托福模拟试题听力 - TPO21 - 讲座3

时间 : 2013-10-13 17:22来源 : VOA官网 收听下载次数 :
下载音频

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 weren’t 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, don’t 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 snake’s 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, that’s 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 wasn’t needed anymore, so it gradually disappeared. There’s 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 that’ll 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.

 

1  What is the lecture mainly about?

A. Methods of analyzing toxic proteins in snake venom

B. Insights about snake evolution provided by venom analysis

C. How snake venom differs from lizard venom

D. Why colubrids are considered nonvenomous snakes



2 Why does the professor review information about the classification of snakes that students probably learned in previous courses?

A. To determine whether the students have enrolled in the appropriate course

B. To stress the usefulness of the classification system for students

C. To present assumptions that have recently been challenged

D. To give an example of a method that she will explain in greater detail



3 According to the professor, what is a major weakness of the classification system that is based on animals' physical characteristics?

A. It can show the relationships only among a small number of animal species.

B. It requires technology that is not widely available.

C. It cannot account for characteristics that first appeared in the recent geologic past.

D. It cannot determine whether similar characteristics developed in similar ways.

 

4  According to the professor, in what way do colubrid snakes differ from other venomous snakes?

A. Colubrids did not evolve from lizards.

B. Colubrids do not use venom to catch their prey.

C. The front teeth of colubrids are much larger than those of other venomous snakes.

D. Colubrids produce a much stronger type of venom than other venomous snakes do.



5 Why does the professor mention the brown tree snake?

A. To support a hypothesis about the evolution of constrictor snakes

B. To support a hypothesis that venomous snakes evolved from constrictor snakes

C. To give an example of a snake species that was never venomous

D. To give an example of a type of snake that can change its color

 

6 What is the professor's attitude toward the results from medical research on snake venom proteins?

A. She is enthusiastic about the drugs that have been tested to date.

B. She is concerned about the side effects of drugs created from snake venom proteins.

C. She doubts that the DNA database will be useful in developing new drugs.

D. She thinks it is too early to tell how successful the research will be.