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题库解析   >   主旨题题型   >   RC439-455
    本题由hyman1提供
117次学习
RC439-455

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 Deses 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.

【OG20-P426-525题】

The primary purpose of the passage is to

    <p>(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</p><p><br/></p><p><br/></p><p>(B)discuss some of the possible cause of the relatively low abundance of migratory nectar-feeding bats in the Sonoran Desert</p><p><br/></p><p>(C) provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert</p><p>&nbsp;</p><p>(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</p><p>&nbsp;</p><p>(E) compare the effectiveness of nocturnal and diurnal pollination for several different species of columnar cacti in the Sonoran Desert</p><p><br/></p>
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答案:
C

A选项(

)通读全文便可以发现 这篇文章并不是在比较两种cacti的适应能力

B选项(错)蝙蝠的数量稀少仅仅是用来解释为什么在 Sonoran Desert’s columnar cacti的适应能力强,而没有说明蝙蝠数量少的原因

C选项(对)通篇文章都是在说为什么这种cacti在白天也可以pollinated

D选项(错)注意关键字眼 “challenge” 文章是在解释为什么白天也授粉的原因,并非chalenge 这个原因

E选项(错)也很扯啦,并没有对比两个时间段授粉的效率啦

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