In the Sonoran Desert of northwestern Mexico and southern Arizona, the flowers of several species of columnar cacti—cardon, saguaro, and...
GMAT Reading Comprehension : (RC) Questions
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 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.
The primary purpose of the passage is to
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| 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. | What it says: Desert cacti used to be pollinated only by bats at night, just like their relatives in tropical Mexico still are today. What it does: Sets up the basic scenario and introduces the main characters (cacti and bats) Source/Type: Factual scientific information Connection to Previous Sentences: This is the opening - establishes our foundation knowledge Visualization: • Past: Sonoran Desert cacti = 100% bat pollination at night • Present: Tropical Mexico cacti = 100% bat pollination at night What We Know So Far: Two regions have similar cacti, bats pollinate them at night What We Don't Know Yet: Why the passage mentions "were once" - something has changed |
| 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. | What it says: In tropical areas, day-time pollinators can't do the job because flowers shut down or close at sunrise. What it does: Explains WHY tropical cacti stick to night pollination only Source/Type: Scientific explanation/mechanism Connection to Previous Sentences: This builds on sentence 1 by explaining the biological reason behind the "bat-only" system in tropical Mexico Visualization: Tropical Mexico Timeline: • Night: Flowers open + receptive → Bats pollinate successfully • Sunrise: Flowers close/become unreceptive → Day visitors fail Reading Strategy Insight: This is helpful detail, not complexity - the author is explaining the logic behind what we just learned |
| 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. | What it says: BUT Sonoran Desert cacti flowers stay open during the day, so bees and birds can pollinate them too. What it does: Reveals the key contrast - shows what has changed since "were once" from sentence 1 Source/Type: Scientific observation/evolutionary fact Connection to Previous Sentences: This directly contrasts with both previous sentences - explains WHY sentence 1 said "were once" and shows how Sonoran cacti differ from tropical ones Visualization: Sonoran Desert Timeline NOW: • Night: Flowers open → Bats pollinate • Sunrise & Day: Flowers STILL open → Bees and birds also pollinate vs. Tropical Mexico (stays same): • Night: Flowers open → Bats pollinate • Sunrise & Day: Flowers closed → No other pollinators Key Insight: We now see the central puzzle - why did Sonoran cacti evolve differently? |
2. Passage Summary:
Main Point:
Sonoran Desert cacti evolved to accept both night and day pollinators because bats alone couldn't reliably pollinate all their flowers - there were too many flowers and too few bats, while day-time pollinators like birds were much more abundant and dependable.