RC question need help.(Confused between B and C)

[littlemrsunshine]

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.
___________________________
The author mentions the "aftermath of the Mount Saint Helens eruption" (the bold part in the last paragraph) 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

I am confused between B and C. I cannot see the difference between them and cannot find a reason to reject each of these two choices. Please help me.

[soumya2022]

hi,
Could someone please assist with this question?
The OA is C.

However, I picked D.

Please explain.

Thank you!
Regards
Soumya

[prashantarora610]

Hi Soumya,

Not sure why you picked D.

Even I narrowed down on B and C, I selected C as one can compare the devastation caused by human intervention with that caused by Mt. HS eruption.

Also, B is incorrect in saying "indicate the extent" since no effort has been made to measure the damage done by human beings.

Hope this helps.

[soumya2022]

Hi Soumya,

Not sure why you picked D.

Even I narrowed down on B and C, I selected C as one can compare the devastation caused by human intervention with that caused by Mt. HS eruption.

Also, B is incorrect in saying "indicate the extent" since no effort has been made to measure the damage done by human beings.

Hope this helps.

Hi Prashant,

I eliminated C since the passage does not mention the rate being a standard for comparison, nor was I able to infer "C" from the data mentioned in the passage. I picked "D" since I inferred that the rate of salmon moving to new streams has been increased due to the event, increased,since usually the rate is low!
Could you please further explain.

Thank you!
Regards
Soumya

[RonPurewal]

Prashant, please kindly leave the explanations to the forum moderators. Thanks.

[RonPurewal]

Soumya"”"”

First, D is not just unjustified, but actually wrong.
The fish didn't stray in greater proportions because their homing instincts were messed up"”"”they strayed because the volcano wrecked their home streams (with "mud and debris")!

Analogy: If a hurricane destroys my house and I say "Now I can't go home anymore", this does not mean that I can no longer find the way back. It means my house was destroyed.

[RonPurewal]

As far as the correct answer, there is evidence on a small scale and on a large scale.

On a small scale, look at the sentence in which the bold part actually appears:
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
They're explicitly comparing the effect of the changes wrought by humans to that of the MSH eruption.

On a larger scale, the entire passage is about what HUMANS have wrought upon the salmon. It's not about "random stuff that affects the salmon".
If they're going to suddenly start blabbing about a volcano, then that example MUST relate in some way to the changes wrought by humans.
If it were something else"”"”e.g., choice D"”"”then it would just be a random piece of randomness, with no good reason at all to appear in this passage.

[soumya2022]

As far as the correct answer, there is evidence on a small scale and on a large scale.

On a small scale, look at the sentence in which the bold part actually appears:
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
They're explicitly comparing the effect of the changes wrought by humans to that of the MSH eruption.

On a larger scale, the entire passage is about what HUMANS have wrought upon the salmon. It's not about "random stuff that affects the salmon".
If they're going to suddenly start blabbing about a volcano, then that example MUST relate in some way to the changes wrought by humans.
If it were something else"”"”e.g., choice D"”"”then it would just be a random piece of randomness, with no good reason at all to appear in this passage.

Got it Sir, Thank you!

Regards
Soumya

[RonPurewal]

Excellent.

[NL]

Dear Ron,

Those were excellent lines of thought! I tried to … imitate you, but failed.

I faced some questions regarding to this passage. I got all wrong!
Why was that? Because my thought was spread out into different directions to find the answers and I didn’t know which way is the right way (to think).

Here are the questions:

1/ The author's argument that increased straying can "lower the overall fitness of subsequent generations" (the last sentence) 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 adaptations

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.


- My thought: So I have a find an assumption. Assumption for what? For “increased straying can lower the overall fitness of subsequent generations”, which must be a conclusion.
--> that means I have to find evidence the author used to support this conclusion. I got stuck at this point.

- Not giving up, I turned to another way: I asked “WHY increased straying can lower the overall fitness of subsequent generations”? It must deal with “gene”. I got stuck again when checking the choices.

Please help me understand what’s wrong with those ways of thinking? What is your thought line when answering this question?


2/ Which of the following does the author mention as support for the view that environmental disturbance 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 activity would differ in their effects from comparable disturbances brought about by natural causes

--> This question seems little vague for me. I thought: Ok, so I have to find something that supports “environmental disturbance caused by human activity could increase straying rates”. Human activities were discussed in the 1st paragraph. Straying rate was discussed in the 2nd one. He then gave evidence, which was the 1980 volcanic eruption, to support the point.

Yes, when I saw the answer, I could make a connection between the answer and the question. But it’s hard to arrive at the answer when, well, I … don’t know the answer.

So, assuming that you don’t know the answer, please tell me your thought process to find the answer.

Thank you so much.

[RonPurewal]

#1

The red part is wrong:

My thought: So I have a find an assumption. Assumption for what? For “increased straying can lower the overall fitness of subsequent generations”, which must be a conclusion.
--> that means I have to find evidence the author used to support this conclusion. I got stuck at this point.

This is exactly what "assumption" DOES NOT mean.

"Assumptions" are things that are not mentioned, but that MUST be true. So, anything that the author actually uses is, by definition, NOT an "assumption".

On the other hand, an "assumption" is something that MUST be true. In other words, if an assumption turns out to be false, the entire line of reasoning is invalidated.

So, make these statements false, and see what happens.

If C is false, then you get "There aren't any streams where local adaptations benefit the salmon."
If that's true, then physical displacement (to a different stream) wouldn't have any negative effects—and so the author would have no basis on which to claim that "straying" is bad for the fitness of the gene pool.

I don't know which other choices you were considering. But none of the other choices will destroy the reasoning if negated.

[RonPurewal]

#2:

This one is asking for something that the author actually mentions. So, it's sufficient to take an inventory of what the author uses as support, and then to look for one of the things in that inventory.

The support for this statement is here:

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

In terms that a 9-year-old would understand:
When MSH erupted, lots more fish were forced to stray.
If humans (rather than nature) mess things up, the effects are likely to be similar.

You need both colors for the argument to work.
Obviously you need the blue part, or else there's no concrete evidence.
But you need the pink part, too. Without the pink part, the MSH eruption and human activity are two completely different things, and there's no reason for us to link them.

Choice E is the pink thing.

You were looking only for the blue thing. Unfortunately, they chose the pink thing.

[NL]

#1

On the other hand, an "assumption" is something that MUST be true. In other words, if an assumption turns out to be false, the entire line of reasoning is invalidated.

So, make these statements false, and see what happens.

If C is false, then you get "There aren't any streams where local adaptations benefit the salmon."
If that's true, then physical displacement (to a different stream) wouldn't have any negative effects—and so the author would have no basis on which to claim that "straying" is bad for the fitness of the gene pool.

--> That’s a fantastic “back-solving” for RC, Ron!
Creative.

So it does deal with gene: lower fitness means their genes are less unique, doesn’t it?
In order to arrive at C, we have to make another assumption that adaptation means the salmons change their genes to adapt to the new environment, affecting the fitness of following generations.

(in fact, long-time adaptation can result in changing gene, but short-term adaptation won’t change gene at all. That's why this question retired soon, I guess :))

Your method is “up going”. I tried to use “down going” method, with which I had to find evidence in order to find a link between the evidence and the conclusion. But I forgot this is a RC question, not CR.

[NL]

#2:

You need both colors for the argument to work.
Obviously you need the blue part, or else there's no concrete evidence.
But you need the pink part, too. Without the pink part, the MSH eruption and human activity are two completely different things, and there's no reason for us to link them.

Thank you. I understood now.

[RonPurewal]

--> That’s a fantastic “back-solving” for RC, Ron!
Creative.

"Negate and see whether the argument is destroyed" is a standard approach to assumption questions. Not sure what is "creative" about it.

The "negation" method has a huge advantage over trying to find assumptions directly: You don't have to pull things out of thin air.
– To "find" an assumption directly, you basically have to conjure it out of nowhere. While not unreasonable (we do this all the time in everyday conversation), this can be hard to do if you don't have much intuition for what a question is saying.
– With the negation method, on the other hand, you always start from choices that are actually there. You'll still need to think carefully about how they affect the argument, but at least you don't have to pull them out of nowhere.