So this lecture, we're going to talk about Evolution. And let me begin by saying a few words about my man here, Charles Darwin. Darwin was able to think through the principles of evolution, without understanding the underlying biochemistry. One of the implications of this, is that the rules of evolution, really don't depend so much on the details on how DNA works. We can actually think of evolution operating not just on selecting for cells on bacteria, or selecting for individuals. But even selecting for ideas, or companies, and we can apply these principles of, of evolution, when we think about life on other planets. So today, I want to begin by talking about the inevitably of evolution. Then turn, and talk a bit about sex, and the role it plays in evolution. And finally, talk about the tree of life. And this is of course a very rich subject. And we only have time to devote one lecture to it. But I want to hit a few of the highlights of this, area and think about some of it's implications, for astrobiology. So, let's begin thinking about the basic principles of evolution. We often think of evolution as, the survival of the fittest. What evolution is doing, is it's selecting for the ability, to survive and reproduce. The key elements for evolution are going to be, we need something that, replicates itself. When it replicates itself occasionally undergoes mutation, so there's some variation. And then the environment will select for whatever variation, is most best able to survive. And reproduce, because those inheritable traits will be passed on. Now there's a rich debate and discussion about thinking about evolution at the level of genes. Do we select for individual genes? Do we select for individuals? Or does it make sense to think about evolution happening sometimes at the group level? People usually focus on evolution at the individual level. But it's an interesting discussion, that out side of what we want to talk about in the course, about what happens if these simpler are more complex levels. As a physicist, I think about evolution as we call effective field theory. What do we mean by this? When I describe the physics of atoms, protons, and electrons, combining, and neutrons combining to make atoms. I don't need to know the details of quarks and gluons. The sub components of protons and neutrons. We'll talk about atoms. I can just talk about protons, and neutrons, and electrons interacting. If I want to understand how atoms work together to make. molecules, I don't need to understand the details of their subcomponents, I can work at the next level of extraction. I can ignore the previous level, and work at a simpler version of the theory. That's what they call, physicists call "Effective Field Theory." If I want to think about. The gas in this room. I don't need a theory that describes the detail properties of how every molecule moves. I can just talk about the density, and temperature and pressure of the gas. The same is true of evolution, I don't need to understand. How molecular biology works. I don't need to understand the complex biochemistry of cells, but get an understanding of the principles of evolution. After all Darwin didn't know about DNA, yet was able to derive the basic principles. And I think we can extrapolate this to other systems. If life uses a different bio chemistry somewhere else. As long as they're individuals that reproduce, and have traits that are passed on, we can think that evolution will happen elsewhere, we just require reproduction. Inheritance of traits, and mutation. In fact, people think about evolution happening even on ideas, things like memes. You can think about how religions evolve and change through time. You can think how philosophies change and political systems. All those things can be thought of as evolving in some ways. And at some level as long as they satisfy these conditions, some of the principles of evolution can be applied. So when we think about life on other planets, and I hope some of you will be doing the final assignment at this point and are starting to think about, how life might evolve on your planet. You don't need to know your life's biochemistry, to think about applying these basic principles. Is life even necessary for evolution? I would argue not. You can think about things like crystal growth, where you can say, if I'm growing crystals. And the crystals that are more stable are better, are able to grow faster, absorb more chemical from the solution. Those crystals will grow, and there will be an evolution like process, that will select for it. We can form things like cellular automoton. These are simple computer games, where you have simple rules that allow things to propagate, and even these simple systems propagating within a computer. We can apply the principles of evolution as long as they undergo this process of inheriting traits. And having some of these traits being selected for. If we make up rules of the game like that, we can apply these evolutionary computer, principles to even to these, computer games. Now, how do these evolutionary principles operate? So the key parts is first we need a variation population properties. These variations can happen during mutation. Or due to sex where we have two individuals mixing, their genetic properties. And as you'll see that's going to be in the next section why sex plays an important role in evolution. But to understand evolution we can just think of prokaryotes and think about mothers and daughters. If we have a mother that has several daughters. If there's some process of either, say, radioactivity induced mutation. Or mutations induced because as the RNA, or DNA copies itself. That there's an error in the copying taking place, so as it copies, the new copy isn't perfect. It deviates in some way. There will be a variation in traits among the daughters. Now most mutations are unfavorable, and these unfavorable mutations are selected against, because this daughter here has some trait that does not help it survive. In fact, makes its survival less likely. And if it's less likely to pass on to survive and reproduce, those traits will be eliminated by evolution. Sometimes you'll have traits. That under a given environmental condition are neither selected for nor against. In this picture here, these two daughters have slightly different traits. But this difference in trait does not affect survival. So both daughters will pass on their traits, but then sometimes later on, it might turn out that this mutation away from the original line, turns out to be favored in it's environment, and this favorable trait will enable it to survive. And the population will evolve. So, this combination of variation, and then selection, are what's essential for driving evolution. Now, if there's no selection process. Then the properties of a species will stay the same, if we have a large enough population. That's the really what's the Hearty Weinberg theorist is telling us and a large population with if, properties aren't selected for or against stay at about the same level. If there's no selection preferring brown eyes, blue eyes, or blue eyes over green eyes. The fraction of a population with a given eye color will stay constant, if the population is large enough. Now when a very small population we have. Very few offspring. Small number statistics come in. And you can have some genetic drift. But overall, we expect changes in population properties to be driven by evolutionary selection processes. Now, we can look at evolution in the fossil record. And, Perhaps classic example is looking at the evolution of horses, where over the last 60 million years they went from a very small horse, that lived in the forest to today's large horse that is evolved to live in grass land, they're larger animals. They're faster, they've evolved to eat grass rather than foliage. And this evolution is a process that we've been able to trace through the fossil record. I'll be it with some missing steps we can watch as we go from this ancient Ancestor of the horse to a larger animal. We can see the change in its bone structure. We can see the change in its teeth, as evolution selects four form. That can best survive in the growing grasslands of the world. And this is sort of the picture, that many of us have in mind when we think about evolution. Usually we think of evolution as something that happens. That's happened to the past, and something we study through the fossil record. But there have been some fascinating experiments, where people can watch evolution happen in the lab. And to me, one of my favorite versions of this is an experiment led by Professor Lensky. Who, for the last 25 years, has been watching evolution happen in his lab. He has been following 12 distinct populations of E.Coli for over 50,000 E.Coli generations. That's long enough that. Every single point mutation that's a mutation that happens at one location in the DNA. Change for the E.coli. Every mutation that can happen probably has happened. And in his experiment he's watched the properties of the E.coli change. He has put the E.coli in a minimal growth medium, one in which they're not going to grow so fast, so that there be a selection process that would enable these species to change, if their traits changed a bit, they would be able to propagate better. And what has happened over time, is that, in each of these 12 separate groups, the cell sizes have grown. As mutations take place independently in each of these groups, that make a better E. coli. To survive in this particular environment, and a form of e coli that is more thick, and what's really interesting is back in 2008, one of the populations had a mutation that let it utilize citrate. This is the stuff that was used to, not as a food source, it wasn't planned as a food source. It was used to as a fixer for the medium of Sudan, and this bacteria that one of them underwent two lucky mutations, that allowed access to a new energy source. This has happened in only one of the samples, and I think this shows the importance sometimes of historical contingency in evolution. Sometimes you have to have the right mutations in order to take advantage of some new niche. Or have the right mutation that you're lucky enough, that when the environment changes, you happen to have some trait that all of the sudden is selected for. Another fascinating place to study evolution, is some of the classic sites like the Galapagos, where you have an isolated place where you can watch populations evolve. A wonderful book about this that I recommend is Jonathan Weiner's Beak of the Fish, Fitch, that follows Peter and Rosemary Grant, two of my colleagues here at Princeton, and looks at how birds have evolved in this very small population. We can study how evolution works. And watch it in progress. No, of course we also get to see evolution in progress, when we look at what happens with viruses and vaccines. Viruses can develop immunity. To vaccines, bacteria that developed immunity to vaccins, to vaccines, and unfortunately, evolution will select for things that survive against our treatments. So, what are the ingredients of evolution? We've talked about one so far. We've talked about the fact that. You want a selection pressure that selects for certain traits. But you also need something that creates a range of traits. You need something that causes mutations, and we've talked about, two processes that generate them. One is transcription errors, when I make copies of my RNA and DNA, mistakes are made. These happen more often in RNA which is why I'm going to look at something like a flu virus the few flu virus under goes fairly rapid mutation, this years flu is different from last year's few flu. Is as multiple transcriptions occur, errors happen and new forms of the flu virus are constantly appearing. Now sometimes transcription errors are in your favor. And there are conditions in which you want to have rapid mutation rate. Other times, you'd like to not suffer such a high mutation rate. And we'll talk about,uh, when we talk about extreme o files, radio durans. Which has a very low mutation rate, and effectively an error-correcting code, because it wants to be able to survive in environments that are very harsh, including things like radioactivity. Most mutations, we should keep in mind, are almost always either neutral,. Are more likely negative. You don't want to start to do what the superheroes do in those movies, swallow that vat of radioactive liquid. The mutations it produces are very likely harmful, probably carcinogenic. It is only the rare mutation. That is going to produce a new trait that will be selected for. What are the evolutionary pressures? The pressures, sometimes they're external changes in the environment. Sometimes external pressures you experience are things like the environment getting hotter. Are wetter, more acidic. They're changes that are happening because of broad geochemical processes. But much of the evolutionary pressure that lifeforms are subject to in our complex ecology, is we're always in constant competition with. Are parasites with a predators and the environment is constantly changing. the wolves are getting faster, we have to evolve in order to survive the wolves. Our prey are getting faster. We have to evolve to keep up with the prey. And, we live in this complex environment in which there's constant competition both within our own species, where we have to be the fittest to survive within our species to propagate our traits. And with other species. And, evolution is not happening in a situation where, what people call the fitness landscape. What traits you need to survive are constant. It's happening in an ever changing environment. I think of these principles as things we can apply not just to evolution, but even when we think about things like companies. You can have a company like, you know, IBM, that did very well when the environment was all about building the biggest computer in the world, the mainframe. But suddenly the rules of the game change, and you start having to compete to build personal computers, and then that might be an environment where a company like an Intel or a Microsoft can thrive, and then the environment changes and you have to start competing to see who can make the best, Product that will work on a cell phone. And the rules of the game is constantly changing, and your competitors are constantly trying to figure out ways in which they can outperform you. And I think of one of the best books on business that I've ever read. Andrew Grove's only the paranoid survive. Talking about his work at Intel, as talking about a process that's a lot like evolution. The rules of the game are constantly changing. The fitness landscape is changing as the other players in the game, are changing the fitness landscape and. Evolution happens in this changing environment. It actually means, if you think about what life would be like on a planet with one species. If there was only one life form, evolution would be much simpler. Probably much slower. There would only be competition between individuals. The predators wouldn't be changing. The prey wouldn't be changing. You'd have a fairly stable fitness landscape. Our world is much richer. Much more complex. Different species evolve at different rates. What's interesting when you look at the evolutionary history really shocking in some ways, to me is given how much the environment is changed the fact that other species are constantly evolving, there are some life forms the horse shoe crab, the tad pole shrimp. That seem to have found a really good niche for themselves, and despite the fact that the world has changed tremendously, the environment itself has changed, temperatures of the planet have changed, other species have changed a great deal, they've kept about the same form. You know, the horseshoe crab has been around at least a hundred times longer than the human species. It's found something it's really good at doing. And it, despite, you know, as the world changes, it's still the right form to have, it seems. And when I think about, the enormous stability of things like a horse shoe crab, it's interesting to contrast it with how rapidly genetic change can happen. Dogs have only been domesticated for. About you know, ten thousand years ago. And most species, most of the breeds of dogs, same species, but most breeds of dogs are very recent, many of them, have appeared in the last 100 years. And. This shows how with the proper, now this is of course done with human selection where humans are making the choices, that made one dog look like a bulldog, another one poodle. And yet another one collie. They were selecting for traits. In their ancestors. But with in a relatively few generations, we are able to radically change the appearance of a given type of dog. This shows how fast evolution can operate. And this is without genetic engineering. Right these various species of breeds of dog arose. Just by human selection of their parents and and selected breeding. Now of course selective breeding and genetic engineering, I always think of as rather close, you know. To shock those of you who don't like genetically modified organisms. Basically every piece of food you've ever eaten was genetically modified by our ancestors, when they selectively bred say, wheat or corn or. Cattle. Those are all products of selective breeding, they're all products of genetic engineering. All right, now that I've, probably going to get a few emails about that comment. Let me now switch, to other thoughts about evolution of ideas... And this is a neat notion that goes back to Richard Dawkins, where he said, we should think about evolution operating on a level the level of memes. And he said, instead of thinking about genes, the unit of transmitting genetic information, he's talked about memes, the idea of transmitting cultural transmission. And since Dawkins is Famous atheists it's fun to apply his ideas to religion and you can think about certain elements in religion being selected for as a base something that will work if you want to keep propagating your religion. Many religions promote the idea of being fruitful and multiplying. If you have a religion that encourages you to bring up children in your religion, and encourages them to have children that idea will propagate. On the other hand if you take the route of the Shakers, and encourage people to not have children. You tend to have much fewer followers. There are things that could be selected for. You can choose different transmissions mechanisms. Mormonism is spreading right now because many missionaries go out. Islam once spread. Through the sort. Very quickly, there are different choices for transmission. And I've wondered sometimes if we think about this at the evolutionary level, when you think about why religion is so pervasive, is perhaps, at some level, the belief in God conveys some evolutionary advantage. I don't think, this is pure speculation. Complete speculation. But, it's interesting to think about selection happening. At the level of ideas, or companies, and the fact that we can apply some of these principles broadly, is why I think we can think about evolution happening at, regardless of what the underlying biochemistry is we think about life elsewhere. So, let's stop for, step back for a second and think about the way evolution acts and I want you to think about the way evolution affects viruses and how they propagate.
