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.[/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