- 2018年05月22日18:09 来源：小站教育作者：小站GRE编辑
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What causes a helix in nature to appear with either a dextral (right-handed, or clockwise) twist or a sinistral (left-handed, or counterclockwise) twist is one of the most intriguing puzzles in the science of form. Most spiral-shaped snail species are predominantly dextral. But at one time, handedness (twist direction of the shell) was equally distributed within some snail species that have become predominantly dextral or, in a few species, predominantly sinistral. What mechanisms control handedness and keep left-handedness rare?
It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable. But left- and right-handed snails are not actually true mirror images of one another. Their shapes are noticeably different. Sinistral rarity might, then, be a consequence of possible disadvantages conferred by these other concomitant structural features. In addition, perhaps left- and right-handed snails cannot mate with each other, having incompatible twist directions. Presumably an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right- and left-handed populations.
But this evolutionary mechanism combining dissymmetry, anatomy, and chance does not provide an adequate explanation of why right-handedness should have become predominant. It does not explain, for example, why the infrequent unions between snails of opposing hands produce fewer offspring of the rarer than the commoner form in species where each parent contributes equally to handedness. Nor does it explain why, in a species where one parent determines handedness, a brood is not exclusively right- or left-handed when the offspring would have the same genetic predisposition. In the European pond snail Lymnaea peregra, a predominantly dextral species whose handedness is maternally determined, a brood might be expected to be exclusively right- or left-handed—and this often occurs. However, some broods possess a few snails of the opposing hand, and in predominantly sinistral broods, the incidence of dextrality is surprisingly high.
Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness. In the case of Lymnaea peregra, studies indicate that a dextral gene is expressed during egg formation; i.e., before egg fertilization, the gene produces a protein, found in the cytoplasm of the egg, that controls the pattern of cell division and thus handedness. In experiments, an injection of cytoplasm from dextral eggs changes the pattern of sinistral eggs, but an injection from sinistral eggs does not influence dextral eggs. One explanation for the differing effects is that all Lymnaea peregra eggs begin left-handed but most switch to being right-handed. Thus the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself.
1. The second paragraph of the passage is primarily concerned with offering possible reasons why
A. it is unlikely that evolutionary mechanisms could discriminate against sinistral snails
B. sinistrality is relatively uncommon among snail species
C. dextral and sinistral populations of a snail species tend to intermingle
D. a theory based on a developmental mechanism inadequately accounts for the predominance of dextrality across snail species
E. dextral snails breed more readily than sinistral snails, even within predominantly sinistral populations
线索很明显在第二段. A选项为误导选项,其与第二段的第一句很像,如果只看这句就做题可能会被误导。但题目问的是"offering possible reasons" why, 这是第二段中间的 Sinistral rarity might....才明说. 此句说造成的sinistral rarity这样的结果可能是也由下面concomitant structural features造成, 所以此句当然也就是那些possible reasons要解释的。
2. It can be inferred from the passage that a predominantly sinistral snail species might stay predominantly sinistral for each of the following reasons EXCEPT for
A. a developmental mechanism that affects the cell-division pattern of snails
B. structural features that advantage dextral snails of the species
C. a relatively small number of snails of the same hand for dextral snails of the species to mate with
D. anatomical incompatibility that prevents mating between snails of opposing hands within the species
E. geographic separation of sinistral and dextral populations
A选项的理由可在最后一段找到; B,C,D,E选项的线索都在第二段,但B选项是advantage 'dextral' snails 而不是sinistral snails. B中的structure feature在文中是用来解释右旋比左旋更有优势。而题目问的是,左旋物种一直保持左旋的原因。即,structure feature可以解释左旋比右旋少,但是不能解释为何左旋物种一直保持左旋，所以选B。