【OG24-524题】The primary purpose of the passage is to_mofa留学圈mofaGRE在线题库系统_GRE在线搜题_GRE题库解析_GRE真题机经_GRE课程_GRE培训机构

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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 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.

The primary purpose of the passage is to

compare the adaptive responses of several species of columnar cacti in the Sonoran Desert with those in the arid tropical regions of southern Mexico discuss some of the possible cause of the relatively low abundance of migratory nectar-feeding bats in the Sonoran Desert provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert 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 compare the effectiveness of nocturnal and diurnal pollination for several different species of columnar cacti in the Sonoran Desert

正确答案： C

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文章大意逻辑
在墨西哥西北部和亚利桑那州南部的索诺兰沙漠，柱状仙人掌花曾经只在夜间由吸食花蜜的蝙蝠授粉，而其在墨西哥南部干旱热带地区的近亲仍是如此。但索诺兰沙漠的柱状仙人掌花日出后仍开放，可由蜜蜂和鸟类等白天访客授粉。为什么会这样？原因可能是吸食花蜜的迁徙蝙蝠数量每年有变化，授粉者不可靠有多种原因。最近数据显示索诺兰沙漠春季时，蝙蝠是特定仙人掌花的授粉者，但仙人掌花数量春季多，蝙蝠种群密度除了在繁殖栖息地附近外较低，且春季白天授粉者数量比夜间蝙蝠多。 文章类型及逻辑简图
文章类型：问题解释型。逻辑简图： In the Sonoran Desert, columnar cactus flowers once only pollinated by night bats, now open after sunrise for diurnal pollinators. Possible reason: Variation in migratory nectar feeding bats' abundance; Pollinators can be unreliable for several reasons. Recent data: Bats are specialists on certain cactus flowers in spring, but low bat density and more diurnal pollinators. 问题类型及做题方法
问题类型为主旨题，做题方法是通过梳理文章整体内容，把握文章围绕索诺兰沙漠柱状仙人掌花授粉者变化原因展开这一核心。 选项分析
A选项：文章重点不是比较索诺兰沙漠和墨西哥南部柱状仙人掌的适应性反应，而是解释索诺兰沙漠仙人掌花授粉者变化，A错误。 B选项：文章不是讨论迁徙吸食花蜜蝙蝠数量低的原因，而是仙人掌花授粉者范围扩大的原因，B错误。 C选项：文章确实是为索诺兰沙漠某些柱状仙人掌花的进化变化（授粉者范围扩大）提供可能解释，C正确。 D选项：文章没有提出挑战特定理论的发现，D错误。 E选项：比较夜间和白天授粉效果不是文章主旨，文章主旨是解释授粉者范围变化，E错误。

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