SAMPLE 12

　　[生物學(xué)類]

　　Flatfish, such as the flounder, are among the few vertebrates that lack approximate bilateral symmetry (symmetry in which structures to the left and right of the body’s midline are mirror images). Most striking among the many asymmetries evident in an adult flatfish is eye placement: before maturity one eye migrates, so that in an adult flatfish both eyes are on the same side of the head. While in most species with asymmetries virtually all adults share the same asymmetry, members of the starry flounder species can be either left-eyed (both eyes on the left side of head) or right-eyed. In the waters between the United States and Japan, the starry flounder populations vary from about 50 percent left-eyed off the United States West Coast, through about 70 percent left-eyed halfway between the United States and Japan, to nearly 100 percent left-eyed off the Japanese coast.

　　Biologists call this kind of gradual variation over a certain geographic range a “cline” and interpret clines as strong indications that the variation is adaptive, a response to environmental differences. For the starry flounder this interpretation implies that a geometric difference (between fish that are mirror images of one another) is adaptive, that left-eyedness in the Japanese starry flounder has been selected for, which provokes a perplexing question: what is the selective advantage in having both eyes on one side rather than on the other?

　　The ease with which a fish can reverse the effect of the sidedness of its eye asymmetry simply by turning around has caused biologists to study internal anatomy, especially the optic nerves, for the answer. In all flatfish the optic nerves cross, so that the right optic nerve is joined to the brain’s left side and vice versa. This crossing introduces an asymmetry, as one optic nerve must cross above or below the other. G. H. Parker reasoned that if, for example, a flatfish’s left eye migrated when the right optic nerve was on top, there would be a twisting of nerves, which might be mechanically disadvantageous. For starry flounders, then, the left-eyed variety would be selected against, since in a starry flounder the left optic nerve is uppermost.

　　The problem with the above explanation is that the Japanese starry flounder population is almost exclusively left-eyed, and natural selection never promotes a purely less advantageous variation. As other explanations proved equally untenable, biologists concluded that there is no important adaptive difference between left-eyedness and right-eyedness, and that the two characteristics are genetically associated with some other adaptively significant characteristic. This situation is one commonly encountered by evolutionary biologists, who must often decide whether a characteristic is adaptive or selectively neutral. As for the left-eyed and right-eyed flatfish, their difference, however striking, appears to be an evolutionary red herring.

　　1. According to the text, starry flounder differ form most other species of flatfish in that starry flounder

　　[A] are not basically bilaterally symmetric.

　　[B] do not become asymmetric until adulthood.

　　[C] do not all share the same asymmetry.

　　[D] have both eyes on the same side of the head.

　　2. Which of the following best describes the organization of the text as a whole?

　　[A] A phenomenon is described and an interpretation presented and rejected.

　　[B] A generalization is made and supporting evidence is supplied and weighed.

　　[C] A contradiction is noted and a resolution is suggested and then modified.

　　[D] A series of observations is presented and explained in terms of the dominant theory.

　　3. The text supplies information for answering which of the following questions?

　　[A] Why are Japanese starry flounder mostly left-eyed?

　　[B] Why should the eye-sidedness in starry flounder be considered selectively neutral?

　　[C] Why have biologists recently become interested in whether a characteristic is adaptive or selectively neutral?

　　[D] How do the eyes in flatfish migrate?

　　4. Which of the following is most clearly similar to a cline as it is described in the second paragraph of the text?

　　[A] A vegetable market in which the various items are grouped according to place of origin.

　　[B] A wheat field in which different varieties of wheat are planted to yield a crop that will bring the maximum profit.

　　[C] A flower stall in which the various species of flowers are arranged according to their price.

　　[D] A housing development in which the length of the front struts supporting the porch of each house increases as houses are built up the hill.

　　5. Which of the following phrases from the text best expresses the author’s conclusion about the meaning of the difference between left-eyed and right-eyed flatfish?

　　[A] “Most striking” (line 3, paragraph 1)

　　[B] “variation is adaptive” (line 2, paragraph 2)

　　[C] “mechanically disadvantageous” (line 7, paragraph 3)

　　[D] “evolutionary red herring” (line 9, paragraph 4)

　　[答案與考點解析]

　　1. 【答案】C

　　【考點解析】這是一道細節(jié)推導(dǎo)題。根據(jù)本題題干中的“most other species”可以把本題的答案信息來源確定在第一段的第三句，從第三句的主從句中可以歸納推導(dǎo)出本題的正確答案C。本句中的“while”一詞的含義是“雖然、盡管”，用來表達事物之間的對照對比。考生在解題時一定要首先學(xué)會審題定位的基本功。

　　2. 【答案】A

　　【考點解析】這是一道文章具體寫作手法題。這是一道比較難的題型，需要考生對文章的敘述結(jié)構(gòu)有深刻的理解。本文第一段介紹了一種現(xiàn)象，第二段和第三段是生物學(xué)家對這種現(xiàn)象的解釋，第四段是本文作者對生物學(xué)家的解釋提出否定�？梢姳绢}的正確選項是A�？忌�在解題時一定要注意文章的發(fā)展脈絡(luò)。

　　3. 【答案】B

　　【考點解析】本題是一道審題定位題型。本題的題干沒有具體給出尋找答案信息的任何線索。在這種情況下考生一定要牢記原文作者的主要意圖。具體來說，本文作者的主要意圖是要否定一些生物學(xué)家對某種生物現(xiàn)象的解釋。相關(guān)的內(nèi)容出現(xiàn)在原文第四段的首句。如果考生能夠認(rèn)識到原文第四段首句在全文中的重要性，就找到了破解本題的方法，再把第三段的尾句和第四段的第二、三句通讀并且仔細理解就會找出本題的正確答案B。當(dāng)然這是一道比較難的題目，需要考生良好的答題素質(zhì)。希望考生在迷失答題思路的時候，一定要多想一想從原文的中心主旨句和每段的段首句設(shè)法破解難題。

　　4. 【答案】D

　　【考點解析】這是一道詞匯理解題。根據(jù)原文中的“cline”一詞可迅速把本題的答案信息確定在第二段第一句。第二段第一句中的“gradual”、“geographic”、“adaptive”和“environmental differences”等詞提供了本題的正確答案D�？忌�在閱讀原文回答問題時，一定要學(xué)會識讀原文中的相關(guān)重要詞語。

　　5. 【答案】D

　　【考點解析】這是一道審題定位題。根據(jù)題干中的“the author’s conclusion”可將本題的答案信息迅速確定在尾段，確切地說就是尾段的最后一句。考生一定要清楚原文作者的結(jié)論一般會出現(xiàn)在全文的尾段。本題的正確答案應(yīng)該是涉及到尾段的選項D。

　　[參考譯文]

　　鲆魚，例如比目魚，是少數(shù)左右兩側(cè)不太對稱的脊椎動物之一(左右兩側(cè)對稱指的是身體中線左右兩邊的結(jié)構(gòu)為鏡象反射)。在成年的比目魚所具有的眾多明顯的不對稱現(xiàn)象中，最令人震驚的是它的眼睛的位置：在成熟以前，一只眼睛會移動，結(jié)果是成年比目魚的兩只眼睛都移動到頭部的同一側(cè)。在大多數(shù)非對稱的物種中，成年個體的非對稱性看起來都是相同的，但星形比目魚種的成員則既可以是眼在左側(cè)的(兩只眼都在頭部的左側(cè))，也可以是眼在右側(cè)的。在美國和日本之間的水域中，星形比目魚的群體組成不斷變化，在美國西海岸附近，50%是眼在左側(cè)的，在美國和日本之間的中間位置，70%是眼在左側(cè)的，而到了靠近日本海岸的地方，則幾乎百分之百都是眼在左側(cè)的了。

　　生物學(xué)家將這種在特定地理區(qū)域范圍內(nèi)的逐漸變化稱為“漸變?nèi)?rdquo;，并針對漸變?nèi)航忉屨f，它強烈地表明這種變化是適應(yīng)性的，是對環(huán)境差異所作出的反應(yīng)。對于星形比目魚來說，這種說法暗示形狀上的差異(同左右兩側(cè)對稱的魚相比)是適應(yīng)性的，日本的星形比目魚之所以眼在左側(cè)，是選擇的結(jié)果，但這又引起了一個令人困惑的問題：兩只眼睛都在一側(cè)比兩只眼睛分在兩側(cè)具有什么選擇性的優(yōu)勢呢?

　　魚只要簡單地轉(zhuǎn)個身，就可以使它的眼睛在一側(cè)的不對稱性所帶來的影響逆轉(zhuǎn)，這是非常容易的，所以生物學(xué)家只好去研究內(nèi)部解剖學(xué)，特別是研究視覺神經(jīng)，以尋求答案。在所有的比目魚中，視覺神經(jīng)都是交叉的，右邊的視覺神經(jīng)連接到大腦的左半部，左邊的視覺神經(jīng)連接到大腦的右半部。這種交叉會引起不對稱性，因為一條視覺神經(jīng)必須會在另一條視覺神經(jīng)的上面，要不就在下面。G·H·帕克對此解釋說，舉例來說，如果一條比目魚的右視覺神經(jīng)在上面而左眼發(fā)生了移動，那么兩條視覺神經(jīng)就會扭曲糾纏在一起，這在結(jié)構(gòu)上可能是不利的。于是對于星形比目魚來說，就不應(yīng)該選擇眼在左側(cè)，因為在星形比目魚中，左側(cè)的視覺神經(jīng)是在上面的。

　　上面這種解釋的問題是日本的星形比目魚種群幾乎全是眼在左側(cè)的，而大自然的選擇則從不推動完全沒有有利條件的種屬發(fā)展。由于其它的解釋被證明同樣也是站不住腳的，所以生物學(xué)家得出了結(jié)論，認(rèn)為眼是在左側(cè)還是在右側(cè)并沒有什么重要的適應(yīng)性差異，這兩種特征在遺傳上與別的一些有意義的適應(yīng)性特征相關(guān)。研究進化的生物學(xué)家經(jīng)常會遇到這樣的情況，他們經(jīng)常必須判斷一個特征究竟是適應(yīng)性的，還是在選擇上中性的。對于眼在左側(cè)的比目魚和眼在右側(cè)的比目魚來說，它們的差異雖然很驚人，但這似乎是進化過程中的一個分散人們注意力的不相干的現(xiàn)象。