OFFICIAL21 According to the professor, what is a major weakness of the classification system that is based on animals' physical characteristics?

[00:00.00]NARRATOR Listen to part of a lecture in a biology class.
[00:09.20]FEMALE 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 ?[00:21.72]OK, venomous snakes are the ones that secrete poisonous substances, or venom—like the snakes of the viper family, or of cobras... [00:28.94]Then there's nonvenomous snakes, like constrictors and pythons...
[00:32.76]Another family of snakes, the colubrids, don't really fit neatly into either category, though.[00:38.67]Colubrids—and you probably learned this, too—although they're often classified with venomous snakes, they're actually generally nonvenomous. [00:48.24]They're classified with venomous snakes because they resemble them, their advanced features, more than the other nonvenomous snakes.[00:55.68]Now...what if I told you that there's a good chance that most everything I just said is wrong? [01:02.67]Well...everything except the part about snakes evolving from lizards.[01:06.77]See, the basic theory about snake evolution has been challenged by a recent study that revealed a whole new understanding of evolutionary relationships for reptiles—y'know, which reptiles descended from which ancestors?
[01:20.72]The researchers studied the proteins in the venom genes of various species of colubrids. [01:26.33]Um ?snake venom is a mixture of proteins...some toxic—poisonous—and some not. [01:32.34] 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.[01:46.27]Traditionally, to understand evolutionary relationships, we looked at various easily observed physical characteristics of animals—their skeleton, the size of their brain...and then classified them based on similarities and differences. [02:01.05] The problem with this method is that characteristics that appear similar may actually have developed in quite different ways. [02:08.71] 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...
[02:20.47] Now—and not everyone will agree about this, [02:24.07]but classification based on DNA seems to be much more reliable. [02:29.09]OK, back to the research.  [02:31.55]The researchers found that venom evolved before snakes even existed ?about a hundred million years before. [02:40.71] Now, a couple of venomous lizards were included in this study, [02:45.10] and the researchers found some of the same DNA in their venom as in the snake's venom. 
[02:50.89]This suggested that the common ancestor of ALL snakes was actually a venomous lizard. Which means that, actually, according to this research, anyway, in terms of the snake's ancestry, there's no such thing as a nonvenomous snake! Not even colubrids.[03:10.43]What separates colubrids from other snakes we've been classifying as venomous is not the lack of venom, but the lack of an effective way to deliver the venom into its prey.
[03:22.17] In most venomous snakes, like vipers and cobras, the venom is used to catch and immobilize the prey—but in colubrids, venom drips onto the prey only after the prey is in the snake's mouth. [03:35.90] So, for colubrids, the venom must serve some other purpose...maybe linked to digesting prey.
[03:42.74]As the different families of venomous snakes evolved, the teeth moved forward, becoming larger ?and the venom became 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.[04:02.21]So if there are no truly nonvenomous snakes, were the so-called nonvenomous snakes—like constrictors and pythons—were they venomous at some point in their evolution? [04:14.47] Well—that's not clear at this point. [04:17.28] Constrictors have evolved to kill their prey by crushing. But perhaps they once were venomous and then...at some point...their venom-producing apparatus wasn't needed anymore, so it gradually disappeared. [04:30.65] There's one species of snake—the brown tree snake—that uses both constriction and venom, depending on its prey... [04:38.89] so—well... it is possible...[04:42.25]So...we have this new concept of snakes?evolution and a new DNA database—all this information on the genetic makeup of snake venom. [04:52.21]And, what we've learned from this has led researchers to believe that venom proteins may have some exciting applications in the field of medical research.
[05:01.47]You see, venom alters biological functions in the same way certain drugs do. And the big benefit of drugs made from snake venom would be that they'd target only certain cells—so maybe that's create fewer side effects. [05:15.75]Now, it sounds far-fetched—venom as the basis for human drugs? [05:20.57]so far, only one protein has been targeted for study as a potential drug? but—who knows, maybe some day...