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
94
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.
Which of the following accurately describes the relationship between the evolutionary and developmental theories discussed in the passage?
选项
A、Although the two theories reach the same conclusion, each is based on different assumptions.
B、They present contradictory explanations of the same phenomenon.
C、The second theory accounts for certain phenomena that the first cannot explain.
D、The second theory demonstrates why the first is valid only for very unusual, special cases.
E、They are identical and interchangeable in that the second theory merely restates the first in less technical terms.
答案
C
解析
The correct answer is Choice C. The third paragraph of the passage argues that the evolutionary theory cannot explain "why right-handedness should have become predominant" and lists some specific reproductive outcomes that cannot be accounted for by this theory. The next paragraph offers an alternative theory(developmental)that seems to do a better job of explaining these outcomes.
转载请注明原文地址:https://kaotiyun.com/show/lwjO777K
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
Normallyastudentmustattendanumberofcoursesinordertograduate,andeachcoursewhichheattendsgiveshimagradewhic
Itiseasiertonegotiateinitialsalaryrequirementbecauseonceyouareinside,theorganizationalconstraints(约束)influencew
Justinwasalwaysprepared.Hismottowas"Neverthrowanythingout,youneverknowwhenitmightcomeinhandy."Hisbedroomwa
Dina:Youlookrun-down,Brian.Brian:Yeah.I’vebeenfeelingundertheweatherrecently.【D8】__________andIstillhaven’tgotte
Thepassageasawholecanbestbecharacterizedaswhichofthefollowing?TheauthorofthepassagequotestheWhiteofficer
Theprimarypurposeofthepassageisto______Theresearchers’conclusionconcerningtheabsenceofdemographicbiaswouldbe
Theauthorindicatesexplicitlythatwhichofthefollowingrecordshasbeenasourceofinformationinherinvestigation?The
DavidBelasco’s1912BroadwayproductionofTheGovernor’sLadycreatedasensationwithascenesetinaChild’scafeteria,ac
随机试题
国库券通常采取的发行方式是()
流式细胞术检测________、________为诊断阵发性睡眠性血红蛋白尿提供直接的证据。
患者,男,34岁。间断喘息10年余。发作时服用氨茶碱症状可缓解。为改善患者的症状,应采取的措施有提示:该患者喘息症状突然加重,查体:双肺满布哮鸣音,心率103次/分。
患者,男,65岁。素体气虚,复感外邪,恶寒较重,无发热,鼻塞流涕,头痛无汗,肢体倦怠乏力,咳嗽咳痰无力,舌质淡,苔薄白,脉浮。诊断为气虚感冒。据此判断气的功能减退主要体现在
A.口淡乏味B.口甜黏腻C.口中酸馊D.口中泛酸E.口苦脾胃湿热则()
下列叙述正确的是()。
作为财务管理的目标,企业价值最大化与股东财富最大化相比,其优点是()。
PowerDesigner是SYBASE公司的产品,属于CASE工具,与PowerDesigner是SYBASE公司的产品还有【】、【】和【】。
函数delete(s,i,n)是作用是从字符串s中删除从第i个字符开始的n个字符,请填空。voiddelete(chars[],inti,intn){intj,k,length=0;while(s[length])
Doyouknowanyonewhofeelsawkwardatapartyorhatestobeinacrowdofpeople?They’reprobablyjustshy,butiftheyget
最新回复
(
0
)