Wisconsin Public Television University Place Wednesday Nite at the Lab  David Baum Darwin's Tree of Life  December 21, 2011 UP668   cc >> Tom Zinnen: Welcome to Wednesday Nite at the Lab. I'm Tom Zinnen. I work here at the Biotechnology Center. I also work for UW Extension Cooperative Extension, and on behalf of those two organizations, thanks for coming to Wednesday Nite at the Lab. We do this every Wednesday night, 50 times a year. Tonight I'm delighted to be able to introduce to you David Baum who's a professor of botany here at UW-Madison. He's also the director of the James F. Crow Institute for the Study of Evolution. He was born in London, England, and obtained a bachelor's degree in botany from Oxford University. He then moved to St. Louis, Missouri, and obtained a PhD in population and evolutionary biology at Washington University. His research interests are in plant evolutionary biology and evolutionary theory. 

His research team conducts both laboratory and field research aimed at understanding the evolutionary diversification of several plant groups and at pinpointing the underlying molecular changes that differentiate plant species. Please join me in welcoming David Baum to Wednesday Nite at the Lab. [APPLAUSE]  >> David Baum: Thank you very much, Tom. And thank you all for coming. My intent tonight is to tell you something very old but try and give it a new spin. So Darwin's Origin of Species is hardly a new piece of work, but what I'm going to try and do today is to tell you a little bit about some relatively new ways to think about Darwin's insights and to kind of shape it, and what I'm going to do is try and connect for you two of his important ideas: common ancestry and the tree of life. So Darwin's most famous work was published in 1859, The Origin of Species, when he was 50 years old, and I thought I would start by sharing with you the very first sentence for The Origin of Species because I think it will shape what I'm going to talk about tonight. So he says, "When on board HMS Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America and in the geological relations of the present to the past inhabitants of that continent." So why did he start The Origin of Species with this sentence? Now, let's just think about a second what he's writing about. He's writing about his voyage on the Beagle. So his voyage on the Beagle took him around the world. 

He started off in Great Britain, he traveled around South America, and went around the world coming ultimately back to Great Britain, to England. And this trip was clearly essential. Before he left he was already interested in the problem of diversity, the problem of where did all these species come from, why do they have the traits that they have, and a very strong case can be made that had he not traveled around the world, he would never had developed this theory. And it's notable the other naturalist who came up with similar mechanisms on the understanding of evolution, Alfred Russel Wallace, also traveled great long distances around the world. So traveling around the world was key, and this sentence explains why it's key. It's key because as he moved around the world he saw geographical patterns, and specifically what he saw that jumped out at him was that when he would go to a place on the planet he would find that the species that occupied that area appeared to have these deep structural similarities to one another. They had some underlying similarity that he interpreted as relatedness. They were seen to be relatives. He also noticed relations between the fossil and the living species that lived in an area. So, for example, in South America, one of the first excavations he did, paleontological excavations he did, involved glyptodons. 

glyptodons are these giant things that we now know are close relatives of armadillos. He noticed the great similarities between glyptodons and armadillos, and he was struck by the fact that armadillos are this uniquely South American group, and the only place in the world that anybody finds glyptodons are in South America. So he was struck by these geographic patterns. And thanks to these observations, he had this great idea, this idea of common ancestry. He concluded that when he looked at organisms, it didn't look like they had all been separately created. It appeared that they had common ancestry. And he first sketched out the idea of common ancestry in a very famous sketch in 1837 that you can kind of see in the background of this slide. So I'm going to be talking a lot about that idea of common ancestry because I feel that it often got lost in the mix. When people talk about evolution currently, people talk a lot about natural selection, they talk a lot about fossils and DNA, but people forget the central importance that common ancestry plays in evolutionary theory, and I'm going to try and spend today emphasizing why we really should focus on common ancestry very clearly. So let's just start. 

What is common ancestry? Common ancestry is a really simple idea, but let me spell it out, nonetheless. So take any set of living species, and I just grabbed five different vertebrate species that are alive today, and play a mental game. Take that organism, an actual individual organism, and go to its parents. And now go to that parent, that organism's parent, the grandparent of the first organism. Now go to the great-great grandparent and the great-great-great grandparent, and keep doing that backwards through time. At some point, those lineages for those separate species will converge on a single common ancestor. So that means that there's one or a set of organisms at a point in time, all of whom are ancestors to both in this example, why can't I find my mouse, to the mouse and the human. So they all trace back to a common ancestor. And if we keep walking backwards through time, of course, we find further common ancestors. This common ancestor that we've come to down here is the last common ancestor of the human, the mouse, and the lizard. 

We walk further down through time, we come to the common ancestor of the frog, the lizard, the mouse, and the human. And on this tree, we ultimately will converge on this ancestor at the bottom here that is an ancestor composed of organisms that are ancestors, lineal ancestors, of all of these living things. There's a simple idea, but it's also a really important idea. It's hard to overestimate the importance of common ancestry for thinking clearly about evolution. And the main reason the common ancestry is so important is that if common ancestry holds, then all the differences that we see between living species had to accumulate over time. In other words, evolution must have happened. You can take any two living species that you wish to think about, if you picture them, and think about all the ways they differ. Now imagine that they do trace back to a common ancestor. All those differences must have accumulated somewhere on the time since common ancestry. The differences in this example between the mouse and the human will either represent changes over here on the lineage to humans or over on the lineage to mouse or both. 

So if common ancestry is true, evolution has to have happened. And the nice thing, as you all see, is that's actually very straightforward to find evidence for common ancestry. In fact, Darwin's claim that evolution happened is really the claim that common ancestry holds. So it's really an essential idea in evolutionary theory. So, in fact, the only way you could entirely deny evolution would be to claim that there were separate ancestry. In other words, if you walk backwards through time, generation by generation from a set of living species, they would all have a separate origin. In that case, evolution might not have happened. But the minute you say that two very different living species share a common ancestor, then evolution happens, substantial amounts of evolution if they're substantially different like a human and a mouse. So it's a very important idea. So, to reiterate, evidence of common ancestry is evidence of evolution. 

So, in The Origin of Species, which was finally published in 1859, Darwin documented evidence for common ancestry, and I'm going to go through some of the evidence in a few minutes. It came from an analysis of the structure of the different organisms embryology, by geography, paleontology, the fossil record, and from classification. But before I can get to the evidence, I just thought I would pose a question, a thought process, which is why did he take so long to publish The Origin of Species? His sketch showing common ancestry was 1837. So why did he wait 22 years from that insight to write down on paper the evidence he saw for common ancestry? And the reason he waited so long was clearly because he felt that he needed a mechanism. It wasn't sufficient to show that evolution happened. He wanted to be able to say how it happened because otherwise he felt people would say, well, okay, it's ridiculous, it can't happen, so there must be something wrong with your evidence. He articulated this very well in the preface to The Origin of Species. I've got another quote here. He said, "In considering the origin of species, it is quite conceivable that a naturalist might come to the conclusion that each species had not been independently created. Nevertheless, such a conclusion would be unsatisfactory until it could be shown how the innumerable species inhabiting this world have been modified, so as to acquire that perfection of structure and co-adaptation which most justly excites our admiration." So, of course, I love Darwin's writing. 

It's beautiful writing, but the idea is very clear here. The idea is that you need a mechanism too. And so he was spending those 22 years trying to develop a clearly defensible mechanism for evolution. And so the consequence of that is that if you look superficially at The Origin of Species, what jumps out at you is natural selection, and I can tell you from experience in classes that if I asked the question, tell me what you think of when you think of Darwin, they always say natural selection. Sometimes they'll say survival of the fittest which was a phrase coined a little after Darwin, but it's the same idea. People rarely say, oh, common ancestry. And I think the reason that he focused on the mechanisms of evolution was almost because common ancestry was so obvious. It was too apparent to him. The minute you see the world through the lens of common ancestry it is about as obvious a thing as you can image. And so I think that's why in The Origin of Species, while the evidence is all in there, it almost takes second place to natural selection and the writing of The Origin of Species. 

And, in fact, he was quite astute in making that decision because it really was natural selection that people tripped up over, and so people, on reading The Origin of Species, scientists were pretty much convinced of common ancestry almost immediately. They were convinced that evolution did happen, but they were very skeptical about the mechanism. Just to give you kind of a contemporary quote, this is Paul Broca, who is well-known for his neurological work, there's an area of the brain named after him, but he wrote in 1866, so just seven years after the publication of The Origin of Species, "The fixity of species is almost impossible, it contradicts the mode of succession and of the distribution of species in the sequence of extant and extinct creatures. It is therefore extremely likely that species are variable and are subject to evolution. But the causes, the mechanisms of this evolution are still unknown." So I think that it's inevitable for the biologist who saw common ancestry became so obvious that the question was all about natural selection. The downside of that is that for 150 years evolutionary biology perhaps is overly focused on the mechanism, and, particularly in an educational context, we haven't placed sufficient emphasis on common ancestry in my opinion. And I'm going to try and explain why I think that's important as we go. So, let's just take sort of a digression, but let's just clarify what is the evidence for common ancestry. And I'm only going to do a broad sweep, broad strokes here, nothing very detailed. And I'm going to use a fair number of historical references. 

I find that useful when I'm telling history. So I think it's Darwin's great grandfather, Erasmus Darwin, obviously he didn't develop a fully fledged theory of evolution or he would be the Darwin we talk about, but he did think a lot about diversity and he wrote this epic poem, sort of medical, scientific poem. But he also thought about the patterns he saw, and the one thing he pointed out, which actually resonates to the modern understanding of evolution, is that when you see deep similarities among organisms, it suggests that they have some degree of relatedness. Of course he's putting it in a more theological framework, but this is the word that he used: "The great Creator of all things has infinitely diversified the works of his hands, but has at the same time stamped a certain similitude on the features of nature that demonstrates to us that the whole is one family of one parent." So it's that one family of one parent, that's a very familiar idea. When we see family resemblances in families, sort of same idea that when we see these deep similarities among species, they tell us that they have some degree of relatedness, some common ancestry. So, similarity, in general, is not particularly good evidence for common ancestry over separate ancestry. The similarity becomes particularly compelling when we have similarities in structure despite differences in function. So if we have two structures that are doing the same thing and they're similar in two different species, you could easily argue that they're structured that way because that's the best way to build an ax. Okay, to build a structure for that function. But when we find something like this where we have very similar structures in the frog forelimb and the bat forelimb and the porpoise forelimb, similar bone arrangements with a similar developmental trajectory, given that these three structures are used for very different functions-- We wouldn't expected them to have similarities, so the fact that they do have similarities, it becomes stronger evidence that they have common ancestry. 

Similarly, we've got sort of a molecular example here in the middle. Here we have two, be them actual representations of protein complexes in bacteria that are used for entirely different functions. The flagellum here is basically a bacterial motor that is used with a little propeller around, bacteria that you can use to swim with. Whereas the type two secretion system which is very similar but is used for completely different structure and function, is used to inject toxins into host cells. So you can just visually see the similarities there. We also have many good cases of structures that show similarity despite a lack of function. We call these vestigial structures. So you can see over here a good example of that. There are these vestigial hip bones in a whale. So whales lack hind limbs. 

So they don't have functional hips but they have these residual hips embedded in the muscle on their flanks. It's sort of shown there on the little picture. So similarity despite functional difference provides the best evidence for common ancestry among similarities. Extending that idea, very strong evidence for common ancestry arises when we find similarities improbable.  [replacing bad mic] Okay, better, no cracking. So we see these kind of patterns all over the place. Cases where we see, for example, similarities despite what appears to be very unintelligent design. So let's give one of my favorite examples. We have a nerve that feeds our voice box that originates from the brain stem, travels down into the chest cavity, underneath aorta, the major blood vessel serving the heart, back up to the voice box. And this odd wiring pattern is seen not just in us but in all mammals. 

So, for example, in a giraffe this nerve runs from the brain stem down, way, way down into the chest, underneath the aorta, back up to the larynx. That's hard to explain without invoking a common ancestor of the mammals. Similarly, the blind spot in our eye is bad wiring again. Our retina is basically wired backwards, and as a result of that, we have a blind spot in the middle of each retina, and that's shared by all vertebrates. Why would that be were it not for common ancestry. And we also see this very clear at the molecular level. So in theory there's an almost infinite number of nitrogenous bases, yet all of life uses the same five, four for DNA and an overlapping four for RNA. Likewise, all of life uses the same 20 amino acids even though there's actually an infinite number of possible amino acids. That seems hard to understand without common ancestry. And the code that matches the DNA to the protein coding sequence to the amino acids is also almost identical across life, even though there's not particular functional benefit to this arrangement. 

So, similarities among organisms are just compelling evidence of common ancestry. Fossils are often misunderstood. The role of fossils as evidence of evolution I think is often misunderstood. But there is an important sense in which they provide evidence for common ancestry which is when we find what are called transitional fossils. So I thought I would just take a second to clarify what that means because I do see a lot of misunderstandings about the nature of fossils. We rarely see in the fossil record continuous series of fossils. Fossil forms gradually morphing one into the other through time, and there are good reasons why we don't see that. Nonetheless, fossils provide very compelling evidence for common ancestry when we find transitional fossils. So what is a transitional fossil? So think of a living group. Most living groups you can think of have a number of distinct features that make them, differentiate them from anything else. 

So here is a distinctive living group represented by this triangle. And here are a whole bunch of traits that are different between this group and everything else. The group of the living mammals you can list at least 10 traits that we have that no other living group has. We have three middle ear bones. We have hair. We have milk, etc. So given that all these traits couldn't have sprung into existence all at once, there must have existed forms in the past that had some but not all of those unique traits. And so whenever we find those transitional forms, that's consistent with the expectations under descent from common ancestry. But you wouldn't predict necessarily finding those forms if there were separate ancestry. So just to kind of give you a concrete example, birds, here's a list of some of the many, many features that are unique to birds. 

And when you look in the fossil record, you find these traits. You find ancestors that had hollow bones but didn't have feathers. You find ones that had hollow bones and feathers but didn't have any of these. You find some like this one. This is an example of a fossil image, an image of a fossil with a very initial wishbone. The wishbone is a fusion of the collarbone. We have two separate clavicles here on either side. The fusion of them into the wishbone is what helps birds achieve the springiness to spring their wings back up, but it evolved in terrestrial dinosaurs halfway up there. Okay, so traditional fossils. I'm going through these kind of different kinds of evidence for common ancestry. 

And then the last one I'm going to talk about tonight, although there are more, but the last I'll talk about is biogeography because this is really where Darwin started. So Darwin's, most famously, when he got to the Galapagos Islands, he was struck by the fact that there was a group of birds there, represented by several species, but they all had these rather striking anatomical similarities. And so when he saw them, he said, "Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species has been taken and modified for different ends." So he could see the biogeographic structure suggested common ancestry. Of course, we know this very different conventional, every day, we know this without really thinking about it. We all know, most people with any biological knowledge know that there's something wrong with this picture. What's wrong with this picture, of course, is that these two species would never encounter one another in nature. They also wouldn't drink Coke, but that's not the point here. So, why is that? Well, you would have thought that a polar bear is very well adapted to a cold, wintery environment, why doesn't it live in the Antarctic? Why is it that penguins, which seem supremely good at living in extremely snowy, cold environments and finding fish to feed on, only live in the southern hemisphere? And the common ancestry, this is relatively easy to understand because penguins, for example, you just have to imagine that their ancestors originated in the southern hemisphere, and they have never had an opportunity to disperse through the warm tropics to the northern hemisphere, the northern frozen zone. Likewise, the bears gave rise to the polar bear on the northern hemisphere. There are many, many such patterns. 

Another one I've illustrated, there are hummingbirds, hummingbirds being wonderfully well adapted, nectar feeders, they're restricted to the New World, to North and South America. Even though they ought, by all logic, to do very well as nectar feeders in Africa or in Asia, they're not there. So biogeography is the last of these major lines of evidence. So, to wrap up that point, common ancestry is extremely apparent, and, as a result, evolution is very obvious when looking at all these aspects of biology. So, what about the tree of life? It was in the title so I want to kind of get to it now. So, Darwin very clearly used a metaphor of the tree of life to organize his thinking and his work by biology. So he said that all the beings of the same class are represented by a great tree. I believe this similarly largely speaks the truth. The great tree of life covers the Earth with ever-branching and beautiful ramifications. So he's basically equating the idea of common ancestry very naturally with the idea of a tree. 

Like a family tree but a tree of relationships among species. So what I want to do now is ask the question, why does the tree metaphor work? Why is that a useful way to think about evolution in the big scale? So, it doesn't actually, you can have common ancestry without a tree. You can have a structure such as the one on the left which is sort of common ancestry but it's sort of like a star burst. There's not really branching within from other branches. You could also have common ancestry with some sort of network of relationships. So why do we use a tree metaphor? Remember that what's distinctive about a tree, within a tree branches diverge and then those branches never come back and fuse again. So once two branches separate, they go on their own way which means, for example, that if you pick two leaves at random on a tree, there is only one path that the squirrel could take to get from one to the other. Whereas, in a network there are many paths between any two points. So why does the tree metaphor work? Why is it appropriate? So to kind of explain this, I want to just take a moment kind of getting you to think about the tree and what it represents. I'm going to walk you through how to think about what the tree actually represents in biology, and I think then you'll see why the tree form is expected. 

So, evolution is all about reproduction. It's all about organisms giving rise, parents giving rise to offspring and so on, and passing on their genetic material to their offspring. So I wanted to start this little sort of vision in a field, and just imagine a small group of plants. Five parents giving rise to five offspring. They're exchanging pollen and giving rise to offspring. And now I want to zoom out a second and just expand our picture. Now we're going to look at, instead of just being a little patch in the field, we're going to think maybe a bigger patch. So now instead of having five individuals, we have 15 individuals, and now instead of looking at two generations, we're looking at five generations. It's already getting a little busy on the slide, and so from here on out let's get rid of the organisms. The organisms are relatively transient. 

It's one of those sad realities that in evolutionary times the organisms are transient, and it's the lines of descent that really matter because it's through the lines of descent that the genetic material is propagated. So I've drawn down here the same diagram, but I just zipped out the organisms. So now let's expand, zoom out even further. Now let's visualize not just this sort of field maybe the size of this lecture hall, let's imagine a field maybe the size of this end of campus. So now we've got, instead of 15 organisms, I think we have about 80 organisms here. Instead of five generations, we've got 80 generations. And you can see that as you start moving out, those connections start blurring into just sort of a mess of lines of descent. And that's what we're looking to communicate here that if you go far enough out you don't see the individual lines, you just see the pattern through which genes are passing. And if you kept zooming out, you would find that this little patch of this plant in this part of campus is part of a braided network of populations connected, disconnected maybe for a period of time, and then maybe connected once in a while by seeds or pollen flow. So this structure shown here is a good way to think of a species lineage. 

It's what a species looks like viewed through time. And then what a tree is such a species lineage that has undergone branches events. So, in this case, starting with one ancestral species lineage we've divided to give rise to here four living species. So that's what the tree is. It's kind of basic but it's worth reviewing it. So why, however, do we get a tree? Why is it not a network? Notice it is a network in here. This is a network. But that is a tree. Why do we get a tree? So I flipped it over to look more like a tree. So time is now running from the bottom to the top. 

And let's ask the question, what happened there? Why did lineages split? So it turns out that we now understand that the main reason lineages split is because they get separated by external forces, geography. So to give you an example, imagine we have a widespread population lineage before this split, before here. And I'm just showing it with little trees. And imagine that there's a canyon that's worn through this population. So the population is now split into two populations, and they're separated far enough that genes are not readily able to flow from one side to the other. If you wait long enough, you expect changes to accumulate on the two sides of the canyon. So some of those changes might be visible as represented here. So what's happening here is that the two lineages are now evolving independently, and if they're independent when a mutation arises in one and it maybe increases in frequency and becomes fixed in one, that mutation is not able to get into the other one and vice versa. So over time those two lineages are guaranteed to begin to separate. So as long as they remain separate they will become more and more different the longer you wait. 

The reason that is significant is that if they wait long enough, if they're isolated for long enough, they will lose the ability to ever fuse again. And that is because the ability for organisms to reproduce sexually is rather complicated. It requires a certain match between the communication systems of the two individuals that are going to reproduce. It requires that their reproductive structures match and fit and it all works. It requires that the zygote produced in the next generation, the genes are compatible, it can develop properly, and that it can be itself able to reproduce. And so I think most of us kind of know intuitively that if you wait long enough like the lion and the tiger, they actually cannot reproduce to make fertile offspring. They can make offspring, they're just not fertile. And if you wait long enough, any two lineages are guaranteed to not reproduce. And we sort of forget this. Here we have squirrel on a tree. 

We all know without even thinking about it they cannot reproduce with one another. But under the thesis of common ancestry, they do share a common ancestor. So clearly what's happened is the inability to reproduced is inevitably acquired over time, and that's why the predominate pattern in evolution is a tree. It's not a network because reproductive isolation arises inevitably given long enough. Okay, so to kind of encapsulate that, the tree for was inevitable because lineages are going to become separated during evolutionary history. And once they're separated, they are almost inevitably going to acquire, eventually acquire an inability to integrate. So to give you a way to think about it, think about languages, the ancestral language for French and Italian was a single language. Those two populations, the Italians and the French, were separated long enough. They were speaking internal to the group. There was no pressure to not speak to the other group, but nonetheless, you come to the present, and a Frenchman and an Italian can't communicate. 

So it's the same basic principle with evolutionary lineages. Inevitably they lose the ability to communicate, to reproduce. >> Except with gestures. >> Yeah. Well, gestures will work anyway. >> Why did this vary from species to species? We know now that polar bears and brown bears can hybrid. And all canines-- >> So the question was why do they vary from species to species. It actually does vary in the sense that the rate in which two lineages lose the ability to interbreed is going to very depending on how complicated their mechanisms of interbreeding are and how prone they are to change. So in some groups, like in plants, we know that plant lineages often retain the ability to interbreed for quite a long time. So you can actually force a cross between almost any two species of orchid even though their last common ancestor might be many millions of years ago, where some other groups very quickly lose the ability to interbreed, like fruit flies are known to be quick. 

Okay, so what we've learned from all this is that there is this tree of life and it is largely a tree. There are a few of these naughty reticulations but it's very tree-like, and that it connects all of life. That pattern I just showed were those little plants in the plot, you keep blowing out, you keep stepping out, zooming further and further out, eventually you get to the entire tree of life that connects every living species around us today. Every living species around us today is, every living organism around us today is connected by an unbroken line of parents, grandparents, and so on, back to common ancestors that lived probably three billion years ago, an unimaginably long period of time. So it's this kind of, to me, really wonderful insight that we have in biology. It explains why we have a science of biology because all of life is part of the same tree. We don't have separate biologies for each kind of organism. We have one biology because we have one common ancestry. And I personally find it very exciting to think of the connectedness, the unity of life, but I can also understand why this is a cartoon that I've been given permission to use which, I think, captures the fact that for many people it's actually an uncomfortable idea, the idea that we share common ancestry with all other life forms. But, as I said, you can flip it around very easily and say it's great, it's useful, it means that research done on a mouse is relative to medicine in humans. 

Indeed, research done on a plant can be relative to humans or bacteria. And so I think it's something that biologists are very comfortable with even if not everybody in the public are comfortable with this idea. Okay, and the exciting thing for me as a practicing scientist in this discipline is that over, particularly, the last 20 years, thanks to the ability to readily sequence DNA, we've made great strides in putting the tree together, in figuring out where each species fits on this great tree. And it is really a triumph of modern science that we can, almost without fail, tell you where a species fits on the tree of life. Okay, so I hope I've showed you so far that there is a tree and why there's a tree and that it was important as a lynchpin for evolutionary thinking. So what I'm going to sort of, my last little section of this talk is really trying to get across the idea of why it's actually important to communicate trees today, and particularly in an educational context. Why is it important that I come out and tell you about trees and tree thinking. The tree metaphor are very old ideas. These are some things you'll find if you look up the tree of life on the Internet. Various mystical ideas. 

But, of course, today the tree of life is used as a catchphrase to refer to this idea of the connectedness of all life through common ancestry. So why is this so important? >> Were those Hebrew words? >> Yes. Yes, they are. That's Hebrew. Appropriate for today, I guess, given that it's Hanukkah. So why is the tree metaphor so important? The main reason that I think it's so important is I would argue that the only way to think clearly about evolution at large is to use the tree metaphor and to deploy what I call tree thinking. And so one of the activities I'm engaged with pretty actively is trying to develop ways to assist teachers to teach tree thinking, which I'll come to shortly. So why is tree thinking important to avoid misconceptions? Well, it tends to be pitted against an alternative metaphor. So the tree metaphor is a way that evolutionary biologists think about evolution. I would contend that most non-evolutionary biologists, their mental image of evolution is not that dissimilar from this other idea called the ladder of life, or scala naturae, which goes back really to the Greeks, to the ancient Greeks, but it proposes that there is this sort of series of living things of different degrees of advancement. 

And so people spend a lot of time, in the 18th century especially, trying to construct these hierarchies of advancement, this ladder of life. And so they would play these games to try to connect things into one unbroken line where you do clever things like you would go from worms to insects. Shellfish, they've got kind of a shell, they link you to reptiles. Snakes conveniently link you to eels which link you to fish which link you to flying fish which link you to birds and so on. But of course the clear theme in all of these ladders of lives was that there were certain organisms that were advanced, superior, and of course in every case among the living things, the top was humans. And in many models you would continue beyond humans up to angels and demons and so on. You could also proceed down below what were perceived to be the lowest form, plants to minerals and non-being. So this is the competing view, and I think you can see if you present it like this it seems very foreign. But nonetheless, this basic way of thinking about evolution as a story of advancement is absolutely embedded in our culture. And it is, I would contend, the way that evolution is conventionally presented. 

So if you look at cartoons about evolution, and there are almost an infinite number of them, they tend to have something like this. They tell a narrative story of advancement, often with a little joke at the end of regression of some kind, but they usually depict organisms that, at least in a cartoony sense, resemble other living organisms. This looks like a living monkey. This might be argued to look like amphibian. But this certainly looks like a living fish. And you can see these all over the place. Here's a number of other examples all showing ancestral apes that look very much like modern apes giving rise to moderns humans. And this is, I think, partly in the culture. I think you probably all recognize it. This is how we tell the story of evolution. 

It's advancement with humans at the end. But it turns out this is an inaccurate way, there's another manifestation of it, even when people draw trees they often add a kind of, editorialize them with a certain property. So you'll see here that here we have this lineage here. There are the vertebrates right at the top there, and these are kind of drawn as shorter branches. They're not actually ancestors but they haven't changed much. And if you actually look at, this is a famous drawing from Haeckel who drew a lot of these beautiful trees. There's mentioned man at the top up there, of course. But if you look along this stem, you find a lot of living things that are perceived to be ancestral to us. We're the top, everybody else is down there. So this is a very -- way of thinking, and the question is, why is ladder thinking so abundant? I'm not going to quiz you on this, but one explanation could be that people just don't really understand evolution and that certainly there is some of that out there. 

I think a very important point, as I do think we have a predisposition to linear narratives, if you open a typical novel, it has a beginning, a middle, and an end. It doesn't have one beginning and 20 ends like a tree does. As I'm going to show you in a moment, tree thinking, I think, is hard. So the bottom line is, I think, that for all these reasons, trees have not permeated into the common understanding of evolution to the extent that they should if we want people to really understand evolution, evidence for evolution, and the connectedness of life. So why is tree thinking so hard? Well, the first thing that is counter-intuitive for many people is that if there is a tree, all tips of that tree, all living species that are around today, are equally evolved. All of them. They've all been around the same amount of time. Take a common ancestor of two living things and you say, well, they had a common ancestor at some point in the past, those two things are equally distant in time from that common ancestor. They've all been equally vetted by selection. So there really is no grounds for looking at any one tip and saying it is more evolutionary advanced than any other. 

I'm not going to read that quote but it just gives an example of a good way of thinking about it, that of course humans are going to put ourselves at the pinnacle because we're humans, but there really isn't a biological basis for doing that. There isn't an evolutionary basis for doing that. I think this is put very clearly by Asa Gray. Asa Gray was the father of American botany at Harvard University, and he wrote in a letter to Darwin, "We have, that I know of, no philosophical basis for high and low." So what he's basically saying is when we look at these organisms that are conventionally interpreted as being primitive, so bacteria, amoeba, lungfish, in fact, in an evolutionary sense, they are no lower than one another nor than Asa Gray himself. All living things are equally the tips of the tree of life. So evolutionary advancement we have to get rid of, and I think people find that hard which is one reason that tree thinking hasn't permeated as readily out of the evolutionary biology community itself. Another implication, or a closely related implication of tree thinking, is that there is no one pinnacle or purpose or privileged point for evolution. Mark Twain, brilliant kind of sardonic way of putting it. If the Eiffel Tower were now represented the world's age, the skin of paint on the pinnacle knob at its summit would represent man's share of that age. And anybody would perceive that that skin was what the tower was built for. 

So that's a challenge to us in our conventional way of thinking. And then it also challenges us in a more biological way which is that it challenges certain conceptions that we might have about organisms and how they are related to one another. So relatedness in every day culture refers to actually how recently you share a common ancestor. Your degree in kinship with your first cousin is greater than with your second cousin because you share a more recent common ancestor, namely your grandparents, with your first cousins than you do with your second cousins, which is your great-grandparents. And, yet, relatedness, when we turn to biology we have a tendency to equate relatedness with similarity, but they're not the same thing. To give you a concrete example here, this is the phylogenetic tree, the evolutionary relationships that are well-established now for the reptiles. So here are the turtles. There are the lizards and snakes. There's the crocodiles, and there are the birds. Now, what this tree says is that the crocodile is more closely related to the bird than it is to the lizard. 

And that's because there's piece of its evolutionary history down here that is uniquely shared between the bird and the crocodile that's not shared with the lizards and snakes. And this is actually supported by a couple of traits including laying and defending nests which is shared between crocodiles and birds. But I think anybody would agree that in terms of similarity, a crocodile is more similar to a lizard, just superficially at least, even though it's more closely related in evolutionary terms to the birds than it is to the lizards. So there's another sense in which trees are challenging, they challenge our preconceptions. So here's a question. I'll do a quick count. I'd actually like you to look at this a second. Here's a tree and this represents relationships. I showed you, basically, this tree earlier. So the question I just want you to think about for a second is given this tree, assume this tree is correct, is a frog more closely related to a trout or a human? Now, I'm going to take a little look out and see what people say. 

If you think it's a trout, maybe you can do a sign like this. If you think it's a human, you can maybe tap your own chest. Okay? And then I can get a sense of the vote here. I'll give you a second to do that. What do you think? Is a frog more closely related to a trout or a human? Okay, I'm see, actually, a mix. I'm seeing both. I can tell you from experience having done this many times in classes and other situations that this audience is a better educator than average audiences. I can tell you that because it was about 50/50. In a typical audience, 80% of people would look at this tree and they would say a frog is more closely related to a trout than to a human. But from an evolutionary point of view, a frog is more closely related to a human than it is to a trout. 

So let me explain that just to give you a feel for how this works. To ask the question of whether the frog is more closely related to the human or the trout, you have to find the common ancestors. So let's first find the common ancestor of the trout and the frog. There it is at the bottom of the tree. Now we can find the common ancestor of the frog and the human. There it is. Since this common ancestor is a descendent of this common ancestor, it's closer to the present, the present's at the top, the time is down here, so this common ancestor is a descendent of this common ancestor, that shows us that according to this tree the frog is more closely related to than human than it is to the trout. Now, my point of showing that exercise is not to teach you how to read trees, although I do like to do that in classes, my point was to show you that for many people it's not obvious how to read a tree. Trees are surprisingly tricky. And this is why I'm a believer in the importance of educating at various levels, college level and at high school and before, children and everybody, to think clearly about trees because it's not easy. 

It takes actual effort to do this, but by doing it, one achieves a much clearer understanding of evolution and avoids some of these important misconceptions. So what I would say looking back over the 150-year story that I've sort of packed into 50 minutes is that it took 150 years for us to actually realize that the common ancestry part of The Origin of Species is really important, and we shouldn't forget it underneath this mess of discussion about natural selection. And that because evolutionary trees are becoming so important in functioning biology, from things like tracking diseases, determining how different traits evolved, that we should be making much more of an effort, and because trees are difficult we should be making a real concerted effort to help people understand evolutionary trees and their place in modern biology. So this is something that I'm engaged with and concerned about, and so me and my group and a lot of colleagues here on campus, we spend quite a lot of time working with teachers, doing workshops for teachers and developing curriculum materials, like pipe cleaners are a great way to introduce the concept of a tree and how to read it and not read it. And my hope is that over the next 10-20 years, we will gradually get good tree thinking introduced not only into the college level but into the K-12 level. The hope being that when I see students coming into an intro biology class at the college level, they will not have this ladder thinking view stuck in there too strongly, and instead they're going to have a greater ability to see that the tree of life is made up of many living tips, all of which have a certain equality to one another, none of which is more advanced or the pinnacle of life, and that evolution is ongoing, that all those lineages are continuing the evolutionary process even now. So that's sort of my hope for the future. So I thought I would just end, I know I've read a lot of quotes, but I just think that I couldn't do better than ending with a quote, the last sentence, I began with the first sentence of The Origin of Species, and I'm going to end with the last paragraph. So this is how Darwin ended The Origin of Species. "It's interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp Earth, and to reflect that these elaborately constructed forms, so different from each other and dependent on each other in so complex a manner, have all been produced by laws acting around us. 

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one, and that whilst this planet has gone cycling on according to the fixed law of gravity from so simple a beginning, endless forms most beautiful and most wonderful have been and are being evolved." So I will leave it there. Thank you for your attention. [APPLAUSE]