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Speciation: Of ligers & men

Hank explains speciation - the evolutionary process by which new biological species arise - in terms of finches, ligers, mules, and dogs.  Created by EcoGeek.

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Video transcript

- You and me, we got some stuff in common. More in common than say, you and my dogs Lemon and Abbie here. For starters you and I are probably, like, the same species and Lemon and Abbie are dogs, which is a different species. As you may have guessed by now, this video is going to be about species but at the very end we're going to talk about dogs, so hang in there 'cause the puppies are comin'. (indie rock music) But before we bust out the puppies, let's talk about people. Our species, Homo sapiens, is the single remaining member of the genus homo. Our buddies Homo erectus and Homo habilis and Homo neanderthalus bought the farm a long time ago. So these days, all us Homo sapiens are pretty different from even our closest living relatives in the animal kingdom, the chimps and bonobos. Humans are a species, a specific type of organism that's different from all of the other types of organisms out there. But what is it that makes us human? Well, we're a specific type of animal called a primate. Monkeys, apes, lemurs, and tarsiers are also primates. Unless you're Sacha Baron Cohen or something, most of us are lacking significant body hair. We're bipedal, meaning we stand on two feet, and we've got these hugenormous brains that allow us to do all kinds of stuff like talk real good and solve complicated problems and write bad poetry during adolescence. And, you know, also, like, figure out how little we can get away with tipping a mediocre waiter at a restaurant without seeming like a total prick. That, my friends, is something that giraffes rarely have to deal with. But being a species is more than just having a bunch of stuff in common. Instead, we describe a species as a group of organisms that can interbreed and produce fertile offspring. Seems pretty simple, right? Two of the same species that can produce blah blah blah blah blah. Hey, pay attention! That last part is important, the two organisms need to be able to produce fertile offspring. It seems like it would be enough for organisms of the same species to be able to make babies, but those babies need to be able make babies, too. Now, it turns out two animals of a different species can sometimes technically have a baby. Take for instance the noble liger, Napoleon Dynamite's favorite animal, which I would know, because I had the very best Napoleon Dynamite costume in the United States for Halloween in 2005. But I didn't just bring up ligers to brag, a liger is what happens when a male lion and a female tiger have a little cub. Only, it's not very little because a liger is generally larger than both of its parents, and ligers are sterile, which leads us to our understanding of what makes a species. Lions and tigers are different species because they don't produce fertile offspring together. We call animals like ligers hybrids, offspring resulting from the crossbreeding of two distinct species. And even though hybridization between two animals is a dead end when it comes to creating a new species, we know that through evolution, or the change in heritable characteristics of a species across generations, new species have formed in the past and they continue to develop all the time. It's tough to nail down every single way that this process that we call speciation can happen, but we know of at least a couple of ways that species evolve into other species, and they both involve one requirement. Reproductive isolation, meaning that two populations of the same species can no longer mate together successfully. Note that I said successfully. One way populations can become isolated from each other is that they can mate, but their offspring aren't fertile or viable, ligers are a good example of this. So are mules, they're the product of a male donkey and a female horse, and unlike lions and tigers, donkeys and horses don't even have the same number of chromosomes. So even though the donkey's sperm can fertilize the horse egg, the mule won't have the genetic instructions it needs to produce its own sex cells. This kind of isolation is called post-zygotic isolation because the parents can form a zygote together, but after that, it's all over for their lineage. Other examples of post-zygotic isolation include pairings of species that always lead to miscarriage or no development of the embryo at all, or things that produce, like, big fetuses that kill the mother at birth. The other type of isolation is pre-zygotic, meaning that the isolation happened between groups of the same species before an egg even thought about getting fertilized. This could include stuff like behavioral changes within a species, like when birds of the same species start singing two different songs to attract mates, or when one group of a species that does all of its business in the daytime gradually becomes nocturnal so that the two groups never end up hanging out at the same time. Pre-zygotic isolation can also be geographic, meaning simply that populations are separated by great distances or physical barriers so that they can no longer get together and bump uglies. When one species diverges into two different species because of geographic isolation, it's called allopatric speciation. Allopatric coming from the Greek for different countries. The two populations of the species end up evolving differently because the conditions are different on each side of this river here. For instance, it might be colder one on side of the river, so the animals on this side grow thicker and more luxurious coats because those guys just do better over there. They probably also put on thicker layers of fat and change their behavior and accumulate a bunch of other possibly random changes. Meanwhile, on the warm side of the river, these animals also accumulate changes and lose some fur, add a bunch of sweat glands. Given enough time and given a complete lack of gene flow between the two populations, thick-coated animals will eventually only be able to breed with other thick-coated animals and sweaty animals with sweaty animals. This propagation of specific traits based on how kick-ass those traits make the animal that has them is called Natural Selection, and a guy named Charles Darwin, or Chuck Darwin, or Chucky D to his friends, was the one who let us know what was up with Natural Selection and how it how can it lead to allopatric speciation. Now, stop me if you've heard this one before, but Darwin visited the Galapagos Islands in the 1830s. So Darwin was obsessed with barnacles, but that did not keep him from noticing the finches, which were actually misidentified by him as grosbeaks on each island, which were all pretty similar to the finches on other islands and very similar to the ones on the mainlands of South America. But they were also obviously their own species. Darwin believed that the process that led to these finches becoming separate species was incredibly slow. So slow that we couldn't actually witness the process, we just had to take his word for it. Now, for a long time after Darwin made these observations, allopatric speciation was the main explanation for how species diverge, but today, we know that that's not the whole truth. Now, we've got lots newfangled DNA testing and other special gadgetry that tells us that one species can diverge into two without being geographically separated. But instead, when they're reproductively isolated in some other way. This is called sympatric speciation, meaning same country speciation, and it also means that it's time for a trip to the chair. (ragtime piano music) So here's a little biological love story for all you romantics out there. Peter and Rosemary Grant, two British evolutionary biologists, they are in fact a married couple, have since the 1970s been spending six months of each year together on a secluded island in the Galapagos, studying Darwin's finches, trying to catch them in the act of evolving. These are, mind you, the same animals that Darwin studied, and the ones that he said were evolving at an imperceptibly slow pace. The island in the Galapagos that the Grants hang out on is called Daphne Major, and when they started their research in 1973, it was occupied by two different finch species, the Medium Ground Finch and the Cactus Finch. But in 1981, another finch arrived on Daphne Major from a nearby island, it was a Ground Finch-Cactus Finch hybrid, and it was a whole lot big than either of the local finches. Its beak was also extra wide, and its song was like a mashup of the jams Ground Finches sang on its home island and the ones sung on Daphne Major. So the newcomer set to work crooning to the local Ground Finch ladies and eventually landed one. The Grants followed the descendants of these two birds for the next 28 years. But after about four generations, Daphne Major experienced a severe drought, which killed many of the finches. There were only two surviving descendants of that one immigrant finch, sort of like cousins of each other, basically, and they mated with each other, and that seems to have set the stage for speciation to occur. The descendants of these two survivors sang a very distinctive song that was learned from their parents, which was different from the other Daphne Major finches. Gradually, as the finch population on the island rebounded, the hybrid finches, the great, great, great, great grandchicks of that one bird began mating exclusively with each other. In December of 2009, the Grants announced that since the drought, the lineage of that one immigrant Ground Finch has been genetically isolated from the other local finches on the island. So that, my friends, is both super romantic and also an excellent example of super quick sympatric speciation in action. Okay, so I promised you puppies. I'm gonna give you puppies. You've probably noticed that, you know, a corgi looks pretty different from a greyhound. They were bred to be different. Corgis were bred to herd animals and guard farmhouses, while greyhounds were bred, you know, mostly to run. Dog breeding kinda takes the natural out of selection, in fact, it's what's what we call Artificial Selection. But it's still a kind of selection. So you've probably wondered what it would be like if a corgi and a greyhound had puppies together because they can have puppies together, even though that's really weird. What's that, Lemon? You're both girls? Oh, well, you'd-- anyway. My point here is that they're the same species, meaning that these dogs, even, like, differenter dogs like an Irish wolfhound and a chihuahua could have fertile offspring together. Like, how? How, how? Would-- How? Various dog breeds are similar enough that post-zygotic isolation isn't an issue. But in a natural setting, a chihuahua-wolfhound pairing would be extremely rare because of the, you know, the difficulties involved in the gettin' it on process. Or pre-zygotic obstacles. You think about it like this, if you were to put a bunch of chihuahuas and a bunch of wolfhounds on an island somewhere, they probably wouldn't breed together, and if they did, the birthing process, at least for the chihuahua mommies, would be-- Aghh! Oh, god! But what this means is that the gene flow between the two groups would stop, and they would become reproductively isolated. Over time, they would become different enough that they could no longer successfully breed together at all, and thus become different species.