We now want to spend a little bit of time reading trees or learning how to read phylogenetic or evolutionary trees. Now in this chapter, it's very unlikely that you're going to have a lot of test questions about interpreting phylogenetic or evolutionary trees, but you are going to see them. So we just want to make sure that when we see them, we know what we're looking at and we can read them in a basic way. So we'll start off here by saying phylogenetic trees, which is just another word for evolutionary trees. This is a way to understand species relationships.
And those relationships are going to be based on common descent, this idea that organisms share a common ancestor. Now when you look at a tree, there's really just two things you're looking at. You're looking at branches and nodes. The branches of the trees, those are the lines of the tree. They represent a population over time as it changes and evolves.
The nodes, well, the nodes are where the branches meet. Those represent a common ancestor of the different populations on the tree. Now there's all different ways that trees are drawn. Sometimes, like the one we have here, organisms are sort of lined up on the side and the tree sort of goes back off to one side. Sometimes the organisms are on the top, the tree comes down.
Sometimes they're even drawn in a circle. Sometimes like this one, the lines meet at points. Sometimes they make right angles. All of that doesn't matter. What we want to focus on when we look at a tree, we're going to say here to read a tree, focus on how branches connect and they connect at the nodes.
All right. So to see what we mean here, we're going to use this phylogenetic tree, this evolutionary tree of the genus Panthera here, the big cats. And we see a common ancestor and we can trace this population through time. So we find this branch. This is that branch, the population of that common ancestor evolving through time.
Well, then we see it splits. Now there are 2 populations evolving through time. We have 2 branches on tree. Well, these branches split again as these populations separate. Here, we see one population that leads to the evolves into the modern jaguar.
We see another population that leads this way and it splits again. One part of that population evolves into the modern lion, and the other group that split off, well, it evolves into the modern leopard. Down at the bottom, we see this other population. Well, it splits. One of those populations that splits off becomes the modern snow leopard, while the other population evolves into the modern tiger.
So those are all our different branches and to find our nodes, we just find where they meet. So here's a node. That's a common ancestor of different populations of this tree. It's a common ancestor of all the cats on this tree. They can trace their branches back to that node.
Alright. We can find different nodes. All of these are different splitting points that are the last common ancestor of different populations on this tree. Now on this tree, we have this arrow here and this arrow represents time. In all these trees, we have this sort of inherent axis moving through time where the organisms that we're looking at are at least alive today or at least alive more recently in time.
And the common ancestor, as you go back through the tree, you're going deeper and deeper into time. So a way to think of this is that relatedness, how closely related organisms are equals how deep in the tree is that common node or in other words, that common ancestor. So to see what we mean, let's look at an example here. Well, take the leopard and the snow leopard. If you want to know how closely related the leopard and the snow leopard are, well, they both have the name leopard in their names.
They both well, they kind of look a little similar. And in space, they're right next to each other on this tree, so you might assume they're closely related. But if we look at how deep in the tree we find the common node, well, I gotta go back all the way down these branches, all the way to find way back here. This first node where that first population split was, that's the last common node that these cats share. They're not very closely related at all.
That node is the ancestor of all the cats on this tree. So we can ask, well, who is the leopard most closely related to? Well, we follow its branch back. We follow it to this node and we see, well, what other population came out of that node? The lion.
So the lion and leopard, they're most closely related to each other. Alright. We're going to practice this type of thinking. More coming up. We'll see you there.
