字幕列表 影片播放 列印英文字幕 Hi. It's Mr. Andersen and welcome to Biology Essentials video number 4. This is on Scientific Evidence for Evolution. Here is a picture of Charles Darwin much later in life. Charles Darwin was a very meticulous scientist. He gathered copious amounts of data and to show that evolution was true and that his mechanism of natural selection was right. Unfortunately he did not have a real good understanding of genetics and DNA obviously was not around back then, but the scientific evidence for evolution right now is pretty overwhelming. Some of the things he could have pointed to were fossil records. This is a baleen whale. If you look at the fin of a baleen whale it almost looks exactly like the bone structure in the human arm, which suggests that both I and the whale have a common ancestor that had bone structure similar like that. Or you could look at the rear legs. Why would a whale have rear legs unless at one point it actually walked on land and evolved from something like that. And so today what I am going to talk about is evidence for evolution. So what is evolution again? It's just changes in the gene pool. But specifically here we're not only talking about microevolution but macroevolution as well. First I'm going to talk about three things that were available to Darwin, and he talked about. Number one is Biogeography, in other words where living things are found. I'll talk specifically about the Galapagos Tortoises. Next he talked about fossils. An example I'll give you is horse evolution over time. Next, homologies or those are characteristics organisms have that show common ancestry. I'll talk about both homologous structures and vestigial structures. And then I am going to talk on a few things that Darwin didn't have available to him. Mainly molecular evidence - DNA is the trump card when it comes to evidence as far as evolution goes. And finally I am going to show you how we can use mathematical models to support our models for how evolution actually takes place. So we've got a lot to do so I better get started. Let's start with where Darwin started, on the Galapagos formulating his ideas. And I have been here as well. And to see these tortoises the first time is pretty amazing. Darwin's idea was that the giant tortoises on the Galapagos had moved from here out to the Galapagos. And so it had to float there, make it someway from the mainland to the Galapagos. And when he looked at the tortoise in Ecuador it looks very similar. There's no predators and so they are able to get very, very big. You can see there are clear differences in the size of the tortoises and more in their structure of their shell. Tortoises that come from really barren areas actually have a longer neck and what's called a saddle shape versus a dome shape. This is one from the highlands of Santa Cruz and this one is from San Cristobal. Now why do we just find tortoises there? Well there was a founding population that actually moved there. Now before Darwin most scientists at that time believed in special creation, that God had placed everything specifically on the island where it was. And Darwin, when he went to the Galapagos and started to see these islands and the similarities but also differences between those he started to realize that life and where life is found, we call that biogeography, is big evidence for evolution. Example, South America used to be connected to Australia. And we had a certain type of mammal there called marsupial mammals. We had eutherian mammals up here on the top. And when those two came together there was a war of the mammals. And eutherians really did fairly well and wiped out a lot of the marsupials that were there. But Australia, as it remained adrift kept those marsupials there. So it would be another example of biogeography. Next we could look at the fossils. It's surprising how much fossil evidence we actually have, because it's hard to create a fossil. The geologic properties required for that are pretty tough. A great example would be in horse evolution. Through time horses have gotten larger and larger and larger as their environment has changed and they move from more of a browsing to more of a grazing kind of animal. But we can see that growth reflected in the change in the fossil record. In other words the teeth become flatter. They have to run faster when they're out on the plains. And so they move from many digits to just one digit. And so we can see this transition. It's not linear, it's branched. In other words if we look at horse evolution there are many dead ends, there are many branch points. But we can see that there is a continuous thread all the way from those first horses to the horses that we have today. So horse evidence, fossil evidence is really good. There's a surprising amount of data on whale and whale evolution if you are interested in that you can take a look. Next is, and Darwin talked about this as well. In fact he actually mentioned this and I've got a big one of these. This is called a Darwin's tubercle, which is like a little, I do not know if you can see that, where your ear is and what it suggests is that we share ancestry with other primates who have this bump there as well. Homology or homologous means that they come from the same ancestry. And so this would be a human arm, this would be a dog leg, this would be a pigeon wing and this would be a fin of a whale. And if you look at it you could definitely if you are an engineer design a wing more efficiently then this wing right here but natural selection started with an organism that had the same bone structure and so it's used that bone structure and it's manipulated it to make the appendage that we have today. Vestigial structures are another thing that I could talk about briefly. Vestigial structures are structures that once had and did something but don't do anything anymore. An example would be goosebumps. I get goosebumps when I get scared or when I get cold, but I don't have a lot of fur, so it doesn't make me large and it doesn't give me much insulation. Wisdom teeth, a primitive tail, all those things are vestigial structures, an appendix, and they point to an ancestor that actually used those but has lost them over time. And so anatomy gives us huge pieces of evidence for evolution. But the one thing that Darwin didn't have was DNA. And so the best way to talk about DNA to start is to talk about the telephone game. If you've ever played the telephone game, essentially what you do is you start with a phrase, let's say, "the fat cat ate the rat". And then this phrase is going to be whispered to the second person in line, who will whisper it to the third person in line. But let's say the third person in line here, we'll call this person yellow, makes a mistake. Instead of saying "the fact cate ate the rat", said "the fat car at the rat". Now that's message, that's the DNA. And so when that DNA is copied again, that mutation is copied on as well. So "the fat car ate the rat". So that goes over here to pink. Pink does a nice job of passing it on but then eventually we get another mutation here. So we get a red mutation. And now it's "the fat car ate the bat" And now "the fat car ate the bat" and finally "the far car ate the bat". So when we get to the end the DNA is very similar but it's changed a little bit. And each of these are a mutation. We could call that the yellow mutation, the red mutation and then the purple mutation. And so life started with one strand of DNA. And all living things on our planet have that same DNA but these mutations have accumulated over time and so they tell us a lot about who's related to whom. In other words if you were to just tell me what the message is and you had this mutation here in yellow but you didn't have the red one, I could kind of place where you are and who you were standing next to. And so if Darwin had evidence related to DNA it would have been a lot, he would have had a better, an easier job convincing other people that he's right. So let's talk about some specific DNA evidence. An example I am going to give you here is the Human FGF2 gene. Over the last 10 years we have started sequencing huge genomes. And a genome, remember is the sequence of all the DNA inside that one living organism. The one I am going to look at is one that makes fibroblasts, so it's helpful in healing of wounds. And so we are going to go to this website (http://uswest.ensembl.org/index.html) right here and we're going to take a look at, let's go right here, we're going to take a look at this gene. So this is the human gene, FGF2. It's found on chromosome 4 and you can tell like how many base pairs, it's 6000 letters long. It tells me where exactly it is. We've learned so much. We could actually look at all the letters in that gene. Each of these red ones here would be an axon, so not really part of it. But again we've go 6000 letters or something. I could look at the whole gene, we've sequenced that, and again this is in homo sapiens, right here. So what we could do is now we could line that up. So this is just like that telephone game. We could line it up next to another organism and we could see how related we are. And so let's do, let's see what would be a good one to do, let's do a platypus. So let's compare that same gene in humans and in a platypus because we've sequenced the platypus genome as well. And what you'll find is that I can look at similarities. And so this would be between homo sapien right here, between us and a platypus we find that there are actually some similarities right here. There're some similarities right here. So we actually share some of these genes with the, portions of this gene with the platypus. But if I keep moving down and down and down and down, we find that there's really not that much in common between me and a platypus. But probably way more between me and a platypus, since it's a mammal, then me and for example a tree. So let's compare something else. Let's find something that's a little closer to home. Let's try a chimpanzee. And compare that gene in us and in a chimpanzee. So let's try that. We're looking at blast data, so this is sequence genome. So now if we look at us and a chimpanzee, you'll found, wow, there's an outstanding amount of overlap between us and them. All the way down that gene there's a huge amount of overlap. And so we can compare that and we can tell who's related to whom and we can also make these evolutionary relationships a little bit more clear. Okay. And so when you hear a chimpanzee and I have 96% of the same DNA, it's because of this. And humans are going to have, you know, 99.9999% of the same DNA but some of that is going to make each of us a little bit different. The last piece of evidence we have is, not so much evidence but it's a way to see how evolution takes place. And that's using population simulation software. And so what we have here is a way to simulate a population. Now remember, if we, let's run this for a second, so this is a population the size of a 1000. We're just looking at one allele here, so we're looking at one gene and its two varieties of the gene. And so if we look at this, this value up here would be my p value, this right here, let me change it to a different color. This would be my q value. And then here are going to be my different genotypes. And so if this is, let's say this is big H and his is little h and maybe we're looking at Huntington's. Then this would be homozygous dominant, this would be heterozygous and this is homozygous recessive. And so we could simulate it again, so let me do that for a second. Let's go back and simulate that again, and so chance is going to take over. So we could get a little bit of genetic drift this time, but the nice thing about population simulation software is it allows us to play around with those 5 different things, those 5 things that are required to maintain this equilibrium. So for example we could play with a bottleneck. So maybe we would want to make a bottleneck here from generation 100 to 120, where the population goes from a 1000 down to 10. Let's try to simulate that and see what happens. So now we've got 1000 in our population but now it's going to change, we're going to have a bottleneck effect right there and so we could study how a bottleneck is going to effect a population over time. Or we could look at mutation or migration or let's try fitness. So let's say if you're homozygous recessive, let's say you have a 50% chance of survival. So we could do a little bit of selection here and we could run it again. Let's try that again, And now we have a p value that's going way up and a q value that's going down. And we have almost the elimination of that homozygous recessive down here. So again, the evidence that Darwin had for natural selection was a ton. The one thing he was missing though was what we now have a really good understanding of now. So that's DNA and it's how DNA is passed from generation to generation. And as it does that it leaves these wonderful footprints that we can track evolution in. And so I hope that's helpful.