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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

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."

Dear @KrishmaB

Regarding Stream B: all fish died --> high or low level of straying does not matter, and new fish will help stream B recolonize. If it is, low level of straying is not crucial for stream B.

Regarding Stream C: high level of straying will cause high level of dillution --> decrease local adaptation of native fish --> decrease overall fitness subsequence generations. OK. If so, low level of straying is crucial for stream C because low level of straying will not cause HIGH DEGREE REDUCTION of local adaptation of native fish, but NOT because low level of straying help stream C recolonize.

Is there something wrong in my understand?

Trung
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[quote="chesstitans"]6. Which of the following does the author mention as support for the view that environmental disturbances caused by human activity could increase straying rates?

Hey chesstitans here is my attempt at why E is the correct answer or Qn 6. Pls note that I am in no way an expert, just an average joe preparing for the GMAT as well.

In the passage the example he presents support for the fact that straying rates increase as a result of environmental disturbances was the one of Mount Saint Helens.
"Straying rates remain relatively low when environmental conditions are stable, but can increase dramatically when streams suffer severe disturbance. The 1980 volcanic eruption of Mount Saint Helens, for example, sent mud and debris into several tributaries of the Columbia River. For the next couple of years, steelhead trout (a species included among the salmonids) returning from the sea to spawn were forced to find alternative streams. As a consequence, their rates of straying, initially 16 percent, rose to more than 40 percent overall."

Later on in the passage, he compares human disturbances to that of the disturbance caused by Saint Helens eruption and mentions that there is no reason to believe that the straying rate effects would be different for either of them.
"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"

This supports option E :
(E) The absence of any reason for believing that disturbances brought about by human activitiy would differ in their effects from comparable disturbances brought about by natural causes
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GMATNinja KarishmaB

Why the answer of question 2 is not choice E? As per the question, we know that the salmon population in the polluted stream has decreased by a bit as the salmon has failed to returned to the natal stream.

Also as per the passage we know, "Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations." In the question, it is mentioned that the stream is polluted, so we can make an inference from that, the stream is somewhat related to environmental degradation. So, the extinction should accelerate in that polluted stream based on the above explanation.

Please help me to clarify my understanding.
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hi can any expert help me with 6th question?
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hi can any expert help me with 6th question?
Check out this post, and let us know if you still have questions!
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Hi voodoochild manishk30. I'm not an expert but I think I can help you guys here.

To answer your first doubt, "Isn't this same as saying "preserving without dilution" i.e. the polluted stream's population can be preserved when there is low-level of straying?" The answer here is a big No, passage only mentions if the straying and spawning is below 15 percent of the number of fish returning, then the adaptations of a particular stream, into which the strayed population enters, will indeed not be diluted, however, this does not imply that the mechanism PRESERVES such dilution. Option B mistakes something (dilution) that does not happen by virtue of just staying in certain limits (below 15 percent straying) to be the main product (which actually is novel gene source and recolonization) of the mechanism.


For Second, If we Negate C: Salmonids of no stream would benefit from particular local adaptions then author's conclusion that increased straying would dilute the native population, degrading the fitness of the genes will be weakened because Author's basis of concluding that there's a causal link b/w increased straying and decreased fitness is that salmonids of some streams do benefit from local adaptions, else if they don't benefit then what would be the point (no causal link) of author's arguing that increased straying dilutes local adaptation, which is Problematic according to her.
voodoochild
Mike,
Thanks for your kind reply. I am not clear, and I am still confused about both the answer choices.

mikemcgarry
B is quite wrong because the passage makes very clear --- when a stream is polluted, straying happens, which means dilution happens. The passage describes clearly the straying that resulted from the Mount St. Helen's eruption, and then around line (70) says that the effect of pollution probably would be about the same as what they saw at Mount St. Helen's. When the stream is polluted, the salmon can't use it to spawn, so they stray.


For the first one, as you have stated above, I think that both of us are on the same page : when the stream gets polluted, the polluted stream's salmon population cannot spawn, and hence they have to find other stream to spawn, leading to straying. Isn't this same as saying "preserving without dilution" i.e. the polluted stream's population can be preserved when there is low-level of straying? Now to extend this idea - why do I think that we can preserve the polluted stream's population? It's because in lines 35-40 it's stated that their genes can be preserved. I am still not able to see why B) is incorrect.

I see dear Douvik's point in that "dilution" is not explicitly stated while discussing 'low levels of straying.' I am not sure whether these are grounds to eliminate B. The passage does talk about high-levels of straying, and has explicitly stated that low-levels of straying is better than high-level straying because of the dilution issue. Can't we infer that the dilution doesn't happen in the case of low-level straying. I am not sure about this. It seems logical to me and within the boundaries of the passage.


mikemcgarry

(E) is a typically GMAT RC distractor, and you fell for the bait. It is a statement that's true in general, but not relevant to the argument. Suppose we said that there was no such thing as a "pristine" perfect place for salmon --- suppose even the environments with zero pollution had natural challenges. Well, then, the salmon in those environments, over the centuries, would adapt to those particular conditions and be quite fit, and then if others stray into their unique stream and interbreed with them, it will reduce the fitness of that population.

For the second one, this is a great 'assumption' question. Let's dissect this further. Conclusion: "Disturbances cause by humans lower the overall fitness of subsequent generation in 'mixed' streams." Premise, as you stated, the salmon in those pristine areas had adaptions particular suited to those areas, which made them quite fit, and along come a bunch of salmon from the polluted stream next door, and they genetically dilute the salmon of that pristine stream, making them less adapted to that unique niche, and hence less fit. I agree 100%. That was my understanding as well. However, I am still not able to see the correctness of C).

No let's negate C - Salmonids in none of the streams benefit from particular local adaptations. In my opinion, this has no effect on the conclusion. The conclusion is about comparing the overall fitness levels between "pristine" population (i.e. native population) and "mixed" population (i.e. native + newly strayed ones). Who cares whether 1% of population benefits from the adaptation or 90% of the native population benefits from the adaptation. We are concerned about the causal link between human activity and the lowering of the overall fitness. It could be possible that the native population is only 1% fit, and the straying would decrease this further down to 0.05%. Do I care about the level of fitness of the native population? No. I am only concerned about the "lowering" of fitness. I am still not clear.

Now let's negate E- (I must admit that while solving this question, I was unsatisfied with all the answer choices and had no option than to choose E) from C and E (50% chance :( ) ) -- I agree that we don't care whether the pristine streams are affected by natural disaster or not. We are only concerned about the link between "human activity" and the lowering of overall Darwinian fitness.



Can you please help me? I am really confused.


Thanks
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Hi experts! Can anyone please help out with Q4? It's mentioned as "Not an official question" but I can't wrap my head around why (C) is a better choice than (B)
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