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
26
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 would serve as an example of "concomitant structural features"(line 13)that might disadvantage a snail of the rarer form?
选项
A、A shell and body that are an exact mirror image of a snail of the commoner form
B、A smaller population of the snails of the rarer form
C、A chip or fracture in the shell caused by an object falling on it
D、A pattern on the shell that better camouflages it
E、A smaller shell opening that restricts mobility and ingestion relative to that of a snail of the commoner form
答案
E
解析
Choice A is incorrect: the passage states that "any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable." Choice B is incorrect, as a smaller population would be the consequence of the disadvantage, not the cause of it. Choice C is incorrect, as damage caused by an external object is not a "structural feature" of a snail shell Choice D is incorrect, as better camouflage is an advantage, not a disadvantage. The correct answer is Choice E: an impaired ability to move around and ingest food would be a disadvantage.
转载请注明原文地址:https://kaotiyun.com/show/YwjO777K
本试题收录于:
GRE VERBAL题库GRE分类
0
GRE VERBAL
GRE
相关试题推荐
Itisdifficulttoimaginewhatlifewouldbelikewithoutmemory.Themeaningsofthousandsofeverydayperceptions,thebases
Inwhatnowseemsliketheprehistorictimesofcomputerhistory,theearlypost-warera(战后时期),therewasaquitewidespreadcon
Shewastheonlymemberofherfamily_________spokeEnglish.
Concerningmoneyoranythingelse,conflictsbetweenhusbandandwifeusuallyreflectapowerstruggle.Conflictsbetweenparent
Nooneknowshowmanlearnedtomakewords.Perhapshebeganbymakingsoundslikethosemadebyanimals.Perhapshegruntedlik
Longbusridesareliketelevisionshows.Theyhaveabeginning,amiddle,andanend—withcommercialsthrownineverythreeor
Thenoticewaswritteninseverallanguages_________foreigntouristsshouldmisunderstandit.
Afriendofmine,inresponsetoaconversationwewerehavingabouttheinjusticesoflife,askedmethequestion:"Whosaidli
Noambitiouspoliticianwillchallengeanelectedpublicofficialrunningforreelectionunlessthatofficialappearsvulnerable
ItispossiblethatNativeAmericansoriginallyhavemigratedtotheWesternHemisphereoverabridgeoflandthatonceexisted
随机试题
请使用VC6或使用【答题】菜单打开考生文件夹prog3下的工程p~og3,其中包含了类Polynomial(“多项式”)的定义。形如5x4+3.4x2-7x+2的代数式称为多项式,其中的5为4次项系数,3.4为2次项系数,-7为1次项系数,2为0次项(常
教师根据成绩对学生进行表扬或者批评,这些强化方式体现了班都拉提出的()
根据《公司法》规定,公司不得收购本公司的股票,但下列情况除外()。
在Word中按()键后,可删除当前光标位置前一个字符。
在德育过程中起主导作用的是()
程序教学模式的理论基础是认知结构学习理论。()
某时刻时针与分针的夹角为60度,问:至少经过多长时间,时针和分针又一次形成60度夹角?
由于创办“快乐之家”而被誉为“第一个新式学校的教师”的意大利人文主义教育家是()
甲、乙双方同意协议离婚。甲因出差,故委托丙去婚姻登记机关代为办理离婚登记手续。根据我国法律,丙()(2012年非法学基础课单选第25题)
Ifasubstanceisdissolvedinwaterorheated,itmay______agas.
最新回复
(
0
)