We see now we can classify reproductive barriers as either prezygotic or postzygotic. Here, we want to take a closer look at those prezygotic barriers. So remember prezygotic, that pre means before and zygotic, well, that refers to the zygote, the fertilized egg. So these are barriers that prevent mating, or if mating does occur, it prevents fertilization. And we're going to look at 5 main types here.

We'll go through them one by one, and we have an example for each. So the first one we're going to call habitat isolation, and this is kind of the most straightforward in my mind. This is when populations are isolated by location, but they just live or breed in different places. And if populations live in different places, they're never going to interact. They're never going to breed with each other.

Right? As we look at habitat isolation, where does that word come from? What we're talking about here, they have different habitats. They breed in different habitats, so they're isolated in that way. Now a classic example of this are the Kaibob squirrels and the Abert squirrels.

These are closely related squirrels that live in very similar habitats, but they are separated by the Grand Canyon. And I don't know if you've ever been to the Grand Canyon, but there is no squirrel that's getting across it. Right? So we have these squirrels here. They may want to mate with each other, but this one says, you live where?

Right? There's a Grand Canyon between them. They're never going to mate. Alright. Our next type of barrier is temporal isolation, and temporal refers to time.

This is when populations are isolated by mating time or season. So in this case, these populations live in the same place or at least have overlapping ranges, but maybe they mate at different times of day or different times of year. And our example here are these spotted skunks. Again, we have the Spilogale gracilis and Spilogale putorius. These skunks mate in different seasons.

Right? So these two are from different species. They may come across each other in the wild, and this one says, it's fall, love is in the air. But this other one says, you mean spring? Right?

They're just not going to mate with each other because they mate at different times of year. Alright. Next, we're going to look at behavioral isolation. And behavioral isolation is when these populations are isolated by mating, wait for it.

Behavior. Right? There we go. Behavior. Right?

So things have to attract a mate in some way. They have to do something to indicate that they want to mate. And if those behaviors are different between populations, even if they could mate with each other, they just don't recognize that that's what the signal is that's being sent out. So an example of this is the southern cricket frog and the northern cricket frog. These have some overlapping ranges, but they have different mating calls.

So even if they could mate together, well, this one's singing la la la or maybe croak, croak, croak, whatever. This other frog just going like, what? I just don't recognize that. They're not going to mate with each other, because they don't recognize that mating behavior. Alright.

Our last two, I think of is a little bit more technical here. The first one we're going to call mechanical isolation. In mechanical isolation, these populations are isolated by anatomical differences. Right? The parts just don't work together. Right?

These organisms have diverged in a way that they just can't mate. Alright. So this is actually kind of common in insects, and so we have an example here from damselflies. It says species of damselfly have reproductive structures that are just shaped differently. And in this image, we see well, these are two different species, male reproductive structures or their damselfly penises that we've drawn there.

And you can see they are shaped very very differently. And so this damselfly, this female, I guess, is coming up and just saying, not going to work. Right? The parts just don't match. Even if they wanted to mate, they don't they can't.

They, anatomically, are different enough that they can't reproduce. Alright. Our final type here, we're going to call gametic isolation, and this is when these populations are isolated by incompatible sperm and egg. Right? So for the sperm to enter the egg, it has to recognize that it's found the egg.

The egg has to recognize that that is a sperm that it wants to let inside and allow fertilization to happen. And if species are too different in certain ways, biochemically, that's just not going to happen. So an example here is with Arabidopsis. This is a classic plant used in genetics, and it says here that adhesion of Arabidopsis thaliana pollen requires a specific biochemical interaction. Right?

So this is a pollen from a foreign species and this flower is saying, don't stick around bud. Right? This might land on the flower, but it's not going to start growing a pollen tube. It's not going to recognize that it's on the flower because the biochemistry doesn't work. The biochemistry never informs that pollen, never informs the flower that they're on, that they're matched up, that they could actually reproduce together.

Alright. So those are our 5 types of prezygotic barriers. We'll go into postzygotic barriers in more detail coming up. Before that, we got examples and practice. I'll see you there.