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505-555 (Easy)|   Long Passage|   Science|         
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Over the last 150 years, large stretches of salmon habitat have been e 21 Sep 2020, 00:42
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imSKR
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Can you please explain how in the below question the answer is D???

2. It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which

(A) pristine streams that are near polluted streams become polluted themselves
(B) the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
(C) the number of salmon in pristine habitats decreases relative to the number in polluted streams
(D) an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
(E) the extinction of the salmon populations that spawn in polluted streams is accelerated
Have you checked out this post? If that doesn't help, let us know what you are struggling with (be as specific as possible!).


It is so disheartened that still I could not choose D over B even after reading the explanations above.

Please check where am I wrong:

Quote:
Look at the second paragraph:

"Although the homing instinct of salmon to their natal stream is strong, a fraction of the fish returning from the sea (rarely more than 15 percent) stray and spawn in nearby streams. Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted."

Low levels of straying are crucial, since the process ( straying process) provides a source of novel genes and a mechanism ( straying mechanism ) by which a location ( any location that is in new location after fish strayed in nearby stream) can be repopulated should the fish there ( nearby stream) disappear.

This location can not refer to old stream because fish has already strayed from there and if fish has already strayed from old location to new location , it can only repopulate at new location, means at new stream,but not in old stream

Understanding:
Fish stray from degraded REGION to pristine REGION
Occasional stray: Helpful because if fish disappear from natal ,fish can be repopulated
If more than 40% stray:local adaptations maybe diluted

Analogy similar to:
If migrants are allowed in country ( if they are in low ratio), migrants can be saved and doesn’t affect existing population

But if migrants’ number is increased at large, it would be a burden on local populations, local environment and some unique characteristics of local population could be diluted.


Quote:
2. It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which
Quote:
(B) the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
particular adaptations ( population of fish that were strayed ) can be survived withuout diltion ( without effecting any danger to local habitat)

Quote:
(D) an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
Problem: degraded stream ( stream is like a route from degraded REGION to PRISTINE region)
By far what we know fish can repopulate if stray quantity is less otherwise effect local population
But in option D the condition is “stream recover”
Stream recover or doesn’t recover, fish population can still be repopulated.


GMATNinja VeritasKarishma VeritasPrepHailey generis
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The passage says:

Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear.

Your understanding is incorrect. This sentence means that straying is crucial because it provides a mechanism by which the current location can be repopulated (it is already populated so it can be repopulated) should the fish here disappear.
Say today some fish stray to other streams. Say after 1 month, the condition of this stream deteriorates such that all the fish in it die. After a few months, this stream is cleaned up and its waters can support life again. Then the fish that had strayed can be brought back here to repopulate these waters.
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Over the last 150 years, large stretches of salmon habitat have been e 21 Sep 2020, 00:52
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Lost in 5th as well,

COnfusion: Salmonids in C are not mentioned LOCALS, what if they are Foreign, then this option can not be valid. But optionA indirectly refers to straying rate more than 40% . please check below my explanation.

Quote:
5. The author's argument that increased straying can "lower the overall fitness of subsequent generations" (see highlighted text) is based on which of the following assumptions?

The condition for lowering the overall fitness of subsequent generations is that the rates of straying should be more than 40 percent overall.


Quote:
(A) A disturbance of salmonid spawning streams caused by human activity could increase the straying rate of affected salmonid populations as much as the aftermath of the Mount Saint Helens eruption did.

optionA says: disturbing by human activity could increase straying rate by more than 40%.If it is less than 40% then overall fitness may not decrease, so it is necessary to have straying rate over 40% and thus should match the assumption( Option A indirectly refers the staying rate is more than 40%)

Quote:
(C) Salmonids in some streams benefit from particular local adaptions.
Option C doesn’t mention these Salmonids are native or foreign. If these salmons are not local then their benefit to local adaptations doesn’t LOWER the OVERALL fitness of subsequent generations.


Kindly suggest GMATNinja VeritasKarishma VeritasPrepHailey generis
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5. The author's argument that increased straying can "lower the overall fitness of subsequent generations" (see hightlited text) is based on which of the following assumptions?

(A) A disturbance of salmonid spawning streams caused by human activity could increase the straying rate of affected salmonid populations as much as the aftermath of the Mount Saint Helens eruption did.
(B) In the streams in which the straying salmonids spawn, these straying salmonids would amount to no more than 40 percent of the total spawning population
(C) Salmonids in some streams benefit from particular local adaptions.
(D) Nonenvironmental factors have no effect on salmonid straying rates.
(E) At least some of the streams in which straying salmonids would spawn are pristine, affected by neither natural nor artificial disturbances.

Passage tells us:
Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted.

So some fish from stream A stray to stream B. Now stream B fish have some local adaptations. If too many fish from stream A come to stream B, those local adaptations could be lost. This could result in lower the overall fitness of subsequent generations.
So the author is assuming that the local adaptations are beneficial. So (C) is correct.

What could increase the rate of straying is irrelevant. The point is what happens after the rate of straying is increased.
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Over the last 150 years, large stretches of salmon habitat have been e 21 Sep 2020, 21:35
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VeritasKarishma
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It is so disheartened that still I could not choose D over B even after reading the explanations above.

Please check where am I wrong:

Quote:
Look at the second paragraph:

"Although the homing instinct of salmon to their natal stream is strong, a fraction of the fish returning from the sea (rarely more than 15 percent) stray and spawn in nearby streams. Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted."

Low levels of straying are crucial, since the process ( straying process) provides a source of novel genes and a mechanism ( straying mechanism ) by which a location ( any location that is in new location after fish strayed in nearby stream) can be repopulated should the fish there ( nearby stream) disappear.

This location can not refer to old stream because fish has already strayed from there and if fish has already strayed from old location to new location , it can only repopulate at new location, means at new stream,but not in old stream

Understanding:
Fish stray from degraded REGION to pristine REGION
Occasional stray: Helpful because if fish disappear from natal ,fish can be repopulated
If more than 40% stray:local adaptations maybe diluted

Analogy similar to:
If migrants are allowed in country ( if they are in low ratio), migrants can be saved and doesn’t affect existing population

But if migrants’ number is increased at large, it would be a burden on local populations, local environment and some unique characteristics of local population could be diluted.


Quote:
2. It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which
Quote:
(B) the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
particular adaptations ( population of fish that were strayed ) can be survived withuout diltion ( without effecting any danger to local habitat)

Quote:
(D) an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
Problem: degraded stream ( stream is like a route from degraded REGION to PRISTINE region)
By far what we know fish can repopulate if stray quantity is less otherwise effect local population
But in option D the condition is “stream recover”
Stream recover or doesn’t recover, fish population can still be repopulated.


GMATNinja VeritasKarishma VeritasPrepHailey generis

The passage says:

Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear.

Your understanding is incorrect. This sentence means that straying is crucial because it provides a mechanism by which the current location can be repopulated (it is already populated so it can be repopulated) should the fish here disappear.
Say today some fish stray to other streams. Say after 1 month, the condition of this stream deteriorates such that all the fish in it die. After a few months, this stream is cleaned up and its waters can support life again. Then the fish that had strayed can be brought back here to repopulate these waters.
Show more

Hi AndrewN Sir,

Sorry to be peculiar .

I am not clear in practical terms how the fish can be brought back to repopulate these waters? You mean some humans would do that? How can they find which fishes were disappeared?

If it is by fish itself, then once they disappeared why would they come back? Even they come back , they would come back in their natural flow without any objective of repopulating.

SajjadAhmad, bm2201 ：
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Over the last 150 years, large stretches of salmon habitat have been e 22 Sep 2020, 12:52
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Hi AndrewN Sir,

Sorry to be peculiar .

I am not clear in practical terms how the fish can be brought back to repopulate these waters? You mean some humans would do that? How can they find which fishes were disappeared?

If it is by fish itself, then once they disappeared why would they come back? Even they come back , they would come back in their natural flow without any objective of repopulating.

SajjadAhmad, bm2201 ：
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Peculiar, imSKR? I myself am a bit of an odd fish, as the saying goes. Go ahead and be peculiar, or, to paraphrase the lyrics of Jimi Hendrix, wave your freak-flag high. If you meant particular, that is okay, too. A little doubt can spur a new stream of thought, one that may lead to progress. To speak to your questions, I took nothing from the passage to indicate that human intervention is necessary for repopulation efforts, apart from reducing the human activities mentioned in the opening line of the passage. No, it seems to be up to the salmon (or salmonids) themselves to repopulate. Why would they come back? I guess you would have to ask the fish. In all seriousness, the passage outlines how some fish intend, through an innate mechanism, to return to their natal stream but go wayward, swimming up the wrong stream and spawning there instead. In other cases, natural disasters or radical changes to the streams brought about by human activity force the fish to enter different streams. Either way, the fish are just being fish, looking for a place to spawn. A stream that has lost life can be repopulated by either wayward fish or displaced fish. So no, their objective is not to repopulate a stream, but simply to spawn the next generation of salmon(ids).

Does that clarify the issue you were having?

- Andrew
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Over the last 150 years, large stretches of salmon habitat have been e 06 Oct 2021, 08:34
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GMATNinja Please kindly help to explain question5

5. The author's argument that increased straying can "lower the overall fitness of subsequent generations" (see hightlited text) is based on which of the following assumptions?

(A) A disturbance of salmonid spawning streams caused by human activity could increase the straying rate of affected salmonid populations as much as the aftermath of the Mount Saint Helens eruption did.

(C) Salmonids in some streams benefit from particular local adaptions.

I chose the wrong option A, and the reason was that we can infer related information from " there is no reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption."➢ For me, such info means that the negaitve impact of human behaviors is comparable to that of natural causes (volcanic eruption), and both kinds of impact can lead to high straying rate, causing adaptations that are present become diluted, in other words, "lower the overall fitness of subsequent generations".

Could you please kindly help to explain why option A is wrong and option C is correct?
Which part I did wrong?

Thank you so so much for your patience and kindness.
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Over the last 150 years, large stretches of salmon habitat have been e 29 Oct 2021, 11:33
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Well0909
GMATNinja Please kindly help to explain question5

5. The author's argument that increased straying can "lower the overall fitness of subsequent generations" (see hightlited text) is based on which of the following assumptions?

(A) A disturbance of salmonid spawning streams caused by human activity could increase the straying rate of affected salmonid populations as much as the aftermath of the Mount Saint Helens eruption did.

(C) Salmonids in some streams benefit from particular local adaptions.

I chose the wrong option A, and the reason was that we can infer related information from " there is no reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption."➢ For me, such info means that the negaitve impact of human behaviors is comparable to that of natural causes (volcanic eruption), and both kinds of impact can lead to high straying rate, causing adaptations that are present become diluted, in other words, "lower the overall fitness of subsequent generations".

Could you please kindly help to explain why option A is wrong and option C is correct?
Which part I did wrong?

Thank you so so much for your patience and kindness.
Show more
For an in-depth look at why (C) is correct, check out this post. Here are some thoughts on why (A) is wrong.

Keep in mind that an assumption is something ABSOLUTELY REQUIRED for a conclusion to hold. Let's apply that test to (A).

The conclusion is that "such a dramatic increase in straying from damaged areas to more pristine streams results in substantial gene flow, which can in turn lower the overall fitness of subsequent generations." By "such a dramatic increase," they must mean the straying increase of 40% caused by the volcanic eruption. Such a high level of straying, the author concludes, will cause gene flow from damaged streams to more pristine streams, which will in turn lower the fitness of the salmon in the more pristine streams.

Note that the conclusion doesn't ABSOLUTELY REQUIRE that the disturbances caused by human activity will increase the straying rate as much as the eruption, as (A) states. All the conclusion requires is that such a high level of straying will cause the outcomes described. So (A) isn't an assumption on which the argument is based.

Regarding (C) -- how exactly does the author make the leap from "gene flow" to reducing "overall fitness?" Earlier in the passage, the author states that gene flow will cause "local adaptations that are present to become diluted." But why should diluting local adaptations reduce the fitness of salmon? To draw that conclusion, we'd have to ASSUME that local adaptations provide some benefit -- otherwise, diluting them wouldn't reduce fitness.

Since that's exactly what (C) says, it's correct.

I hope that helps!
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Over the last 150 years, large stretches of salmon habitat have been e 05 Dec 2021, 00:21
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Hi GMATNinja,

I have read all the previous posts but am still a bit unsure about the second question.
I got the option (D) by mapping and eliminating all other options, but the wording in the option (D) seems problematic to me. I know that a change of even one single word can greatly change the reasoning in GMAT, so I would like to know whether my doubt makes sense.

The passage clearly says that "Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear," so I know that the failure of some salmon to return to their home streams provides a way by which another streams can be repopulated, if the fish there disappear.

But, the condition in the option (D) is "should the stream recover," which the passage does not mention clearly.

I know that previously a few members or experts, while analyzing this condition, made inferences that the stream should have be restored first so that the new fish that stray and arrive in the stream can survive there.

But, after reading many RC passages regarding biology, I find it hard to believe that such inference could be made with complete certainty. The disappearance of the first group of fish could be attributed to many reasons other than environmental damage, such as an increase in its predators. In this case, the condition in the option (D) "should the stream recover" seems out of place.

Even if we assume that the first group of fish disappeared due to environmental damage, there is still a chance that the second group of fish can survive in the damaged waters, even though the stream has not recovered, as different species of animals have different tolerance for environmental pollution (I presume that this is is a common sense.) In this case, the condition in the option (D) "should the stream recover" seems also out of place.

The second question is an inference question, so we need to find an answer whose information is implied by the passage. I just do not think the condition "should the stream recover" works here.

Hope you can clarify my doubt if you are available. Thank you very much.


VRC000460-02
2. It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which

(A) pristine streams that are near polluted streams become polluted themselves
(B) the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
(C) the number of salmon in pristine habitats decreases relative to the number in polluted streams
(D) an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
(E) the extinction of the salmon populations that spawn in polluted streams is accelerated
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Over the last 150 years, large stretches of salmon habitat have been e 10 Dec 2021, 19:17
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Question 2


GraceSCKao
Hi GMATNinja,

I have read all the previous posts but am still a bit unsure about the second question.
I got the option (D) by mapping and eliminating all other options, but the wording in the option (D) seems problematic to me. I know that a change of even one single word can greatly change the reasoning in GMAT, so I would like to know whether my doubt makes sense.

The passage clearly says that "Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear," so I know that the failure of some salmon to return to their home streams provides a way by which another streams can be repopulated, if the fish there disappear.

But, the condition in the option (D) is "should the stream recover," which the passage does not mention clearly.

I know that previously a few members or experts, while analyzing this condition, made inferences that the stream should have be restored first so that the new fish that stray and arrive in the stream can survive there.

But, after reading many RC passages regarding biology, I find it hard to believe that such inference could be made with complete certainty. The disappearance of the first group of fish could be attributed to many reasons other than environmental damage, such as an increase in its predators. In this case, the condition in the option (D) "should the stream recover" seems out of place.

Even if we assume that the first group of fish disappeared due to environmental damage, there is still a chance that the second group of fish can survive in the damaged waters, even though the stream has not recovered, as different species of animals have different tolerance for environmental pollution (I presume that this is is a common sense.) In this case, the condition in the option (D) "should the stream recover" seems also out of place.

The second question is an inference question, so we need to find an answer whose information is implied by the passage. I just do not think the condition "should the stream recover" works here.

Hope you can clarify my doubt if you are available. Thank you very much.


VRC000460-02
2. It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which

(A) pristine streams that are near polluted streams become polluted themselves
(B) the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
(C) the number of salmon in pristine habitats decreases relative to the number in polluted streams
(D) an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
(E) the extinction of the salmon populations that spawn in polluted streams is accelerated
Show more
You're right that this is an inference question -- in other words, the correct answer choice will NOT be stated directly in the passage. So, you can't use the fact that (D) isn't directly stated in the passage to eliminate (D).

Think about the role of the words "should the stream recover." Here, they LIMIT the circumstances in which an environmentally degraded stream could be repopulated by straying.

The passage tells that low levels of straying mean that a "location can be repopulated should the fish there disappear." There are no limitations placed on why the fish disappeared in the first place, or what happened to the environment after that.

From this, you could make all kinds of inferences -- for example, if the fish disappeared because of over-fishing, then you could infer that straying would allow the location to be repopulated once fishing is limited again in the area.

Equally, you could infer that if the fish disappeared because of environmental degradation, then straying would allow the location to be repopulated once the environment recovers. This is what (D) tells us. Sure, it's just one circumstance out of many possible circumstances. But that limitation is totally fine -- we can still infer (D), so that's the correct answer to question 2.

I hope that helps!
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Over the last 150 years, large stretches of salmon habitat have been e 16 Nov 2022, 23:06
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2. It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which

(A) pristine streams that are near polluted streams become polluted themselves
(B) the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
(C) the number of salmon in pristine habitats decreases relative to the number in polluted streams
(D) an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
(E) the extinction of the salmon populations that spawn in polluted streams is accelerated
Show more

@KarishmaB explanation:

The passage says: Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear.

This sentence means that straying is crucial because it provides a mechanism by which the current location can be repopulated (it is already populated so it can be repopulated) should the fish here disappear.

Say today some fish stray to other streams. Say after 1 month, the condition of this stream deteriorates such that all the fish in it die. After a few months, this stream is cleaned up and its waters can support life again. Then the fish that had strayed can be brought back here to repopulate these waters.[/quote]

Passage tells us: Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted.

So some fish from stream A stray to stream B. Now stream B fish have some local adaptations. If too many fish from stream A come to stream B, those local adaptations could be lost. This could result in lower the overall fitness of subsequent generations."

GMATNINJA explanation:
[b]GMATNinja[/b]
"Think about the role of the words "should the stream recover." Here, they LIMIT the circumstances in which an environmentally degraded stream could be repopulated by straying.

The passage tells that low levels of straying mean that a "location can be repopulated should the fish there disappear." There are no limitations placed on why the fish disappeared in the first place, or what happened to the environment after that.

From this, you could make all kinds of inferences -- for example, if the fish disappeared because of over-fishing, then you could infer that straying would allow the location to be repopulated once fishing is limited again in the area.

Equally, you could infer that if the fish disappeared because of environmental degradation, then straying would allow the location to be repopulated once the environment recovers. This is what (D) tells us. Sure, it's just one circumstance out of many possible circumstances. But that limitation is totally fine -- we can still infer (D), so that's the correct answer to question 2."

Hello GMATNinja and @KrishmaB
From your explanations for the meaning of "Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted.", I understand as follows:

- A location (or environment/ stream/ habitat) had degraded because of some event (for example environmental degradation), so the fish in this location disappeared. But then this location recovered.

- Next, a low rate of new salmon fish stray to this location (instead of returning their natal stream because of this stream was degraded). Because of LOW RATE, this location is recolonized/ repopulated.

I am confused: If the fish in this location disappeared, there is no fish. Therefore, the new salmon fish do not interbreed with any existing fish. (interbreed with whom?) Therefore, low rate or high rate of straying does not matter with dillution in this location. In other words, any rate of straying can help this location to be recolonized.

Please help me to clear my mind.

Thank you very much
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Over the last 150 years, large stretches of salmon habitat have been e 16 Nov 2022, 23:29
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KarishmaB


2. It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which

(A) pristine streams that are near polluted streams become polluted themselves
(B) the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
(C) the number of salmon in pristine habitats decreases relative to the number in polluted streams
(D) an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
(E) the extinction of the salmon populations that spawn in polluted streams is accelerated

@KarishmaB explanation:

The passage says: Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear.

This sentence means that straying is crucial because it provides a mechanism by which the current location can be repopulated (it is already populated so it can be repopulated) should the fish here disappear.

Say today some fish stray to other streams. Say after 1 month, the condition of this stream deteriorates such that all the fish in it die. After a few months, this stream is cleaned up and its waters can support life again. Then the fish that had strayed can be brought back here to repopulate these waters.

Passage tells us: Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted.

So some fish from stream A stray to stream B. Now stream B fish have some local adaptations. If too many fish from stream A come to stream B, those local adaptations could be lost. This could result in lower the overall fitness of subsequent generations."

GMATNINJA explanation:
[b]GMATNinja[/b]
"Think about the role of the words "should the stream recover." Here, they LIMIT the circumstances in which an environmentally degraded stream could be repopulated by straying.

The passage tells that low levels of straying mean that a "location can be repopulated should the fish there disappear." There are no limitations placed on why the fish disappeared in the first place, or what happened to the environment after that.

From this, you could make all kinds of inferences -- for example, if the fish disappeared because of over-fishing, then you could infer that straying would allow the location to be repopulated once fishing is limited again in the area.

Equally, you could infer that if the fish disappeared because of environmental degradation, then straying would allow the location to be repopulated once the environment recovers. This is what (D) tells us. Sure, it's just one circumstance out of many possible circumstances. But that limitation is totally fine -- we can still infer (D), so that's the correct answer to question 2."

Hello GMATNinja and @KrishmaB
From your explanations for the meaning of "Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted.", I understand as follows:

- A location (or environment/ stream/ habitat) had degraded because of some event (for example environmental degradation), so the fish in this location disappeared. But then this location recovered.

- Next, a low rate of new salmon fish stray to this location (instead of returning their natal stream because of this stream was degraded). Because of LOW RATE, this location is recolonized/ repopulated.

I am confused: If the fish in this location disappeared, there is no fish. Therefore, the new salmon fish do not interbreed with any existing fish. (interbreed with whom?) Therefore, low rate or high rate of straying does not matter with dillution in this location. In other words, any rate of straying can help this location to be recolonized.

Please help me to clear my mind.

Thank you very much
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We are not talking about the same stream here.
Consider three different but neighbouring streams.
Stream A which is native to our fishA and is doing well.
Stream B gets overly polluted and all our fish pop dies there. Subsequently, stream B is cleaned.
Stream C is surviving but has high levels of mercury. The fish there have adapted to these high levels.

Now when fishA return from the ocean back to stream A, there may be some low level of straying into stream B. This is good for stream B because it will have fish again now. This explains "Low levels of straying are crucial, since the process provides a mechanism by which a location can be repopulated should the fish there disappear"

What if while fishA are returning, there is high level of straying to stream C? They breed with fishC (who are immune to high levels of mercury ) and produce fish that are not immune to these mercury levels. Most fish in stream C could then die. This explains "Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted."
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Over the last 150 years, large stretches of salmon habitat have been e 04 Oct 2025, 05:15
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Hi GMATNinja KarishmaB GMATNinjaTwo,

I have read all the earlier posts, but I still have doubt in Question 4

Quote:
4. [Not An official Question]The author mentions the “aftermath of the Mount Saint Helens eruption” most likely in order to

(A) provide an example of the process that allows the repopulation of rivers whose indigenous salmon population has become extinct
(B) indicate the extent to which the disturbance of salmon habitat by human activity in one stream might affect the genetic structure of salmon populations elsewhere
(C) provide a standard of comparison against which the impact of human activity on the gene flow among salmon populations should be measured
(D) show how salmons’ homing instinct can be impaired as a result of severe environmental degradation of their natal streams
(E) show why straying rates in salmon populations remain generally low except when spawning streams suffer severe environmental disturbance
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I can see why option (C) is correct but I am unable to see why (B) and (E) are incorrect.

Please let me know what I am missing here.
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Over the last 150 years, large stretches of salmon habitat have been e 07 Oct 2025, 01:21
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Hi GMATNinja KarishmaB GMATNinjaTwo,

I have read all the earlier posts, but I still have doubt in Question 4



I can see why option (C) is correct but I am unable to see why (B) and (E) are incorrect.

Please let me know what I am missing here.
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piyushnagre99 If I may help here:

Why (B) is incorrect:

The critical issue with (B) is that the Mount Saint Helens example is about a NATURAL disaster (volcanic eruption), not human activity. Read option (B) carefully: "indicate the extent to which the disturbance of salmon habitat by human activity..."

The Mount Saint Helens example doesn't directly indicate anything about human activity—it's about a volcanic eruption. The passage USES this natural disaster example to make inferences about what MIGHT happen with human disturbances (paragraph \(3\)), but the example itself doesn't "indicate" the extent of human impact. It serves as an analogy or comparison point, not a direct indication of human activity effects.

Why (E) is incorrect:

This is trickier. Option (E) accurately describes WHAT the example demonstrates (that severe disturbance increases straying rates from \(16\%\) to over \(40\%\)). However, the question asks WHY the author MENTIONS it—what's its rhetorical purpose in the passage?

The key is in paragraph \(3\): "Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, there is no reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption."

The author mentions Mount Saint Helens not just to show that disturbances increase straying (that's what it demonstrates), but to establish a COMPARISON or STANDARD for understanding human-caused disturbances. The purpose is comparative—it's the "standard of comparison" mentioned in option (C).

Key Distinction:
  • Option (E): Describes the content of what the example shows
  • Option (C): Describes the rhetorical purpose of why the author includes it

Trap Answer Recognition for "Author Mentions X in Order To" Questions :
  • If an option describes WHAT an example shows but not WHY it's mentioned \(→\) likely wrong
  • If an option mischaracterizes the example's subject matter (like calling a natural disaster "human activity") \(→\) definitely wrong

I hope this clears up your confusion! Feel free to ask any follow-up questions you may have on this one.
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