We continue thinking about building phylogenetic trees, we now need to introduce a concept and that's because, well, we're going to say here that phylogenetic trees are built using the concept of parsimony. So parsimony, we're going to define here. Parsimony is this concept just that the simplest explanation is probably correct, right? Now this you can take outside of biology. And in general philosophy, this is often referred to just as Occam's razor.

Now, Occam was a philosopher, and what he said was you want to take your explanation and shave it down to its simplest form because, again, that simplest explanation is probably the correct one. Now when we're looking at phylogenetic trees, we can take what we call a maximum parsimony approach, and that just says that a tree that represents the fewest number of evolutionary changes is most likely. It's probably the correct tree if you had the least number of evolutionary changes, or we can think of character state changes happening on that tree. Now the way this actually works in modern biology is you'll take your data, you'll put it in a computer. It might be thousands of characters, you know, if you're looking at DNA evidence, and that computer is going to build all sorts of very similar phylogenetic trees.

And then it's gonna use the concept of parsimony to figure out which one of those very similar trees is the most likely explanation. Alright. Now, to sort of model how this is gonna work, we're gonna compare 2 very similar trees here with these characters and character states that we've looked at before. So here our trees show. Well, we have a fish, a frog, a mouse, a platypus, a pigeon, and a crocodile.

It's the same organisms we've looked at before and in the same organism in both trees here. And the difference between these trees, well, the tree on the left, we see that the mouse and the platypus are shown as more closely related to each other. Whereas, this tree on the right, we're kind of testing this hypothesis that, well, maybe the platypus is more closely related to the bird in the crocodile. All right, now figure this out. We're gonna map our character state.

So here are our character states that we're going to be looking at. So let's start, and every time we think that there should be a character state change on our tree, we're going to map it. To see how this works, I'm going to use a green dash for the ability to lay eggs. Now laying eggs is actually an ancestral trait, so I don't need to map that character state change on here because that comes before the root of this tree. But we know that mice don't lay eggs, right?

So that's also a character state change. So I can put a dash on this lineage that leads to the mouse, right? Because on that lineage, it lost the ability to lay eggs or evolve the ability to have, you know, internal fertilization, whatever you want to say. Now on this tree over here, same thing, lineage leading to mice. I'll put this dash because we have a character state change.

The mouse doesn't lay eggs. Alright. Now we can keep on going for all of our other traits here. We have 4 limbs. I'll mark that one in red.

Well, the ancestor of all 5 of these organisms on both trees, that's where we think that 4 limbs evolved. So I can put that character state change on that branch that leads those 5 organisms. We have the amnion. The amnion, I'll mark in this sort of yellow, orange, or gold color here. Remember, the amnion is this layer that surrounds the embryo in these 4 organisms.

So I find the branch that leads to those 4 organisms, and I put that character state change on my tree. I'm gonna do the gizzard. Gizzard here in this blue. Gizzard, the structure that grinds up food and the bird and the crocodile, so it would have evolved in this ancestor that leads to those organisms on this tree. And then we have the mammary glands.

The mammary glands, these milk-producing organs. Now on the left, that's present in the mouse and platypus, so I find the branch that leads to them, and I have the character state change where we gain the ability to make milk, gain the mammary glands there. But then on this tree over here, well, I have the mouse and the platypus, but they don't share an immediate common ancestor with each other. So how is this going to work? Well, now I could say that maybe the mammary glands evolved independently in each organism, right?

That's one possible explanation. That would be 2 character state changes. Now another explanation, that's what I'm gonna draw in here. It's still gonna be the same number of character state changes. It's gonna be 2.

We could have evolved mammary glands in the ancestor of all of these organisms, And then maybe mammary glands were lost in the lineage leading to the bird and the crocodile. Again, not saying this is what happened. We're just testing this hypothesis. Okay, so that's a possibility. So which tree is the correct tree then?

Or which do I think is the correct tree? Well, I'm gonna count my number of evolutionary changes. Well, this one on the left, I see 1, 2, 3, 4, 5 character state changes. So I'm going to put a 5 here, 5 evolutionary changes. My tree on the right, 1, 2, 3, 4, 5, 6, 6 evolutionary changes.

Alright. Therefore, by the concept of parsimony, I can reject this tree. I think the tree on the left there is more likely to be the correct tree because it is more parsimonious. There are fewer evolutionary changes. Alright.

Now in modern biology, we still use the concept of parsimony, but we understand that things can be a little more complex than just counting the number of changes. So we use more advanced statistical methods, things like maximum likelihood. Now maximum likelihood sort of builds into our model and just sort of recognizes that some changes we're going to say are more likely than others, right? So it's not just as simple as counting the number of changes. We want to make sure that the more likely changes are happening more often.

Now, for example, we know that certain mutations. We know that certain mutations are more likely than others. So for example, a mutation from a cytosine to a thymine, we know that is more likely than a mutation from a cytosine to a guanine, from a C to a G. And that's just due to the chemistry and the structure of DNA. So we can build that into our model and say, what we actually think is more parsimonious might be a few more evolutionary changes if those evolutionary changes are more likely to happen. So again, remember the idea of parsimony, go with the simplest explanation, and you can take this again outside of biology.

If you're trying to win an argument with a friend, you've got a new 50¢ word to use. Alright. More after this. I'll see you there.