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

Question

Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: mining, livestock grazing, timber harvesting, and agriculture as well as recreational and urban development. The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine 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 populations. Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations. Although some rivers have been recolonized, the unique genes of the original populations have been lost.

Large-scale disturbances in 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 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. 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.

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 . 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 .   1 
 VRC000460-01 
 1. The primary purpose of the passage is to

(A) argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative
(B) correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity
(C) compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations
(D) differentiate the particular effects of various human activities on salmon habitats
(E) describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction

2 
 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

3 
 VRC000460-03 
 3. According to the passage, human activity has had which of the following effects on salmon populations?

(A) An increase in the size of salmon populations in some previously polluted rivers
(B) A decline in the number of salmon in some rivers
(C) A decrease in the number straying salmon in some rivers
(D) A decrease in the gene flow between salmon populations that spawn in polluted streams and populations that spawn in pristine streams
(E) A decline in the vulnerability of some salmon populations to the effects of naturally occurring habitat destruction

4 
 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

5 
 VRC000460-05 
 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.

6 
 VRC000460-07 
 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?

(A) The existence of salmon populations in rivers where the elimination of salmon habitat by human activity had previously made the fish extinct
(B) The results of studies measuring the impact on straying rates of habitat loss caused by human activity
(C) The potential for disturbances in one environment to cause the introduction of novel genes into salmon populations in neighboring areas
(D) The weaknesses in the view that the extinction of entire salmon populations is the only mechanism by which human destruction of salmon habitat reduces genetic diversity in salmon
(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

jaiswalamrita · Accepted Answer

Q10:
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

As given in line 20 “Although some rivers have been recolonized, the unique genes of the original populations have been lost.” This refers that some streams have been initially “DE SALMONed” i.e Salmons fail to return to their natal streams in order to spawn because of some environmental degradation of the stream but somehow the stream was restored and finally recolonized. That’s exactly what option D says (If the stream recovers, an environmentally degraded stream could be recolonized).

Q11:
According to the passage, human activity has had which of the following effects on salmon populations?

a. An increase in the size of salmon populations in some previously polluted rivers
b. A decline in the number of salmon in some rivers
c. A decrease in the number straying salmon in some rivers
d. A decrease in the gene flow between salmon populations that spawn in polluted streams and populations that spawn in pristine streams
e. A decline in the vulnerability of some salmon populations to the effects of naturally occurring habitat destruction

As given in opening lines “Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: Line mining, livestock grazing, timber harvesting, and agriculture as well as recreational and urban development. The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine ones;” This clearly refers that human activity caused decline in number of salmons in some river (B)

Q12:
The author mentions the “aftermath of the Mount Saint Helens eruption” (lines 73-74) 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

As given in last para “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”, author is clearly comparing the effect of human activity with that of environmental degradation.

a. provide an example of the process that allows the repopulation of rivers whose indigenous salmon population has become extinct –  Not an example of any process of repopulation 
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 –  not talking about any particular stream 
c. provide a standard of comparison against which the impact of human activity on the gene flow among salmon populations should be measured -  YES 
d. show how salmons’ homing instinct can be impaired as a result of severe environmental degradation of their natal streams –  Clearly out 
e. show why straying rates in salmon populations remain generally low except when spawning streams suffer severe environmental disturbance –  Clearly out

mikemcgarry · Answer

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. 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 :-)

souvik101990 · Answer

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.

voodoochild · Answer

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
Voodoo Child

) ) -- 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

mikemcgarry · Answer

Dear tae808 , I'm happy to respond my friend. :-)

A " habitat " is the environment in which a living thing is designed to live, the place where it naturally thrives. For a deer, the habitat is a forest. For a giraffe, the habitat is the Serengeti. For a salmon, the habitat, for at least part of its life cycle, is a stream. Many things live in a healthy stream, so for these animals, it is a habitat. Does this make sense? Mike

Luckisnoexcuse · Answer

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?

(A) The existence of salmon populations in rivers where the elimination of salmon habitat by human activity had previously made the fish extinct
(B) The results of studies measuring the impact on straying rates of habitat loss caused by human activity
(C) The potential for disturbances in one environment to cause the introduction of novel genes into salmon populations in neighboring areas
(D) The weaknesses in the view that the extinction of entire salmon populations is the only mechanism by which human destruction of salmon habitat reduces genetic diversity in salmon
(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

Can experts here help me understand the process to reach answer in this one.
I was able to remove only A and ended up selecting C

GMATNinja · Answer

This exactly what is described in the last paragraph. There is no reason (i.e. "absence of any reason) to suspect that the effect (i.e. "disturbances brought about by human activity") would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption (i.e. "natural causes").

The preceding paragraph describes and quantifies how the Mount Saint Helens eruption (a natural cause) increased straying rates. Although no one has quantified changes in straying rates caused by  human  disturbances, there is no reason to believe that the consequences of human disturbances would be much different than the consequences of the eruption (a natural cause).

I hope that helps!

ucb2k7 · Answer

-----------------------------------------------------------------------------------------------------------------

Hi  mikemcgarry

Please explain Q2,4,5 and 6. I ended up marking these wrong.

Thanks,

ucb2k7

GMATNinja · Answer

ucb2k7 , this is certainly a challenging passage, but can you try explaining your selections for the questions that you asked about? Sometimes, attempting to explain your thought process can lead you to the correct answer. Also, I know that the always-amazing  mikemcgarry  (Hi Mike!) receives numerous requests for help every day, and I imagine that he'd give priority to questions that are a bit more specific and thoughtful. The more information you provide, the easier it is for all of us to help!

As discussed in our  RC Guide for Beginners , try reading the passage again and focusing on structure and purpose, rather than worrying about every little detail:

See if that approach helps improve your understanding of the passage, and then give the questions another shot. Then, as I recommended above, try explaining your thought process for the ones that you miss.

Question #4 does not appear to be from the GMAT Official Practice questions, so I wouldn't worry about that one.

Good luck!

MagooshExpert · Answer

Hi ucb2k7 ! Carolyn from Magoosh here

Mike can't respond right now, but I'd be happy to answer your questions

However, as GMATNinja mentioned, it would be really useful if you could make your questions a bit more specific. Please try to explain your thought process for each question, and mention what exactly you're having trouble understanding. That way we can provide better, more specific explanations! Thanks!

Carolyn

Manas1212 · Answer

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

Diya52 · Answer

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?

warrior1991 · Answer

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

KarishmaB · Answer

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.

dcummins · Answer

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.

GMATNinja · Answer

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!).

mSKR · Answer

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

mSKR · Answer

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

mSKR · Answer

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

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

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

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