首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
What causes a helix in nature to appear with either a dextral("right-handed" or clockwise)twist or a sinistral("left-handed," or
What causes a helix in nature to appear with either a dextral("right-handed" or clockwise)twist or a sinistral("left-handed," or
admin
2015-05-25
44
问题
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.
The second paragraph of the passage is primarily concerned with offering possible reasons why
选项
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
答案
B
解析
The first paragraph ends with the question "What mechanisms control handedness and keep left-handedness rare?" The second paragraph attempts to answer this question: perhaps, the paragraph suggests, sinistral rarity is a consequence either of natural selection working on "concomitant structural features," or of difficulties in mating for left-handed snails. Thus Choice B is correct. The paragraph starts by dismissing the idea that evolution could be working against left-handedness itself, but Choice A is incorrect because the paragraph is not concerned with offering reasons for that dismissal. The paragraph does not even suggest that opposite-handed populations tend to intermingle(Choice C)or that dextral snails breed more rapidly(Choice E), and it does not discuss developmental mechanisms(Choice D).
转载请注明原文地址:https://kaotiyun.com/show/ewjO777K
本试题收录于:
GRE VERBAL题库GRE分类
0
GRE VERBAL
GRE
相关试题推荐
Imagineaschoolthatexpecteditsstudentstobecomeliteratewithoutanyformalinstruction.Mostparentswouldbealarmedby
Itisdifficulttoimaginewhatlifewouldbelikewithoutmemory.Themeaningsofthousandsofeverydayperceptions,thebases
Inwhatnowseemsliketheprehistorictimesofcomputerhistory,theearlypost-warera(战后时期),therewasaquitewidespreadcon
Nooneknowshowmanlearnedtomakewords.Perhapshebeganbymakingsoundslikethosemadebyanimals.Perhapshegruntedlik
Thenoticewaswritteninseverallanguages_________foreigntouristsshouldmisunderstandit.
AreportconsistentlybroughtbackbyvisitorstotheUSishowfriendly,courteousandhelpfulmostAmericansweretothem.To
Afriendofmine,inresponsetoaconversationwewerehavingabouttheinjusticesoflife,askedmethequestion:"Whosaidli
Fiftyapplicantsforajobweregivenscoresfrom1to5ontheirinterviewperformance.Theirscoresareshowninthetableabo
Eachof5positivewholenumbersisatleast8.Theaverage(arithmeticmean)ofthese5numbersis24.Iftheaverageoftwooft
Inanattempttoimprovetheoverallperformanceofclericalworkers,manycompanieshaveintroducedcomputerizedperformancemo
随机试题
______道德观认为,能给行为影响所涉及的大多数人带来最大利益的行为才是,善的。
下列除哪项外,均可引起血清钾增高()
首次产前检查的时间应在
①通货膨胀必然会降低人们的实际收入,从而降低人们的生活水平。②通货膨胀会造成国民收入的重新分配。③菲利普斯曲线反映了通货膨胀与失业率之间的相互替代关系。④拉弗曲线表示税率与总税收之间的关系。以上说法中正确的是()。
某股份有限公司已向银行贴现的商业承兑汇票,不包括在资产负债表中的“应收票据”项目中。()
科学家们预言,21世纪将是生物学世纪。生物学世纪将在三个领域中到来:医药、环境整治和农业。以清除污染为例,新的研究表明,在进化的过程,自然界曾反复地从微生物中增加或减少基因簇,就像调试计算机时增加或减少程序一样。既然自然界能这样做,今天的操纵者也
Agingbabyboomersaredeterminedtofighttheagingprocess.Theyspendmillionsofdollarsayearon【C1】________theyperceive
Anumberofrecentbookshavereworkedsubjects,formsandwritingtechniques.Today’schildrenreadstoriesaboutdivorce,deat
NexusWireYourUltimateLinktotheWorldPOSITIONSAVAILABLE:Toupholdourreputationasoneofthemostwidelyreadnewsmaga
Musiccomesinmanyforms;mostcountrieshaveastyleoftheirown.【C1】____.theturnofthecenturywhenjazzwasborn,Ameri
最新回复
(
0
)