We said that allopatric speciation occurs when the two populations just live in different places. So now we want to take a closer look at how that can happen. So we'll redefine it here to start allopatric speciation. We're going to say these diverging populations are geographically isolated from each other. In other words, they have that habitat isolation.
They reproduce in different habitats. We're going to see two basic ways this can happen. That's dispersal and vicariance. So dispersal, well, that's just the movement of individuals from one place to another. And we have an image illustrating this.
We have these butterflies living on the mainland here, and we have this island with no butterflies. And it looks like some of these butterflies make it to this island. Maybe they're blown there in a storm, who knows why? But now they're stuck on that island. So the first step we're going to say here is that the individuals, some group of individuals disperse.
They disperse and they colonize some new habitat. Well, now in this new habitat, well, there's water between them that apparently they can't get across very easily. So there's reproductive isolation. Importantly, there's that genetic isolation. There's no gene flow between these two populations.
And if there's no gene flow, there's nothing keeping these populations genetically similar to each other. So the populations diverge. And we have that illustrated here by the butterflies becoming different colors. Now importantly for speciation, the idea is that if this goes on long enough, that divergence happens for long enough, these populations are going to be no longer capable of breeding in the wild if they ever do come in recontact. Alright.
So that's dispersal. Now let's look at vicariance. Vicariance is going to be a physical splitting of a habitat. Alright. So to illustrate this, we have this field of flowers here.
And right in the middle of the field of flowers, well up pops a mountain range here. Alright? So now these flowers are stuck on either side of this mountain range. So a vicaritous event is going to start with an event, either a change in habitat in some way or maybe a geological event that splits the habitat. Now once the habitat is split, in this case, these flowers are stuck on other sides of the mountain, right?
We have that reproductive isolation, that genetic isolation. There's no more gene flow. That there's no more gene flow, there's nothing keeping these populations genetically similar. They're going to evolve in different directions and these populations are going to diverge. And we have that illustrated again through them changing colors.
But what's really important is that if this goes on long enough, these populations are going to be no longer capable of breeding in the wild if they ever do come in recontact. Now you might look at these and say, I don't really get what the big deal is here. Right? You're still just splitting the population. What's really the difference between the two?
Well, that's true. Right? They're very similar, but you're going to have very different implications for how these populations might evolve going forward. So for example, in dispersal, well, you're much more likely to get the founder effect, where a small population, right, loses a lot of genetic diversity. Now when populations disperse in this way through a dispersal event, they're also likely to land on an island that's very different from the habitat that they were in.
And that might require some real evolution and adaptation to live in that new place. That's going to be very different from a vicariance event, where you're much more likely to get larger populations, larger starting populations for both populations. And they're not actually physically moving anywhere, they're just staying where they are. So again, these are very similar, but they do have important implications for how you might think these populations will evolve going forward. Alright.
We'll look at this more in example and practice problems going forward, and you don't want to miss it.