What causes a helix in nature to appear with either a dextral (right-h

Question

What causes a helix in nature to appear with either a dextral (right-handed, or clockwise) twist or a sinistral (lef-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 lef-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 lef- and right-handed snails are not actually true mirror images of one another. Teir shapes are noticeably different. Sinistral rarity might, then, be a consequence of possible disadvantages conferred by these other  concomitant structural features.  In addition, perhaps lef- and right-handed snails cannot mate with each other, having incompatible twist directions. Presumably an individual of the rarer form would have relative difculty in fnding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right- and lef-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 lef-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 lef-handed—and this ofen 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 lef-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 lef-handed but most switch to being right-handed. Tus the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself.    1 
 1. Which of the following would serve as an example of “concomitant structural features” ( Highlighted ) that might disadvantage a snail of the rarer form?

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

2  
 2. 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

3  
 3. Which of the following accurately describes the relationship between the evolutionary and developmental theories discussed in the passage?

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 frst cannot explain.
D. The second theory demonstrates why the frst is valid only for very unusual, special cases.
E. They are identical and interchangeable in that the second theory merely restates the frst in less technical terms.

4  
 4. 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

Sajjad1994 · Answer

Passage Summary

The passage addresses the question of what determines the dextral (clockwise) or sinistral (counterclockwise) shape of snail shells and discusses two possible explanations of the distributional patterns of shell shapes in snail populations: the evolutionary theory and the developmental theory. Evidence against the evolutionary theory is presented, and the developmental theory is offered as a more plausible explanation.

Sajjad1994 · Answer

OE

1. Which of the following would serve as an example of “concomitant structural features” (     Highlighted     ) that might disadvantage a snail of the rarer form?

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. 
 
  Answer: E

Sajjad1994 · Answer

OE

2. The second paragraph of the passage is primarily concerned with offering possible reasons why

The first paragraph ends with the question  “What mechanisms control handedness and keep lef-handedness rare?”  Te 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 difculties in mating for lef-handed snails. Thus  Choice B  is correct.

The paragraph starts by dismissing the idea that evolution could be working against lef-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). 
  
  Answer: B

Sajjad1994 · Answer

OE

3. Which of the following accurately describes the relationship between the evolutionary and developmental theories discussed in the passage?

The correct answer is  Choice C.  Te third paragraph of the passage argues that the evolutionary theory cannot explain  “why right-handedness should have become predominant”  and lists some specifc 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. 
  
  Answer: C

Sajjad1994 · Answer

OE

4. 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

The question asks about possible reasons why a  “sinistral snail species might stay predominantly sinistral” ; this would happen if the offspring of a sinistral species is largely sinistral.  Choices A, C, D,  and  E  give plausible reasons for why this might happen: either genetic predisposition for sinistrality (Choice A) or lack of reproductive competition from dextral individuals (Choices C, D, and E). Therefore, the correct answer is  Choice B : having structural features that advantage dextral snails would tend to reduce the number of sinistral individuals in a species and thus to eventually transform a sinistral species into a dextral one.

Answer: B

What causes a helix in nature to appear with either a dextral (right-h

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