字幕表 動画を再生する 英語字幕をプリント Hi. It's Mr. Andersen. And welcome to the Unit 2 review. In this podcast I'm going to talk about speciation. We're going to talk about speciation, extinction, mechanisms of speciation. But before we get started we should define what speciation is. Speciation is essentially a biological process by which new species arrive. And so all life on our planet started with what we call the last universal common ancestor. All other species that we have on our planet then arose through speciation. So the formation of new species. And so let's start by talking about what phylogenetics are. Phylogenetics are basically the evolutionary history of species and organisms on our planet. So this right here would be an example of a, let me get a different color, a phylogenetic tree. And so basically time is going to go in this direction on a phylogenetic tree. And so it's going to start with an ancestor of all the different organisms on this tree. This one happens to be about whales. But every time we see a branch point like that or if we have branch point up here, then we eventually have two new species. So this would be a speciation event. Each of these junctions then on a phylogenetic tree is simply going to be a common ancestor. And so what is this point here actually mean? It simply means there was a common ancestor between the southern minke and northern minke whale at sometime in the history. Now I don't want you to memorize a lot of phylogenetic trees, except this one over here. This is the phylogenetic tree of all life on our planet. And so basically we have the three domains of life. And those are bacteria, archaea and then eukaryotes. And so if we were to look at this tree what we find is that there's a really early branch point. In other words a branch point between the bacteria on one side. And then the archaea and eukarya on the other side. What does that mean? Well you're more related to an archaea then to a bacteria. And so we had a branch point here. And then we had another branch point, this one broke down into the archaea and then into the eukarya over here. And so basically this is the phylogenetic tree of all life on our planet. We've never found life on our planet that doesn't fit into one of these three domains. And so basically looking at this tree we can say this point right here is the point at which that last universal common ancestor existed. Now could have there have been earlier life before this branch point? For sure. It just didn't leave any kind of a fossil record or it didn't leave any ancestors. And so that last universal common ancestor will come back to later in the podcast. So let's talk about speciation, specifically in relation to extinction. And so speciation again we say is the formation of new species. Extinction is going to be the opposite of that. In other words when species when species leave. In other words if we're looking at time in this direction, so this is a specific type of a saurapod, a type of dinosaur. Basically this would be ancient past and this is time moving in this direction. So every time we see a branch point like that, that's a speciation event. Every time it comes to an end, so right here it comes to an end, that would be extinction. And so all of the organisms we have on our planet today either form through speciation or will disappear through extinction. Two big things that I want to talk about in relation to speciation and extinction are number one, mass extinctions. Mass extinctions are going to be extinctions where it's not just one species getting wiped out but a number of species getting wiped out at the same time. So the extinction of the dinosaur is an example of a mass extinction. And we're pretty sure that that was caused by an asteroid impact just off the Yucatan peninsula, in the Gulf of Mexico. And so we had this massive extinction of a number of species at that time. Now was it the asteroid impact or climatic changes after that? We'll probably figure that out. But that's a mass extinction. We've had like five mass extinctions. And you're lucky enough to be right in the middle of probably the biggest mass extinction of all. Humans are making climate changes that species simply can't adapt quickly enough to. Okay. So I said there are two things I wanted to talk about. One is mass extinctions. The second is something called adaptive radiation. So adaptive radiation is when we have just one species that branches into a number of species very very quickly. And so example. Let me give you a couple of examples. When Pangaea formed and all the continents came together we had a giant desert in the middle of the super continent. And it was bad for life. So we had a mass extinction. As the continents started to break apart then we had all of these little niches between the continents. And so we had adaptive radiation of dinosaurs. The dinosaurs did really well and filled all of those niches. Okay. That was followed by a mass extinction of this asteroid impact. Which in turn was followed by adaptive radiation of mammals filling those niches that were once filled by dinosaurs. And so adaptive radiations can be large scale. For example the exploitation of mammals of these new niches that were vacated by dinosaurs. Or it could be even at the local level. So those first finches that landed on the Galapagos Islands adaptively radiated to fill all of those niches on the islands. Next thing is the artificial selection lab. We haven't done this yet. But in the artificial selection lab what we'll do is we'll use these Wisconsin fast plants. And we're basically going to choose traits that we want in the offspring. And then we're going to set those crosses up. And so we'll use a bee stick where you take a bee and you put it on a stick. It's a dead bee, but you can transfer pollen from one plant to another. You then get seeds and you can choose the characteristics that you want. So is this artificial selection? Yeah. Because we're making the choices. Not natural selection. Now all of these creatures were made, at least their characteristics were made through artificial selection as well. So dogs are wolves if you look at the DNA of dogs and wolves its essentially the same thing. But humans have selected for traits that they wanted over time. And through that we've been able to create species. If you were to show somebody, they didn't know about dogs, a chihuahua and a great dane, and say these are of the same species, they would say you're nuts. And that's because we've changed them so much. Bent them a little bit to our will. What are some mechanisms by which speciation can occur? Well if we're talking geographically, let me give you an example of that. And so let me try and draw the United States. This is really bad. There we go. So there's a species of meadowlark that lived right across here in North America. So it lived in these mid kind of latitudes. During the last ice age we had a, let's get a different color, so we had the ice sheet move down right down like this and fill up that center part of North America. So now what we had was meadowlarks that were over here. We had meadowlarks that were over here. And those were isolated geographically. Now there are two types of geographic isolation. One is allopatric. At least our book talks about. Allopatric is when those different populations are separated geographically in different lands. Sympatric I'll get to in just a second. Okay. So we had our meadowlarks that were separated by this ice sheet. Now the ice you know eventually melted and retreated. And after it had done that then we had these two populations, the western and the eastern meadowlark. Now during this period of time they developed different songs. So they had different behavior. And so even though in this margin we'll probably have eastern and western meadowlarks in the same area at the same time, since the males impress females with their songs, they can't communicate anymore. And so this would be how species are formed. I'd said that sympatric speciation is different. Allopatric speciation is when you're in different lands. Sympatric speciation is when you're in the same land. And so that's when we have a population. And then we get a new population within that population. That generally occurs in plants. And it generally occurs using polyploidy where we have a mistake. Where an increase in the number of chromosomes creates a brand new species. And so that would be a genetic polymorphism. So when we have a change in the chromosome number and it creates something brand new. It happens a lot in plants. Rarely in animals. Now the two things I haven't mentioned are peripatric and parapatric. And I say those exactly the same way. That's when organisms move into a niche, nearby niche that maybe isolated or it maybe even attached. But now they start adapting to their local environment. And so the forest elephants of Africa moved into the forest. Moved into the niche. And so they're getting a different appearance and eventually will create a brand new species. Okay. So those are geographic. Four types of geographic isolation. Behavior isolation, I kind of mentioned is when you're behavior keeps species from breeding. Temporal isolation is, I always remember the T stands for time. That's when they mate is going to create new species. In other words if this frog breeds in the spring and this one in the fall, even though the could form hybrids, they're not going to. And then mechanical isolation could be mechanical isolation of the anatomy. For example these snails. Some of them are turned clockwise and some are counter-clockwise. So their sex parts can't quite get together. But mechanical isolation could be isolation of, you know, even the egg and the sperm can't get together. And so that would be another thing that can create brand new species. Okay. If we talk about natural selection within species, now we're starting to talk about changes in a bell shaped curve. And so in anything, pretty much every trait that we have, unless it's caused by one or even a couple of genes, it's going to give you a phenotype that is a bell shape curve. So this could be skin color in humans. It could be height in humans. It could be length of thumb in humans. But we're essentially made up of a bunch of these bell shaped curves. And so if you ever have selection on one side of that, you can push a bell shaped curve in one direction or the other. We can squeeze a bell shaped curve. We can make it tighter on the sides. And basically there's three things that we can do to a bell shaped curve. First one of these is called directional selection. That's where we're actually moving it in one direction. The quintessential example of this is the Galapagos finches that the Grants were studying on Daphne Major. So basically they measured all of the beaks of the birds on Daphne Major. They collected all of the birds that they could possible catch. They found 751 birds. And this is the average beak depth. Okay. Now they had a massive drought. So they had a bird apocalypse. And so they came back in 1978. There were 90 birds that survived. So almost all of them had died. But the bell shaped curve you can see had switched over here to the bigger side. And the reason why is the beaks were now able to open seeds that they couldn't open before. What happened to the birds that weren't able to open up those seeds? They died. So they died and that's why we see the bell shaped curve moving in this direction. And so remember in natural selection it's not like organisms are changing the way they are. They can't do that. They're either dying or surviving based on the characteristics they have. And so the population can change. Or the population can evolve over time. Disruptive selection is when we have, oops, is when we have either pressure pushing them apart. Or it could be drawing them to the sides. So we have a pressure here that is removing individuals that are in the middle. And so an example of that could also be found in the Galapagos finches. And so why do we have so many different types of beaks in the Galapagos finches? Well each of those birds are modified so they can feed on a specific seed. And so once those first finches got to the Galapagos, they landed and flew to different islands. And they were able to adapt to that specific climate. And then finally we can have stabilizing selection. An example of stabilizing selection is when we squeeze the bell shaped curve together. An example I always give is babies. If a baby weighs one pound it would never be able to survive. If it weighed 21 pounds it would not survive and it would take mom with it. And so basically babies weigh about 7 pounds. Because there's pressure on either side. Eliminating babies at the extreme. A specific type of natural selection that puzzled scientists for a long time is in the amazing shows of like the peacock or the songs of birds or the colors or the butterfly or the huge antlers in an elk. Basically what's going on here is sexual selection. So it's not nature making a choice as to who survives. It's females making a choice as to males. So essentially she's checking out this peacock. She's looking at his feathers. And she's making a judgement call. If he isn't able to produce feathers, isn't able to produce the correct number of eye spots, he probably can't produce fertile offspring as well. And so whenever females make a choice that's when we get this weird dimorphism, this change between males and females. Kind of finishing up in the beginning. So basically how did life on our planet, before all of this speciation take place, how did it form? Well a lot of scientists are thinking that it is through abiogenesis. In other words it came from non-living material. Now we know that life just doesn't spring from nothing. Especially today when oxygen is present and it's going to break down chemicals quickly. But the famous Miller-Urey experiment, what they did is they basically created the early earth's atmosphere. They got rid of oxygen. Included water, methane, ammonia. And they added a shock to kind of simulate lightning. And what they were able to do is produce the building blocks of life. Amino acids, nucleotides. And so they were able to make the genes that would be found in this last universal common ancestor. Today we see an area where that might be in the stromatolites that we find in this area kind of in this tide pool kind of an area. It's similar to some of the first fossils that we found on our planet. But maybe life was delivered here in a meteorite from Mars or from a different planet. We don't know. And we'll probably never know what that first ancestor looked like, that first cell. But we have a not of theories that kind of point us in the right direction. So I want to finish with kind of a walk through time as far as life goes. I don't expect you to memorize all the different periods of time. But I do want you to know the progression. And so if we look back the earth formed about 4.6 billion years ago. For most of that first part it was just a ball of magma. And so it was just a molten ball of magma. So life couldn't have existed on it at all anyway. But basically around 4 billion years the first life on our planet formed. And that was prokaryotic life. And so that was, let me get a different color, so basically the first life on our planet was that. It had genetic material on the inside. Probably similar to a bacteria. But it was a simple cell. So this is the first life we have on our planet. If we play the clock forward, photosynthesis start here. We start to get an appreciable oxygen in the atmosphere which actually adds pressure to species that aren't designed to work well with oxygen. And so our next big thing that happens, so here is prokaryotic life. Next thing we have is eukaryotic life. So eukaryotes show you know like 2 billion years ago. Now eukaryotic cells are going to be different than prokaryotic in two ways. Number one they have a nucleus. And they've got the genetic material inside there. But they also have all of the other parts of a cell. So they have like golgi apparatus. They have endoplasmic reticulum. They have mitochondria. They have lysosomes. And so they have all these organelles that are separated by membranes. Now how did we go from prokaryotic cells to eukaryotic cells? Well two ways. Number the membrane started to fold in on itself. And so we got what's called an endomembrane system. That's how you get an golgi apparatus. That's how you get lysosomes. But we also had mitochondria. So mitochondria were probably bacteria that moved into cells. Chloroplasts the same way. So now we had eukaryotic cells. If we play the clock forward, we eventually get to multicellular life. And then it's awhile before we get to animals, plants, mammals. And then humans show way up in the end. So multicellularity comes when we have cells working together for a common purpose. Why is it that we don't see animals and plants moving onto land until much later? It's because the atmosphere was not really formed yet. Until we had an ozone, until we had a defensive atmosphere, life had to exist in the oceans where it was protected from that. And so you stand at kind of a unique time. It's not been long that humans have been around. But we have had some huge impacts on our climate that are probably going to impact species and lead to that maybe sixth mass extinction. And so that's speciation in a condensed form. And I hope that's helpful.