Understanding Hybrid Zones and Speciation

Hello 199 students, this is Dr. Halfhill and we are up to lecture 3 of chapter 24. Our first lecture we introduced what is the biological species concept, that species can only cross with each other and not cross with anyone else and produce viable hybrids. We put forward two different modes in which speciation could occur. We introduced allopatric speciation and sympatric speciation. And today we will finish up this chapter by talking about what happens when two species are adjacent to one another and begin to form hybrids, and these areas are going to be called hybrid zones. And then we're going to end with the question, so how fast does speciation actually occur? So a hybrid zone is a region where two different species come in contact with one another and potentially mate and form hybrids. And so what a hybrid zone allows us to look at is to see, so what happens between those two species and whether or not the two species are going to come together and form one species, or are they going to stay apart from one another? An example of a hybrid zone is this. So in one part of Europe and Asia, you have frogs that have yellow bellies. So we're going to call them yellow-bellied frog. And in another region, you have ones that have red bellies, so we'll call them. red-bellied frog. And so there are locations in the environment where these two species are going to come across one another and they have the possibility of mating. And so when they get a mortgage with one another, they have the capacity of forming hybrids. And what a hybrid zone is going to let us to do is study this process in action. One thing that we'll observe is that hybrids are often going to have reduced fitness in comparison to the original types. So let's look at an example of a hybrid zone. So here we are in the mountains that are going to be separating Europe from Asia. So on the Asia side, you generally see red-bellied frogs, hence the orange. And in Europe, you generally see the yellow-bellied frogs, and so they're on that one side. And so the red line, which is actually pretty thick, represents the hybrid zone, where these two species can cross with one another and make hybrids. Now, what we really want to look at is the lower right hand side. Let's look at this crazy graphical representation where on the X axis, it represents the distance from the hybrid zone. And so we're talking in kilometers. So these frogs definitely interact with. with one another over a big part of the range. And then on the Y-axis, you have an allele frequency. So an allele frequency, this is either P or Q. So the upper part of the line represents one of the alleles being at high frequency. And so you're probably thinking that in this particular case, that upper part is mainly, let's say, the yellow-bellied frog. And then on the lower part, perhaps where you see a real low allele frequency with that part of the line, that's going to be the alleles associated with red belliness. And then you have that area right in the middle that has a very severe slope. Now, that right there represents the hybrid zone, where you end up with intermediate individuals that are going to be hybrid between the two consistent lines of the different alleles. And so... In this hybrid zone, there are going to be several outcomes that are going to happen to the two different species. The first outcome is that there is going to be some sort of reduction in fitness of the hybrids, and you will strengthen the reproductive barriers between the two species. They will continue to micro-evolve away from one another to the point where you will absolutely continue to have two different species. is if the species end up being pretty related to one another, there could be a weakening of the reproductive barriers and then the two populations come back together and form a single species. And then the last thing that can happen is going to be continuing to form hybrids. The two species never come back together and they just continue to have that hybrid zone in the middle. Let's look at a challenging graphical representation. So the first thing, you have these yellow circles. Now each yellow circle represents a population. So in this case, we have four populations. Now the blue circles, they represent individuals within those populations. So in the populations, they're crossing with one another. And so these four are all the same species. There can be gene flow between the populations, hence the black arrows. What that's saying is that they're crossing with one another. Now all of these are absolutely the same species. So then what happens next is that there's some barrier to gene flow that occurs. Perhaps this population here becomes isolated. There is a change in the flow of a river that isolates one population on the other side. a forest becomes fragmented, there's some reason that this one population, the one that's shown on the top line, becomes isolated. Now, it is going to go through microevolution as it goes away, and so the color change going from blue to pink represents the change in allele frequencies over time. Now, the lower arrow... This represents the original species where there's still multiple populations, they're still in contact with one another, and so allele frequencies are not going to change a lot. They're going to generally remain the same. So then over a long period of time, you could then have a hybrid zone between the species that became, the group that became isolated from the original. And so here is where the three different modes could occur. The first outcome would be reinforcing, where in this particular case, the hybrids would not be very viable, where what you see is that the green in the center, they become less and less in regards to the number of hybrids over time. And so this would be true speciation occurring, where the top line would become a different species from the bottom. Or... you could have fusion. Now, this would be in the hybrid zone. If the hybrids were viable and could start crossing with both the isolated population and the original, the whole isolated population could just fall right back into the original gene frequencies. And you would say that this isolated population fused back with the original. Or the last thing that could end up having would be stability. Now, in the stability scenario, both original species are going to maintain their identity, and you continue to form hybrids. An example of this would be the donkey-horse-mule situation, where the top line could represent donkeys, the bottom line could represent horses, and they're able to continue to form hybrids, and the two species are never going to come back. together and fuse into one and they will just stably remain isolated from one another with the capacity of forming hybrids over long periods of time. So this is how hybrid zones help us understand what happens to isolated populations. They're either going to be reinforced as being separate, they're going to come back together and fuse into a single population, or they will remain stably isolated from one another until something large happens. So we've introduced the biological species concept, we've introduced different methods in which speciation can occur, we can use hybrid zones to help us understand how populations are either going to remain stably isolated, reinforced, or are they going to fuse back together. And then the last thing that we'll talk about in this chapter is ask the question, so how fast does speciation occur? So I get a call from my mom about every year where she's struggling with thinking about evolution. She goes, Matt, why don't I see new species popping up all the time? If evolution is occurring the way that you seem to think that it does, why don't I see new things all the time? And of course I say, mom, you're not looking around everywhere, step one. And step two, it takes a long period of time, way longer than a single human life. Darwinian evolution is a long process, much longer than what any single human is generally going to see. And if we start to look at the fossil record, we see a couple trends that can help us understand this. One thing that we seem to see is that new species appear pretty rapidly in... a geographic level. stratum. So when you're looking at sedimentary rock, the species is the same, the same, the same, and then all of a sudden a new form pops up very quickly. And then as you look above that new form emerging, then you generally see it be unchanged for long periods of time until it finally disappears, which means it becomes extinct in that particular location. And so this is the fossil evidence that... new forms seemingly appear rapidly and then remain unchanged over long periods of time, and then they go extinct. Two scientists, a guy named Niles Eldridge and Stephen J. Gould, they looked at the fossil record and suggested a model for how fast speciation occurred. What they termed punctuated equilibrium is that big changes happen in the environment. quickly. And when that big change occurs, there would be a rapid period of speciation and then long periods of time where the environment generally stays the same. So in punctuated equilibrium, a species will appear generally quite rapidly. Now, another model that has been used is going to be called gradualism. So in gradualism, You would expect there's an ancestral species at the bottom of this tree, and the species would gradually micro-evolve away from one another over long periods of time. And you would see a slow change over time going from the original butterfly to the red version that is on the right and the black version that's on the left. Now, the fossil record does not support this. gradualism in many cases. What it often sees is something more like this, where in punctuated equilibrium, suddenly a new red version would occur over a relatively rapid amount of time, and then you would end up seeing no change over long periods of time. And so in the left side of this tree, the new black version emerged relatively quickly. quickly and then remain unchanged. So in punctuated equilibrium, you end up seeing rapid change and then periods of stasis. So the last thing that we'll talk about here is macroevolutionary changes happen through many small microevolutionary changes. So macroevolutionary change is going to be the accumulation of thousands and thousands of small... speciation events over long periods of time. And in many instances, the evolution of novel biological forms occurs in a series of steps. And so this is as far as we are going to go with this chapter this time. I appreciate it. And this is the end of chapter 22, 24.

And today we will finish up this chapter by talking about what happens when two species are adjacent to one another and begin to form hybrids, and these areas are going to be called hybrid zones. And then we're going to end with the question, so how fast does speciation actually occur? So a hybrid zone is a region where two different species come in contact with one another and potentially mate and form hybrids. And so what a hybrid zone allows us to look at is to see, so what happens between those two species and whether or not the two species are going to come together and form one species, or are they going to stay apart from one another? An example of a hybrid zone is this.

So in one part of Europe and Asia, you have frogs that have yellow bellies. So we're going to call them yellow-bellied frog. And in another region, you have ones that have red bellies, so we'll call them. red-bellied frog.

And so there are locations in the environment where these two species are going to come across one another and they have the possibility of mating. And so when they get a mortgage with one another, they have the capacity of forming hybrids. And what a hybrid zone is going to let us to do is study this process in action.

One thing that we'll observe is that hybrids are often going to have reduced fitness in comparison to the original types. So let's look at an example of a hybrid zone. So here we are in the mountains that are going to be separating Europe from Asia. So on the Asia side, you generally see red-bellied frogs, hence the orange.

And in Europe, you generally see the yellow-bellied frogs, and so they're on that one side. And so the red line, which is actually pretty thick, represents the hybrid zone, where these two species can cross with one another and make hybrids. Now, what we really want to look at is the lower right hand side.

Let's look at this crazy graphical representation where on the X axis, it represents the distance from the hybrid zone. And so we're talking in kilometers. So these frogs definitely interact with. with one another over a big part of the range.

And then on the Y-axis, you have an allele frequency. So an allele frequency, this is either P or Q. So the upper part of the line represents one of the alleles being at high frequency.

And so you're probably thinking that in this particular case, that upper part is mainly, let's say, the yellow-bellied frog. And then on the lower part, perhaps where you see a real low allele frequency with that part of the line, that's going to be the alleles associated with red belliness. And then you have that area right in the middle that has a very severe slope. Now, that right there represents the hybrid zone, where you end up with intermediate individuals that are going to be hybrid between the two consistent lines of the different alleles. And so...

In this hybrid zone, there are going to be several outcomes that are going to happen to the two different species. The first outcome is that there is going to be some sort of reduction in fitness of the hybrids, and you will strengthen the reproductive barriers between the two species. They will continue to micro-evolve away from one another to the point where you will absolutely continue to have two different species.

is if the species end up being pretty related to one another, there could be a weakening of the reproductive barriers and then the two populations come back together and form a single species. And then the last thing that can happen is going to be continuing to form hybrids. The two species never come back together and they just continue to have that hybrid zone in the middle.

Let's look at a challenging graphical representation. So the first thing, you have these yellow circles. Now each yellow circle represents a population. So in this case, we have four populations. Now the blue circles, they represent individuals within those populations.

So in the populations, they're crossing with one another. And so these four are all the same species. There can be gene flow between the populations, hence the black arrows.

What that's saying is that they're crossing with one another. Now all of these are absolutely the same species. So then what happens next is that there's some barrier to gene flow that occurs. Perhaps this population here becomes isolated.

There is a change in the flow of a river that isolates one population on the other side. a forest becomes fragmented, there's some reason that this one population, the one that's shown on the top line, becomes isolated. Now, it is going to go through microevolution as it goes away, and so the color change going from blue to pink represents the change in allele frequencies over time.

Now, the lower arrow... This represents the original species where there's still multiple populations, they're still in contact with one another, and so allele frequencies are not going to change a lot. They're going to generally remain the same. So then over a long period of time, you could then have a hybrid zone between the species that became, the group that became isolated from the original.

And so here is where the three different modes could occur. The first outcome would be reinforcing, where in this particular case, the hybrids would not be very viable, where what you see is that the green in the center, they become less and less in regards to the number of hybrids over time. And so this would be true speciation occurring, where the top line would become a different species from the bottom.

Or... you could have fusion. Now, this would be in the hybrid zone. If the hybrids were viable and could start crossing with both the isolated population and the original, the whole isolated population could just fall right back into the original gene frequencies. And you would say that this isolated population fused back with the original.

Or the last thing that could end up having would be stability. Now, in the stability scenario, both original species are going to maintain their identity, and you continue to form hybrids. An example of this would be the donkey-horse-mule situation, where the top line could represent donkeys, the bottom line could represent horses, and they're able to continue to form hybrids, and the two species are never going to come back. together and fuse into one and they will just stably remain isolated from one another with the capacity of forming hybrids over long periods of time. So this is how hybrid zones help us understand what happens to isolated populations.

They're either going to be reinforced as being separate, they're going to come back together and fuse into a single population, or they will remain stably isolated from one another until something large happens. So we've introduced the biological species concept, we've introduced different methods in which speciation can occur, we can use hybrid zones to help us understand how populations are either going to remain stably isolated, reinforced, or are they going to fuse back together. And then the last thing that we'll talk about in this chapter is ask the question, so how fast does speciation occur?

So I get a call from my mom about every year where she's struggling with thinking about evolution. She goes, Matt, why don't I see new species popping up all the time? If evolution is occurring the way that you seem to think that it does, why don't I see new things all the time?

And of course I say, mom, you're not looking around everywhere, step one. And step two, it takes a long period of time, way longer than a single human life. Darwinian evolution is a long process, much longer than what any single human is generally going to see.

And if we start to look at the fossil record, we see a couple trends that can help us understand this. One thing that we seem to see is that new species appear pretty rapidly in... a geographic level. stratum.

So when you're looking at sedimentary rock, the species is the same, the same, the same, and then all of a sudden a new form pops up very quickly. And then as you look above that new form emerging, then you generally see it be unchanged for long periods of time until it finally disappears, which means it becomes extinct in that particular location. And so this is the fossil evidence that...

new forms seemingly appear rapidly and then remain unchanged over long periods of time, and then they go extinct. Two scientists, a guy named Niles Eldridge and Stephen J. Gould, they looked at the fossil record and suggested a model for how fast speciation occurred. What they termed punctuated equilibrium is that big changes happen in the environment. quickly.

And when that big change occurs, there would be a rapid period of speciation and then long periods of time where the environment generally stays the same. So in punctuated equilibrium, a species will appear generally quite rapidly. Now, another model that has been used is going to be called gradualism.

So in gradualism, You would expect there's an ancestral species at the bottom of this tree, and the species would gradually micro-evolve away from one another over long periods of time. And you would see a slow change over time going from the original butterfly to the red version that is on the right and the black version that's on the left. Now, the fossil record does not support this. gradualism in many cases. What it often sees is something more like this, where in punctuated equilibrium, suddenly a new red version would occur over a relatively rapid amount of time, and then you would end up seeing no change over long periods of time.

And so in the left side of this tree, the new black version emerged relatively quickly. quickly and then remain unchanged. So in punctuated equilibrium, you end up seeing rapid change and then periods of stasis.

So the last thing that we'll talk about here is macroevolutionary changes happen through many small microevolutionary changes. So macroevolutionary change is going to be the accumulation of thousands and thousands of small... speciation events over long periods of time.

And in many instances, the evolution of novel biological forms occurs in a series of steps. And so this is as far as we are going to go with this chapter this time. I appreciate it.

And this is the end of chapter 22, 24.

Transcript for:Understanding Hybrid Zones and Speciation

Transcript for:
Understanding Hybrid Zones and Speciation