Biologist: Species with broad geographic ranges probably tend to endur

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

egmat what would be appropriate prethinking in this question?

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.

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.

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.

­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 ­