The concept of homology, that organisms share traits because those traits were present in their common ancestor, is going to be an incredibly important idea for understanding evolutionary relationships. But there's also going to be some specific types of homology that you're probably going to need to know about. So here we're going to go through some of those. We're going to talk about vestigial structures, embryologic homology, and molecular homology. Alright.

Well, let's start with vestigial structures. This is something you've probably heard about before. The idea of a vestigial structure is a trait that has lost most or all of their ancestral function. Alright. There are some traits when you see them.

It's just very clear it's a reduced form or not as functional as it once was in the ancestor. Now importantly, that doesn't mean that it has no use. It's just clearly a reduced function. So some great examples of this, right?

The pelvic girdle and femur in some snakes. Right? Snakes don't have legs. Why do they have a pelvic girdle? Wings on an ostrich.

Right? An ostrich can't fly. Now it does flap those wings around sometimes, but it's very clear that it's descended from birds that had flying wings, and it can't fly. And my favorite example is going to be human goosebumps. Right?

We see here an image of goosebumps. Right? If you get cold or scared or excited, the hairs on your body stand up on end. But why do they do that? Well, if you had fur, right, standing the hair up on end, that's going to increase the warmth of that fur coat, or it could be used as a threat display.

Think of, like, a dog growling. The hair is standing up on the back of their neck. Right? But it doesn't do much for us because, well, I don't have very much hair. I don't have enough hair to keep me warm when it stands up on end, and I don't have enough hair to be threatening when it stands up on end.

Alright. Our next example here is going to be embryologic homology. Now, embryologic homology is one that people realize fairly early in the study of evolution. And this is just this idea that organisms share traits as embryos that are not shared in adults. Right.

So we can see here we have an embryo of a fish, a bird, a reptile, and a mammal. And for how differently these organisms look as adults, they look pretty similar as embryos. And we can even call out specific traits, like the pharyngeal arches and the post-anal tail. Right? We see here these little bumps here, these are all the pharyngeal arches.

Sometimes they're called gill slits, but technically that's correct because all these organisms don't have gills. But early in development, all these organisms have these sorts of same bumps, and then they go on to develop into sort of different but related structures in the adult organisms. They also all have tails, even though not all these organisms have tails as adults. So why would that be? Well, we said that evolution, right, it works on existing variation.

It doesn't just sort of blow up body plans and start all over. And changes in an early developing embryo, that's probably going to cause a lot of changes down through development as that organism turns into an adult. Where small changes sort of at the end of development, they're less likely to sort of blow things up, and those are the type of traits that evolution is more likely to work on. So these body plans, these early developments are well, it's remarkably stable through evolutionary time, and so you can look at things that have really distant common ancestors, like these organisms we see here, and they still share traits as embryos. Alright, our final one here is going to be molecular homology.

And really, in modern biology, this is the big one. Right? If you see evolutionary trees today, if it's not based on fossils, then almost certainly it's based on molecular homology. And this is just the idea that DNA and protein sequences are more similar in related organisms. Alright.

So we have some DNA sequences here. We have the sequences of a lion, a tiger, and a house cat. But we know the lion and the tiger are more closely related, and we can look at the DNA sequences. Their DNA sequences are more similar. Why?

Well, we have these stretches of DNA here that are identical between these two cats, the lion and the tiger, because that is almost certainly the sequence that was in their common ancestor. Now the cat, there's some parts that you see that are similar because these things do have a common ancestor if you go back far enough, but there's more differences because, sent with modification, it's been longer since these things shared a common ancestor. Alright. So with homologies, remember, homologies instruct us as to how things are related to each other evolutionarily. And there's lots of different ways that we can see homology.

Alright. With that, I'll see you in the next video.