hey Rebekah

lets read the question once more.

Which of the following does the author mention as support for the view that environmental disturbances caused by human activity could increase straying rates?

Now ask yourself a question, how author can support his claim? by providing examples right ? so in which paragraph did author put an example ? in the second paragraph, the last sentence: see below

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.

note it is not a environmental disturbance caused by human. it is caused by volvanic eruption, which skyrocketed rates of straying to more than 40%


now lets move on to the third pararagraph. see below first sentence

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.


so author mentions here that environmental disturbances caused by volcanic eruptions is equivalent to the environmental disturbance caused by human.


hence correct answer is E

The absence of any reason for believing that disturbances brought about by human activitity would differ in their effects from comparable disturbances brought about by natural causes

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

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
_________________

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.