We've been thinking about reproductive isolation, and we've talked about prezygotic barriers. Well, now we want to focus in on postzygotic barriers. Remember, postzygotic, post means after, and zygotic refers to the formation of the zygote or the fertilization of the egg. And so postzygotic barriers, we're going to say these are incompatibilities that decrease, and I'll indicate that with a down arrow. They decrease the fitness of the offspring or that hybrid.

Right now, when these incompatibilities arise, sort of how far down the line after fertilization those incompatibilities show up, that's going to give us our different types of postzygotic barriers. We're going to have 3 of them. So let's go through them one by one. The first way that you could see this is through reduced viability. Now viability is just the ability to survive.

So in reduced viability, we're going to say those hybrids are just less likely to survive compared to non-hybrids. Well, if the hybrids don't survive, you can't get gene flow, and if you can't get gene flow, you're reproductively isolated. So our example here are birds of this genus, Fasedula. In this genus, there are some species that interact in the wild and sometimes will mate with each other, and they will have fertilized eggs.

But those hybrid offspring of these different species of the Physidula, they are just less likely to hatch. So these birds are sitting on this nest, but those eggs, they're not hatching. And if the eggs don't hatch, you don't get hybrids, you don't get gene flow, those species again, they're reproductively isolated.

Alright. Well, sometimes the hybrids do survive and can quite often they do. But when they do, you often see something called hybrid sterility. And well, in hybrid sterility, the hybrids are viable. They survive in fact, sometimes are quite healthy, but they can't reproduce. Right? They are sterile.

Now our example that we've been using here is the classic horse and the donkey mating to form a mule, right? The horse and donkey create a mule. That mule is strong. It's a really healthy animal, but it's sterile. And if an animal is sterile, it can't mate with its sort of parent species.

There's going to be no gene flow. The horse and the donkey are reproductively isolated. Alright. Well, what if a hybrid is fertile? Well, then you might see hybrid breakdown.

In hybrid breakdown, we're going to see that first generation hybrid, it's going to be healthy, and it is going to be fertile. But it's the further generations, it's like the second generation that we're going to see that decrease in fitness as I'm indicating there with that down arrow. Now our example for this is going to be cotton. And there's actually 2 species of cotton that are grown for textiles, things like clothes. And you can make them, and when you do, you get a hybrid, and that hybrid is quite healthy.

But the second generation hybrids, if you mate that hybrid, it has offspring that are weak and sterile. We're going to say here, they are feeble, they're unhealthy, and they are infertile. Right? And so if the second generation hybrids are infertile, if they don't survive again, you're blocking gene flow. You have reproductive isolation.

Now, I just want to note, you don't have to have just one type of reproductive isolation. You might have a prezygotic barrier, even multiple prezygotic barriers. And if those are overcome, well, then there's also postzygotic barriers. But to be called different species, we have to have at least one form of pre or postzygotic barrier. Alright?

We'll look at this more in example and practice problems. I'll see you there.