Official 33 Set 5

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  • Q2
  • Q3
  • Q4
  • Q5
  • Q6
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Notothenioids

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  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
  • Q6
What is the lecture mainly about?
  • A. How various proteins function in notothenioids

  • B. How notothenioids became the dominant type of fish in the Southern Ocean

  • C. An example that contradicts the theory of adaptive radiation

  • D. Changes in ocean habitats caused by continental drift

显示答案 正确答案: B

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    NARRATOR: Listen to part of a lecture in a biology class.

    FEMALE PROFESSOR: [responding to a student’s comment]…Ways in which animals adapt to their environment are often quite ingenious, actually.And, as an example of this, lemme tell you about a fish … a-a group of fish known as the notothenioids.

    There's over 90 known species of notothenioids, and they inhabit both shallow and very deep waters-mostly around Antarctica.Many are fairly small, though the largest species can weigh up to 150 kilograms.Notothenioids can be identified by their large eyes, which are covered by a thick, insulating layer of clear tissue.This tissue protects their eyes from freezing.Remember, the freezing point of ocean water-saltwater-is lower than for freshwater-negative 1.9 [one-point-nine]degrees Celsius … so it can get a lot colder for fish in an ocean, say, than in a river or lake.

    So … this means that the ocean waters around Antarctica are cold enough to freeze most types of fish. But notothenioids don't freeze. In fact, they thrive.They account for some 95 percent of all fish in the Southern Ocean-the ocean that surrounds Antarctica.

    So, how unusual is that …to have a single family of fish dominating an entire ocean?I mean, think of, say, tropical or temperate marine environments,which have incredibly diverse fish populations.Coral reefs, for example, support over 4,000 types of fish,along with sponges … crustaceans … and many other organisms.

    So, exactly when-and how-did the notothenioids come to dominate the Southern Ocean?Well, around 30 million years ago, the waters around Antarctica were a lot warmer than they are today.Um … at that time, Antarctica was connected to South America, which means that warm air, from the north, could flow southward and heat up the Antarctic waters.Because the water around Antarctica then was relatively warm, it supported many types of fish, and we know this from fossil evidence.But the 90 or so species of notothenioids that exist today didn't exist at all back then ….In fact, only one ancestral notothenioid species existed.

    But somewhere between five million and fourteen million years ago,two major changes took place.First, what we call a chance mutation-a tiny genetic change-occurred in that one notothenioid species.Its DNA allowed for the production of a special protein,a protein that prevents the fish from freezing.The way this-this antifreeze protein-works is:it binds to any ice crystals that form inside the fish.This binding action prevents the ice crystals from growing larger … and this is what prevents notothenioids from freezing.

    Now, at that time, the waters the notothenioids inhabited were still not freezing cold. So the protein didn't really make a difference, as far as the fish's survival.But this would change because, in the same period of geologic time, there was a shift in the Earth's continental plates; continental drift caused Antarctica to move apart from the landmass of South America and to drift into the southern polar region.This resulted in a powerful water current encircling Antarctica,which prevented the Antarctic waters from mixing with warmer water… so the Southern Ocean-isolated from that warm airflow from the north-cooled down-drastically-to the kinds of subfreezing temperatures we associate with it today.

    Now, most fish species couldn't survive in this frigid environment,and they became extinct.But that one notothenioid species, with its unique ability to produce that antifreeze protein,thrived. It had virtually the entire Southern Ocean to itself- so … well, there was little or no competition for food or space.You might think of it as, um …[searching for a metaphor] as a—a kind of … an ecological vacuum; … and the notothenioids exploited it fully.The species migrated into different habitats throughout the Southern Ocean,and its population increased dramatically, with various subpopulations migrating into different parts of the ocean.

    Over time, these subpopulations, in all those different habitats … well, they developed very different physical traits; they adapted to survive in their particular ecological niche—[explaining the term] their … their position within a particular ecosystem.We call this type of species diversification within a species adaptive radiation.And what adaptive radiation is: is [slowly, stating a definition] an evolutionary process by which a parent species rapidly undergoes changes resulting in various new species, in order to fill multiple ecological niches.So, in the case of the notothenioids, that single species started colonizing empty habitats to such an extent that it evolved into a broad range of new species-the 90 or so notothenioid species that we have today.[slight pause] So, lemme switch to adaptive radiation with regard to another species that’s also been very successful ….

  • 旁白:听一段生物学课程。

    教授:其实,动物适应所处环境的方式通常都是非常具有独创性的。举个例子,让我给你们讲讲一种鱼吧,这类鱼称为南极鱼类。

    南极鱼类一共超过90种,它们栖息在浅水和深水域,主要在南极洲附近。其中很多南极鱼都相当小,尽管最大的鱼体重能达到150千克。南极鱼可通过它们的大眼睛被辨认出来,这些眼睛被一层厚厚的,绝缘的透明组织所覆盖。这些组织保护它们的眼睛不被冻伤。记住,海水的结冰点,即盐水,是比淡水低的,零下1.9摄氏度。所以,对鱼来说,在海里比在河里或湖里冷多了。

    这就意味着,南极洲附近的海水非常冷,足以冻住大多数鱼类,但南极鱼则不会。事实上,它们非常兴旺。南极鱼在南部海域鱼类里占几乎95%,即南极洲周围海域。

    这多么不寻常啊?一种鱼类主宰了整片海域。我是指,想想热带或温带的海洋环境,那儿的鱼非常多样。比如说珊瑚礁,就养活了超过4000种鱼,还有别的海绵生物、甲壳类生物和其它生物。

    那么,到底是什么时候,以及通过什么方式,南极鱼类统治了南方海域呢?在大约三千万年前,南极洲周围水域比现在暖和多了。那时候,南极洲连着南美洲,也就是说,从北方来的暖空气可以一直流向南方,加热南极洲水域。因为那时候南极洲附近水域相对暖和,也就给很多鱼类提供了生存环境,我们是从化石证据中知道这些的。但现在存在的90多种的南极鱼类在那时候还没出现呢。实际上,(那时候)只有一种古老的南极鱼。

    但在一千四百万年前到五百万年前之间,发生了两个重大变化。第一个是我们称为的偶然突变,一个微小的基因突变发生在那种古老的南极鱼中。它的基因产生了一种特殊的蛋白质,这种蛋白质能保护南极鱼不被冻住。这种抗冻蛋白工作的原理是:它会附在鱼类体内的每一个冰晶上。这种粘附能防止冰晶变大,这就能保护南极鱼不受冻了。

    在那时候,南极鱼所在水域还没有那么冷,所以这种蛋白对于鱼类生存来说,并没有起很大作用。但这很快就会改变,因为在同一地质时期,地球的大陆板块发生了变化,大陆漂移导致南极洲从南美大陆中分离出来,漂流到南极区域。这导致南极洲被一股强水流环绕,阻止了南极洲水域和温度更高的水融合。所以,南部海域与来自北方的暖气流隔绝了,大幅度降温,达到了低于结冰点的温度,也就是今天的温度。

    大部分鱼类在如此寒冷的环境下都无法生存,很快灭绝了。但那种南极鱼,有了独一无二的制造抗冻蛋白的能力,茁壮成长。基本上整个南部海域都是它的了!所以...那儿几乎没有关于食物和生存空间竞争。你大概能把它当成……一种生态真空。南极鱼充分利用了这一点。这种鱼迁徙到不同的栖息地,遍布整个南部海域。鱼类数量大幅增加,发展出很多不同的亚种,迁徙到海洋的不同区域。

    久而久之,这些在不同栖息地的亚种……它们发展出了差异很大的生理特性。它们适应了在自己特定的生态位生存,即它们……它们在某个特定生态系统中的位置。我们称这种种类物种多样化为“适应性辐射”。适应性辐射是什么呢:它是一种进化过程,母种迅速经历了许多改变,导致了新物种出现,来填补更多的生态位。所以,在南极鱼的例子中,单一物种开始在空旷的栖息地大量繁殖,以至于它进化出了形形色色的新物种—也就是我们今天拥有的90多种南极鱼。让我(把话题)切换到另一个非常成功的物种,它也有适应性辐射。

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    题型分类:主旨题

    原文定位:

    Ways in which animals adapt to their environment are often quite ingenious, actually. And, as an example of this, lemme tell you about a fish … a—a group of fish known as the notothenioids...

    They account for some 95 percent of all fish in the Southern Ocean-the ocean that surrounds Antarctica...

    So, exactly when-and how-did the notothenioids come to dominate the Southern Ocean?

    选项分析:

    开门见山指出动物适应环境的方法很有才,举个鱼的例子吧。然后后面都在讲这个鱼是怎么做到的了

    对应B选项

    其它选项不是主要论述内容

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