Certain groups of Asian snails include both “left-handed” and “right-handed” species, with shells coiling to the left and right, respectively. Some left-handed species have evolved from right-handed ones. Also, researchers found that snail- eating snakes in the same habitat have asymmetrical jaws, allowing them to grasp right-handed snail shells more easily. If these snakes ate more right-handed snails over time, this would have given left-handed snails an evolutionary advantage over right-handed snails, with the left-handed snails eventually becoming a new species. Thus, the snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species.


Which of the following would, if true, most strengthen the argument that asymmetrical snake jaws helped drive left-handed snail evolution?    


In one snake species, the snakes with asymmetrical jaws eat snails, while the snakes with symmetrical jaws do not eat snails.    

Some species of Asian snails contain either all right-handed snails, or all left-handed snails.    

Anatomical differences prevent left-handed snails from mating easily with right-handed snails.    

Some right-handed snails in this habitat have shells with a very narrow opening that helps prevent snakes from extracting the snails from inside their shells.    

Experiments show that the snail-eating snakes in this habitat fail more often in trying to eat left-handed snails than in trying to eat right-handed snails.    

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考题讲解

情景：一种亚洲蜗牛分为“左撇子”和“右撇子”。一些左撇子是从右撇子进化来的。研究人员也发现吃蜗牛的蛇的下巴不对称，这种下巴让蛇吃右撇子更方便。因此，左撇子有了这种优势后慢慢进化成一个新物种。所以，蛇的不对称的下巴促进了左撇子蜗牛这一物种的出现。

推理：要加强的结论：蛇的下巴不对称→左撇子蜗牛出现。

选项分析：

A选项：下巴不对称的蛇吃蜗牛，下巴对称的蛇不吃蜗牛：与下巴对称的蛇无关。

B选项：一些亚洲蜗牛的物种要不然全是左撇子要不然全是右撇子：无关。我们要找的是蛇的下巴和左撇子蜗牛的出现之间的关系。

C选项：解剖上的差异阻止左撇子蜗牛和右撇子蜗牛交配：无关。

D选项：一些右撇子蜗牛的壳有一个狭窄的额开口，会阻止蛇把蜗牛从壳里取出来：无关。

E选项：正确。实验证明吃蜗牛的蛇比起吃右撇子蜗牛更不容易吃左撇子蜗牛：加强了蛇在左撇子蜗牛这一物种出现上起到的作用。

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