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- [Voiceover] In any discussion of biology or discussion of evolution, the idea of a species will come up over and over again. And we have a whole separate video on species. But the general idea, or the mainstream definition of a species is a group of organisms that can interbreed, interbreed, and produce fertile offspring, fertile, fertile offspring. So for example, in this picture right over here, you have a bunch of species of both modern elephants and previous, or now nonexistent species, that are related to modern elephants. But today on earth you have Asian elephants and you have African elephants, and they are each a species. An Asian elephant can interbreed and produce fertile offspring with another Asian elephant, and an African elephant can interbreed and produce fertile offspring with another African elephant, but they can't do it with each other. An Asian elephant and an African elephant cannot get together and interbreed to produce fertile offspring. We know, people have actually tried this. But the next question, or the most obvious question, and this is one of the central questions of evolution, is, well, how do you get these species? We see drawings like we have on the right, on the left here we have actually some of Darwin's original drawings showing this evolutionary tree, showing how over and over again we have this branching from a parent species into two, I guess you could say different child species. You see this here with the elephants. At some point, the Asian elephant and the African elephant shared a common ancestor, and it was also, based on this diagram, a common ancestor of the mammoth. And you go even further back, it's the common ancestor of this species that I'm not familiar with, the Anancus. And you can keep going back. But how does this tree branch? How do you actually get speciation? How does the variation within a population, within a species get so extreme and in some ways so separate from each other that they can no longer interbreed and produce fertile offspring? Well, there's a couple of ways to think about it. The most obvious way that you could imagine this happens, or maybe the most intuitive way that you could imagine this happening, is through geographic separation. And the technical term for speciation, which is the formation of new species, so speciation, actually, let me just write it this way, the technical term for speciation due to geographic separation is allopatric. Allopatric speciation. So speciation is just how, the formation of new species. And "allo" comes from the word "other," and "patric" comes from the root or the word "homeland." So it's really talking about other geographies, or other homelands, or geographic separation. And one commonly cited example here are the antelope squirrels. So if you go to the, if you were to go to the American Southwest a long time ago, before the Grand Canyon was a canyon, when the Colorado River was just kinda going through and wasn't a major barrier, there was a parent species, an ancestral species to both of these characters that lived on both sides of the river. And on different times of the year, it was able to get across the river, it's able to, the squirrels on the north and the squirrels on the south were able to interbreed and produce fertile offspring, so they were all one species. But over time, the Colorado River started to erode more and more soil and rock, and so this became what we now consider to be the Grand Canyon. And so over time, this became a very significant geographic barrier. No longer could, before they could travel across, but once it became the Grand Canyon, it became very difficult or impossible for them to travel across. And so now you have these two different populations. They have the same parent species, but they're now geographically isolated. And since the creation of the, or while we have the creation of the Grand Canyon, since it became very hard or impossible them to cross, you've now had enough, both genetic drift, also natural selection, these are the evolutionary processes that we've talked about, where the Harris' antelope squirrel, which lives on the south side and is right over here, it's this picture, and the white-tailed antelope squirrel, which lives on the north side. Even though they look quite similar, as you can see from these pictures, they have now diverged enough that they are different species, that they no longer will be able to interbreed and produce fertile offspring. So it's fairly intuitive how allopatric speciation can work. Geographic separation, no longer can interbreed, and over time their genes change through natural selection and genetic drift. But what about situations where they stay in the same place, where theoretically, they could get together, they could interact. Could you still have speciation? And the answer is yes. And that form of speciation, where you are still in the same geography, that is called sympatric speciation. Let me write that down. Sympatric speciation. Speciation. And examples of sympatric speciation are a little bit less obvious, or a little bit less intuitive, but there's an example that people believe is sympatric speciation happening before our eyes. So this species, the technical term Rhagoletis pomonella, I know I'm mispronouncing it right over here, this is native to North America, and before European settlers brought apples to North America, they hung out and they laid their eggs and their maggots were inside of, or they leveraged the hawthorn fruit right over here. So they would go to the hawthorn trees, and they would lay inside of the haw, they would use the hawthorn fruit for their, to lay their eggs, and for their young to kind of consume. But once the European settlers came and introduced apples into North America, a certain, I guess you could say a subgroup, of Rhagoletis pomonella started to leverage the apples, so started to lay their eggs and their eggs and their maggots would grow inside of the apples. And they've actually now diverged, not into fully different species now. In theory, they can still interbreed and produce fertile offspring. But they don't tend to do it any more. That it tends to be, even though they're in the same geography, and it's not hard to fly from the hawthorn tree to the apple tree, they don't tend to do it. And because of this behavioral divergence, that some decide to go to the apple, some decide to stay at the hawthorn, they actually are now developing different traits that are selected for depending on that, I guess you could say initial preference, or that initial bias for which fruit they want to use to lay their eggs in. So for example, the ones that are in the apple tree, they now have, their breeding cycle is more aligned with the growing season for apples, while the ones in the hawthorn tree, their breeding cycle is more aligned with what, for the growing cycle for the hawthorn. And so biologists believe that this is an example of sympatric speciation happening before our eyes, that if we were to wait a few hundred more years, possibly a thousand years or more, that this will diverge into two different species that will no longer be able to interbreed and produce fertile offspring. Another example of sympatric speciation, which is a little bit more wild in some ways, it's a little bit more out there, but this would be an example with plants. So as we learn in other Khan Academy videos, organisms like human beings, and in fact many sexually reproducing organisms, they're diploid organisms. They have two sets of chromosomes. For example, human beings have two sets of 23 chromosomes, for a total of 46 chromosomes, 23 from your mom, 23 from your dad. And so we are diploid organisms. And in general, there are errors that occur during reproduction and errors that occur during meiosis that can lead to polyploidy, where an organism can have more than two sets of, or can start, or a potential organism could have more than two sets of chromosomes. In the animal kingdom, that doesn't work out too well. Usually, that does not produce a viable embryo, a viable zygote. But in the plant kingdom, this is, it tolerates it a little bit more. So you could have a situation where you have a diploid plant, and through meiosis, through an error in meiosis, instead of producing haploid egg and sperms, it produces diploid egg and sperm, which then are able to get together to form a tetraploid plant, so a plant that has four sets of chromosomes instead of two sets of chromosomes. And then once that tetraploid plant exists, it might only be able to reproduce with other tetraploid plants versus the diploid plant. So you have the diploid plants here, when meiosis is working "properly," and I'll put that in quotes, 'cause maybe, you know, arguably this error is what helps for a speciation sometimes, especially in the plant kingdom, it is, it can produce this haploid egg or sperm. The tetroid plant would then, would, through its meiosis, its ploidy, I guess you could say halves, when it goes to the egg or the sperm, but you're now going to have a nonviable or infertile triploid plant, because the separation won't happen properly in meiosis for this egg, if it's even viable. So all of a sudden this tetraploid plant is now a, you've had speciation occur. It could be viewed as a new species. And you could think about things like this happening as a potential, and we don't understand all of it, we don't understand how all of the speciation that we now observe has actually occurred, but you can even imagine as this being a mechanism for why you have an increase in the number of chromosomes in certain species versus others. So hopefully this is starting to answer some questions, and hopefully even introduce more questions, 'cause this is a very exciting topic, about how we get species from, more species, how do we get the diversity from parent species or ancestral species.