In the Sonoran Desert of northwestern Mexico and southern Arizona, t

Question

Line 
 
 In the Sonoran Desert of northwestern Mexico and southern Arizona, the flowers of several species of columnar cacti—cardon, saguaro, and organ pipe—were once exclusively pollinated at night by
 
 (5) 
 
nectar-feeding bats, as their close relatives in arid tropical regions of southern Mexico still are. In these tropical regions, diurnal (daytime) visitors to columnar cactus flowers are ineffective pollinators because, by sunrise, the flowers' stigmas become unreceptive
 
 (10)  
 
or the flowers close. Yet the flowers of the Sonoran Desert cacti have evolved to remain open after sunrise, allowing pollination by such diurnal visitors as bees and birds. Why have these cacti expanded their range of pollinators by remaining open and receptive in daylight?

(15) 
 
 This development at the northernmost range of columnar cacti may be due to a yearly variation in the abundance—and hence the reliability—of migratory nectar-feeding bats. Pollinators can be unreliable for several reasons. They can be dietary generalists
 
 (20) 
 
whose fidelity to a particular species depends on the availability of alternative food sources. Or, they can be dietary specialists, but their abundance may vary widely from year to year, resulting in variable pollination of their preferred food species. Finally, they
 
 (25) 
 
may be dietary specialists, but their abundance may be chronically low relative to the availability of flowers.
 
 Recent data reveals that during spring in the Sonoran Desert, the nectar-feeding bats are specialists feeding on cardon, saguaro, and
 
 (30) 
 
organpipe flowers. However, whereas cactus-flower abundance tends to be high during spring, bat population densities tend to be low except near maternity roosts. Moreover, in spring, diurnal cactus-pollinating birds are significantly more abundant in
 
 (35) 
 
this region than are the nocturnal bats. Thus, with bats being unreliable cactus-flower pollinators, and daytime pollinators more abundant and therefore more reliable, selection favors the cactus flowers with traits that increase their range of pollinators. While data suggest
 
 (40) 
 
that population densities of nectar-feeding bats are also low in tropical areas of southern Mexico, where bats are the exclusive pollinators of many species of columnar cacti, cactus-flower density and bat population density appear to be much more evenly
 
 (45) 
 
balanced there: compared with the Sonoran Desert's cardon and saguaro, columnar cacti in southern Mexico produce far fewer flowers per night. Accordingly, despite their low population density, bats are able to pollinate nearly 100 percent of the available flowers.
 
   1 
(Book Question: 439)
 1. The primary purpose of the passage is to

A. compare the adaptive responses of several species of columnar cacti in the Sonoran Desert with those in the arid tropical regions of southern Mexico
B. discuss some of the possible causes of the relatively low abundance of migratory nectar-feeding bats in the Sonoran Desert
C. provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert
D. present recent findings that challenge a particular theory as to why several species of columnar cacti in the Sonoran Desert have expanded their range of pollinators
E. compare the effectiveness of nocturnal and diurnal pollination for several different species of columnar cacti in the Sonoran Desert

2 
(Book Question: 440)
 2. According to the passage, which of the following types of nectar-feeding pollinators is likely to be an unreliable pollinator of a particular cactus flower?

A. A dietary specialist whose abundance is typically high in relation to that of the flower
B. A dietary specialist whose abundance is at times significantly lower than that of the flower
C. A dietary generalist for whom that flower’s nectar is not a preferred food but is the most consistently available food
D. A dietary generalist for whom that flower’s nectar is slightly preferred to other available foods
E. A dietary generalist that evolved from a species of dietary specialists

3 
(Book Question: 441)
 3. According to the passage, present-day columnar cacti in the Sonoran Desert differ from their close relatives in southern Mexico in that the Sonoran cacti

A. have flowers that remain open after sunset
B. are pollinated primarily by dietary specialists
C. can be pollinated by nectar-feeding bats
D. have stigmas that are unreceptive to pollination at night
E. are sometimes pollinated by diurnal pollinators

OG 2019 ID's
RC00633-01
RC00633-02
RC00633-06

zvazviri · Accepted Answer

A is incorrect because of one word only: sunset. The passage says the flowers remain open after SUNRISE (during the day) not after SUNSET (during the night).
E. The passage clears states that "Yet the flowers of the Sonoran Desert cacti have evolved to remain open after sunrise, allowing pollination by such diurnal 
 visitors as bees and birds."

GMATNinja · Answer

Question 2 
 
The passage does mention preference explicitly in lines 22-24:

Or, they can be dietary specialists, but their abundance may vary widely from year to year, resulting in variable pollination  of their preferred food species.

However, the role of preference is implied throughout the passage.

The distinction between "generalist" and "specialist" begins with the premise that animals pollinate  food species  (in other words, animals pollinate what they feed on). 
  Some pollinators (the generalists) pollinate whatever food species are available, while other pollinators (the specialists) do have a special preference, which can only be satisfied by a narrow range of food species.

So how we do we eliminate choice (C)? Well, here's the author says when describing reasons what could make a pollinator unreliable:

Pollinators can be unreliable for several reasons. They can be dietary generalists  whose fidelity to a particular species depends on the availability of alternative food sources.

According to the passage, a dietary generalist will be faithful to a particular food species if it's available. They don't have special food preferences. If they're feeding on a certain type of nectar, but an alternative food sources become available, then the generalists will begin pollinating that alternative food source instead. Is that what choice (C) says?

Nope. (C) tells us that this flower  is the most consistently available food  for that generalist. If that's true, then we'd expect the generalist to be a  reliable  pollinator for this flower, because the generalist pollinates whatever is available.

But choice (C), as it's written, states that the generalist is likely to be an  unreliable  pollinator for this flower.

This contradicts the expected behavior of generalists -- so we must eliminate it.

I hope this helps!

GMATNinja · Answer

Passage breakdown 
In the first paragraph (P1), the author  asks a question :

Why have cacti in the Sonoran desert evolved to open their flowers during the day?

In the second paragraph (P2), the author  provides one potential explanation for the question raised in P1:

The cacti may have evolved because of annual variations in the reliability of pollinators. 
 The author then lists three reasons why a pollinator may be unreliable.

In the third paragraph, the author  provides data that supports the explanation given in P2

Bats are unreliable, so the cacti have evolved to allow birds to pollinate them during the day

For more on this process, check out  this article  and  our live RC videos .

Explanations for individual questions 
 
 Question 2
 
  Part 1  
  Part 2    
 Question 3
 
  Part 1  
  Part 2

roadrunner · Answer

Very good passage. All correct but took 8 minutes to solve :shock:

The primary purpose of the passage is to A. compare the adaptive responses of several species of columnar cacti in the Sonoran Desert with those in the arid tropical regions of southern Mexico B. discuss some of the possible causes of the relatively low abundance of migratory nectar-feeding bats in the Sonoran Desert C. provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert D. present recent findings that challenge a particular theory as to why several species of columnar cacti in the Sonoran Desert have expanded their range of pollinators E. compare the effectiveness of nocturnal and diurnal pollination for several different species of columnar cacti in the Sonoran Desert Straight forward C. End of first paragraph pitches it clearly and rest of the passage tries to answer the question. (Book Question: 440) According to the passage, which of the following types of nectar-feeding pollinators is likely to be an unreliable pollinator of a particular cactus flower? A. A dietary specialist whose abundance is typically high in relation to that of the flower B. A dietary specialist whose abundance is at times significantly lower than that of the flower C. A dietary generalist for whom that flower’s nectar is not a preferred food but is the most consistently available food D. A dietary generalist for whom that flower’s nectar is slightly preferred to other available foods E. A dietary generalist that evolved from a species of dietary specialists (Book Question: 441) According to the passage, present-day columnar cacti in the Sonoran Desert differ from their close relatives in southern Mexico in that the Sonoran cacti A. have flowers that remain open after sunset B. are pollinated primarily by dietary specialists C. can be pollinated by nectar-feeding bats D. have stigmas that are unreceptive to pollination at night E. are sometimes pollinated by diurnal pollinators

hoangphuc · Answer

I don't know why answer A in Question 3 is wrong
Passage clearly states that  "Yet the flowers of the Sonoran Desert cacti have evolved to remain open after sunrise"

aragonn · Answer

P1 - some cacti evolved from night time to day time pollination.
P2 - reasons given for situation defined in p1.

The primary purpose of the passage is to 
A. compare the adaptive responses of several species of columnar cacti in the Sonoran Desert with those in the arid tropical regions of southern Mexico - p2
B. discuss some of the possible causes of the relatively low abundance of migratory nectar-feeding bats in the Sonoran Desert - P2/2
C. provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert - Most accurate among all.
D. present recent findings that challenge a particular theory as to why several species of columnar cacti in the Sonoran Desert have expanded their range of pollinators - challenge a theory is not there, its a simple explanation.
E. compare the effectiveness of nocturnal and diurnal pollination for several different species of columnar cacti in the Sonoran Desert -- Passage is certainly not about this.
------------------------------------------------

(Book Question: 440)
 According to the passage, which of the following types of nectar-feeding pollinators is likely to be an unreliable pollinator of a particular cactus flower? 
Lines to read - cactus-flower abundance tends to be high during spring, bat population densities tend to be low except near maternity roosts.
B. A dietary specialist whose abundance is at times significantly lower than that of the flower - correct.

------------------------------------------------
(Book Question: 441)
 According to the passage, present-day columnar cacti in the Sonoran Desert differ from their close relatives in southern Mexico in that the Sonoran cacti

E. are sometimes pollinated by diurnal pollinators - the flowers of the Sonoran Desert cacti have evolved to remain open after sunrise, allowing pollination by such diurnal visitors as bees and birds.

T1101 · Answer

Dear Verbal Expert,

even after reading the discussion I am still not able to fully understand Question 2... I understand why B is correct, but I do not understand why C is incorrect. Is it because of the "not a preferred food"? The passage never talks about explicitly about preference.

Thank you in advance!!!

(Book Question: 440)
According to the passage, which of the following types of nectar-feeding pollinators is likely to be an unreliable pollinator of a particular cactus flower?
A. A dietary specialist whose abundance is typically high in relation to that of the flower
B. A dietary specialist whose abundance is at times significantly lower than that of the flower
C. A dietary generalist for whom that flower???s nectar is not a preferred food but is the most consistently available food
D. A dietary generalist for whom that flower???s nectar is slightly preferred to other available foods
E. A dietary generalist that evolved from a species of dietary specialists

felixduan320 · Answer

Thanks for sharing this. A few things. Wouldn't the term "generalist", which suggests that they can pollinate more than one flower, indicate that a generalist would pollinate according to preference, and not abundance? It would make more sense for a specialist to prioritize abundance, since they can only feed on a select few flowers, whereas a generalist has more choices and can therefore be more picky. This would make Answer C more correct.

In addition, Answer A specifically mentions "at times significantly lower", whereas the excerpt in the passage quotes "their abundance may be CHRONICALLY low relative to the availability of flowers". It seems to be the passage is suggesting the only chronically low abundance would be a reliability issue, but if it happens once in a while ("at times"), the effects are not significant enough to warrant the "unreliable" label.

Let me know if the above makes sense. Based on the logic above, I am still struggling to understand why B is correct and not C. Thank you!

GMATNinja · Answer

The passage tells us that a generalist can be an unreliable pollinator because its "fidelity to a particular species depends on the  availability  of alternative food sources."

This tells us that a generalist doesn't really have a preference for which plant it eats -- it just eats whatever is most readily available. Let's say that out of plants X, Y, and Z, only plant X is available. Generalists will just eat plant X and be happy to do so! Now say that plants Y and Z move into the area -- the generalist doesn't remain faithful to plant X, but eats whatever is most convenient. Plant X will not get as much attention from the generalist -- so from plant X's perspective, the generalist is an  unreliable pollinator  when other plants are available for it to feed on.

The main point is that the generalist doesn't choose what it eats based on preference -- it chooses based on availability.

Take another look at (C):
 
Based on the discussion above, we know that generalists decide what to eat based on availability. So, it will reliably eat (and therefore pollinate) "the most consistently available food." This is essentially the opposite of answer choice (C), so (C) is incorrect.

To understand why (B) is correct, take a look at the two reasons identified in the passage to explain why a specialist may be an unreliable pollinator:
  "their abundance may vary widely from year to year, resulting in variable pollination of their preferred food species." OR 
 " their abundance may be chronically low relative to the availability of flowers."

Now take a look at answer choice (B):
 
You are correct to point out that (B) doesn't align well with a case in which the specialist population is "chronically low." However, this answer choice does fit nicely with the other case, in which the specialists' "abundance may vary widely." Because (B) specifically tells us that the specialist's abundance is "at times significantly lower than that of the flower," we can say that it is not consistently available to pollinate all of the flowers. Therefore, this particular specialist is an unreliable pollinator, and (B) is correct.

I hope that helps!

Tracy95 · Answer

According to the passage, present-day columnar cacti in the Sonoran Desert differ from their close relatives in southern Mexico in that the Sonoran cacti
A. have flowers that remain open after sunset
B. are pollinated primarily by dietary specialists
C. can be pollinated by nectar-feeding bats
D. have stigmas that are unreceptive to pollination at night
 E. are sometimes pollinated by diurnal pollinators

=> I didn't choose (E) because  I have concern over the word "sometimes" . Why does the choice include "sometimes"? What I get from (E) is that Sonaran Dessert cacti are not regularly pollinated by diurnal visitors while the sentence "Yet the flowers of the Sonoran Desert cacti have evolved to remain open after sunrise, allowing that the act of pollination as a fact, which is permanent." seems to consider that pollination  a permanent fact .

GMATNinja · Answer

Sorry for the delay!

Yes, the flowers of the Sonoran Desert cacti have evolved to remain open after sunrise, and that  capability  doesn't come and go. But just because the cacti can remain open after sunrise does not necessarily mean that those cacti are ALWAYS pollinated by diurnal pollinators. What if the diurnal pollinators are only around certain times of the year?

The passage only really tells us about the spring:

"... whereas cactus-flower abundance tends to be high during spring, bat population densities tend to be low except near maternity roosts. Moreover, in spring, diurnal cactus-pollinating birds are significantly more abundant in this region than are the nocturnal bats. Thus, with bats being unreliable cactus-flower pollinators, and daytime pollinators more abundant and therefore more reliable, selection favors the cactus flowers with traits that increase their range of pollinators."

We can certainly infer that the Sonoran Desert cacti are pollinated by diurnal pollinators in the spring, but we don't know what happens the rest of the year. That's why "sometimes" is appropriate in choice (E).

I hope that helps!

saby1410 · Answer

Hi,
 ChiranjeevSingh   VeritasKarishma   GMATNinja   AnishPassiTGC   GMATNinja Two
They can be dietary generalists whose fidelity to a particular species depends on the availability of alternative food sources. Or, they can be dietary specialists, but their abundance may vary widely from year to year, resulting in variable pollination of their preferred food species. Finally, they
may be dietary specialists, but their abundance may be chronically low relative to the availability of flowers.

In the above line what role does Contrasting word But plays. What is contrast in the line.
how they arrive to tell that they may be dietary specialists( Finally, they may be dietary specialists)
What is the role of BUT in last sentence and What purpose this line(but their abundance may be chronically low relative to the availability of flowers.) serves.

KarishmaB · Answer

Look at a simpler example first:

Following are the reasons you shouldn't trust him to help you:
1. He may be a bad person.
2. He may be a good person but he may be busy with helping others.
3. He may be a good person but he may not know what to do.

On the same lines, the passage is listing the various reasons why pollinators can be unreliable.

1. They can be dietary generalists (they eat anything) whose fidelity to a particular species depends on the availability of alternative food sources. (so if they get some other food sources, they may not pollinate and hence that makes them unreliable)

2. They can be dietary specialists, but their abundance may vary widely from year to year, resulting in variable pollination of their preferred food species.

This is in contrast to point 1 above. They could be dietary specialists (not generalists) such that they would eat the nectar of this cacti only so they would be reliable, but their numbers may vary from year to year so that would make them unreliable.

3. They may be dietary specialists, but their abundance may be chronically low relative to the availability of flowers.

Another reason could be that their number could be lower than what is required to pollinate all flowers. So this would make them unreliable.

sagarsangani123 · Answer

Dear Experts,

For the correct answer choice in this question, there seems to be a confusion. The passage states that   In these
tropical regions, diurnal (daytime) visitors to columnar cactus flowers are  ineffective  pollinators because, by sunrise, the flowers' stigmas become unreceptive or the flowers close  . The word 'ineffective' doesn't mean that the columnar cacti in southern Mexico are never pollinated by diurnal pollinator. Right? Then there is no point in comparing the frequency of pollination by diurnal pollinators of the two regions. But, the correct answer choice "are  sometimes  pollinated by diurnal pollinators?" seems to do that.

Kindly help resolve.

GMATNinja · Answer

Question 3 
 
The first sentence of the passage should make this more clear:

"In the Sonoran Desert of northwestern Mexico and southern Arizona, the flowers of several species of columnar cacti—cardon, saguaro, and organ pipe—were once  exclusively  pollinated at night by nectar-feeding bats,  as their close relatives in arid tropical regions of southern Mexico still are."

From this, we know that the columnar cacti in Southern Mexico are ONLY pollinated at night by nectar-feeding bats, and NEVER pollinated by any daytime creatures. Cacti in the Sonoran Desert, by contrast, have adapted to allow for diurnal pollinators.

So, we can compare the frequency of diurnal pollination -- the cacti in Southern Mexico are never pollinated in the day, while the cacti in the Sonoran Desert are sometimes pollinated in the day.

(E) is the correct answer to question 3.

I hope that helps!

waytowharton · Answer

In question 440, Why is Option B i.e. "Are pollinated primarily by dietary specialist" incorrect? We are given in "Southern Mexico" bats pollinate 100% of available flowers while in "Sonoran Desert" pollinating birds are more abundant than nocturnal bats.
Please help  abhimahna   GMATNinja   VeritasKarishma

KarishmaB · Answer

According to the passage, present-day columnar cacti in the Sonoran Desert differ from their close relatives in southern Mexico in that the Sonoran cacti
A. have flowers that remain open after sunset
B. are pollinated primarily by dietary specialists
C. can be pollinated by nectar-feeding bats
D. have stigmas that are unreceptive to pollination at night
E. are sometimes pollinated by diurnal pollinators

We need to find the diff between cacti in Sonoran desert and those in Southern Mexico:

A. have flowers that remain open after sunset
Both are pollinated by nocturnal bats so they both have flowers open after sunset.

B. are pollinated primarily by dietary specialists
In Sonoran desert, both bats and birds pollinate. We know that bats are dietary specialists but we are not given whether birds are dietary specialists or not (whether they have only specific food sources or a wide variety of food sources). What we know is that the flowers allow both to pollinate. It is possible that birds are also dietary specialists. 
In southern Mexico, we know that bats pollinate and they are dietary specialists.
Hence we can't say that this is a diff between the two.

C. can be pollinated by nectar-feeding bats
Both can be.

D. have stigmas that are unreceptive to pollination at night
Both are pollinated at night so their stigmas are receptive at night.

E. are sometimes pollinated by diurnal pollinators
Correct. Birds are diurnal pollinators. In Sonoran desert, birds pollinate too. In Southern Mexico, only bats pollinate.

Answer (E)

avigutman · Answer

Video solution from Quant Reasoning starts at 47:20 Subscribe for more: https://www.youtube.com/QuantReasoning? ... irmation=1 https://www.youtube.com/watch?v=Qyd5onrqTgc

woohoo921 · Answer

KarishmaB 
I am a bit confused on how the passage refers to "nectar-feeding bats" and then just "bats". Why wouldn't the GMAT repeat the "nectar-feeding" part, or is that just to be assumed at this point?

Also, to clarify for #527 (according to the passage, present-day columnar cacti in the Sonoran Desert differ from their close relatives in southern Mexico in that the Sonoran cacti), is the support for Choice E found here: "Yet the flowers of the Sonoran Desert cacti have evolved to remain open after sunrise" --> in that the "yet" indicates the difference?

avigutman

In the Sonoran Desert of northwestern Mexico and southern Arizona, t

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In the Sonoran Desert of northwestern Mexico and southern Arizona, t

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