托福听力TPO21原文 Lecture 3

托福听力 TPO21 原文 Lecture 3下面就让小编来为大家介绍一下托福听力 TPO21 原文中 Lecture 3 的文本内容吧，大家要好好把握，这些都是非常有价值的材料，同时，大家也可以登录前程百利论坛进行 TPO 练习辅导，希望能够给准备托福听力的同学带来帮助。TPO21 Lecture3 Biology(Snake Evolution)Professor：Probably back in some previous biology course you learned that snakes evolved from lizards, and that the first snakes werent venomous and then along came more advanced snakes, the venomous snakes. Ok, venomous snakes are the ones that secrete poisonous substances or venom, like the snakes of the viper family or cobras. Then there is non-venomous snakes like constrictors and pythons. Another family of snakes, the colubrids, dont really fit neatly into either category though. Colubrids, and you probably learned this too, although they are often classified as venomous snakes, they are actually generally non-venomous. They are classified as venomous snakes because they resemble them, their advanced features more than the other non-venomous snakes.Now, what if I told you that there is a good chance that most everything I just said is wrong? Well, everything except the part about snakes evolving from lizards. See, the basic theory about snake evolution has been challenged by a recent study that revealed a whole new understanding of evolutionary relationship for reptiles, you know, which reptiles descended from which ancestors. The researchers study the proteins in the venom genes of various species of colubrids. Emm. snake venom is a mixture of proteins, some toxic, poisonous, and some not. By analyzing the DNA, the genetic material of the proteins, the researchers could focus on the toxic genes and use them to trace the evolution of snake venom, and from this, the evolution of snakes.Traditionally, to understanding evolutionary relationships, we looked at various easily observed physical characteristics of animals, their skeleton, the size of their brain, and. and then classify them based on similarities and differences. The problem with this method is that characteristics that appear similar may actually have developed in quite different ways. For example, some venoms are chemical-based, and others are bacteria-based, so they clearly had to have developed along different routes and may not be as closely related as we thought.Now, and not everyone will agree about this. The classification based on DNA seems to be much more reliable. Ok, back to the research. The researchers found that venom evolved before snakes even existed, about a hundred million years before. Now, a couple of venomous lizards were included in this study. And the researchers found some of the same DNA in their venom as in the snakes venom. This suggested that the common ancestor of all snakes was actually venomous lizard, which means that actually, according to this research, anyway, in terms of the snakes ancestry, there is no such thing as a non-venomous snake, not even colubrids. What separates colubrids from other snakes we have been classifying is venomous, is not the lack of venom, but the lack of an effective way to deliver the venom into its prey. In most venomous snakes, like vipers and cobras, the venom is used to catch and inmoblize the prey; but in colubrids, venom drips onto the prey only after the prey is in the snakes mouth. So for colubrids, the venom must serve some other purpose, maybe linked to digesting prey. As the different families of venomous snakes evolved, the teeth moved forward, becoming larger, and the venom becoming stronger, so the evolution of the obvious venomous snakes, like cobras and vipers, is about the evolution of an efficient delivery system, not so much the evolution of the venom itself. So, if there are no truly non-venomous snakes, were the so-called non-venomous snakes, like constrictors and pythons, were they venomous at some point in their evolution? Well, thats not clear at this point. Constrictors have evolved to kill their prey by crushing, but perhaps they once were venomous, and then at some point their venom-producing apparatus4 wasnt needed anymore, so it gradually disappeared. Theres one species of snake, the brown tree snake, that uses both constriction and venom, depending on its prey. So, well, it is possible.So, we have these new concepts of snakes evolution and a new DNA database, all these information on the genetic makeup of snake venom. And what we have learned from this has led researchers to believe that venom proteins may have some exciting applications in the field of medical research. You see, venom alters biological functions in the same way certain drugs do, and the big benefit of drugs made from snakes venom would be that they target only certain cells, so maybe thatll create fewer side effects. Now, it sounds far-fetched5, venom is the basis for human drugs. So far, only one protein has been targeted for study as a potential drug, but who knows, maybe someday.教授：可能在之前的生物课上你们学习到蛇是由蜥蜴进化而来，而且起初的蛇是没有毒的，后来才出现了更高级的蛇，也就是毒蛇。毒蛇就是指那些分泌有毒物质或者毒液的蛇，比如蝰蛇及眼镜蛇。还有像蟒蛇、蚺蛇这样的无毒蛇类。还有一类蛇-黄颌蛇，并不能将其进行严格的分类。黄颌蛇你应该也学习过，虽然通常被分到毒蛇类别中，但事实上它们大致是无毒的。之所以将其划分到毒蛇类，是因为它们更像毒蛇，而且它们具备一些比其他无毒蛇类更先进的一些特点。那么现在，假如我告诉你们，我刚才讲的多数的理论很有可能都是错误的，那会是怎样呢?呃，除了蛇是从蜥蜴演化而来以外，其余部分也许都是错误的。蛇进化的基本理论受到了最近一项研究的动摇。这项研究揭示了一个关于爬行动物进化关系全新的理解，你们知道，就是什么爬行动物由何祖先演变而来。研究人员研究了各种黄颌蛇的毒液基因。恩，蛇毒是由蛋白质构成的混合物，有的有毒，有的则没有毒。通过对 DNA 及蛋白质中的基因物质的分析，研究人员就能专注于有毒基因并用其找出蛇毒的进化轨迹，这样，也就找到了蛇的进化源。传统意义来讲,为了了解动物的进化关系, 我们要看动物各种各样容易观察的体型特征,它们的骨骼, 大脑的大小, 然后, 呃, 然后再根据其相同点及不同点来进行分类. 这种方法的弊端就是表面上的相似特点也许实则是以不同的方式进行进化发展的. 比如, 有些毒液是以化学元素为基础的, 有些则是以细菌为基础的, 因此很显然它们是在以不同的路径进行进化发展的, 也许并不如我们所想的那般关系紧密.而现在不是每个人都同意这个观点。基于 DNA 进行分类看似是更有说服力的。好，那再回到研究的话题上来。研究人员发现毒液在蛇还未存在的时候就已经进化了, 大约是在一亿年前. 现在, 两种毒蜥蜴也已经被囊括到研究中来. 而且研究人员在毒蜥蜴体内也发现了一些和毒蛇同样的毒液. 这就意味着所有蛇类的祖先都是毒蜥蜴, 也就是说, 根据这项研究, 关于蛇的祖先这个问题来讲, 根本就没有无毒蛇之说, 包括黄颌蛇. 我们将黄颌蛇与其他蛇类鉴别开来是通过它们的毒液, 并不是说它没有毒液, 而是看它是否用毒液来作为一种有效的方式进行捕猎. 多数的毒蛇, 像蝰蛇及眼镜蛇,