In biology, we're often talking about groups of organisms, and we have a set of terms that allows us to understand exactly how the members of that group are related to each other on a phylogenetic tree. Those terms are monophyletic, paraphyletic, and polyphyletic, and those words always refer to types of groups. Now, you probably recognize right away those words are kind of long and sound pretty similar, so it could be easy to get them mixed up. We're going to go through them very clearly here, and we'll practice it going forward so we can get it all straight.
Now, what we're doing here really is cladistics. Cladistics is when we classify organisms focusing on their common ancestor. So if you are putting organisms in groups based on a phylogenetic tree, you're doing cladistics. The groups that you want to make when you're looking at that tree, we typically want to find clades, and we've defined a clade before as an ancestral taxon and all of its descendants.
And we're going to say that by definition, a clade is a monophyletic group. But now we want to look at monophyletic groups a little bit more closely and compare them to our other types of groups to understand why you're going to use each term. Let's again start with the monophyletic group.
A monophyletic group, well, mono means 1, and fila, that comes from the Greek that means tribe or group. So this literally means a one group group. A monophyletic group, that's a clade, and that means that this is the ancestor and all of its descendants. As we talk about this, we're going to be using the same phylogenetic tree here.
Let me just introduce you to it. So we're showing some mammals here. We're not really going to be interested in the mammals though. We also have on this tree are reptiles and birds.
So you see here the turtles, the lizards, the birds, and the crocodile. We see how they're related to each other. In blue, we have all the branches here that belong to this monophyletic group, and this group we call the sauropsida. Right now, you probably haven't heard of that group, sauropsida, before, and that's because we normally talk about reptiles and birds. But if we're talking about a clade or a monophyletic group, well, reptiles aren't a monophyletic group.
If you're talking about the monophyletic group that includes reptiles, you have to include birds. So if we go back to this common ancestor here, we see that the turtles, the lizards, and the birds and crocodiles are all descended from that common ancestor. So Sauropsida is our monophyletic group, and the way that I tell that, my little trick here is I'm going to try and circle the organisms in the group that I'm interested in. So I'm interested in here, well, the sauropsida, the turtles, the lizards, the birds, and the crocodiles and their common ancestor.
So when I circle that, my circle should only cross my tree one time. It should cross the branch leading to that common ancestor, and everything else should be safely inside that circle. If that's how you circled your organisms, that's a monophyletic group.
But maybe you're interested in reptiles. So what are reptiles? Well, reptiles are going to be an example of a paraphyletic group. Now, para means parallel or beside.
So this is literally a beside group group, and this is when a group includes that common ancestor, but only some of the descendants. So again, now reptiles, that's the turtle, lizard, and the crocodile, but not the birds. So when I go to circle this group, well, the first thing I want to know, was the common ancestor a reptile? Well, the common ancestor is here on this tree, and yes, that was a reptile.
So when I go to circle it, I'm going to circle that common ancestor. I'm going to circle the turtle, the lizard, but I got to dive in and I got to cross the tree again to go around the bird, and then go back around the crocodile, and I finish my circle there. That's what it looks like when you circle a paraphyletic group. You have to cross another branch to leave one of the organisms out of your tree. One of the ways you can sort of think about it is that the birds have kind of evolved out of this group.
They have enough changes to them that we call them something different. So, for example, things that reptiles have in common that birds don't, reptiles have a 3-chambered heart, they're cold-blooded, reptiles have scales instead of feathers.
So, it is reasonable to talk about reptiles as a group and birds separately, but we just have to be really clear it's a paraphyletic group. That lets people know either to leave something out of this group to make it a group. Our final grouping here is going to be a polyphyletic group.
And well, poly means many. So this literally means a many group group, and that's when your group does not include the common ancestor. So if you make a group and the common ancestor of the organisms in that group aren't included in the group, that means that whatever you're grouping them by is probably an analogous trait.
That's a trait that evolved independently. It wasn't shared in that common ancestor. So for example, I might be interested in swimming reptiles. On this tree, that includes the turtles and the crocodiles, and that may be interesting to study because they may do some things differently and some things the same, and you may want to understand how reptiles adapt to this sort of semi-aquatic lifestyle.
So when I circle this tree, I'm going to circle around the branch that leads to the turtles, around the crocodiles. But then I have to come back around the birds and lizards, and I do not include that ancestor because this common ancestor right here on the tree, that was not a swimming reptile. Again, by just saying, hey, this is a polyphyletic group, I understand you're talking about a group that you've put together that has some things in common, but they're analogous traits that they have in common.
They weren't shared by that common ancestor. So we can see then that talking about para- and polyphyletic groups, that can be useful groupings of organisms. There are reasons why you want to talk about those types of groups. But we want to be really clear that only monophyletic groups should be used in modern systematics.
When you are naming organisms, putting them in groups with scientific names, we always want to make sure that those are monophyletic groups, or in other words, that they represent a clade. We'll practice this more going forward.
See you there.
