OG In the Sonoran Desert of northwestern Mexico

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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. 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. 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?

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
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.
Recent data reveals that during spring in the
Sonoran Desert, the nectar-feeding bats are
specialists feeding on cardon, saguaro, and
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
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
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
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
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.

439) 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

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

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

Q439: C
Q440: B
Q441: E

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by NandishSS » Tue Sep 19, 2017 7:55 pm
Hi Ceilidh/Dave,

How to deal with this long passage?

439) 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

OA is C but are we discussing evolutionary changes? Isn't it about the pollinated. How does it happen in lack of availability?

Can you pls explain Q2 & Q3?

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by NandishSS » Tue Apr 17, 2018 6:11 pm
Bumping for experts review

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by GMATGuruNY » Sat Apr 21, 2018 2:51 am
NandishSS wrote: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
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?

The rest of the passage attempts to answer why Sonoran Desert cacti have evolved to expand their range of pollinators.
Only C correctly describes this purpose:
provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert.

The correct answer is C.
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by GMATGuruNY » Tue Apr 24, 2018 4:00 am
AbeNeedsAnswers wrote: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
From the passage:
Pollinators can be unreliable for several reasons.
They may be dietary specialists, but their abundance may be chronically low relative to the availability of flowers.
Nectar-feeding bats...being unreliable cactus-flower pollinators, and daytime pollinators more abundant and therefore more reliable.


According to the excerpt above, daytime pollinators are more abundant than nectar-feeding bats and thus are more reliable than nectar-feeding bats.
In other words, nectar-feeding bats are less reliable because they are LESS ABUNDANT, exemplifying one reason that pollinators can be unreliable:
Their abundance may be chronically low relative to the availability of flowers.
This information supports B:
A dietary specialist whose abundance is at times significantly lower than that of the flower.

The correct answer is B.
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by GMATGuruNY » Tue Apr 24, 2018 4:46 am
AbeNeedsAnswers wrote:
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
From the passage:
During spring in the Sonoran Desert...diurnal cactus-pollinating birds are significantly more abundant.
Compared with the Sonoran Desert's cardon and saguaro, columnar cacti in southern Mexico produce far fewer flowers per night
, with the result that bats are able to pollinate nearly 100 percent of the available flowers.

Whereas nearly 100% of columnar cacti are pollinated by BATS (which pollinate only at night), other types of Sonoran cacti are pollinated by CACTUS POLLINATING BIRDS (which are DIURNAL and thus pollinate during the day).
This information supports E:
Their close relatives...are sometimes pollinated by diurnal pollinators.

The correct answer is E.
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