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Biology: Building a Cladogram


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About this Lesson

  • Type: Video Tutorial
  • Length: 12:30
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 134 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: The Evolution of Life on Earth (34 lessons, $64.35)
Biology: Classifying Earth's Organisms (4 lessons, $8.91)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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So if you were a systematist trying to do a taxonomy, what would you do? How do we build these unbelievably complex trees? The answer is we build them one twig at a time. How do we do that? Let's take a look. Here's what I would do. If somebody said to me, "Okay, you're the Martian. Land on the planet. Make a phylogenetic tree, would you?" Well, the first thing I would do is I would do a situation where I broke organisms down into the smallest groups I possibly could, depending on what I wanted to classify. So I would gather the organisms. So the first thing I would do in building a cladogram, one of these branching trees that are based on evolution. What I would do is I would gather the organisms in question. That would be number one.
Then the second thing I would do is I would determine homologies. Let's review a little bit about what a homology is. Homologies are structures that something has in common. That's what a homology is. There are different kinds of homologies, and I'd have to differentiate. For example, there's one called a "general homology." The general homology would be a homology that's common to all the organisms in my little dish that I'm checking out, or in my zoo or in my cage or in my room. So a general homology would be common to all of them.
Now, I want you to think about this for a second and keep this in perspective to all the organisms in question. What does that tell you about a general homology? Well, if they all have it, and they're all evolutionarily linked--remember, these organisms are organisms you think are evolutionary linked. You probably wouldn't be a good taxonomist if you were saying, "Okay, let's get a chimp, a bird, a reptile, a salamander, and an earthworm and put them all in there together," because you're not looking closely enough at their physical traits. So remember, we're thinking they're evolutionarily linked. Well, if this homology is common to all of them, then it must be the most ancient trait. It must be the ones that they all have in common, and was therefore down in their common ancestor, so that's why it's common to all. It is the most ancient of the traits.
On the other hand, there are what we call "special homologies," and special homologies are only shared with a few, so they're common to a few. These would be the homologies that branched off the most recent, and that's going to be important. We'll see that when we build our tree. So I'm looking at homologies. Now, you've got to be careful of something. Beware of homoplasy. Bad. Now, what that means is this. This is when structures co-evolve and they may have the same function, but they are completely different structures. And remember, function is one thing. Function can evolve over here from one set of genes, and the same function can evolve over here from another set of genes, and these genes might be completely unrelated. One example that I like to use is wings. You use this in all levels of biology. A bat's wing is made out of bones and tissue and muscle like a bird's wing. Well, if a bat's wing is similar to a bird's wing, and then we take a look at an insect's wing... Take a look at this wing, it's made out of scales and it's made out of membrane, and there's no muscle and there's no bone. This is a homoplasy. This is homoplastic, and we sometimes refer to that as an analogous structure--analogous meaning same function, different structure. So you've got to be careful of that when you're building these homologies to be careful and make sure that you do what you're supposed to do in terms of physical traits.
Well, that being said, we've got our homologies, we've got our critters, we're comparing their physical structures. Now we want to determine derived traits. Now we want to determine which traits differ from the ancestral traits. So we've determined what organisms are related, now we want to see how they're different. So we definitely want to get a group of ancestral traits and then work that into what we call "derived traits," traits that have been derived.
And then last, but not least, we want to determine something called the "out group." We want to look at this group of things that we've kind of lumped together and say which ones are out there, and we're going to call that the "out group." Which one is the most different than all the others? This is kind of like our control, if you will. This is kind of like our comparator, the thing we're going to compare things to. Think about it. If it is the most different, that means it probably branched off from the others earlier, so it's the most ancient branch. See? And therefore, it's the one that branched off the first. And that is how you start your tree. Your first fork of the trunk is going to start with your out group. See how logical this is? And look what you're doing. By limiting the number of traits, you're able to build this same tree and then go to each little branch and work on that as you add more organisms to it.
Let's do one together. Let's do a simple one. Let's take a look at a group of organisms--chordates, if you will. We'll talk about that at another time, why they're called chordates. What I have here is primates, mice, birds, lizards, salamanders, fish, and the hagfish. Now, you look at all these things and you say, "Okay, well what do all of these things have in common?" Why would some be one group and others be another group? What we have to do is up at the top of the chart we're going to keep track of their derived traits. Watch the way we order these things. This is important. I'm using one, two, three, four, five, six derived traits. I'm going to order these things according to the most they have in common to the least they have in common--or actually, I'm going to start down here and you'll see what I mean. Watch how we're going to build this.
So I have these in a particular order. Before you guys came here I had all these critters out on this table and I made this chart and I'm going to fill this chart out for you. Here's the bird we used. The other creatures have all climbed away. Let's take a look now. As I look through these things what I'm going to find is that--I'm going to write on this chart here, and I'm going to uses minuses and pluses. Hagfish--no jaws. This is my out group. He has literally, as you're going to see, nothing in common with all of these except some basic anatomical body structures that makes me say he belongs to these. But fish have jaws. Jaws, jaws, jaws, jaws, and primates have jaws.
Now, what about lungs? Well, hagfish don't have lungs, fish don't have lungs, but salamanders do, reptiles do, bird do, mice, and primates. What about claws and nails? Well, hagfish sure don't have claws and nails, and fish don't, and guess what? Salamanders don't, but reptiles do, birds do. So look what I've chosen. These are characteristics I've chosen on purpose.
Now, we come to an interesting one which we'll talk about later. None of these three obviously have feathers. They don't have features--lizards. Birds have feathers, but you and I don't and mice don't, but we do have fur. But birds don't have fur. So this little divergence right here is kind of interesting. We're going to have to talk about that. And we know no fur there. And certainly mammary glands, the ability to feed your young with milk we've determined is a big one, and mice and chimps both have that. So if you follow my pluses--see what I've got? I've got this nice little accumulation--almost so I can build a branching tree.
So let's take a look at my branching tree. I'll try to go back and forth with this, but watch the way this is going to work, because we're going to build a cladogram. Notice what this is going to develop for us. Since this is evolutionary, this branching tree is going to be temporal. It's going to give me time. It's going to be relative time, but it's going to be time. So let's take a look.
So remember what we did. Here's our common ancestor and the very first place we forked was with the out group, the hagfish. And the very first thing we asked was, jaws. And so the hagfish went off on the jaw list group, and now we come to a fork--and I'm only using right angles here just so it fits. This is not suggesting, "Boom, overnight jaws." This is a temporal thing, and there's no relationship in this particular cladogram to height and time. That's important. This is just to show relative diversity. So you can't say, "Oh, look..." Well, I've got these all the same size, and that's a good thing, but in this particular one we're not using real time in our vertical axis, just relative time. And there's nothing on the horizontal axis.
So then we have jaws, and then look what we did. We said, "Lungs," and the fish branches off at the lung branch. So along we go to the fish, and the fish is its own phylogenic group, it's own taxon, and now we fork here, and you remember the next fork was the lungs, and we forked there. And then we forked at the claws and nails because what we realized is that--again, look at it. Everything to the right of this is going to have claws and nails. Right? Everything to the right of jaws is going to have lungs and claws and nails. You see? And I said to you before--just this one last thing, "What's up with the feathers?" Well, apparently what happened--we could say one of two things--we could say, "Ah, after claws and nails something developed feathers and then the feathers turned into fur." But sometimes we've just got to use our heads here. Our logic tells us that there was probably another branch right there, and at that branch we went into the furry guys and in this branch we went into the feathery guys, and then last, but not least, our fur division--also, we talked about mammary glands. And then, if we kept going... Just one last thing. Suppose we wanted to keep going along this branch? So we wanted to take these two and lump them with other things. Now we could start talking about, "Well, let's get all the furry, mammary glandy things running around in here and let's check them out." So pop up a couple of kangaroos, we'll bring in some humans, we'll have a great big party, and we'll phylogenetically tree that one out, too. Logical, in essence, simple, and what a great way to keep yourself organized.
The Evolution of Life on Earth
Classifying Earth's Organisms
Building a Cladogram Page [2 of 2]

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