The Structure of Evolutionary Theory

Transcription

Congratulations! This is our last episode of our section on Evolution and Genetics, which puts us at the halfway mark of CrashCourse Biology. So far we've learned about DNA, genetics, natural selection, how cells multiply, populations, speciation, replication, respiration, and photosynthesitation. I'm so proud of you. But I couldn't let this section end without discussing the iscussion that everybody can't help but discuss these days: Evolution. It's a thing. It's not a debate. Evolution is what makes life possible. It allows organisms to adapt to the environment as it changes. It's responsible for the enormous diversity and complexity of life on Earth, which not only provides organisms with sources of food and some healthy competition. It also gives us some truly awesome stuff to marvel at. And even though evolution makes living things different from one another, it also shows us how we're all the same. All of life, every single thing that's alive on the Earth today, can claim the same shared heritage, having descended from the very first microorganism when life originated on this planet 3.8 billion years ago. There are people who will say that this is all random- It's not. And that this clumsy process could not be possible for the majestic beauty of our world. To them, I say, well at least we agree that our world is beautiful but, well you're probably not going to enjoy the rest of this video. To me, there are two sorts of people in the world, those who are excited about the power and beauty and simplicity of the process of evolution, and those who don't understand it. And somehow, I live in a country where only 40% of the population believes that evolution is a thing. The only possible reason for that that I can accept is that they just don't understand it. It's time to get real, people. First, let's understand what we mean when we talk about the theory of evolution. Evolution is just the idea that gene distribution changes over time, which is an indisputable fact which we observe all the time in the natural world. But the THEORY of evolution is a large set of ideas that integrates and explains a huge mass of observations from different disciplines including embryology, paleontology, botany, biochemistry, anatomy and geophysics. In every day language, the word "theory" means "hunch" or even "hypothesis." But in science, a theory is an idea that explains several phenomena at once. Thus, The theory of evolution is a bunch of ideas that explain many things that we, as humans, have observed for thousands of years. It's the theory that meticulously and precisely explains the facts, and the facts are indisputable. So let's spend some time going through the facts, and how evolution explains them all so well. First, fossils: The fossil record shows that organisms that lived long ago were different from those that we see today. Sounds obvious, but two hundred years ago it seemed a little bit crazy. When scientists first started studying dinosaur fossils in the 1820s, they thought that all dinosaurs were basically giant iguanas. That's why the first fossil dinosaur was named Iguanodon. It wasn't until the fossils of two-legged dinosaurs started showing up in the 1850s that scientists had to grapple with the idea that organisms of the past were somewhat similar to ones today like, dinosaurs were reptiles, but many of them took on a diversity that's barely recognizable to us. And of all those ancient not-really-iguanas were all extinct, either dying out completely or evolving into organisms that survive today, like birds. Fossils make it clear that only evolution can explain the origin of these new kinds of organisms. For instance, fossils taught us that whales used to walk. Whales are cetaceans, a group of mammals that includes porpoises and dolphins, and biologists long suspected that whales descended from land mammals. Partly because some modern whales still have the vestigial remnants of a pelvis and hind-limb bones. But it wasn't until recently, the 1990s and 2000s, that the pieces really came together. First, paleontologists discovered fossils of DOR-oo-dons, cetaceans that had different skulls from modern whales but still had the same vestigial leg bones. Then they found even older fossil remains of another cetacean that actually had hind legs and a pelvis. The pelvis wasn't fused to the backbone like ours is, so it did swim like a whale, but more importantly, it still had ankle bones And they were ankle bones that are unique to the order that includes bison, pigs, hippos and deer. So by following these clues left behind in fossilized bones, paleontologists were able to track the origin of whales back to the same origin as bison and pigs. This leads us to another series of facts that evolution explains: Not how animals were different, but how they are incredibly similar. Last week we talked about Carl Linnaeus and how he classified organisms by their structural similarities. Well he didn't know anything about evolution or genetics, but when he began grouping things in this way, he hit upon one of evolution's most prominent clues: homologous structures. The fact that so many organisms share so many finely detailed structures shows us that we're related. Let's go back to the whale. Like my dog, Lemon, and me, the whale has two limbs at the front of its body, its front flippers. And so does this bat, its wings. Inside our limbs we all have the very same structure: one longish bone on top, connected to two thin bones at the joint, followed by a cluster of small bones called the carpals, and then our fingers, or digits. We each use our forelimbs for totally different purposes: the bat flies, the whale swims, Lemon walks and I... you know, jazz hands! Building limbs like this isn't the most efficient way to swim or fly or walk. Our limbs have the same structure because we descended from the same animal, something like this more-gan-uh-cah-don here, which, yeah, has the same forelimb structure. In the first stage of our existence, every vertebrate looks almost exactly the same. Why? Because we're all descended from the same initial vertebrates. So our structures are the same as other mammals and other vertebrates, sure, but it also turns out that our molecules are the same as, like, everything. In fact, if we were ever to find life on Mars or something, the sure fire way of knowing whether it's really extra-terrestrial is to check and see if it has RNA in it. All living things on our planet use DNA and/or RNA to encode the information that makes them what they are. The fact that we all use the same molecule itself suggests that we are all related, even if very distantly. But what's more, by sequencing the DNA of any given creature, we can see precisely how alike we are. The more closely related species are, the more of the same DNA sequences they have. So the human genome is 98.6% identical to that of the chimpanzee, our closest evolutionary relative, and fellow primate. But it's also 85% the same as a mouse. And I wonder how you're going to feel about this, about half of our genes are the same as in fruit flies, which are animals, at least. So, just as your DNA proves that you descended from your parents, your DNA also shows that you descended from other organisms and ultimately, from that one prokaryotic microorganism 3.8 billion years ago that is the grandparent of us all. Now when it comes to species that are very similar, like say, marsupials, their distribution around the world or their biogeography, is also explained extraordinarily well by the theory of evolution. Animals that are the most similar, and are the most closely related, tend to be found in the same regions, because evolutionary change is driven in part by geographical change. As we talked about in our speciation episode, when organisms become isolated by physical barriers, like oceans or mountains, they take their own evolutionary courses. But in the time scales we're talking about, the geographical barriers are much older, and are often even the result of continental drift. So, marsupials. You know about marsupials. They can be found in many places, but they aren't evenly distributed around the world. By far the highest concentration of them is in Australia. Even the majority of mammal fossils in Australia are marsupials. So why is Australia rife with kangaroos, koalas and wombats while North America just has, opossums? Fossils show us that one of marsupials' earliest ancestors found its way to Australia before continental drift turned it into an island 30 million years ago. More importantly, after Australia broke away, placental mammals like us evolved on the main landmass and quickly outcompeted most of the marsupials left behind, in what would become North and South America. So, very few marsupials remain in the Americas, while Australia has been drifting around like some kind of marsupial Love Boat. Darwin's finches are another example of biogeographical evidence As he wrote in The Origin of Species, Darwin observed that different species of finches on separate Galapagos islands were not only similar to each other but were also similar to a species on the South American mainland. He hypothesized that the island finches were all descendants of the mainland finch and changed over time to be more fit for their environments, a hypothesis that genetic testing has since confirmed. Now, you'll remember, I hope, a few weeks ago, when I told you about Peter and Rosemary Grant, the evolutionary biologists/lovebirds who have studied Galapagos finches since the 1970s. One of their greatest contributions came in 2009 when studying finches on the island of Daphne Major. They discovered that the offspring of an immigrant finch from another island and a Daphne Major finch had become a new species in less than 30 years. This is just the latest example of our fourth body of evolutionary evidence: direct observation of evolution. The fact is, we have seen evolution take place in our own lifetimes. One of the fastest and most common changes we observe is the growing resistance to drugs and other chemicals. In 1959, a study of mosquitos in a village in India found that DDT killed 95% of the mosquitos on the first application. Those that survived reproduced and passed on their genetic resistance to the insecticide. Within a year, DDT was killing only 49% of the mosquitos, and it continued to drop. The genetic makeup of the mosquito population changed because of the selective pressures caused by the use of DDT. But it's not just tiny changes in tiny animals, we've also observed larger animals undergoing some pretty striking changes. In 1971, for instance, biologists transplanted ten Italian wall lizards from one island off the coast of Croatia to another. Thirty years later, the immigrant lizards' descendants had undergone some amazing, fundamental changes like, even though the original lizards were mainly insect eaters, their digestive systems had changed to help them exploit the island's most abundant food source: plants. They actually developed muscles between their large and small intestine that effectively created fermenting chambers, which allowed them to digest vegetation. Plus, their heads became wider and longer to allow them to better bite and chew the grasses and leaves. These are all great examples of microevolution, allele frequency changes that happens rather quickly and in small populations. Macroevolution is just that microevolution on a much longer time scale. The sort of thing that turns hippos into whales is a lot harder to observe for a species that, 200 years ago, thought dinosaurs were big iguanas, but part of the power of the human mind is being able to see far beyond itself and the time scales that our own individual lives are limited to. And I for one, am pretty proud of that. Let's all at least agree that the world is a beautiful and wonderful place. And life is worth studying and knowing more about, and that's what Biology is. If you want to go back and watch parts of this video again please click on the annotations in the little table of contents over there. If you have questions for us, please leave them on Facebook or Twitter or in the YouTube comments below. Thanks to everybody who helped put this together. And we'll see you next time.