Understanding and building phylogenetic trees | High school biology | Khan Academy

When we look at all of the living diversity around us, the natural question is, "Well, how related are the different species to each other?" If you put that into an evolutionary context, relatedness should be tied to how recently two species shared a common ancestor. What we're going to try to do in this video is construct a tree showing how different species evolved from common ancestors.

We're going to do it based on some of these observable traits that we see, but this is going to be a huge oversimplification. I'm only doing it with five species and five very simple traits, as we'll see or as we'll talk about in future videos. This can be done in a much more complex way, and that's what biologists would do. They would look at much more than five traits, and they would look at molecular evidence, in terms of protein differences and DNA differences, to really start to build out what we call a phylogenetic tree.

So let me write this down; that's what we're going to create: the phylogenetic tree. "Phylo" comes from the Greek for group or kind or tribe, and then "genetic" comes, you know, related to the word "genesis." How do these things come about? How do the different groups or tribes, or in this case, how do the different species come about?

Well, when you're trying to make one of these trees, it's important to realize that this is a hypothesis, but you're always trying to come up with the simplest hypothesis that can explain the observations that you actually see. When we look at these, at least the species that we have listed here, it looks like there's one that is more different than all the other ones. The lamprey here does not have any of these five traits that we are observing. So, this we would call the outgroup; the lamprey is the outgroup.

A lot of times when you need to construct a phylogenetic tree, they might provide you with something that is clearly an outgroup. Here, it doesn't have any of these observable traits. Sometimes if we're looking at genetic differences, it might have the largest number of genetic differences relative to everything else. It makes sense that the simplest hypothesis is that its common ancestor is most distant into the past with everything else.

So let me start to draw this tree. I am going to put deep into the past, so deep into the past, there is a branching out point where you have the common ancestor of the lamprey and everything else we see here. Eventually, you have that common ancestor, and there are many, many species along the way, and eventually, we get a lamprey in present time.

In present time, the next thing to think about is: all right, well, how did everything else end up branching? What's common about everything else that maybe wasn't common about the lamprey? One common thing is we see that everything else, at least that we have listed here, has jaws. It’s reasonable to say, all right, we have this common ancestor between the lamprey and everything else at this branching point right over here, and then it branched off into multiple species. One of those species must have evolved jaws, so let me put jaws right over here.

So, jaws right over there! Jaws are considered a derived trait. This ancestral species at this root did not have jaws, we're assuming, but at some point, they evolved and they stuck around because they proved to be favorable in certain environments, or it could have even been things like genetic drift; who knows? But I'm guessing that it was favorable in certain environments.

Now, let's see if we can classify everyone else. So now out of the four, let’s actually cross out the lamprey just for simplification since we've already classified that character. Now, of everyone else, we've already thought about everyone having jaws, so now let's go to the next most common trait.

Actually, let me cross out the jaws too, just to keep things simple so I can do that a little bit cleaner. So, I'm going to cross out the jaws, and now let’s see. The most the next most common trait is—are the lungs! But not every species that we have left has lungs. The sea bass does not have lungs; it does not breathe air the way that animals that live outside of the water breathe.

So, the next point of divergence must be between the sea bass and everything that we have left over. Let me draw that. Once again, I said must be, but this is a hypothesis; I think it's a reasonable hypothesis. So let me draw that. This is the sea bass, and there's a common ancestor between the sea bass and everything else: the antelope, the bald eagle, and the alligator.

At some point, that common ancestor diverged into multiple species, and one of those child species must have evolved lungs. So lungs must have evolved at some point, but we're assuming that that wasn't on this lineage for the sea bass. Once again, I'm just trying to find the simplest explanation. There might have been some situation where maybe lungs evolved and then went away at some point; you reverted to an ancestral form.

But we like to go with the simplest explanation. This is a property that biologists will also often call parsimony. Actually, let me write this down: parsimony, which in everyday language means cheap. If someone tells you you're parsimonious, it's a nice-sounding word, but it means that you are cheap.

Parsimony in this context says, "Hey, we're trying to be cheap with complexity. We're trying to be as simple as possible in our explanation of what's going on."

But anyway, let's go back to what we were doing. We've already considered; we have already talked about the sea bass here, and we have already talked about lungs. All right, so what do we have left? We have to talk about the antelope, the bald eagle, the alligator, and gizzard and fur.

It looks like the bald eagle and alligator have gizzard; the antelope has fur. Oh, and actually, we haven't talked about the bald eagle and feathers as yet either.

All right, so it is possible. Let’s make the next thing between—well, we could do it this way. Once again, I’m trying to do this in real time. Something that seems so… let’s make a branch here, and let’s say that is the branch for—let's say that's the branch for the bald eagle. I’ll say "be eagle," that's the branch for the bald eagle.

Let’s see if I can construct one that will explain the differences between the bald eagle, the antelope, and the alligator. Well, the bald eagle and the alligator have something in common; they have a gizzard in common. So let me make a branching point; let’s make them a little bit closer than the bald eagle is to the antelope, so let me do that.

Let me put the alligator there, and then I'm going to talk about when we get these derived traits. So, that is the alligator. Obviously, I could have written the alligator on this side and the bald eagle on that side, or I could have rotated—I could rotate at any one of these branching points.

What we would have left is the antelope, and let’s see if I can account for all of these derived traits. Antelope! All right, so we have the common ancestor of the sea bass, the bald eagle, the alligator, and the antelope right over here.

We have a branching point; at some point, lungs—we are hypothesizing—evolved in this branch. And then this branch—let’s say that this branch—this is the common ancestor between the antelope, alligator, and bald eagle, and a common ancestor of the bald eagle and alligator: they have to get the gizzard.

So let’s put the gizzard down right over here; this is where the gizzard—this is our hypothesis. Doing that same color: so that’s the gizzard, gizzard right over there. Everything that descended from that ancestor that had the gizzard—well, they’re going to have gizzards; that’s what we’re assuming.

But once again, that can be lost; this is a hypothesis. And so we have accounted for the gizzard. Let me cross that out; so we have accounted for the gizzard.

Now, let’s see; we have to account for the feathers, and the bald eagle is the only one that has feathers. So let me put that here. So at some point, you have a common ancestor of an alligator and a bald eagle; it branches off into multiple species, one of which gets feathers.

Once again, I—you know, that could have branched off into many, many things because we know that the bald eagle isn’t the only species with feathers. But the bald eagle for sure is a species that has feathers.

And let’s see; so we've accounted for the feathers. Now, feathers—and now we just have to account for the fur, the fur of the antelope. We don’t know where this could have happened; we might want to look for more evidence to come up with a good hypothesis.

But, someplace along this right branch, we could put the fur. And so there you have it; this is actually a reasonable phylogenetic tree. I practiced the biologic—the practice of parsimony to come up with the simplest explanation, but there are more complicated explanations, and we don’t know.

Some of those more complicated explanations could very well be true, but from this, we have a very quick and easy graphical representation of how related different species could be and where they share common ancestors.

So, the bald eagle and an alligator, based on this phylogenetic tree, we would say are more related than a bald eagle is to an antelope. They have a more recent common ancestor right there than both of their common ancestors with the antelope, and that would make them more related.

If we were doing this for real, we would want to look at genetic evidence and look at the various proteins and say, "Okay, does that back this up? Do bald eagle and alligator's DNA have more in common with each other than they do with the DNA, say, of an antelope?"

Especially once you get complex, there could be many different explanations, and we just want to get more and more and more evidence to keep refining our phylogenetic trees.