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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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Q 19

Basically the last paragraph says thta for a relative stable population , which has some sort of oscillation around a certain parameter, the scientists can infer and interpret the cycles

For irregularly fluctuating populations - it is quite difficult to catch the meaning behind and the reasons

19. Which of the following statements can be inferred from the last paragraph?

A For irregularly fluctuating populations, doubling the number of observations made will probably result in the isolation of density-dependent effects.

No where in the last part of the passage is mentioned the red part

B Density-dependent effects on population dynamics do not occur as frequently as do density-independent effects.

The comparison in the red part is not made in the last p.

C At present, ecologists do not understand any of the underlying causes of the density-dependent effects they observe in populationdynamics.

To extreme as assessment. They understand part of the phenomenon and part is still obscure

D Density-dependent effects on growth parameters are thought to be caused by some sort of biochemical "signaling" that ecologists hope eventually to understand.


For populations that remain relatively constant, or that oscillate around repeated cycles, the signal can be fairly easily characterized and its effects described, even though the causative biological mechanism may remain unknown.

biochemical "signaling" \(\neq\) biological mechanism



E It is sometimes possible to infer the existence of a density-dependent factor controlling population growth without understanding its causative mechanism.


the signal can be fairly easily characterized and its effects described, even though the causative biological mechanism may remain unknown.

We do know the effects but not always the causes

E is correct.

Ps: this was very challenging
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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I took 15 mins to read and solve ...
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
1
In the last question(23),
Is the answer (E) derived due to the sentence "But it now SEEMS clear that...".
So, does the word SEEMS make no proper inference of the conclusion?
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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Apparently yes. tht verb could indeicate inference but always it depends on the context.

In the last question, you have to see the passage as a whole and what it conveys

And all elements lead us to D

Hope this helps
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
Carcass

Hello!

Would you please help me to fin out how can we ruled out option A from question 21?

(A) roughly constant population levels from year to year

Where can I find the prove that the passage talks about it?

Regards!
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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FCOCGALVAN wrote:
Carcass

Hello!

Would you please help me to fin out how can we ruled out option A from question 21?

(A) roughly constant population levels from year to year

Where can I find the prove that the passage talks about it?

Regards!


The great variety of dynamic behaviors exhibited by different populations makes this task more difficult: some populations remain roughly constant from year to year; others exhibit regular cycles of abundance and scarcity; still others vary wildly, with outbreaks and crashes that are in some cases plainly correlated with the weather, and in other cases not.

Pretty net the answer in the first paragraph: some populations remain roughly constant from year to year;

So A is wrong because we have in the passage
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
Can you please explain 1 and 4?
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
Since there are 7 questions for this passage, is it safe to say that finishing all of them in under 14 minutes is fine?
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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vrushab97 wrote:
Since there are 7 questions for this passage, is it safe to say that finishing all of them in under 14 minutes is fine?


Time management for verbal reasoning

https://gre.myprepclub.com/forum/gre-time- ... tml#p57079
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
How much time can I devote in the exam to such a big question? While practicing it took around 14 minutes in total.
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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Scarecrow wrote:
How much time can I devote in the exam to such a big question? While practicing it took around 14 minutes in total.


see the link above your reply

All is explained there. Learn to go through the board.

We do have crazy good material and is also fun to discover always new resources :blushing: ;)
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
23. In the passage, the author does all of the following EXCEPT

(A) cite the views of other biologists
(B) define a basic problem that the passage addresses
(C) present conceptual categories used by other biologists
(D) describe the results of a particular study
(E) draw a conclusion

Correct answer is D BUT doesn't the first line of last paragraph:

Quote:
In order to understand the nature of the ecologist's investigation, we may think of the density-dependent effects on growth parameters as the "signal" ecologists are trying to isolate and interpret, one that tends to make the population increase from relatively low values or decrease from relatively high ones, while the density- independent effects act to produce "noise" in the population dynamics.


Isn't the author basically describing the results of a study? If no, what exactly is this sentence doing? What is the function of this sentence?
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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Quote:
17. The author of the passage is primarily concerned with


Keep 2 things in mind for primary purpose questions:

1/ EVERYTHING IN the answer must ACCURATELY DESCRIBE THE PASSAGE. Anything unsupported makes an answer WRONG.
2/ The answer doesn't have to—and SHOULDN'T—get EVERY fact from the passage!. Don't expect to see every single specific point.


Let's look at the choices one at a time:

Quote:
A discussing two categories of factors that control population growth and assessing their relative importance


"Two categories of factors" are there: density-dependent and density-independent.

"Assessing relative importance" is also there: The author ends up saying that density-dependent factors are ultimately the only "signal" that determines the long-term population levels of a species—and thus immeasurably more important than the density-independent factors, which statistically act like "noise".

This answer is completely supported.


Quote:
B describing how growth rates in natural populations fluctuate over time and explaining why these changes occur


Neither half of this choice is supported.

This part doesn't happen because the author doesn't give any specific characterizations of HOW growth rates fluctuate (exponentially? in sine waves or other seasonal cycles? etc).
The only mathematical spec mentioned by the author, in fact, concerns what DOESN'T happen: namely, that the population would increase/decrease without bound ••IF•• there were only density-independent factors—an observation used to disprove that hypothesis!

And this doesn't happen either. In fact, the third paragraph is pretty emphatic about stating that, in many cases, nobody even knows the most important reasons why populations stabilize at certain levels.


Quote:
C proposing a hypothesis concerning population sizes and suggesting ways to test it


The point that density-dependent factors are dominant could reasonably be called a hypothesis about population sizes. However, there are no suggested experimental procedures for testing it—and in the last paragraph, the author makes it clear that the necessary work has already been done to support this hypothesis.

That there's no mention at all of the two categories that the author spends at least half the entire passage explaining is also a major issue here.


Quote:
D posing a fundamental question about environmental factors in population growth and presenting some currently accepted answers


[color=#00786a3]This[/color] doesn't happen. The author treats some ideas that turn out NOT to be acceptable (i.e., the possibility that there might be only density-independent factors, which would lead to the contradictions mentioned above), and finally lands on only one idea that's the accepted truth according to actual research.


Quote:
E refuting a commonly accepted theory about population density and offering a new alternative


This part is inaccurate because the author describes the accepted refutation of the notion that there could be only density-independent factors. This idea is NOT "commonly accepted" at the time of writing; the work to refute it has already been done.

Similarly, the 'alternative' isn't new—THIS is the current consensus.


Answer is A.
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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Carcass wrote:
18. It can be inferred from the passage that the author considers the dichotomy discussed in the second paragraph to be


"Inferences" need to be rigorously supported by the actual passage content.

This question is about how the author regards a certain theory. The second paragraph is entirely dedicated to summarizing that theory; the author does not opine on it in that paragraph.
Therefore, the part we want is at the beginning of the next paragraph—back to speaking in the author's own voice:
This dichotomy has its uses, but it can cause problems if taken too literally.

We want an answer choice that basically says this, again, with a slightly different choice of words.


Quote:
A applicable only to erratically fluctuating populations


Not supported—the author says no such thing. The issue (according to the author) is not that the theory only applies to certain cases; it's that it's impossible for one of the two dichotomous states to entirely describe the reality of ANY case.


Quote:
B useful, but only if its limitations are recognized


This choice is readily recognizable as the same thing the author is saying above. Therefore, this is the correct answer.


Quote:
C dangerously misleading in most circumstances


Both parts of this choice are unreasonably extreme.

This part: The stuff in the third paragraph does suggest that the dichotomous model may lead people to underestimate and/or underemphasize the importance of density-dependent factors, but the author goes nowhere near as far as to say that it is "dangerously misleading".

This part: The author does not quantify the relative numbers of cases where the model of a strict dichotomy might be deceptive vs. those where it broadly applies in ways that agree with intuition.


Quote:
D a complete and sufficient way to account for observed phenomena


The whole point is that the dichotomous model is NOT this. Fully the last two paragraphs are devoted to showing why it's not.


Quote:
E conceptually valid, but too confusing to apply on a practical basis


Not a word of this choice is supported by anything appearing anywhere in the text of the passage.



It's B.
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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Carcass wrote:
19. Which of the following statements can be inferred from the last paragraph?


As usual, we need something that's supported rigorously. "If the author says those things, then she/he will DEFINITELY also say..."


Quote:
A For irregularly fluctuating populations, doubling the number of observations made will probably result in the isolation of density-dependent effects.


The author says: For irregularly fluctuating populations, we are likely to have too few observations to have any hope of extracting the signal from the overwhelming noise.

This statement contradicts choice A: If all we have to do in such cases is gather twice as many data points, then that means there's LOTS of "hope of extracting the signal"!


Quote:
B Density-dependent effects on population dynamics do not occur as frequently as do density-independent effects.


The author makes no quantitative comparisons of this sort at all—neither between the two items in this answer choice, nor between regularly varying and erratically fluctuating populations.

The only somewhat relevant statement in the paragraph is the last, which states:
all populations are regulated by a mixture of density- dependent and density-independent effects in varying proportions
If we're quantifying the statement in choice B by counting numbers of populations, then it's definitely false—because, according to this statement, the numbers of populations are exactly the same (since both types of effects obtain in ALL populations). If we're quantifying it instead by counting the effects themselves, then "varying proportions" leaves the entire spectrum of possibilities wide open, from possibilities that would agree with choice B to those that would disagree with it.


Quote:
C At present, ecologists do not understand any of the underlying causes of the density-dependent effects they observe in population dynamics.


Much too extreme.
The author says that "the causative biological mechanism may remain unknown" for certain cases, but there's no basis on which to say that scientists don't know even one such reason for anything.



Quote:
D Density-dependent effects on growth parameters are thought to be caused by some sort of biochemical "signaling" that ecologists hope eventually to understand.


What a strange answer choice.
"Signal", in this passage, refers to a statistical phenomenon—i.e., an illustrative mathematical pattern that's strong enough to overcome a background of random fluctuations ("noise"). Biochemical signaling is something else entirely, to such an extent that this choice almost seems to have been erroneously taken from the wrong passage altogether.


Quote:
E It is sometimes possible to infer the existence of a density-dependent factor controlling population growth without understanding its causative mechanism.


This statement captures the meaning of this sentence from the passage text:
For populations that remain relatively constant, or that oscillate around repeated cycles, the signal can be fairly easily characterized and its effects described, even though the causative biological mechanism may remain unknown
...in which "the signal" refers to the density-dependent effects that ultimately determine population size in the long term.



It's E.
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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Quote:
20. According to the passage, which of the following is a true statement about density-dependent factors in population growth?


The question means exactly what the words say: We want something that's stated in the text.


Quote:
(A) They ultimately account for long-term population levels.


This choice effectively summarizes the main point made in the passage. It's stated most explicitly at the end of the third (next-to-last) paragraph—but also supported with full rigor (i.e., without the uncomfortably weakening use of the word "usually" at the end of ¶3) by the following language in the fourth and last paragraph:
we may think of the density-dependent effects on growth parameters as the "signal" ecologists are trying to isolate and interpret, one that tends to make the population increase from relatively low values or decrease from relatively high ones, while the density- independent effects act to produce "noise" in the population dynamics

This answer is therefore supported.


Quote:
(B) They have little to do with long-term population dynamics.


This choice exactly contradicts the main point made about density-dependent factors, which is that they are ultimately the determining factors whereas the other (density-independent) factors are just statistical noise in the long run.


Quote:
(C) They are always more easily isolated and described than those that are density-independent.


The word "always" should make you extremely skeptical of this choice right from the start; that's quite a statement to support!

Beyond the inappropriately extreme phrasing, this choice contradicts at least two statements in the passage saying that these effects can be difficult or even impossible to describe:
their cause may be correspondingly hard to determine (from ¶3),
the causative biological mechanism may remain unknown (from ¶4).


Quote:
(D) They include random environmental events.


This description applies to density-independent effects, which are exactly the factors that we are not asked to consider here. (The author describes density-independent effects, near the end of paragraph 2, as rates buffeted by environmental events).


Quote:
(E) They contradict current ecological assumptions about population dynamics.


Most certainly not so—the author concludes the passage by stating that they are at work for EVERY species!
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Re: As Gilbert White, Darwin, and others observed long ago, all [#permalink]
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Quote:
21. According to the passage, all of the following behaviors have been exhibited by different populations EXCEPT


This question asks for a population effect that has NOT been observed. Anything that the passage says HAS been observed can be eliminated.

Quote:
(A) roughly constant population levels from year to year


• ¶1 mentions populations [that] remain roughly constant from year to year.
Relatively steady populations are described in ¶2.
populations that remain relatively constant pops up again in ¶4.
Eliminate.



Quote:
(B) regular cycles of increases and decreases in numbers


¶1 mentions the existence of populations that exhibit regular cycles of abundance and scarcity. Also, populations ... that oscillate around repeated cycles are mentioned in ¶4. Eliminate.



Quote:
(C) erratic increases in numbers correlated with the weather


In ¶1 the author mentions that populations have been found whose numbers vary wildly, with outbreaks and crashes that are in some cases plainly correlated with the weather. Eliminate.


Quote:
(D) unchecked increases in numbers over many generations


"Increasing without bound" is mentioned in ¶3 as an absurd, counterfactual result that we WOULD see if there were populations driven only by density-independent factors.
Nothing in the passage suggests that any such thing has been seen—and, moreover, the author leverages the very absence of such phenomena to make the main point of the passage.


Quote:
(E) sudden declines in numbers from time to time


This is the meaning of "crashes" with respect to population figures. At the end of ¶1, the author mentions populations that 'crash' from time to time. Eliminate.



It's D.
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