نطق سوم

Listen to part of a lecture in a zoology class.

Professor, we were talking about the principles of classification, how some types of animals are related to other animals and similarities between different kinds of animals, but I was reading the textbook last night, and it mentioned that there are related to each other even though they have some major dissimilarities. Could you talk about that a little bit?

Sure. And I’m glad you brought that up. But first out of curiosity, are any of you from the southwestern part of the United States? No? Well, then I’m probably explaining something a little new to most of you because when you asked about that, the example that jumps to my mind is one type of lizard the Gila monster.

Gila monsters pretty much all live in the desert areas of the southwestern United States and in Northern Mexico, parts of Northern Mexico. The Gila monster looks and acts a lot like other lizards but in one key way it seems different.

I mean, Gila monsters would be classified as lizards because, well, the Gila monster and all lizards share certain characteristics. Some of the features of the skull, the jaw, and other parts of the skeleton, all lizards inherit these features from a common ancestor.

That’s why they are classified as lizards. Most lizards also have things like four legs, a long tail, skin covered in scales, let’s see, a triangular head, and, yes, Michelle?

You know, there’s actually an illustration of one on, let’s see, page 12 of our handouts.

Thanks Michelle. Let’s all take a look. Basically it’s a fairly large creature, I guess about two feet long. It has, well, you can see those spots or blotches all along its body. Mike.

You said that Gila monsters are different from most other lizards, but what you just described and the picture, well, it’s like lots of other lizards.

Right. Yes, as I said, Gila monsters are like other lizards, but they have one significant difference. In contrast to nearly all other lizards, Gila monsters are venomous.

Venomous? You mean poisonous like snakes?

Well, yes, in fact, Gila monsters are a lot like some snakes in that way, but snakes don’t have all those other characteristics I mentioned before. But many snakes, many kinds of snakes are venomous. So are Gila monsters more closely related to snakes or to snakes or to lizards?

Snakes?

No. Lizards. It’s true that Gila monsters and snakes do have that one major characteristic in common, and of course snakes and Gila monsters are both reptiles, but what helps us decide where the Gila monsters belongs. Well, and this is the point I want to stress here. Similar features in different species may occur because they were inherited from a common ancestor, then those features are a sign that the species are related, but they may also evolve independently and they may even evolve for completely different reasons, different purposes.

And that’s what’s happening here, not only did the venom system of snakes and Gila monsters evolve independently but they also function quite differently from each other. Snakes use their venom to hunt and subdue prey for food, but Gila monsters just, they basically use it for self-defense. You’ll only see a Gila monsters use its venom if it’s attacked or if it feels threatened in some way.

You see, Gila monsters, when they sense danger, they first try to hide, to mix in with the environment. The colors of their skin with all its spots and blotches blend well with the colors of their surroundings. But sometimes their camouflage can fail. So, then, for defense, they use their venom. So getting back to the point, it would be a mistake to classify snakes and Gila monsters in the same category just because they are both venomous.

Oh, there’s another interesting thing about Gila monsters and their venom. There is a chemical in their venom that’s being used in a new drug to help fight type II diabetes. And the experiments with this new drug, well, this could really represent an important medical breakthrough. It’s worth noting that this chemical is only found in the venom of Gila monsters. It doesn’t occur in snakes and it doesn’t occur in any other lizard.