Allopatric and Sympatric Speciation - Online Tutor, Practice Problems & Exam Prep

Speciation occurs through allopatric and sympatric mechanisms. Allopatric speciation involves populations evolving in different geographic areas, leading to reproductive isolation via dispersal or vicariance. In contrast, sympatric speciation occurs within the same location, often through polyploidy or disruptive selection, where reproductive isolation is crucial. Polyploidy results in organisms with extra chromosome sets, creating barriers to reproduction. Disruptive selection favors divergent phenotypes, necessitating mechanisms like mate choice for isolation. Understanding these processes is essential for grasping evolutionary dynamics and biodiversity.

Allopatric and Sympatric Speciation

We've seen how all different types of reproductive isolation can maintain different species, but here we want to sort of layer on top of all these two ways that we can think about speciation occurring. We can have allopatric speciation and sympatric speciation. Now, for both of these, we're going to go into more detail on them coming up in future videos. Here, we just want to introduce the two so we can generally understand the difference between them. Alright.

So, let's start just by remembering that speciation requires reproductive isolation. But broadly, we can say that reproductive isolation or speciation happens in two ways, either allopatrically or sympatrically. Now, allopatric speciation, we're going to say, is when species evolve in different locations. In other words, they evolve with habitat isolation. Remember, habitat isolation is when those populations reproduce in different habitats.

So, for allopatric speciation, the first step of speciation is that the population in some way splits, and those two populations are now living in different places. So for me, it's really kind of conceptually easy to understand how that could lead to speciation. Right? If you live in different places, you're reproductively isolated, they can evolve in different ways and become different species. And in fact, that is the more common type of speciation.

But you can also have sympatric speciation. Now, sympatric speciation is when those species evolve in the same location. In other words, without habitat isolation. Now we have other types of reproductive isolation, but for me at least, it's harder to understand how those could enter into a population that's all living in the same area and at least in the past has been reproducing with each other.

But it does happen. Alright? So we're going to be using these words a lot. So we want to just break down the root so that we're familiar with them. Alright?

So, allopatric - allo, that root allo means other or different, I'm going to say. And patric refers to country. It's the same root as the word patriotic. Now here, we don't literally mean country; we just mean region.

So allopatric species are species that live in different regions. Allopatric speciation occurs when the populations live in different regions. Sympatric, well, sim means the same. And again, Patrick refers to country, or we can think of it as region. So sympatric species are species that live in the same or at least overlapping regions.

Sympatric speciation occurs when the two populations are living in the same region. Again, we'll look at how both of these occur in more detail coming up in future videos. It'll be a hoot. See you there.

True or False: if false, choose the answer that best corrects the statement.

The difference between allopatric and sympatric speciation is how long it takes for populations to split into separate species.

• A. True.
• B. False; the difference between allopatric and sympatric speciation is if pre- or postzygotic barriers are used to create reproductive isolation.
• C. False; the difference between allopatric and sympatric speciation is how the species respond to recontact in a hybrid zone.
• D. False; the difference between allopatric and sympatric speciation is whether or not it involves a geographical barrier.

Allopatric Speciation

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.

Allopatric and Sympatric Speciation Example 1

Our example tells us that the map below shows the ranges of the common chimpanzee or Pan troglodytes in orange, and the bonobo or Pan paniscus in purple. Now also shown on the map is the Congo River. Right? So let's just check out this map real quick. We see in orange, right, that's the range of the common chimpanzee there.

Kind of tucked up right next to it there, we see the range of the bonobo there in purple, and the Congo River is running right between the two. Alright. It says that both species live in similar habitats within the tropical rainforest environment. The two species are thought to have split from a common ancestor approximately 1.5 to 2 million years ago. We want to use this information, information from the map, to answer the following questions.

Alright. So a) here says, are these two species sympatric or allopatric species? What do you think? Well, I'm going with allopatric. Remember, allopatric, we broke that word down.

Allo means other or different. Patrick refers to the country or region. So these species just live in different areas. They are separated by that river. They don't have overlapping ranges.

b) says, what specific type of reproductive isolation do you think likely led to and is most responsible for maintaining the two species? Alright. So what type of reproductive isolation do we see with allopatric speciation? Well, we call that habitual isolation.

Remember, they reproduce in different habitats. Right? And well, that makes sense because chimpanzees and bonobos, they can't swim. Right? So they can't get across this river.

There is a barrier that is separating them, so they are reproductively isolated. Alright. Our question c) though says if a vicariance event led to the formation of these two species, make a prediction about the age of the Congo River and how bonobos and common chimpanzees ended up on opposite sides of the river. Alright. Think what does vicariance mean and what would that imply about the age of this river?

Well, vicariance means that the population was sort of living where you find it now, but it was physically split by something. And in this case, I would assume that would be the formation of the river. So this, if this was a vicariance event, well these populations split, it says 1.5 to 2 million years ago, I'd expect the river to be 1.5 to 2 million years old. That's my age of the Congo River.

Alright. Well, how did they end up on opposite sides of the river? Well, I'm just going to say that the river, the formation of the river, split the population. Right? It was one population.

The river formed. It ran sort of right through the middle of it. They can't swim. Now they're reproductively isolated. Now that's classic vicariance right there.

Alright. Well, d) then says, if a dispersal event led to the formation of these two species, make a prediction about the age of the Congo River and how bonobos and common chimpanzees ended up on opposite sides of the river. Alright. So what would your prediction be if this was a dispersal event? Well, if it was dispersal, I'm going to assume that the river is older than 1.5 to 2,000,000 years.

So I'm just going to put greater than 2 million years old. Well, how did they end up on opposite sides of the river? Well, I don't know where they started. I don't know which is sort of the original population, and I don't actually know how they got across, but I'm just going to say that some individuals crossed. Individuals crossed the river.

Alright. Who knows how they got across because they can't swim, but somehow they did. Alright. What's the real answer? Well, we don't know for sure.

Scientists think this was probably a vicariance event and the age of the river is about 1.5 to 2 million years old, though other scientists do disagree with that interpretation. Alright? Alright. So with this, we got more practice to come. I'll see you there.

Which of the following would be most likely to lead to speciation by vicariance?

• A. Volcanoes in the ocean create new islands.
• B. A change in the environment leads to a grassland becoming a forest.
• C. A small group of organisms are washed to sea during a storm and land on a new island.
• D. A change in climate creates a desert that divides a large grassland.

The organisms of the Galapagos islands have become famous for people studying their processes of speciation. Which statement best describes how speciation most likely occurs on islands like the Galapagos?

• A. Speciation likely occurs through vicariance as the organisms that reached the islands split off from mainland populations.
• B. Speciation likely occurs through dispersal as groups of organisms reach new islands and adapt to local conditions.
• C. Speciation likely occurs through sympatric speciation as organisms diverge in different ecological niches.
• D. Speciation likely occurs through hybridization as new organisms interact with resident organisms on islands.

Sympatric Speciation

We've now seen how allopatric speciation can occur when populations are living in different areas, but now we want to look at sympatric speciation. Remember, sympatric speciation is when new species form in the same geographic area. Now this, at least for me, is a little harder to wrap my head around because, well, how do you get reproductive isolation when organisms are in the same place? And for that reason, because it's harder to get that reproductive isolation, sympatric speciation is much less common than allopatric. But there are two ways that we can say that this can happen.

We're going to say sympatric speciation can happen through polyploidy and disruptive selection. Let's talk about polyploidy first. Now, polyploidy, I'm just going to say it's a little bit more technically difficult to wrap your head around, so we're going to go into it in more detail in an upcoming video. But this is when there's a mutation that results in more than 2 complete sets of chromosomes. Right?

When we're thinking of eukaryotes, we're normally thinking of diploid organisms that have 2 complete sets of chromosomes. Well, there are some eukaryotes that are tetraploid. They have four complete sets of chromosomes. Strawberries are octaploid. They have eight complete sets of chromosomes.

Now when species that have different sets of chromosomes try to mate, well, the offspring usually either aren't viable or they're sterile, and so that's a form of reproductive isolation. Now this is going to be much more common in plants, and we'll see why it's more common in plants when we go into the technical details about how this works in an upcoming video. Alright. But now let's look into disruptive selection because we can figure out how this works pretty straightforwardly, I think. Alright.

Disruptive selection, we said, is selection for divergent phenotypes. Right? You have one population and selection is sort of selecting for two different morphs that both have high fitness or two different behaviors, something like that. Now you can sort of imagine how that could lead to speciation, but I want to put the brakes on for a second because it's really important to remember that we require a mechanism for reproductive isolation. And in fact, I would say that reproductive isolation is more important than the disruptive selection.

Reproductive isolation, when things are in the same area, this could be something like mate choice. Right? Organs, for whatever reason, are just choosing to mate with things that maybe look like them or have similar behavior, or maybe something like microhabitat niches. Right?

Maybe these organisms live in the same broad area, but, you know, they live on different plants, and so they're only interacting with certain organisms within that population. Or maybe one becomes more active in the morning while the other is more active at night. So while they're, again, in the same broad area, they're only interacting with certain organisms of their species, and there is a reproductive barrier there. Alright. So to think this through, how this works, we have this little image here.

We have this fish. Here we have some disruptive selection. We have a green and a yellow morph in this population, two types. And we see in this first image, they're just sort of mating randomly.

You see the green and the yellow fish mating with each other. The yellow and the yellow are paired off. The green and the green are paired off. Right? It just sort of seems random.

That will not lead to speciation, no matter how strong that disruptive selection is. But if we get some mate choice, or for whatever reason, these fish of the same type are only breeding with each other right here. Now the yellow fish are only mating with themselves. The green fish are only mating with the green fish. If that happens and that goes on for a long time, well, you can get speciation.

We see here that those differences have become greater. We've actually created different species. Okay. So again, just to remember, disruptive selection, the important thing here is actually the reproductive isolation. How is that going to happen so that those divergent phenotypes can actually become their own species?

Again, we'll look at polyploidy in more detail soon. But before that, we've got an example and practice problems. Check them out.

Strong disruptive selection doesn’t always create new species. What other requirement must be met for speciation to occur?

• A. There must also be reproductive isolation of the divergent phenotypes for speciation to occur.
• B. There must be no stabilizing or directional selection on other traits for speciation to occur.
• C. There must also be chromosomal differences between the groups for speciation to occur.
• D. There must be random mating in the population for speciation to occur.

Within the Northeast Pacific, two types of killer whales (Orcinus orca) are found. Resident killer whales feed mostly on salmon, while the transient population feeds mostly on marine mammals. While these populations spend a lot of time in the same waters, they do not interbreed, likely because they use different systems of calls to communicate. Some scientists believe that these two populations of orca are undergoing speciation. What type of speciation is occurring, and what type of reproductive barrier do you suspect is reinforcing the speciation?

• A. Allopatric speciation with behavioral isolation.
• B. Allopatric speciation with habitual isolation.
• C. Sympatric speciation with behavioral isolation.
• D. Sympatric speciation with habitual isolation.

Polyploidy

We now want to take a closer look at polyploidy. Polyploidy, as we said, is technically more complex. So we want to go through it more slowly here. It's unlikely that you'll need to be responsible for all these technical details, but we want to show them to you just so that you understand how it works. Remember that polyploidy, as we said, involves having extra sets of chromosomes.

If your normal cell is diploid, it has 2 sets of chromosomes. A tetraploid cell, which is a polyploid cell, will have 4 sets of chromosomes, twice the normal amount. Strawberries are octaploid; they have 8 complete sets of chromosomes. When a cell becomes polyploid, it can create a new species in a single generation. This occurs because having extra sets of chromosomes leads to speciation. Species with different numbers of chromosomes trying to reproduce together may create sterile hybrids, if those hybrids are viable at all.

To see how polyploidy works, let's consider two basic types: autopolyploidy and allopolyploidy. Starting with autopolyploidy, the prefix 'auto' means self. Autopolyploidy is created through a cell division error within a species or within a single organism. This is rarer, but let's explore how it works. Consider a diploid cell with 2n = 6 chromosomes. If this cell experiences a cell division error, the total number of chromosomes might double, resulting in a tetraploid cell with 4n = 12 chromosomes.

This organism, when it undergoes meiosis, will produce diploid gametes (2n = 6) instead of the typical haploid gametes. The challenge is finding a compatible mate since no other organisms have diploid gametes. However, if this organism can self-fertilize, the diploid gametes will fuse, recreating a tetraploid zygote (4n = 12). This new zygote can grow into an organism and reproduce, thereby creating a new species with a new number of chromosomes in a single generation. This ability to self-fertilize is why polyploidy is more common in plants, though it can occur in other organisms.

Turning to allopolyploidy, 'allo' means different. Allopolyploidy arises through hybridization between two different species, followed by a cell division error. Consider two species: Species 1 is diploid with 4 chromosomes, and Species 2 is diploid with 6 chromosomes. Through meiosis, these species produce normal haploid gametes. The gamete from Species 1 has two chromosomes, and from Species 2, three chromosomes.

When these gametes fuse, they may form a hybrid, likely a sterile one because the parental chromosomes differ in number. This hybrid isn't technically diploid because the chromosomes from each parent don't pair properly, resulting in a genome where n = 5. If a cell division error occurs afterward, we end up with a diploid organism that has 10 chromosomes, forming an allopolyploid. This allopolyploid can self-fertilize. Here, we have complete DNA sets from two different species and two of each chromosome, effectively doubling the DNA while maintaining a diploid state.

While these details might be more than you need to know, here’s what you probably really need to remember: Autopolyploidy results in polyploid organisms within a single species. Allopolyploidy starts with a hybridization event followed by a cell division error, leading to extra DNA. More lessons and practice are coming up, and I'll see you there.

Allopatric and Sympatric Speciation Example 2

Our example here tells us that plant breeders developed seedless watermelons by creating polyploid plants. Normally, watermelons, or Citrullus lanatus, are diploid, with 2n=22, but breeders create watermelons that are tetraploid, with 4n=44, by treating plants with a known mutagen that prevents the segregation of chromosomes during mitosis, creating cells with twice as many chromosomes. These plants could then be self-fertilized to create the new tetraploid breed. The plants grown from matings between these tetraploid plants and diploid plants are then sterile and therefore grow no seeds and, well, note they are delicious to eat. Right?

So, if you've ever had a seedless watermelon, which you probably have because that's mostly what they sell nowadays, right, you've been having a hybrid between a tetraploid and a diploid breed of watermelons. Alright. So here it asks, as described, are tetraploid watermelons allopolyploids or autopolyploids? Alright. So what do you think?

Well, let's remember those roots here. 'Allo' means other or different, 'Auto' means self. So, allopolyploids start with hybridization between two species, whereas an autopolyploid just starts with a mutation within one species. And that's what happens here. The species are treated with a mutagen, so we get that mutation. There is no hybridization event that starts this out, so these are autopolyploids.

Regarding the question, "How?" I'm just going to say there was no hybridization to create the polyploids. Alright. Now, diploid and tetraploid watermelons are typically considered the same species. Based on the information above, however, by which species concept could it be reasonable to consider them different species? What do you think? I'm going with that good old biological species concept. Right? These things are clearly reproductively isolated.

They have postzygotic reproductive isolation. Their hybrids are sterile. That meets the biological species concept definition of a different species. Now, we typically call them the same species because people just did this in the lab. It's not something that evolved in nature, and we did it just so that we could have yummy seedless watermelons. But by the biological species concept, this matches the criteria for reproductive isolation, and it's reasonable to call them different species. Alright. For that, more practice after this. I'll see you there.

Polyploidy creates individuals or gametes with novel numbers of chromosomes. To mate, these individuals must mate with other organisms that are also polyploid. Would this lead to polyploidy being more common in animals or plants, and why?

• A. Animals. Animals tend to have mobile sperm, so the gametes can travel farther to find compatible eggs.
• B. Plants. Many plants can self-fertilize, and gametes from the same plant will have the same number of chromosomes.
• C. Animals. Animal species are more likely to have individuals that are only one sex.
• D. Plants. Plant ovules tend to be larger than animal eggs meaning they can better accommodate the extra DNA.

What is the difference between autopolyploids and allopolyploids?

• A. Allopolyploids can only reproduce through self-fertilizing, while autopolyploids are able to cross-fertilize.
• B. Allopolyploids are descended from a single species, autopolyploids begin as hybrids between two species.
• C. Autopolyploids are descended from a single species, allopolyploids begin as hybrids between two species.
• D. Autopolyploids can only reproduce through self-fertilizing, while allopolyploids are able to cross-fertilize.