Now you need to find something that means the conclusion MUST follow from the other information in the argument (or, a sufficient assumption).

To be honest, there are very, very few official GMAT questions that ask you to find a sufficient assumption. So, it's really not worth your time to memorize a specific approach to this kind of question.

It is, however, extremely valuable to develop a habit of reading each question exactly as it is written. If you try to take shortcuts and react to just one word in the prompt, then you're likely to waste time answering the wrong question.

I hope that helps!

An effective approach to high level Assumption questions entails more than merely identifying the conclusion and evidence (premises) of the argument; it also entails identifying the relevant and irrelevant parts of the argument. Why? Because irrelevant parts of an argument should be ignored.

Of the three sentences in the argument above, one of them is irrelevant. How do we know this? Because it supports the EVIDENCE of the argument and not the CONCLUSION. Why is this irrelevant? Because evidence is assumed to be true - any support for evidence must by definition be irrelevant. No one cares why evidence is true because it’s assumed to be true.

But how do we know that a specific sentence in the argument supports the evidence but not the conclusion? By asking: WHY?

The conclusion is the third sentence. WHY? At first glance, it would appear that both the first and second sentence answer this question. However...

Ask: WHY is the first sentence true? Because of the second sentence.

Ask: WHY is the third sentence true? Certainly because of the first sentence, but NOT because of the third sentence.

WHY is it likely that the proportion of species with broad ranges tends to gradually increase with time? Because species with broad geographic ranges probably tend to endure longer than species with narrow ranges.

WHY do species with broad geographic ranges probably tend to endure longer than species with narrow ranges? Because the broader a species’ range, the more likely that species is survives the extinction of populations in a few areas.

Since the second sentence supports evidence from the first sentence already assumed to be true, the second sentence must be irrelevant and should be ignored.

Why should irrelevant information be ignored for high level Assumption questions? Because the correct answer to these questions most often contain what I call Unique Elements: ideas/concepts found within the RELEVANT parts of the stimulus that are discussed nowhere else in those RELEVANT parts.

In the argument above, the idea “to survive the extinction of populations” is certainly discussed only once, but is not a Unique Element because its found within an irrelevant part of the argument.

Focusing only on the first and third sentences, the two Unique Elements would be “endure longer” (from the evidence) and “proportion of species... gradually increase with time”.

Answers that discuss “(not) enduring longer”: D, E

Answers that discuss “proportion gradually increasing (or decreasing)”: A, D

The only answer that discusses both Unique Elements: D

Yup...

Posted from my mobile device

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?

more likely to survive the extiction is a small area → more likely to survive?

P : Broader a species' range → more likely to survive extiction in a few areas.
C : Species with broad range tend to endure longer.
C : Proportion of species with broad ranges will gradually increase.

A : Extinction does not occur in global level
A : The endurance level of species with broad geographic range against extiction peril is not weaker than that of species with narrow range.
A : There is no other factor that makes a species with broad geographic range more vulnerable.

A. There are now more species with broad geographic ranges than with narrow geographic ranges.
-> Irrelevant. Current status cannot be the explanation since the possibility that curresnt status was due to other factors cannot be eliminated.

B. Most species can survive extinctions of populations in a few areas as long as the species’ geographic range is not very narrow.
-> Weakens, Given the option B is true, the conclusion stated in the argument can only be true if the species with narrow range is equivalent to species with very narrow range. However, since no information is given to determine such assumption, option B cannot be the answer.

C. If a population of a species in a particular area dies out, that species generally does not repopulate that area.
-> Irrelevant. Option C has to be false for the conclusion to be true.

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. As mentioned in the argument, having broad geographic range tends to help species endure longer, the conclusion stands given the option B is true.

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.
-> Irrelevant. Option E only provides information of surviving the extinction in a few areas but does not provide information in the long term as option D does.