Over the last 150 years, large stretches of salmon habitat have been e

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.

Hi GMATNinja
I got the above question wrong and while articulating my confusion here , I ended up with a better reasoning.(As you have mentioned this in one of your previous posts here)

Here is my reasoning :

The author says INCREASED straying could lower the fitness of the subsequent generations(presumably in the foreign habitat.

So Does that mean the author thinks "Normal Straying increases/benefits the local fitness?"

If so, then C would be the answer.

Please tell me whether my reasoning is correct. Or please add on to it if my should have been better

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.

I am trying to understand the answer to this question and i read an explanation by mikemcgarry in a previous post. So as per my understanding "lower the overall fitness of subsequent generations" this is caused by straying of salmon from damaged areas to more pristine areas(as mentioned in the last paragraph) and high rates of straying can dilute local adaptations.(as mentioned in second paragraph) Hence we can infer that salmonids in some streams benefit from particular local adaptations. Can someone please confirm if my thinking is correct?

VeritasKarishma GMATNinja VeritasPrepHailey generis

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

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."


Some salmon do not return to their natal stream. They stray to nearby streams. Low levels of straying are crucial since they provide a mechanism to repopulate if the fish disappear in the nearby streams.

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
(D) an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
So if the old population of some nearby stream disappears because of degraded stream, straying can repopulate if the stream recovers.

Passage map: Overall the passage describes genetic diversity as another unintended consequence of severed environmental disruptions to Salmon Habitats
P1: to describe the extent of the problem and non-obvious
p2: To argue that large scale disturbances in one locale impact another
p3: To state the potential human cause of straying

Q1
E is correct - quite clear from the 1st - 2nd sentence in P1
A - no conventional explanation therefore incorrect
B - no common misunderstanding therefore incorrect
C - no, not to compare, but to argue that humans can have the same effect
D - no, the argument routinely refers to one environmental effect and humans as one potential cause of this effect

Q2
Inference -refer back and infer.
We are told that some salmon stray to other streams and that this straying is crucial as it provides a source of novel genes and a way by which a location, presumably the disturbed one, can be repopulated if the fish in that disturbed population disappear.

We are then asked to infer what straying provides:
A - it does not state anywhere that pristine streams become polluted by straying populations -incorrect
B - no, we are in fact told the opposite - that dilution occurs.
C - no. We aren't told anything on the population numerical impact
D - Yes. As we are told, VERY INDIRECTLY, "a location can be repopulated".
E - No. We have no basis for this statement.

Q3
Detail - refer back.
We are told of the impact on human activity in the first and last paras. The obvious impact is the numerical effect: decrease in population of polluted environments.
A is incorrect- we are only told that the populace decreases in polluted rivers, nothing of an "increase" in "previously polluted" streams
B is correct - this statement - " the decline in the number of salmon in SOME rivers" (the polluted ones) is the stated effect
C is incorrect because we aren't given a quantitative link between straying and human impact
D is incorrect because we are in fact told the opposite in the last para - "substantial gene flow"
E is incorrect because we are told that humans' increase the vulnerability of salmons by increasing the likelihood of their populace straying

Q5
Which of the following must be true in order for the argument to be true? That's the question.
The argument: A dramatic increase in straying from damaged streams to more pristine streams increases gene flow, lowering the fitness of subsequent generations.

A - This is not required for the argument to be true because humans could SIMPLY cause straying e.g. 1-2% straying rate and the fitness of subsequent generations of the salmon impacted could still be lowered
B - Again, this quantity is not necessarily assumed in arguing that the overall fitness decreases
C must be true because if it weren't true then it wouldn't matter if salmons inter-bred. The argument is that "fitness" (or how good something is) essentially decreases, so if decreasing is bad, what must be true? Keeping genes pure must be true.
D is incorrect - NO - humans are PART of the cause, not ALL of the cause
E is incorrect - no this is the opposite. What's inferred by E is that some of the straying salmon DONT decrease the fitness.


Q6
The potential for humans to impact straying is discussed in P3 by the statement - "although NO ONE has QUANTIFIED changes in the rate of straying as a result of...humans" "there is no reason to believe that the effect would be qualitatively different".

A is not mentioned or supported
B is incorrect because no such studies exist
C is mentioned in the passage, but it is not mentioned in support of the argument that humans increase or cause straying rates. For this reason, C is incorrect.
D is incorrect. Firstly it is the weakness in the view that humans ONLY decrease populations - the whole premise of the passage is that HUMANS DO OTHER THINGS i.e. "increase Straying". Second, "extinction" generally isn't supported. Third, destruction actually INCREASES genetic diversity (mixing).
E is incorrect - The way it is worded can be off-putting. But E essentially states that the "absence" of any "reason (evidence)" does not negate the argument that humans cause straying.

Sorry for not replying sooner, Manas1212! I think you are both on the right track here...

The passage specifically tells us 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."

But the very next sentence says, "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." According to the author, high rates of straying are bad because they dilute any local adaptations that are present. Okay, that sounds reasonable enough... but what if those local adaptations do not actually benefit the salmonids in any way?

For example, maybe a local population has developed a certain color in response to certain characteristics of the local water. If that change in color doesn't actually help the salmon at all, then it would be an example of a local adaptation that does not actually benefit the salmonids.

But if salmonids in some streams DO benefit from particular local adaptions, then passing those traits to future generations would obviously improve the overall fitness of future generations. Now if a bunch of non-local salmonids come in and diluate those adaptations, a lower percentage of the next generation will have that beneficial adaptation. That would of course lower the overall fitness of future generations.

It might seem a bit ridiculous at first, but unless we assume that salmonids in some streams benefit from particular local adaptions, then we can't argue that increased straying would be harmful to the fitness of future generations. That's why (C) is correct!


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:



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.




particular adaptations ( population of fish that were strayed ) can be survived withuout diltion ( without effecting any danger to local habitat)


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

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.



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




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%)


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

For question 2: and 5th
COnfusion:
5th: what if salmonids are not LOcals, how can they benefit. C
A : indirectly says : 40% + condition
https://gmatclub.com/forum/over-the-las ... l#p2622667

for 2nd: after fish disappear, it will repopulate in new stream , it doesn't matter old stream stays polluted or gets cleaned
https://gmatclub.com/forum/over-the-las ... l#p2622625

please help on this

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.

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.

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 ：

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

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.

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!

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

Question 2

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!