TOEFL Listening: ETS-TOEFL听力机经 - 68A846DKX46N481X

Lecture: Animals containing toxins: Narrator: Listen to part of a lecture in a biology class. Professor: As I said, Darwin described evolution as a random process, one that's triggered by accidental variations. Now, based on this, biologists had assumed two things. First, that evolution is unpredictable. We can't predict how any particular species will evolve. And second, the replication of evolves in changes doesn't happen. That any particular mutation that occurs in one species is unlikely to be repeated in another. Well, some recent researches suggest that that second assumption is wrong. Um, the type of animal you probably heard about, the pufferfish. Ok, the pufferfish, as you may have actually considered a delicacy in many parts of the world, but you need to be careful. This fish contains deadly poison called tetradotoxin. Now, tetradotoxin is a hundred times more deadly than pc cyanide and is not destroyed by cooking. So if a pufferfish isn't prepared exactly right, well, let's just say, a risky undertaking. Anyway, the pufferfish isn't unique in containing tetradotoxin. Tetradotoxin is also found in other animals, like in certain newts in North America and Japan, pending kinds of frogs in central and South America and in Bangladesh, and also in a certain type of snail. So it's actually pretty widespread both geographically and across animal species. And that some interesting questions because you see except for these animals, animal toxin in general are, they're specific to a particular group of animals. For example, we know that there are lots of poisonous snakes, right? And there are also lots of poisonous scorpions, but snake venom are very different from scorpion venom. They're very different types of poisons. And that's the typical case. But with tetradotoxin, we have a whole disparate group of animals – some fish to frogs, to snails, all of whose bodies contain identical toxin. So how can such a wide variety of species contain the same poison? Well, notice those animals whose bodies contain tetradotoxin. I didn't say their bodies produce tetradotoxin. I would be very sure if they did, if they each synthesized or produce this poison. It's a pretty complex molecule that involves several steps, chemical steps to create. But in fact, there's evidence that they don't produce it. Because pufferfish or Japanese newts for condominium frogs are raised on special diets or in bacteria free. They're non toxic. They don't end up containing any tetradotoxin at all. So this means it's their environment that makes them toxic. They must either obtain the toxin from their natural food chain or from the environment. Okay, so one mystery solved. We have an idea about how they come to contain toxin. But another mystery takes its place. If the toxin is in their bodies, why doesn't it kill them? If we want to understand this, we have to examine how tetradotoxin works. Ok. So all animals have membranes surrounding their nerves and muscle cells, right? And normally, ions of sodium can travel through their channels across these membranes, and sodium ions travel across these channels. Well, what tetradotoxin does is it blocks the normal movement of sodium ions. And when sodium ions don't reach nerves and muscles, these nerves and muscles become paralyzed, which leads to respiratory failure, and in some cases, death. But in japanese newts and condominium frogs ion movement isn't blocked by tetradotoxin. It says that in some points in the evolution of these animals, certain generic mutations occurred that block tetradotoxin from interfering with sodium ion channels. And individuals in these different species which inherited these mutations were favored and reproduced. As a result, different groups of animals independently acquire the same mutations, making them immune to tetradotoxin. And certain predators may have evolved resistance in the same way. So there are snakes in the western United States, garter snakes that feed on our highly toxic newts. And at least three species of garter snakes who prey on the newts have independently evolved immunity to tetradotoxin. And these snakes underwent the same genetic mutations that contain resistance to tetradotoxin, as pufferfish did. Namely mutations that prevent tetradotoxin from blocking the movement of sodium ions. So we've got parallel mutations across different species, indicating that evolutionary processes are more replicable, more likely to repeat themselves than previously thought.