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We humans like to get our heads around all of the complexity around us by classifying things. And you could imagine there's no more obvious thing to classify than all of the living things around us, than all of the life that surrounds us. So what I want to start talking about is, how do we classify all of the life around us? And this is more often generally referred to taxonomy. But the most basic question you have when you look at all the life around, you start to see similarities between some of these living things. You see, obviously this thing right over here is more similar to the things that look like it, than it does to the grass behind it, or to that tree. And so we start saying, well maybe I should group this thing right over here into a group with other things like it. And that very most building block of how we classify all of the living things around us, is putting them into buckets called species. So for example, this is one particular animal, but we see other animals that seem to look like it, and so we say they're all part of the species of lions. And this animal, it's one animal, and there's other animals that have stripes, but some might be fatter, or taller, or skinnier, or whatever else, darker, or lighter, but we say they're similar enough, that we call them all tigers. We call all the animals, even though they might be a little bit bigger, or skinnier, or fatter, or lighter, or darker, we call all of them-- they look similar to this thing right over here-- we would call this a donkey. We would call the things that we think are like this animal right here, a horse. Now, that might seem like a pretty straightforward way to think about it. Oh, everything that looks kind of like this character right here is a lion, anything that looks kind of like this character right here is a tiger. But that, by itself, is not a good enough definition for a species. Things that look like each other or things that act like each other, because what we'll see is that there's some things that could be very different, at least in and how they look or act, but are actually closely related. And we'll talk about what it means to be closely related. And then we can see things that look very similar, that they have similar structures or they have similar behavior, like for example bats and birds, but they are actually all very, very distantly related. So we need a more exact definition for species than just things that look like each other, or just things that act like each other. And so the most typical definition for species are animals that can interbreed. And when we say interbreed, literally they can produce offspring with each other, and the offspring are fertile. Which means that the offspring can then further have babies, that they're not sterile, that they're capable of breeding with other animals and producing more offspring. And to show an example of this, this right here is a male lion. You find a male lion and a female lioness and most of the time they will be able to have offspring, and those offspring can go and mate with other lions or lionesses, depending on their sex, and then they can have viable offspring. So it seems to work out pretty well for lions. Same thing is true of tigers. Now, it does turn out that if you get a male lion and a female tigress they can breed. They can breed and they can produce offspring. And their offspring-- which was made famous by Napoleon Dynamite, he was kind of fascinated by, these are kind of fascinating animals. Their offspring is called a liger. You get a male lion breeding with a female tiger you produce a liger, which is a hybrid, it's a cross between a lion and a tiger. And they're fascinating animals. They're actually larger than either lions or tigers. They are the largest cats that we know of. But these ligers cannot be referred as a separate species. Or you can't say that lions and tigers are the same species, because even though they are able to interbreed, their offspring, for the most part, is not fertile, is not able to produce offspring. There have been one off stories about ligers being mated with either a lion or a tiger, but those are one off stories. In general, ligers can't interbreed. And in general, this combination isn't going to produce offspring that can keep interbreeding or that are fertile. So that's why we say that lions and tigers are different species. And that liger, we wouldn't even call it as a species at all. We would actually call it a hybrid between two species. Now the same thing is true-- and actually you might be asking yourself, well, this was a male lion and a female tigress, what if we went the other way around? What if we had a female lioness and a male tiger? In that case, you would produce something else called a tiglon or a tiglon, I actually don't know how to pronounce that. And that is a different hybrid that has slightly different properties than a liger. I encourage you to look up what a tiglon is. Similarly, you give me a male donkey. And donkeys are clearly a species by themselves, because if you give me a male donkey and a female donkey they can reproduce, produce another donkey, and then that donkey can mate with other donkeys to produce more and more donkeys. So not only can a donkey interbreed with another donkey, but that product, that child donkey, can keep interbreeding with other donkeys. Similarly, horses, they can interbreed and produce fertile offspring. But if you give me a male donkey and a female horse they can mate and they can produce a mule. But once again, like the ligers, mules are not, at least as far as I know, mules they're not fertile. Mules cannot produce further offspring. They cannot interbreed with each other. And because, even though donkeys and horses can breed and produce mules, their offspring aren't fertile. We don't consider donkeys and horses part of the same species. And we would consider mules, like a liger or a tiglon, we would consider them a hybrid. So these are all hybrids, or we would call cross. In general, the word hybrid is used when you have two things, two different types that are somehow coming together, somehow having a combination. And once again, like the case with the tiglon, you might say, well, what if I had a female donkey and a male horse? And then you would actually produce something called a hinny, which isn't as common as a mule. And people like to use mules, they're actually very good work animals because they have some of the good properties of both donkeys and horses. Hinnies are less common, but once again, it is possible. And they have different properties than mules. And I do want to emphasize this idea. Because when we started off we just tried to think about, well, how can we classify things? And we said, hey, maybe things that look and act similar, we can call a species. And maybe things that look and act different, we shouldn't call them species. But I want to show you a very typical case, one that's really all around us all the time, where this definition-- animals that can interbreed and the offspring are fertile-- really does seem to become much, much more important than just some notion of animals that look alike or animals that act the same. And the best example of that is with dogs. As I said, this is a very typical species here, because dogs-- and I just took a sample of some of the different types of breeds of dogs-- they can look very, very different. It's obvious, look at the difference between these dogs. For example, this little chihuahua here and this dog right over here. Obviously, they're size-wise, their look, and even how they act are much, much more different than maybe how this donkey would act relative to this horse, or how this lion would act relative to this tigress. And they obviously look very different, they have completely different sizes, but these two things actually can interbreed-- although for these two in particular, it seems like the mechanics would get kind of difficult-- but assuming they get over the mechanical hurdles they could interbreed and produce fertile offspring. Same for these two characters, same for these two characters over here. And because of that, even though all the different breeds of dogs-- and most of this is really due to humans' doings of trying to breed for a specific traits-- even though they look so different, and even though they act so different, because they can interbreed, and they could produce fertile offspring, we consider all of these things to be members of the same species.