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Over the last 150 years, large stretches of salmon habitat

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Over the last 150 years, large stretches of salmon habitat [#permalink] New post 15 Oct 2012, 16:28
Complete discussion is provided at the below mentioned link
over-the-last-150-years-large-stretches-of-salmon-habitat-135658.html#p1103703



Over the last 150 years, large
stretches of salmon habitat have
been eliminated by human activity:
Line mining, livestock grazing, timber
(5) harvesting, and agriculture as well
as recreational and urban devel-
opment. The numerical effect is
obvious: there are fewer salmon
in degraded regions than in pris-
(10) tine ones; however, habitat loss
also has the potential to reduce
genetic diversity. This is most
evident in cases where it results
in the extinction of entire salmon
(15) populations. Indeed, most
analysts believe that some kind
of environmental degradation
underlies the demise of many
extinct salmon populations.
(20) Although some rivers have
been recolonized, the unique
genes of the original populations
have been lost.
Large-scale disturbances in
(25) one locale also have the potential
to alter the genetic structure of
populations in neighboring areas,
even if those areas have pristine
habitats. Why? Although the
(30) 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
(35) 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
(40) repopulated should the fish
there disappear. Yet high rates
of straying can be problematic
because misdirected fish may
interbreed with the existing stock
(45) to such a degree that any local
adaptations that are present
become diluted. Straying
rates remain relatively low when
environmental conditions are
(50) stable, but can increase dramati-
cally when streams suffer severe
disturbance. The 1980 volcanic
eruption of Mount Saint Helens,
for example, sent mud and debris
(55) into several tributaries of the
Columbia River. For the next
couple of years, steelhead trout
(a species included among the
salmonids) returning from the
(60) 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
(65) overall.
Although no one has quantified
changes in the rate of straying
as a result of the disturbances
caused by humans, there is no
(70) reason to suspect that the effect
would be qualitatively different
than what was seen in the
aftermath of the Mount Saint
Helens eruption. Such a dra-
(75) matic 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.

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

[Reveal] Spoiler:
OA D

Why is B wrong? When high straying happens, dilution is the issue. Hence, when low straying happens, dilution is not an issue at all -- what B) says. Also lines (30), " Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be (40) repopulated should the fish there disappear. " --- implies genes are preserved -- and as the next sentence says "dilution happens" in case of high straying. Can't we infer that in case of low straying, no dilution happens and the genes are preserved?

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

a) a disturbance of salmonid spawning streams caused by human activtity will increasing 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 total spawning population

c) Salmonids in some streams benefit from particular local adaptations

d) Nonenvironmental factors have no effect

e) At least some of the streams in which straying salmonids would spawn are pristine, affected by neither natural nor artificial disturbances.

[Reveal] Spoiler:
OA C


Why is E) wrong? Also what support do we have for C) To me, "lowering the overall fitness" implies less likely to pass Darwin's "survival of the fittest" Hence, such adaptations are in fact bad -- 180 to what the OA is :( To me, the conclusion is that straying because of human effects causes bad things to Salmon in pristine streams. What's the support? Straying causing the dilution of the gene pool. I couldn't find that answer choice.

Thoughts?
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Re: Over the last 150 years, large stretches of salmon habitat [#permalink] New post 17 Oct 2012, 09:17
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voodoochild wrote:
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


[Reveal] Spoiler:
OA D

Why is B wrong? When high straying happens, dilution is the issue. Hence, when low straying happens, dilution is not an issue at all -- what B) says. Also lines (30), " Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be (40) repopulated should the fish there disappear. " --- implies genes are preserved -- and as the next sentence says "dilution happens" in case of high straying. Can't we infer that in case of low straying, no dilution happens and the genes are preserved?

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.

voodoochild wrote:
The author's argument that increased straying can "lower the overall fitness of subsequent generation" (see highlighted text) is based on which of the following assumptions?
a) a disturbance of salmonid spawning streams caused by human activtity will increasing 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 total spawning population
c) Salmonids in some streams benefit from particular local adaptations
d) Nonenvironmental factors have no effect
e) At least some of the streams in which straying salmonids would spawn are pristine, affected by neither natural nor artificial disturbances.


[Reveal] Spoiler:
OA C


Why is E) wrong? Also what support do we have for C) To me, "lowering the overall fitness" implies less likely to pass Darwin's "survival of the fittest" Hence, such adaptations are in fact bad -- 180 to what the OA is :( To me, the conclusion is that straying because of human effects causes bad things to Salmon in pristine streams. What's the support? Straying causing the dilution of the gene pool. I couldn't find that answer choice.

You're correct in your understanding of "fitness" --- we are talking about Darwinian fitness here.

The passage says "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." In other words, 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. That passage is enormous support for (C)

(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.

Does all this make sense?

Mike :-)
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Re: Over the last 150 years, large stretches of salmon habitat [#permalink] New post 17 Oct 2012, 06:22
Expert's post
Hey in the first question, we are asked to provide an "inference" which MUST be true according to the passage!
Also, the passage says nothing about dilution during low levels of straying. It just says that with low levels of straying, there is a solid mix of new genes which could be good for the area. That is the are could or could not benefit from low straying. It becomes crucial if the population of fishes decline in the other area which could be increased by moderate level of straying. (please note that the new population could or could not be diluted: not indicated in the passage). It just means that low levels of straying could save a dying fish population.
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Re: Over the last 150 years, large stretches of salmon habitat [#permalink] New post 18 Oct 2012, 21:21
Mike,
Thanks for your kind reply. I am not clear, and I am still confused about both the answer choices.

mikemcgarry wrote:
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 wrote:

(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
Voodoo Child
Re: Over the last 150 years, large stretches of salmon habitat   [#permalink] 18 Oct 2012, 21:21
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