When we first started talking about species, we said that there is no one way to define a species. But then we talked about one way, the biological species concept, quite a bit. Here, we want to look at some other species concepts, and we're going to compare them to the biological species concepts and understand why you may want to use them. Now just to be clear, there are more than 3 other species concepts out there. We're looking at these 3 because they're relatively commonly used and they're ones that you may be responsible for.
First, let's look at the biological species concept and understand what's good and potentially bad about it. Well, the biological species concept, we're going to say is based on reproductive isolation. That's how we define different species. And right we have an illustration for that. We have our horse, our donkey, and of course, when they mate, they make that sterile mule. Well, that sounds like a great species concept, except what if you're interested in asexual organisms? The biological species concept is entirely based on what happens when individuals from 2 populations are they able to mate or reproduce with each other? That's not how asexual organisms reproduce. So, it just doesn't apply to asexual organisms. I'm also going to put a negative sign next to fossils. If you're digging up bones, you're interested in the history of life on Earth and what species existed in the past. Well, you dig up a bone, how do you know what that bone could have potentially mated with? There's just no way to know that. So why do we use it? Well, I'm going to say it's the most widely used, which is why it's what you're going to be responsible for. And that's really because it is conceptually simple. If we say, and we did say this, that reproductive isolation is required for speciation, well, it makes sense then to define a species based on whether they actually are reproductively isolated.
What's also negative about this though, in my mind, is that it can be difficult to apply to natural populations. Imagine you are a biologist, you're walking through the woods, you see a beetle, and you're like, Hey. I think that's a new species. To define it as a new species by the biological species concept, you have to know everything that that beetle could potentially mate with and what happens when it does. That's a lot to know about an organism before you're allowed to call it a new species.
So that brings us to our next species concept, the morphological species concept and morphological species concept. It's based on morphology, which is really just the shape and structural features of organisms. And to illustrate this, we have some skulls that looks like from different species from human evolution. Well, those skulls should key you in as to why this is a particularly good species concept. In some cases, you can definitely use it for fossils. If you're digging up bones, really that's all you have to go on, is the shape and structure. And you're going to group things together that have similar or the same shape and structure. You'll call those things the same species. What about asexual organisms? Can you use it for them? Heck yes, they got shape and structure. And I'm going to say a plus to this is that it's really just generally easy to apply. You just need to study the morphology and see if this organism is unique. You don't need to know other things about its biology. You don't need to know who it could potentially mate with. Now that leads to a negative though, is that it can be subjective. I may study the morphology and say, I think this is different. It's a new species. Another expert may look at it and say, I think this is kind of different, but not different enough to be a new species. And well, the only way to settle that is really just to argue it out, I guess. Now also, I want to note that morphology and reproductive isolation don't always align. There's some species that just have a ton of variation within them. Think of dogs. A chihuahua and a Great Dane are very clearly the same species, but they have very different morphology.
Now on the flip side, there's other populations that very clearly have strong reproductive isolation, but structurally, they're nearly identical. And by the morphological species concept, you might not recognize them as different species. And that brings us to our next species concept here, the ecological species concept. Well, that's going to be based on ecology. We're going to say here species are defined by their interaction with the environment. And to illustrate this, we have wolves and coyotes here. And wolves and coyotes really by any species concept, they're different species. But there is gene flow between these two species. Not a lot, but it's there. Now that might surprise you because they live such different lives. They interact with their environment so differently. They have different social structures. They eat different food. They live in different places. Right? And so if that's what you're interested in, if you're interested in how organisms interact in their environment, how they live their lives, maybe it makes sense to base your species concept on that. As we go through our little check boxes, can you apply this to asexual organisms? Absolutely. They interact with their environment. Can you apply it to fossils? Not really. Right? You can infer things about how organisms interacted with their environment from the fossils, but that's really just based on morphology. So what do I like about this? Well, it's that it emphasizes that adaptation to an ecological niche. And again, if that's what you're interested in, if you're interested in how organisms interact with their environment, well, then maybe it makes sense to base your species on that.
A negative though, well, it does not directly consider reproductive isolation. Now there's other cases, different from the wolves and coyotes where there's actually a lot of gene flow between populations, But those populations still live in different places and have local adaptation to those areas, and so are going to live very different lives. If there's that much gene flow though, does it make sense to call them different species? Again, that's it's a judgment call.
For our last one here, I'm actually just going to remove myself from the screen and move over a little bit here. So here we are going to be talking about the phylogenetic species concept, and here we really want to take our tree thinking approach to species. Here we're going to say the phylogenetic species concept, we're going to call a species the smallest evolutionarily distinct population. And so we're going to build a phylogeny, build an evolutionary tree like this, And those branches, if it's its own branch, we're going to call that a species. Now really, this is the only concept. This is the only one that we've built a tree for. So really, that is what makes this unique. We're really taking that tree thinking approach. But what makes it actually so useful today, you can build trees on lots of different data. But one thing that it's really easy to build phylogenetic trees on, I'm going to jump down a little bit here and say that it can be based solely on DNA sequence data. And DNA sequence data is really, really easy to get nowadays. You can get DNA sequence data for organisms that you know nothing about. You can build a tree, and you can decide whether things are their own species based on this concept.
Now if we go back to our check boxes, can this be used for asexual organisms? Heck yes, it can. Can it be used for fossils? I'm going to put, like, a little check here. You can build phylogenetic trees for fossils. Absolutely. But it's difficult to get enough resolution from fossil data to get clear branches on a tree down to the species level sometimes. Our final negative here. What people don't like about this species concept is that sometimes it breaks populations into more species than is helpful. Right? If you're looking at a group and, you know, they live in the same or similar environments, they interact with those environments in the same or very similar ways, they look the same, but you make your tree and there's a whole bunch of branches there. Does it make sense to call them different species? Is that helpful to the question you're asking? Well, again, I guess it depends on the question you're asking. So again, for all of these, there's some good times and some not as good times to use different species concepts. Remember, not everything applies in all cases. We'll practice these going forward, and I'll see you there.
