If speciation is the process of taking one species and breaking it into two through reproductive isolation, what happens when those two populations, those two new species come back in contact with each other? That's what we want to talk about here. We're talking about hybrid zones. We're going to define hybrid zones as just areas where members of, we'll say different species, different species mate and they produce hybrids. Right?
Pretty straightforward. Now if they're making hybrids and those hybrids are fertile, that means that gene flow is possible in hybrid zones. And gene flow, we can think of kind of as the enemy of speciation. Speciation creates genetically distinct populations, different species. Well, gene flow makes populations more genetically similar to each other.
Alright. Now this is possible if we're going to say there is incomplete reproductive isolation after secondary contact. And we've bolded secondary contact here, because that's a phrase you're likely to see. Now to think about what we mean here. We have our red and our blue butterflies.
These are newly evolved species that evolved in allopatry in different areas. But now for whatever reason, their ranges have expanded, and there's this area where their ranges are overlapping. Some of them are in sympatry. We call that secondary contact. The question is, well, what happens now?
Alright. There are a few different options here. Let's go through them. The first thing that can happen, we can call fusion. Well, these two species could fuse back into one.
That could happen if there is high gene flow. If there's a lot of gene flow, it's going to counteract that speciation process. We're going to say it erases the species boundaries. Now that can happen if there are a lot of hybrids.
If there are many hybrids with high fitness, at least relatively high fitness, high enough fitness so they can go back and mate with the parent populations and have that gene flow. Now that we're going to say will reverse. It reverses speciation, and we can see that in our image here. Right? Where we started with the red and blue butterflies.
Now there's no clear boundary. It's fused back into a single species. Right? Well, another option is stability. You could have a stable hybrid zone.
That can happen if you have limited gene flow, and specifically limited gene flow, we're going to say here, out of the hybrid zone. Well, that can occur if you have a really well-defined hybrid area. Well, if you have a really well-defined hybrid area that limits the amount of gene flow and those species can remain, there's still enough reproductive isolation. We're going to say here despite those hybrids being formed. And to illustrate this, right, we still have the red butterflies and the blue butterflies where they overlap.
We have this area with very clear hybrids. But as long as gene flow is generally going into the hybrid zone, and there isn't a lot of gene flow going back out of the hybrid zone, well, that gene flow isn't going to be enough to make these species back into one. It's not enough to fuse them. You get that stable hybrid zone. Alright.
The final option here, we're going to call reinforcement. Look at this. I'm actually just going to move myself over on the screen here. And reinforcement, we're going to say natural selection eliminates gene flow.
Now that can happen if the hybrids have low fitness. Right? If hybrids have low fitness, there's going to be really strong selective pressure for organisms to recognize their own species and only mate with their own species because it's a waste of time to mate with the other species and have these low fitness offspring. Now that is going to increase, we'll say, prezygotic barriers, because prezygotic barriers are those barriers that prevent hybrids from being formed in the first place. So the outcome here, we see we have a very clear distinction between these two species.
Now they still may be living in sympatry, but they are not mating with each other. And in fact, where they are sort of close to each other or encountering each other, we can see here if anything, they become more different from each other. They get these strong prezygotic barriers that we've indicated the colors being more intense there. Where they're more in allopatry, where they're not encountering each other, they might not have those prezygotic barriers because there's no selection for them because they're in allopatry. They're never encountering each other.
They're never making hybrids. All right. So the big idea here, right, when you have these hybrid zones, what's going to happen is going to depend on how much gene flow exists. Alright. We'll practice this more going forward.
It'll be fun.
