Biology: Construct a Phylogenetic Tree of Animals

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How, when faced with this unbelievable cacophony of animals... That's a great word. I don't think it means what I meant it to mean, but anyway, when faced with this Noah's Ark of animals, if you will, how do we classify these things? Where do we start splitting stuff up? You know what? It may surprise you. Remember, this is phylogeny we're talking about here. We're not talking about just anatomical classification, so we have to look at a lot of things.
Now remember, the first thing you do in a phylogeny is find a common ancestor. Well, we're already kind of stuck because we believe that animals came from an ancestral protist like creature, a multi-cellular colonial flagellate. What does that mean? Well, even today there are multi-cellular colonial flagellates. They are creatures that have many cells, but the cells are not dependent upon each other, so they're colonial, but there are many of them, and they roll around in balls. If you want to look up one on the web sometimes, look up volvox. Volvox is very cool. I'm not saying great-grandma was volvox, okay? But what I'm saying that it was something like this that was multi-cellular that took in its food, because that's is the beginning of what an animal is. Before we start phylogenizing the animals, we probably should talk about what makes something an animal.
So the first thing I have to do if I'm this Martian that just landed on the planet and I say, "If you want me to make up a phylogeny, tell me what an animal is." Here's what I would tell you. I would say that an animal takes in food, therefore has ingestion. So when you're looking for animals, go look for things that take in food. But there's more. Look for things that use proteins for structure support, because fungi take in food. But you want to look for proteins for structural support. What that means is you don't want cell walls. You want things like collagen, that supportive protein that makes up so much of the protein in your body. Look for nerves and muscles.
Now, you might find some of the animals don't have nerves and muscles, but most of them will. But that's okay because we need an out group anyway. Number four, I want you to look at their embryological development patterns. That's going to be important because you're going to find out that animals have a very unique way of developing, so you want to look at their embryological development patterns. They have very discernable differences. So that's what you're looking for to make it an animal.
And so you go out and you collect your ark of creatures, and you say, "We've got a problem here." We've got things like this and we've got things like this. By the way, one of these two is real. We have things like this and we've got things like this and we've got things like this. This is just too hard to do. How am I going to classify all these things? That's why we have phylogenies, folks, because we're going to do a sequential breakdown, one right after the other, of a branching tree, trunk, branches of the trunk, branches of the branches of the trunk, eventually getting down to twigs, twiglets, and everything else.
So we now have to determine how to build this phylogenetic tree, this gigantic cladogram, if you will, of the animals. And so we're going to work with our first group, and we're going to call that the out group. And this, you know, I love that this is an animal. This sponge is an animal. How cool is that? It just sits there on the bottom of the ocean looking like not much of at. And yet, when you look at it microscopically you find cells that ingest. You find a diversity of function in the cell, so it is a true multi-cellular organism, not a colony. You find a protein called spongeon that holds it all together. All right, you don't find nerves and muscles, so we'll make it our out group. But the bottom line it is the out group of the animals. And so when we start to take a look at our animal phylogeny we see that the very first branch, right here, right down at the bottom, is going to be... So here's our ancestor, our multi-cellular protist, and let's put in--and we literally are up here and so we've defined multi-cellularity. So everything on this branch is multi-cellular.
And it's at this point where I am right now where we're going to take our out group and we're going to come up with a group called the porifera, the porous creatures, the sponges. That is our first division. Now, what's so special about them? We're going to call those the parazoa. It's a descriptive term. It's a phylogenetic term. It is not a taxonomical term. The parazoa. They belong in their own phylum. Remember, kingdoms are divided into phyla. Kings play chess on fiberglass stools. Kingdom, phylum, class, order, family, genus, species. King Phillip came over from Grecian shores. You get it.
So this phyla porifera really is divided phylogenetically as a parazoan. Zoa--animal, para--kind of. It's a "kind of" animal. So we have now divided the parazoan to those that have true tissues. So there's no true tissues in the parazoa, but this division here does have true tissues, and we're going to call that the eumetazoa, an evolutionary term meaning "after the metazoa." Trust me on that one. And what we have here are tissues. So we have tissues in the eumetazoa. So we've already divided these things up, and now everything here is going to have tissue.
And so we come to our next fork in the tree. And look what we've done. We've seemed to have forked off a group of organisms called the cnidarians and the ctenophorans. If I had a cnidarian or a ctenophoran here you'd recognize it, because it would look something like this. I `m sure you're saying, "Wow, that's a jellyfish," and you'd be right, or a coral. OR the cone jellies, which look very much like jellyfish to the untrained eye, but they're not. And you'll notice that these branch off right here, and then everything else goes to the right. Why is that?
That brings us to the concept of symmetry. To our left we have a type of symmetry called "radial symmetry," and to our right we have a type of symmetry called "bilateral symmetry." Let me tell you a little bit about that. A creature with radial symmetry is like a can of soup. It has no sides to it. It has an axis that goes right up through the middle. I often tell my students it will have no head. There's no head to a jellyfish. This jellyfish doesn't have a head, but it has a body plan, and that's what symmetry is all about. Symmetry is body plan. So this has a radial body plan. What that means is that its body radiates out from a central axis, so it's like a soup can, as opposed to, say, a soup spoon, which we can call bilateral, but I'll get there later. So it has this thing and it has this radiating symmetry out. So something like a jellyfish, radial symmetry.
Well, what's bilateral? Well, just look at the word. Bilateral means two-sided. And so we have a two-sided symmetry. So if you take something like, say, a spoon--and I only use this analogy because I used soup can. It'll have two sides to it. Take something like me if you don't buy my soup spoon. I have two sides. My body is divided, generally--I mean, if you were to draw an axis, a plane, right down my body here, you would see a bilaterally symmetrical body. Two nostrils, two eyes, two halves to my brain. I have a body plan, a line right down my head where my palate joins together. You can even feel that line up in the top. I go right down. All of my organs, even my heart, two sides. So I am bilaterally symmetrical.
That's not all. I'm not the only bilaterally symmetrical creature on this chart. According to this chart, so are things like flatworms, and so are things like clams, and so are things like worms. So that's very interesting, except for one thing. Before we go on I have to throw one thing at you. The idea here is that sometimes symmetry is not the only thing you look at. Because if you take a look at starfish, we have starfish on the bilaterally symmetrical division. Don't get confused there. You're going to see that there are other things that take priority. Sometimes we have to use our heads.
Speaking of heads, let me tell you that you have a head, and there's a reason for that. You are bilaterally symmetrical, and that is called "cephalization." Without bilateral symmetry you never would have had cephalization, and you probably wouldn't understand a thing I'm saying because I bet you there's no jellyfish listening to me right now.
The Evolution of Life on Earth
Evolution of the Animal Kingdom
Constructing a Phylogenetic Tree of Animals: Animal Development Page [2 of 2]