The Biological Species Concept - Online Tutor, Practice Problems & Exam Prep

The biological species concept defines species based on reproductive isolation, which prevents gene flow between populations. This isolation can be categorized into prezygotic barriers, such as habitat, temporal, behavioral, mechanical, and gametic isolation, and postzygotic barriers, including reduced viability, hybrid sterility, and hybrid breakdown. Understanding these concepts is crucial for studying speciation and biodiversity, as they illustrate how different species evolve and maintain distinct genetic identities.

Biological Species Concept

We said that there is no one way to define a species. In fact, scientists use different definitions depending on what they're studying. Now we call these different definitions species concepts, because they're different conceptual ways that we could think about how we group organisms. Now while there are many species concepts, we're going to be talking about the biological species concept in a lot of detail here. And that's because you are very likely to be responsible for this, and it is also the most commonly used species concept.

So let's see how we define species under the biological species concept. We're going to say that different species are reproductively, reproductively isolated from each other. Alright? So our test here is this idea of being reproductively isolated. Well, what does that mean?

Well, reproductive isolation, we're going to define as the existence of a barrier of some sort that prevents gene flow between species. Right? Gene flow, remember, makes populations more similar to each other. And being different species, we're saying that these two groups are clearly different things. So we need to block gene flow to call these things different species.

Now we're going to have sort of a 2 part test, therefore, to see if things are different species. And while our populations need to pass one of the two parts of this test, the first part of the test is, do they mate with each other? So we're going to say that different species have no potential to interbreed in nature. Right? So for example, here we have humans and we have chimpanzees.

They're a closely related species, but I know they are different species because given the opportunity, a human and a chimpanzee are just never going to mate with each other. They are just not interested. We know this because humans and chimpanzees have been living in the same areas of Africa for a million years, and they have never interbred. Right? So these things are clearly reproductively isolated from each other.

They're different species. Alright. But what if species do try to mate with each other? Well, then it takes us to the second part of the test. If they do try to mate, well, we're going to say that they do not produce either viable or fertile offspring.

Now viable means able to survive, fertile means able to reproduce. So the classic example here, we have the horse and the donkey or Equus ferus and Equus asinus. Given the opportunity, these species will mate with each other. And when they do, well, they produce a viable offspring. They produce a mule, this horse-donkey hybrid.

In fact, mules are quite strong. They're great farm animals, but you may know mules are sterile. They're not fertile. And so that blocks gene flow because this mule, this hybrid can't go back and mate with either parent species and pass those genes. Right?

So the horse and the donkey, clearly different species. They are reproductively isolated from each other. Alright. I want to emphasize reproductive isolation here one more time, because I want to say, well, that's our test for whether things are different species. But I also want to say that speciation, the process itself of becoming different species, can only occur if there is reproductive isolation.

So the process of speciation starts with populations becoming reproductively isolated, and then we're going to use reproductive isolation as our test to see whether things are different species under the biological species concept. So we're going to be talking about reproductive isolation quite a bit going forward. I think it'll be fun. I'll see you there.

The Biological Species Concept Example 1

We're going to be taking a look here at that classic example of natural selection, the peppered moths. So let's take a look. It says peppered moths are common in the United Kingdom. They can be either light or dark in color, and the two color morphs interbreed. Now before the industrial revolution, virtually all moths were light-colored to camouflage with lichen.

During the industrial revolution, when the trees were covered in soot, darker moths became more common. Now we have an image here showing those light and dark color morphs on those different backgrounds, and the questions we have here are, well, part a says based on this description, would you consider the dark moths to be a new species? And then part b says why or why not? So what do you think? Well, I'm going to say definitely no.

I would not consider them a new species. Now the moths here, it's a classic example of natural selection. We see a lot of change in these moths, but speciation isn't just about change. It's about the potential to interbreed. Right?

And it says right here at the beginning, the two color morphs interbreed. So why or why not? I am going to say there is no reproductive isolation. Right? That reproductive isolation or reproductive barriers, that is the basis for the biological species concept.

It's not present in this example, so it's not a new species. Alright. For that, more practice to come. We'll see you there.

Which of the following is the primary criteria for defining species based on the biological species concept?

• A. Ability to produce viable hybrids.
• B. Geographical isolation.
• C. Reproductive isolation.
• D. Similarity in appearance.

Giraffes live throughout large regions of Africa. Historically, only one species of giraffe has been recognized, but many scientists argue that giraffe populations actually represent four distinct species. What piece of evidence would best determine whether different giraffe populations should be considered different species according to the biological species concept?

• A. Whether or not there are regular identifiable physical characteristics that make the populations distinct.
• B. Whether or not the populations have historically interbred or are capable of producing offspring.
• C. Whether or not the populations interact with the environment differently in regular and meaningful ways.
• D. Whether or not there are regular differences in reproductive traits.

Types of Reproductive Isolation

We said that the biological species concept is sort of centered around this idea of reproductive isolation, but reproductive isolation can work in different ways. And so here, we're going to say that it can be broken into 2 basic types of barriers. Now going forward, we're going to look at both of these in more depth and see how these can be subdivided even more. But for now, we just want to sort of set up the stage here, understand what the two basic categories are. Well, those categories are prezygotic barriers and postzygotic barriers.

Let's first look at prezygotic. So pre lets us just look at this word, prezygotic. Pre means before, and zygotic, that refers to the zygote. Remember, a zygote is a fertilized egg. So these are barriers that happen before fertilization.

Right? So we'll write that here before fertilization. So a prezygotic barrier, well, it might prevent mating. Or mating may happen, but it's going to hinder fertilization. It's going to keep the sperm and the egg from joining in some way.

Alright. So we have an example here. We have the eastern and western spotted skunk. Honestly, I can't tell the two apart. It doesn't really matter.

These are skunks that live in North America. In some places, they have ranges that overlap. But one of them has a summer mating season, and one of them has a winter mating season. Right? So even in these places where these skunks maybe even come in contact with each other, they're not mating at the same time, so there's no chance of gene flow.

They are reproductively isolated because there is no chance for fertilization. Alright. Well, that's prezygotic. What about postzygotic? Well, post means after and, again, after the zygote.

So this barrier well, this barrier happens, we're gonna say here, after fertilization because remember that zygote is a fertilized egg. Alright. So for postzygotic barriers, you get a hybrid. The egg gets fertilized, but those hybrids either do not survive or they don't reproduce. And in general, you can just think of this.

These hybrids are going to have very low fitness because of those. Well, so our classic example that we'll be using here, right, that horse and that donkey, they do mate with each other. They get a zygote, and it can even grow up to be a mule. But that mule is sterile, and so there's no chance for gene flow. Alright.

So that's an example of hybrid sterility, but you also could have reduced viability. We're gonna have other examples of this as well. Okay. So again, both of these we're gonna go into more detail coming up. But just remember, the distinction here is, does the zygote get made?

Does the egg get fertilized? That's going to be our dividing line, and, well, we'll see what happens on either side of that line coming up.

The Biological Species Concept Example 2

Our example introduces us to H. davidii and H. fortii, which are two closely related species of orchids. So, we're talking about flowers here. For each statement below, if the statement represents a prezygotic barrier, write "pre" on the line. If the statement represents a postzygotic barrier, write "post" on the line.

If the statement does not represent a reproductive barrier, write "none" on the line. Let's remember the distinction between prezygotic and postzygotic barriers:

• Prezygotic barriers occur before fertilization.
• Postzygotic barriers occur after fertilization.

Let's check out our options here:

A) Both flowers are found in the same region.

Well, if they're found in the same region, that does not sound like a barrier to reproduction, so we should write none.

B) When fertilized by pollen of the other species, hybrids fail to produce seeds.

This involves the creation of hybrids, and since it's a problem that occurs after the zygote is formed, it is a postzygotic barrier. The hybrids are sterile, indicating a barrier to successful reproduction.

C) H. davidii and H. fortii are both pollinated by a single pollinator, the hawk moth.

A single pollinator implies potential cross-pollination between the two species, so it is not a barrier. Write none on the line.

D) Both flowers bloom in July.

If they bloomed at different times, that might be a barrier, but since they bloom simultaneously, there's no barrier here. Thus, the answer is none.

E) The species' unique flower structures release pollen on different body parts of the pollinator, so pollen is not transferred between the two species.

Since the pollen isn't transferred, there's no opportunity for fertilization between the two species, indicating a prezygotic barrier.

With that, we conclude our analysis. More practice after this and give it a try.

Which of the following describes a postzygotic barrier?

• A. Hybrids between Muscovy and Mallard ducks are often larger than either parent species but are sterile.
• B. Songbirds are only attracted to songs made by birds of the same species.
• C. Jaguars live in South America, while leopards live in Africa and Asia.
• D. Different flower species often bloom at different times of the year, preventing cross-pollination.

Prezygotic Barriers

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.

The Biological Species Concept Example 3

Our examples tell us that there are two closely related species of iris, Idomestica and Idichotima, and we can actually see them right down here in our image. Idomestica is always shown in orange, and Iris dichotima is always shown in purple. It then says that these flowers have overlapping ranges and flowering seasons. They use the same pollinators and deposit pollen on the same body parts of honeybees that visit them. We want to use the graph below to determine how these species may be reproductively isolated.

Alright, well right here I see a number of ways that they are not reproductively isolated, right? So how are they? Let's take a look at this graph. I'm going to scroll down quickly so that we can see the actual questions we need to answer, and also we see here the study that this data came from. In the graph, we see here the number of honeybee visits on the y-axis going from 0 to 18, and on the x-axis, we see time of day from 7 hours to 23 hours or 7 AM to 11 PM.

And then we also see the flowering time, so the flowering time of Idomestika there in orange, and the flowering time of Idicotiba in purple when those flowers are open during the day. Alright. So taking a look at that, what type of reproductive isolation do you think this is? And how do you know? Well, when I look at it, right, I notice that these flowers are open at different times of day.

Now there is a time when they're both open, but what do you notice with the bee visits? Right? The bees are visiting the flowers at different times of day. They're visiting Idomestika in the morning. They're visiting Iris dichonema in the evening, so there's no chance for cross-pollination because there is a temporal difference in when the bees are visiting.

So what type of reproductive isolation is that? It's temporal isolation. Remember, temporal refers to time. How do I know? Well, it's because the bees visit the flowers at different times. Alright. With that, we answer the question. We got more practice after this. Check it out. Like always, it'll be fun.

Abalone, a type of sea snail, are broadcast spawners, meaning they release their gametes into open water, where the sperm and egg drift until they meet and fertilization occurs. The sperm of the red abalone does not recognize the eggs of the pink abalone and vice versa due to biochemical incompatibilities. This is an example of what type of reproductive barrier?

• A. Mechanical isolation.
• B. Habitual isolation.
• C. Gametic isolation.
• D. Behavioral isolation.

Postzygotic Barriers

We've been thinking about reproductive isolation, and we've talked about prezygotic barriers. Well, now we want to focus in on postzygotic barriers. Remember, postzygotic, post means after, and zygotic refers to the formation of the zygote or the fertilization of the egg. And so postzygotic barriers, we're going to say these are incompatibilities that decrease, and I'll indicate that with a down arrow. They decrease the fitness of the offspring or that hybrid.

Right now, when these incompatibilities arise, sort of how far down the line after fertilization those incompatibilities show up, that's going to give us our different types of postzygotic barriers. We're going to have 3 of them. So let's go through them one by one. The first way that you could see this is through reduced viability. Now viability is just the ability to survive.

So in reduced viability, we're going to say those hybrids are just less likely to survive compared to non-hybrids. Well, if the hybrids don't survive, you can't get gene flow, and if you can't get gene flow, you're reproductively isolated. So our example here are birds of this genus, Fasedula. In this genus, there are some species that interact in the wild and sometimes will mate with each other, and they will have fertilized eggs.

But those hybrid offspring of these different species of the Physidula, they are just less likely to hatch. So these birds are sitting on this nest, but those eggs, they're not hatching. And if the eggs don't hatch, you don't get hybrids, you don't get gene flow, those species again, they're reproductively isolated.

Alright. Well, sometimes the hybrids do survive and can quite often they do. But when they do, you often see something called hybrid sterility. And well, in hybrid sterility, the hybrids are viable. They survive in fact, sometimes are quite healthy, but they can't reproduce. Right? They are sterile.

Now our example that we've been using here is the classic horse and the donkey mating to form a mule, right? The horse and donkey create a mule. That mule is strong. It's a really healthy animal, but it's sterile. And if an animal is sterile, it can't mate with its sort of parent species.

There's going to be no gene flow. The horse and the donkey are reproductively isolated. Alright. Well, what if a hybrid is fertile? Well, then you might see hybrid breakdown.

In hybrid breakdown, we're going to see that first generation hybrid, it's going to be healthy, and it is going to be fertile. But it's the further generations, it's like the second generation that we're going to see that decrease in fitness as I'm indicating there with that down arrow. Now our example for this is going to be cotton. And there's actually 2 species of cotton that are grown for textiles, things like clothes. And you can make them, and when you do, you get a hybrid, and that hybrid is quite healthy.

But the second generation hybrids, if you mate that hybrid, it has offspring that are weak and sterile. We're going to say here, they are feeble, they're unhealthy, and they are infertile. Right? And so if the second generation hybrids are infertile, if they don't survive again, you're blocking gene flow. You have reproductive isolation.

Now, I just want to note, you don't have to have just one type of reproductive isolation. You might have a prezygotic barrier, even multiple prezygotic barriers. And if those are overcome, well, then there's also postzygotic barriers. But to be called different species, we have to have at least one form of pre or postzygotic barrier. Alright?

We'll look at this more in example and practice problems. I'll see you there.

The Biological Species Concept Example 4

This example tells us that around the world, people have bred domestic cattle or Bos taurus with other local related species. In Nepal, it is common to breed cattle with domestic yaks or Bos grunniens. The offspring, called Zho, are usually larger and stronger than either parent species and contain desirable traits from both. Male Zho are usually sterile, but the females are fertile. Nevertheless, the females are seldom bred because the second generation hybrids tend to be smaller and weaker compared to both yaks and domestic cattle.

Alright, with that information, we need to answer these questions. A says, based on this description, should yaks and domestic cattle be considered different species? Yes or no? What do you think?

Well, I'm going to go with yes. Alright. I definitely see forms of reproductive isolation between these two species. So knowing that, let's move on to B. B asks if yes, is the barrier prezygotic or postzygotic?

As described in this passage, I see postzygotic barriers. Right? There may be prezygotic barriers between these two groups, but here we have mated them. Now what happens? So, we have made hybrids. So now we are looking at those postzygotic barriers. The question here for C then is if a pre or postzygotic barrier is present, specifically what type of pre or postzygotic barrier is it.

If more than one type is described, list all types of barriers. Alright. So what do you see? Well, I see the males are sterile, which is a form of hybrid sterility. Alright. But is that it though? What I also see is that the females, well, they are usually fertile, but they are seldom bred because the second generation hybrids tend to be smaller and weaker. Well, that sounds like hybrid breakdown.

So, we see two forms of postzygotic barriers here: hybrid sterility and second generation hybrid breakdown. With that, more practice after this. Give it a try.

Which of the following pairs correctly matches the postzygotic barrier with its definition?

• A. Hybrid sterility: reduced fitness of hybrids after the first generation.
• B. Hybrid breakdown: hybrids are unable to reproduce.
• C. Gametic isolation: hybrid offspring are sterile.
• D. Reduced viability: decreased fitness of hybrid offspring.

“Ring species” are species that are distributed across a habitat in a ring-like distribution. Every population is able to breed with neighboring populations, but where the ring closes, the populations at the two “ends” do not interbreed. One example of a ring species is Ensatina escholtzii, a species

of salamander in California. The species began as a single population in the North, and spread South on either side of the California Central Valley. Each color on the map shows the distribution of different named population of E. escholtzii. Hybrids created by interbreeding of the populations marked

with the circle are sterile. If hybrid sterility is present, why do you think these populations are still considered the same species?

• A. To be considered separate species there must be pre- and postzygotic barriers.
• B. Because each population interbreeds with its neighbor, gene flow is possible throughout the entire species.
• C. This is a postzygotic barrier; prezygotic barriers are necessary for speciation.
• D. To become distinct species the populations would need to be geographically isolated.