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Lecture1 听力原文

Listen to part of a lecture in a botany class.

Professor: So, continuing with crop domestication and corn, or, um, maize as is often called,  obviously it’s one of the world’s most important crops today.  It’s such a big part of the diet in so many countries and it’s got so many different uses that it’s hard to imagine a world without it. But because it doesn’t grow naturally, without human cultivation, and because there’s no obvious wild relative of maize, um, well, for the longest time researchers weren’t able to find any clear link between maize and other living plants. And that’s made it hard for them to trace the history of maize.

Now, scientific theories about the origins of maize first started coming out in the 1930s, one involved a plant called teosinte.Teosinte is a tall grass that grows wild in certain parts of Mexico and Guatemala. When researchers first started looking at wild teosinte plants, they thought there was a chance that the two plants, um, maize and teosinte, were related. The young wild teosinte plant looks a lot like the corn plant. And the plants continue to resemble each other, at least superficially, even when they’re developed.

But when the scientists examined the fruits of the two plants, it was a different story. When you look at right corn, you see roll upon roll of juicy kernels, um, all those tiny little yellow squares that people eat. Fully-grown teosinte, on the other hand, has a skinny stock that holds only a dozen or so kernels behind a hard, um, almost a stone-like casing. In fact, based on the appearance of its fruit, teosinte was initially considered to be a closer relative to rice than to maize.

But there was one geneticist, named George Beadle, who didn’t give up so easily on the idea that teosinte might be, well, the parent of corn. While still a student in the 1930s, Beadle actually found that the two plants have very similar chromosomes, very similar genetic information. In fact, he was even able to make fertile hybrids between the two plants. In hybridization you remember, the genes of two species of plants are mixed to produce a new third plant, a hybrid. And if this offspring, this hybrid, is fertile, then that suggests that the two species are closely related genetically. This new hybrid plant looked like an intermediate, right between maize and teosinte. So Beadle concluded that maize must have been developed over many years, ah, that is a domesticated form of teosinte.

Many experts in the scientific community, however, remained unconvinced by his conclusions. They believed that with so many apparent  differences  between  the two plants, it would have been unlikely that ancient, the pre-historic peoples could have domesticated maize from teosinte. I mean, when you think about it, these people lived in small groups, and they had to be on the move constantly as the seasons changed. So for them to selectively  breed, to have the patience to be able to pick out just the right plants and gradually over generations, separate out the durable nutritious maize plant from the Beadle teosinte that easily broke apart, it’s a pretty impressive feat.

And you can easily see why so many experts would have been skeptical. But as it turns out, Beadle found even more evidence for his theory when he continued his experiments, producing new hybrids, to investigate the genetic relationship between teosinte and maize. Through these successful experiments, he calculated that only about 5 specific genes were responsible for the main differences between teosinte and maize. The plants were otherwise surprisingly similar, genetically.

And more recently, botanists have used modern DNA testing to scan plant samples collected from throughout the western hemisphere. This has allowed them to pinpoint where the domestication of maize most likely took place. And their research took them to a particular river valley in southern Mexico. They’ve also been able to estimate that the domestication of maize most likely occurred about 9000 years ago. And subsequent archaeological digs have confirmed this estimate.

In one site, archaeologists uncovered a set of tools that were nearly 9000 years old. And these tools were covered with a dusty residue, a residue of maize as it turns out. Thus making them the oldest physical evidence of maize that we found so far.

题目

1.What is the lecture mainly about?

A.A research study that compares wild and domesticated plants.

B.Problems with a commonly held hypothesis about the origin of teosinte.

C.Reasons why wild plants are usually unsuitable for agriculture.

D.The process used to identify the ancestor of a modern crop.

2.What evidence seemed to indicate that maize and teosinte are not related?

A.Young teosinte plants do not physically resemble young maize plants.

B.Preliminary DNA evidence indicated that teosinte was related to rice.

C.Maize and teosinte usually grow in significantly different climates.

D.Maize and teosinte have very different types of kernels.

3.Why does the professor discuss hybrids?

A.To explain how a geneticist confirmed that maize was widely grown 9,000 years ago.

B.To indicate the earliest method used by geneticists to identify plant origins.

C.To explain a method used to demonstrate a link between two plant species.

D.To describe how geneticists distinguish between wild plants and domesticated plants.

4.What was most researchers' initial view of George Beadle's theory  about teosinte?

A.They accepted it but questioned the evidence cited.

B.They rejected it because of conflicting archaeological evidence.

C.They questioned it because it implies that ancient farmers were sophisticated plant breeders.

D.They questioned it because genetic research was viewed with skepticism at that time.

5.What did Beadle conclude about maize and teosinte?

A.Both plants lack particular genes that are common in most domesticated plants.

B.Both plants have particular genes that enable them to adapt to varying climates.

C.Only a small number of  genes are responsible for the differences between the two plants.

D.The genetic composition of both plants is very similar to that of rice.

6.According to the professor, why was the discovery of stone tools important?

A.It proved that teosinte was simultaneously domesticated in multiple locations.

B.It helped to confirm the period in which maize was first domesticated.

C.It suggested that maize required farming techniques that were more complex than experts had previously assumed.

D.It provided evidence that maize plants were used for more purposes than experts had previously assumed.

答案及解析

1.正确答案：D

题目解析：本题出处是：Professor:But because it doesn’t grow naturally, without human cultivation, and because there’s no obvious wild relative of maize, um, well, for the longest time researchers weren’t able to find any clear link between maize and other living plants. And that’s made it hard for them to trace the history of maize. 教授在开头的时候提到，我们很难去追溯玉米这种植物的起源。后面的讲座内容也都是关于对于玉米起源的种种说法的。选项 D 合适。选项 A 说是比较野生植物和驯化植物的研究，选项 B 说是关于大刍草的起源假设的问题，选项 C 是说为什么野生植物不适合农业，都不合适。选择 D。

2.正确答案：D

题目解析：本题出处是:Professor: But when the scientists examined the fruits of the two plants, it was a different story. When you look at right corn, you  see roll upon roll of juicy kernels, um, all those tiny little yellow squares that people eat. Fully-grown teosinte, on the other hand, has a skinny stock that holds only a dozen or so kernels behind a hard, um, almost a stone-like casing. In fact, based on the appearance of its fruit, teosinte was initially considered to be a closer relative to rice than to maize. 这里说了虽然玉米和大刍草外观上相似，但是它们的果实有很大的不同， 因此科学家认为他们应该不怎么可能有亲缘关系。选项 D 合适。选项 A 说年幼的两种植物不相似，错误，年幼的两种植物也是相似的，前文提到过；选项 B 说早期 DNA 证据说明大刍草和稻谷有关系，无中生有；选项 C 说这两种植物长在不同的气候下，无中生有。选择D。

3.正确答案：C

题目解析：本题出处是：Professor: In hybridization you remember, the genes of two species of plants are mixed to produce a new third plant, a hybrid. And if this offspring, this hybrid, is fertile, then that suggests that the two species are closely related genetically.这里教授说了，如果两种植物可以杂交产生能繁育后代的新物种，则其亲缘关系就比较近。而玉米和大刍草就可以，所以它们有比较密切的联系。选项 C 正确。选择 A 为了解释玉米在 9000 年前被广泛种植，选项 B 说为了说明最早的鉴别植物起源的基因方法，选项 D 是说如何鉴别野生植物和驯化植物，都不合适。选择 C。

4.正确答案：C

题目解析：本题出处是：Professor: Many experts in the scientific community, however, remained unconvinced by his conclusions. They believed that with so many apparent differences between the two plants, it would have been unlikely that the ancient, the pre-historic peoples could have domesticated maize from teosinte. I mean, when you think about it, these people lived in small groups, and they have to be on move constantly as the seasons changed. So for them to selectively breed, to have the patience to be able to pick out just the right plants and gradually over generations, separate out the durable nutritious maize plant from the Beadle teosinte that easily broke apart, it’s a pretty impressive fate.  这里教授说，一开始很多科学家都不相信 Beadle 的结论，因为他们觉得考虑到古代人动荡的生活方式及驯化野生植物的时间长度、难度和复杂度，古代人不大可能把大刍草成功驯化成玉米。选项 C 合适。选项 A 说他们接受观点但是质疑证据，选项 B 说他们因为这个结论与考古学发现相矛盾而反对它，选项 D 说他们因为当时基因研究被质疑而怀疑这个结论，都不合适。选择 C。

5.正确答案：C

题目解析： 本题出处是： Professor: Through these successful experiments, he calculated that only about 5 specific genes were responsible for the main  differences between teosinte and maize. The plant were otherwise surprisingly similar, genetically. 这里 Beatle 最终得出的结论是，有 5 个基因导致了玉米和大刍草之间的差别，它们之间其他大多数的基因都是相似的。选项 C 正确。选项 A 说两者都缺某一驯化植物里普遍有的基因，选项 B 说两者都有使它们适应不同气候的基因，选项 D 说两者的基因和稻谷的基因都很像，都不合适。选择 C。

6.正确答案：B

题 目 解 析 ： 本 题 出 处 是 ： Professor:And subsequent archaeological digs have confirmed this estimate. In one site, archaeologists some uncovered a set of tools that were nearly 9000 years old. And these tools were covered with a dusty residue,   a residue of maize as it turns out. Thus making them the oldest physical evidence of maize that we found so far.这里教授提到石头工具就是为了说明，石头工具上面的一层玉米残渣说明那时候的人就已经把玉米驯化成功了。选项 B 正确。选项 A 说证明了大刍草在多个区域同时被驯化，选项 C 说玉米需要更加复杂的种植技术，选项 D 说玉米植株的用途比专家所假设的更多，都不合适。选择 B。