Reading Comprehension

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.

1. 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

2. The author's argument that increased straying can "lower the overall fitness of subsequent generations" (see highlighted 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 adaptataions
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

3. 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 activity would differ in their effects from comparable disturbances brought about by natural causes

Please provide answer with explanation.

We discussed this passage in one of our recent online classes. It was then that many students told me that official answers to many questions belonging to this passage are not available online. Sharing the answer key and explanations here in case it benefits someone in future.

OA: 1D 2C 3E

Answering each question in a new post to increase readability and 'quote'ability.

1. Some salmon do not return to their natal streams to spawn. This leads to ____
A: Why would pristine streams become polluted because some salmon do not return to their natal streams? Incorrect.
B: Look at lines 36-37. Whether the level of straying is low or high, some dilution of genes does happen. Option B says that salmon population can be preserved without dilution - incorrect.
C: Why would the number of salmon in pristine streams decrease because some salmon do not return to their natal streams? Incorrect.If anything, this number should increase because salmons from polluted streams would now come to pristine streams.
D: Correct.
E: Not necessarily true - the salmon from polluted streams would now come to pristine streams. So we can't say that they would become extinct faster.

2. Treat this question like a Critical Reasoning 'Find the assumption' question. i.e. if you negate the right answer, the argument must break down.

Argument: If straying increases, overall fitness of subsequent salmon generations decreases.

We are making a connect between straying and fitness. Let's try to draw the intermediate connections:
If straying increases, non-native salmon will come to pristine streams and interbreed with the local salmon.
Therefore, gene dilution will happen. But the conclusion is that the fitness of subsequent local salmon goes down.

Option C provides the missing connect about genetic dilution - it draws from lines 41 to 47 when it says that local salmon benefit from particular local adaptations. Hence, their dilution of their genetic makeup will lead to a reduction in their fitness. Thus, option C is the right answer.

3. This is a straightforward 'according to the passage' type of question. The answer to this question can be found in lines 66 to 74.

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