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 six. This is on phylogenetics. And phylogenetics is essentially the evolutionary history or the evolutionary family tree of organisms. If you look on this page we've got a number of pictures, we've got a number of pictures of cetaceans, so a bunch of whales. So the goal of phylogenetics is to create a phylogenetic tree. In other words a tree that shows who is related to who. In other words, is the humpback whale most related to the gray whale or to the minke whale or to the fin whale. And so phylogenetics is actually a really fascinating area right now because with all of the DNA evidence that we have we're able to put together a wonderful picture. And our goal is that the phylogeny will match taxonomy. In other words we can give names to organisms based on who they're related to. But the more you learn about this the more you realize that all life is very very similar. We have similarities between all of it. So let's get going on phylogenetics. And so basically speciation is when one organism or one population or one group eventually diverges and so they can't interbreed any more. It's the simplest way to think about speciation. And so phylogenetics and phylogenetic trees require speciation to have occurred. There are a number of different phylogenetics trees. The one that we'll talk a lot about are called cladograms and they use what are called clades. And so it's just a specific type. But a phylogenetic tree shows the evolutionary history of an organism. Now how do we figure that out? Well we could use all the tools at our disposal. I am going to talk about two specifically today. One is morphological. Morphological is the structure that you have. And the other is molecular. And so morphologically I am going to talk about hearts and how hearts have changed over time as they've required more and more things. And then the last is molecular. In other words how do we use DNA to figure out who is related to whom. So those are phylogenetic trees. This is the evidence that we use to figure it out. And then at the end I am going to talk more about cladograms and how you create a cladogram. And so let's start with the biggest phylogenetic tree of all, phylogenetic tree of life. So if this is life down here, so if life began you know 3.6 billion years ago, it's diverged into all these different lineages. And so this right here would be a phylogenetic tree. A phylogenetic tree of life. Now the one thing I want to point out and Darwin was the first person to do this, is that whenever you have a tree that suggests that there's common descent. And what does that mean? Well bacteria, archaea and eukarya, since they're all on the same phylogenetic tree, it means that they all came from that common ancestor. And so every time we have a branch point on here, so what does this suggest, that branch point right there suggests where that tree diverged into the eukarya and then the archaea that we have. And so the idea of descent is a long one, but the more evidence that we gather the more we realize that Darwin was right on. Now we have to figure out who is related to who. And so let's start at evidence that we have. So the evidence that we can use to make a phylogenetic tree, let's start with the first one and is morphological. Morphological are the structures that you have. And so this is a phylogenetic tree of vertebrates. And so we've got early vertebrates, we've got time periods over on the side, but we're most interested in the mammals. We've got birds, reptiles, amphibians, fishes. And so how did scientists figure out who's related to who? Well, we can choose one characteristic and then we can trace that through time. We can look at one thing and see how it's changed over time. So a perfect example would be the heart. The heart began in fishes as a two chambered heart. A two chambered heart really just has one valve. In other words, the blood is going to flow in this direction and then there's a valve that opens in this direction, so once the blood moves through it, it can't come back in. And so it's just a muscle that has a valve on the inside of it. And so what's the function of the heart? Well in a closed circulatory system, in other words, insects don't use a closed circulatory system, they use, their blood just goes everywhere in through the tissues. But in a closed circulatory system, in a fish, the blood, which in blue in this case just means that it is deoxygenated, is going to go through the gills, and then it's going to be oxygenated, so it's red, and then its going to go through the body and the tissues in the body, and then it's going to drop off that oxygen and then it comes back to the heart again. And so a two chambered heart, a better way to think about what a two chambered heart is, is it's simply a single loop. So we just have this one loop through the gills and back to the body. And for fishes that works great. The problem is that as we move on to land there are quite a few more constraints as you move, especially as you move towards being like a warm blooded organism. And so the constraints get heavier and heavier and so it's okay to have a two chambered heart, works great if you're a fish, but as we move on to land then it has to modify itself. And so again we just have this one loop. The major problem is that once it goes through the gills you loose a lot of the pressure. And so you loose the pressure and so it's hard to move that through the rest of your body. Works great if you're floating in water, but as you move on to land, we don't have that pressure. Okay, so let's go to a three chambered heart. Now three chambered heart arrives in the amphibians. And so things like a frog have a three chambered heart. So they've got three delineations. We've still got that loop that goes through the lungs and mostly in amphibians it actually goes through the skin where they pick up oxygen. But you see that we now have a problem here. We're not losing that pressure, in other words we're able to pump the blood to the skin and the, excuse me, we are able to pump it to the skin and the lungs and then we have a separate loop that goes through the body, so we still don't have to deal with that pressure. But the problem comes in right here, and that is that we have a mixing of the oxygenated and deoxygenated blood. And so it's purple. Now is that a problem? Well it's a problem if you are anything above, or spend more time on land then amphibians do. And so it works great for amphibians, but you have a mixing of oxygenated and deoxygenated blood. And so if you look at this loop, it's a double loop, but if you look at it and say okay, now let's move up to the reptiles and now we have to move more blood and we don't want as much of this mixing here because we're going to lose a lot of the oxygen. Well think about it as an engineer, how could you solve this? Well a three chambered heart works like this and what it does is it has a septa that's built right down here in the middle of the heart and that septa separates the deoxygenated from the oxygenated. It still has a little bit of mixing of the blood but that works great because they're cold blooded critters and so as they move that body, that blood around their body they can actually keep themselves a little bit warmer. But that's a three chambered heart and as we move on to endothermy, as we move on to birds and mammals, that just doesn't cut it. And so we think that birds and mammals both evolved this independently and you can see on here that birds branched off from reptiles and mammals branched off earlier from a common ancestor. And so we eventually have the arrival of the four chambered heart. What's the four chambered heart do? Well you can see that that septa that went right down the middle has completely closed off. So we don't have any mixing of the oxygenated and deoxygenated blood. And so birds and mammals have this morphological change and they did it because they're endothermic. In other words they require a constant body temperature. And so we can trace this morphological evidence through the organisms and we can say who's related to who. In other words is we have a three chambered heart that's shared by everything above here, that means that on our phylogenetic tree we want to at least put those on the same branch. Next I want to talk about molecular data. So molecular data is looking at the DNA. So looking at the genetic code. And so this is a study that was done in 2009. And what they were trying to figure out is where metazoans fit and who is related to whom. And every time I have a new biology book I find that this is actually organized a little bit differently. But we have this group down here of the, so the jellyfish and the sponges down here on the bottom. And then we have this group up here which contains things like us. And so scientists weren't sure if this branched off early or if these branched separately. And so what they did is they gathered a huge amount of DNA evidence. And so you can see here that this was a very large study done on a number of different families a number of different groups of animals. And they looked at mitochondrial DNA, proteins, ribosomal RNA. They looked at a number of different things and they figured out, this is very recent, that this branch and this branch are actually sister branches. The branch up here that makes us and the branch that makes the jellyfish have kind of separated a long time ago and they have been evolving since then. And so this is a great way, this would be a phylogenetic tree that we can use molecular evidence to answer a problem. But you have to gather a huge amount of data before you can actually do that. And on here you can see that they actually have an out group which is group of fungi which is not an animal, but it's a way that we can actually make comparisons to that molecularly and then we can figure out the connections. Okay. Last thing I said I would talk about are cladograms. Cladogram uses what is called a clade. And a clade, if I remember right comes from I think Latin. It means a branch. And so a clade is simply a group that has an organism and all, and I would circle the word all, of it's descendants. And so this right here is a clade, because it has this organism and all the descendants that come from that. Where this, the orange, is going to be a clade because it has this organism and it has all of the descendants that come from that. But green, right here, we would not call this a clade and the reason why is that you would have this organism right here and all of these descendants but you're missing a number of them over here. And so that's not a true clade. And so cladograms are going to, it's the definitive answer of who's related to whom. But you use two things. You use molecular and DNA for sure, evidence, but the other thing that you're going to use are something called synapomorphies. And a synapomorphy is going to be a characteristic that's shared by all of those in the clade. So a couple of good ones as we look through the fossil evidence of dinosaurs, we branch all the dinosaurs into two groups. The ornithischia and saurischia. And these are all, saurischia if I remember right means bird hipped and ornithischia means lizard hipped. And so it's a way to branch these groups and so a synapomorphy would be this characteristic. In other words, saurischia is going to be in this hip structure, is going to be shared by everything in this clade. And so the goal is to have a clade that has similar characteristics and it also doesn't leave anything out. A real example that you're probably familiar with would be reptiles. Reptiles is a silly term because reptiles used to be this blue area right here. It contained things like turtles, crocodiles and birds we left out of that. And so if you look at this, this group is what's called paraphyletic and so reptiles as a group was paraphyletic. It had all of these descendants but it lacked the birds. And we now know that birds are apart of this group. And so the goal of a cladogram is to create what are called monophyletic groups. Monophyletic would be this, yikes, would be this yellow group right here because it contains this and all of the descendants of that, that move from here. So polyphyletic means you have groups that come from different areas. So if we were to put mammals and birds together in one group that would be polyphyletic. And paraphyletic is when you have some organisms but not others. And so what's the goal? The goal of a cladogram is to figure out all of life. So we put all of life on branches and we figure out who's related to whom. And then, hopefully we can use a naming system and classify all of life. It seemed like a daunting task at one time but molecular evidence is giving us an in roads to that and it's a really hot topic as far a biology goes today. So that's phylogenetics and I hope that's helpful.