We've been practicing reading phylogenetic trees, but now we want to take a look at how you actually build them. Right? How do we know that two organisms are linked together on a phylogenetic tree? How do we infer this complex branching pattern connecting all these branches at different nodes? Well, starting off very simply, we're just gonna say that phylogenetic trees are built by looking at traits or characters, right?

So traits or characters, we can use these two words synonymously. A trait, you know, it might be something morphological like, you know, a lump on a bone or it might be something like DNA evidence. But what we're gonna say here is that to build a tree, what we're looking for are shared characters, and that's because shared characters suggest that organisms are more closely related. If two organisms share a character, we're gonna assume that character was in its common ancestor. Alright.

So if we look at here, we though we can take these shared characters and break them into two broad groups, our shared derived characters and our shared ancestral characters. Now we're gonna define those and see which one of those we want to really focus in on when building trees, but first, I want to introduce you to our file generic tree here. We'll just be using this for an example. We're connecting here the fish, the frogs or amphibians, mice, platypus here, a bird or a pigeon there, and a crocodile. Now I just wanna be clear.

This is a tree that is already made. Right? So we are looking at this tree. What we're talking about here is how would you build this tree? How would you know to link certain things on this tree?

Alright. Well, we're gonna say you would use share derived characters. A shared derived character is a trait that is present in more than one taxon. Right? That's that shared part, and we're gonna say that it evolved within the tree.

We're gonna say that evolved since the root of this tree. That's that idea of this being a derived character. Now we're gonna say that the shared derived characters, that's what you wanna use to build trees. So for example, in mammals, how did we know to link the mouse and the platypus together? Well, they share some characters that are derived.

For example, mammary glands and hair. Right? Both of these organisms have mammary glands and hair, so they are shared. Now, are they derived? Well, did the root of the tree, the organism that was alive at the root of this tree way back there, did that organism have mammary glands and hair?

I don't think so. Right? So therefore, I can infer that the mammary glands and hair evolved in the common ancestor of the mouse and the platypus, so I would feel comfortable linking them together. In other words, I think that those traits, those characters, evolved on this branch here in my tree. Now we have to compare that or contrast that rather with shared ancestral characters.

A shared ancestral character, that is a trait that was present at the root of the tree. So for example, all of these organisms have bones. Right? They all have bones. That's a shared character.

Most of them, you know, everything except the mouse, they all lay eggs. It's another shared character. But when I look at this tree, I go back to the root. I think that that organism at this root, I think it had bones and I think it laid eggs. So that's not informative to me.

Right? It doesn't tell me that these things are more that certain ones of these organisms are more closely related to each other. It just tells me that all of them have a common ancestor, but I knew that. Right? I was gonna link them all on the tree anyway.

I knew that somewhere they have a common ancestor. So these shared ancestral characters, yeah, they're shared, but they're not helpful to me for interpreting who is more closely related to each other because they are present at the root of the tree. Now just one quick note, you may look at that egg-laying and be, hey, what about the mouse? The mouse doesn't lay eggs, So is not laying eggs, a derived trait in that mouse? Yes, it is.

But it's not gonna be helpful to us because it is not a shared derived character. In this tree, the mouse is the only thing that does not lay eggs.