Now I want to take a little bit of time to look at developmental genes and gene regulation. Because when we're thinking about adaptive radiation, we just want to know, well, how do things evolve so quickly? How do they differentiate from each other so quickly? Well, what we want to start off by saying is that many evolutionary changes are changes to gene regulation. Now I know when I think about evolution, I'm often thinking about a mutation to a coding region in a protein.
But what we're saying here is that a lot of mutations are to the regulatory regions or to a regulatory gene itself. Now why that's important is because that means that what we're doing in evolution is not always, you know, evolving new proteins. We're just changing how much proteins are used or when they're used in development. And this is important because small changes, we're going to say early in development, well, those can have really large impacts going forward. Right?
If cells are using different proteins early in development, that can have big implications for how that organism develops going forward. Alright. So now to think about this, I want to bring back this idea of homology. Remember, a while ago we looked at vertebrate limbs, and we have here a bat wing, a human arm, and a whale fin. And we can see that they have the same underlying structure.
Right? They have the same bones. So what's different about them? What's different is how the growth is regulated. Right?
In this bat wing, these bones grow long and thin. In the whale fin, they grow thick and relatively short. And, you know, we see another difference, right, in the bat wing. Right? We're just going to grow more skin between them.
And the whale fin, you know, the tissue is going to grow thicker and and more tough. But again, we're not inventing whole new body parts, whole new structures. We're just growing things at a different rate. Now that idea we call heterochrony. Hetero means different and crony refers to time.
So what this says is that changes in the rate or the timing of development, that's going to be responsible for a lot of the evolutionary changes that we say that we see rather. Different structures, we're going to say they are not evolved new every time. Different structures generally just come from different growth rates. All right. So as we're thinking about gene regulation, we need to call a very specific type of regulatory gene, and these are called homeotic genes.
Homeotic genes are genes that determine the specific organization of body parts in organisms. And these are going to be transcription factors, genes that control the expression of other genes, and lots of different organism patterns. So for example, in plants, there are homeotic genes that control you know, what parts of the flower develop into different things. So, you can imagine though, you know, there's a homeotic gene that says, hey, develop into a petal. Well, that means that changes in these homeotic genes, they can cause large changes in a short time.
Right? You can imagine a change that homeotic gene could cause that flower to develop very differently. Now in animals, we have a very specific group of homeotic genes that we need to call out and talk about. They are called the Hox genes. Hox genes control animal body plans.
Now not all animals have Hox genes, but a lot of those animals that we see bursting out of the scene in the Cambrian have them. So we're going to say here, they are first seen in these Cambrian phyla. And one idea is that, well, these Hox genes had to evolve probably just before the Cambrian. And then this set of genes allowed for the diversification of animals as part of that Cambrian explosion because these genes determine the body plan in a head to tail orientation. And the idea here is well, we can see these Hox genes drawn out in a couple phyla here.
We have the mollusks, that snail there, an arthropod, that fly, and we have chordates with that mouse there. And what we see, well each colored box here represents a Hox gene. And you can see in that in all these organisms, well, there's the same order of genes in the chromosomes. We basically have the same genes in the same order on the chromosome. Now it's not exactly the same.
There are some duplications. You can see, you know, one's missing there. You can see in vertebrates. This entire set of genes has been duplicated a couple times, so we have a lot more Hox genes. But they're basically there in all these organisms doing the same thing.
So what are they doing? This first one, well, it sends out a signal. When it's transcribed, it sends out a signal and say, hey, cells. You're in the front of the head. And then going down each one during development sends out that signal that you're in a different body part going down head to tail, and this last one, it sends out the signal.
Hey cells, you're in this segment that's going to become the end of the tail. Now that idea, this tells organisms how to develop. That means that these animals have really detailed information as to where those cells are in the body plan. That allows those cells to develop into more complex bodies. And as we see the vertebrates here, well, we have a lot of Hox genes.
That means that the cells in a vertebrate body plan have extremely detailed information as to exactly where they are in the body that allows them to evolve into a very complex body. So here when we think about the Cambrian, these new genes, well, this allows for complex bodies to evolve because we have detailed information in the body during development about where cells are. But again, because changes in regulatory genes can cause kind of big changes to the organisms, that means that changes to these Hox genes can cause really big changes in animal body plans. And that's exactly what we see during the Cambrian explosion. Alright.
We'll look at this more, an example and practice problem to come. I'll see you there.