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| 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. |
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
答案：E
