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KittyDoodles
Joined: 21 Jan 2020
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17 Jul 2022, 10:22
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AlexTheTrainer
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
29 Jul 2022, 07:32
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KittyDoodles
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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09 Aug 2024, 04:03
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STEP #1 -> IDENTIFY THE QUESTION TYPE
Let us read the question stem to identify the question type.
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.
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 timeSHADOW 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.
Let us read the question stem to identify the question type.
Quote:
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
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.
