The Nature of Evolution: Selection, Inheritance, and History

Evolutionary tree / OneZoom

Lecture by Dr. Stephen Stearns / 01.12.2009
Edward P. Bass Professor of Ecology and Evolutionary Biology
Yale University


Biological evolution has two big ideas. One of them has to do with how the process occurs, and that’s called microevolution. It’s evolution going on right now. Evolution is going on in your body right now. You’ve got about 1013th bacteria in each gram of your feces, and they have enough mutations in them to cover the entire bacterial genome. Every time you flush the toilet, you flush an entire new set of information on bacterial genomes down the toilets. It’s going on all the time.

Now, the other major theme is macroevolution. This process of microevolution has created a history, and the history also constrains the process. The process has been going on for 3.8 billion years. It has created a history that had unique events in it, and things happened in that history that now constrain further microevolution going on today.

That’s one of the tricky things about evolution. It has many different scales. My wife always gets frustrated with me. She says, “Well when did that happen?” I say, “Oh not too long ago, only about 20 million years.” And, you know, that’s what happens when you become an evolutionary biologist, you zoom in and out of deep time a lot.

History of Evolutionary Studies

Portrait of Alexander von Humboldt, by Joseph Karl Stieler, 1843, oil on canvas / Schloss Charlottenhof, Potsdam

So where did this idea of evolution come from? Well, there are always ideas. You can go back to Aristotle and find elements of evolutionary thought in Aristotle. But really it’s a nineteenth century idea, and in order to see how it developed let’s go back to about 1790 or 1800; so at the end of the Century of the Enlightenment.

At that point, if you were to ask a well-educated person living in a Western culture how old the world is, they would say, “Oh thousands of years.” And if you were to ask them, “Well, where did all these species on the planet come from?” they would say they were all created just the way they look now and they’ve never changed. And if you asked them, “Have there ever been any species that went extinct?” they would say, “No, everything that was created is still alive and can be found somewhere on the planet.”

So when Alexander von Humboldt, who was certainly a creature of The Enlightenment, sets out to explore South America, he thinks that he might encounter some of those strange fossils, that the French have been turning up in the Paris Basin, on top of Tepuis in Venezuela. So he really thought that there was a lost world. Of course, Arthur Conan Doyle later wrote a novel about that. But these guys actually thought, “Hey, I go to Venezuela or I go to the Congo, I might meet a brontosaurus.” That was what they thought at that time.

They thought that adaptations were produced by divine intervention. They did not think that there was a natural process that could produce anything that was so exquisitely designed as your eye. We now know that your eye is in fact very badly designed, but it looked pretty good to them. What’s wrong with your eye?

The nerves and the blood vessels are in front of the retina. The light has to go through the nerves and the blood vessels, to get to the retina. The octopus has a much better eye.

Photograph of Charles Darwin; the frontispiece of Francis Darwin’s The Life and Letters of Charles Darwin (1887) has the caption “From a Photograph (1854?) by Messrs. Maull. And Fox. Engraved for Harper’s Magazine, October 1884. / Wikimedia Commons

Okay, now by the time that Darwin published his book in 1859, people thought that the world is very, very old; how old they weren’t sure. We now know about four-a-half billion, but at that point, based on the rate of erosion of mountains and on the saltiness of the ocean, assuming that the ocean had been accumulating salt continuously, and that it hadn’t been getting buried anywhere, which it does, people thought hundreds of millions of years. They weren’t yet in the billions range, but they thought hundreds of millions.

They knew that fossils probably represent extinct species. That was Cuvier’s contribution. He did it for mammal fossils in the Paris Basin. Geoffrey Saint-Hilaire had had a big debate with Cuvier about homology, and that was in 1830. By the way, it was one that many people throughout Europe followed very closely–this was a very, very key intellectual topic at the time–and it was about homology. Basically it was about the idea that Geoffrey Saint-Hillaire had had that if my hand has five fingers then–and a bat’s wing has five fingers and the fin of a porpoise has five fingers–that that indicates that we all got those five fingers from a common ancestor, and therefore we are related because we had a common ancestor.

So you could see that in 1830. That’s before Darwin publishes his book. Okay? Then of course we have the idea that adaptations are produced by natural selection; and we owe that to Darwin. And I will run through the process he went through between 1838 and 1859 very briefly. This is one of the most important ideas about the nature of life, and therefore about the human condition, that’s ever been published, and I strongly recommend that, if you have a chance, read The Origin of Species. Darwin actually was quite a good writer. It’s Victorian prose, so it’s a little bit like reading Dickens. But it’s good stuff, he has a nice rolling style.

How did he come to it? Well Darwin was a med school dropout. Went to Edinburgh, didn’t like med school; loved beetles and became passionate enough as a naturalist to become known, as a 22-year-old young man, as a guy who might be a good fellow to have on an expedition. And the British Admiralty was sending Fitzroy around the world to do nautical charts and Darwin got on the ship.

So at an age not very much greater, or perhaps even a bit younger than some of you, Darwin sets off. He’s 22 years old. He wants to know how species form. He has set himself that goal. So he’s ambitious. He’s set a clear goal. The goal is to solve one of the most pressing problems that biology has at that time: where do species come from?

Now the stimulus that he has is in part from Charles Lyell, the geologist, who had discovered deep time, and that convinced Darwin that there would’ve been enough time. He stops in Argentina. In the banks of a river in Argentina he can see giant fossil armadillos, and then right on top of that same bank he can see the current armadillos walking around, up on top of the bank. There they are; the live ones are right above the fossil ones. They look the same but–I mean, they look similar–but they’re not the same. So there’s some connection there.

He gets on a horse in Chile and he rides up into the Andes and he sees marine fossils lifted thousands of feet above sea level; clearly some dynamic process is going on that had lifted those marine fossils up. He doesn’t know about continental drift yet–right?–but there the fossils are.

In the harbor at Valparaiso he sees the effects of an earthquake that had happened just before they arrived. It was a big one. It was probably as large as the earthquake that recently caused the big tsunami in Indonesia–so it was probably an 8.5, 8.6 earthquake–and it had caused an uplift in the harbor of maybe 50 feet. So he began to see the world as dynamic. Things hadn’t always been the way they are.

Then he goes to the Galapagos, and please navigate the Galapagos website and have a look at some of these differences. The thing that Darwin noticed is that the mockingbirds are different on the different islands. If you go to the Galapagos what you’ll notice is that if you land on Espanola, the mockingbirds really want your water supply, and they will hop onto your head or your knee to try to get at your water supply. But, in fact, the mockingbirds all look a little bit different on the different islands, and that’s what Darwin noticed.

He could also see that that the marine iguanas look a bit different, and the land iguanas look different. Interestingly, he didn’t notice the differences in the finches, until he got back to England and gave his collection to the British Museum, and the ornithologists at the British Museum came in and said, “Hey Darwin, do you realize that the finches on these islands are different?” And that was when he began to really see how many differences could accumulate, how rapidly, when you take a migrant from Central America and put it on an isolated archipelago.

HMS Beagle in the Straits of Magellan at Monte Sarmiento, reproduction of R. T. Pritchett’s frontispiece from the 1890 illustrated edition of The Voyage of the Beagle. / The Popular Science Monthly, Volume 57, London

So he goes back to London. He’s been onboard ship for about four years. He has a problem with seasickness. He never again sets foot on a ship. He doesn’t want to go near the water after being four years on this ship. He had a few issues with the captain too, Fitzroy, but mainly it was that he had a very bad upset stomach onboard the Beagle.

He reads the Reverend Malthus on population growth. Malthus’s book had come out in 1798. Malthus said basically that populations grow exponentially but agriculture grows linearly. Therefore populations will always outstrip their resource base. This convinced Darwin that all organisms are in a competitive struggle for resources, and that that must inevitably be the case. He saw very clearly how powerful reproduction is at generating exponential population growth. We will come back to that in the ecology portion of the course.

And we now know that organisms are in competition really essentially not just over food resources, they are in competition over anything that will get their genes into the next generation. So that can be competition for mates. It can be competition for nesting sites, competition for food; lots of different things. But at any rate this primed Darwin’s thinking. So he writes down the idea of natural selection. It comes to him in 1838; it’s in his notebooks in 1838.

Basically, I’ll run through natural selection in a minute. It’s a deceptively simple idea because the mechanism looks so simple, but the consequences are so wide ranging. Darwin recognized what the consequences were. And he didn’t publish immediately. He did other things. He went off and he worked five or six years on barnacles. He wrote down lots of ideas about things unrelated to natural selection, and he wasn’t really jogged out of this until a letter arrived in 1858 from Alfred Russel Wallace, a young British naturalist who had, in a fit of malarial fever, had the same idea, in Indonesia.

And Wallace knew that Darwin had been thinking about these things, and he sent Darwin a letter. And at that point Darwin, British gentleman as he was, had to decide whether he would do the sort of gracious, honorable thing and let Wallace have the idea, or do the honest thing, which, his colleagues knew, was that he had already had the idea. And what they decide upon is that they will do a joint publication.

So if you go to the Biological Journal of the Linnaean Society for 1858, which is in the Yale Library, you can look up the back to back papers by Alfred Russel Wallace and Charles Darwin in which the idea of Natural Selection is laid out. And then Darwin rushes his book into print. So he has been working on a book that was probably going to be about 1200 pages long, and instead he publishes an abstract of it, which he calls “The Origin of Species”, which is about 350 pages long. And it sells out on the first day, sold all 6000 copies on the first day, and has remained in print ever since.

That’s The Beagle. Darwin slept in a hammock in the captain’s cabin, at the back of the ship, which rocked horribly. And that’s essentially all I want to do about the development of the idea of Evolution. Basically what I did was I wanted to give you the feeling that there was somebody like you who went out and knew what a deep problem was, and happened to have the luck to get into a special situation where they were stimulated, and came up with an idea that changed the world. No reason it can’t happen again.

Conditions for Natural Selection

Lamarck’s giraffe illustration of natural selection / Wikimedia Commons

So now I’m going to give you a brief overview of microevolution and macroevolution. Here’s Natural Selection; here’s Darwin’s idea. If, in a population, there is variation in reproductive success–what does that mean? Would everybody in the room raise their hand if they’re an only child? Look around. There are about five or six. How many of you come from families with two children? Lots. How many with three? Quite a few. How many with four? Quite a few, but not as many as there with only children. Anybody with five? Yes, a couple. Anybody with six? No. If we were, by the way, in the nineteenth century, at this point there would still be lots of hands going up.

What you’ve just seen is the amount of variation in reproductive success represented by the families in this room. Variation in reproductive success basically means that different families have different numbers of offspring, or different individuals have different numbers of offspring. Then there has to be some variation in a trait.

Lots of variation in height in this room. So we got lots of variation in reproductive success; lots of variation in height. There has to be a non-zero correlation between reproductive success and the trait. On this particular trait there’s been some research. Turns out that taller men have more children. I don’t know whether that’s just an NBA effect or what that is but it turns out to be true in many societies.

So there is a non-zero correlation between the reproductive success and the trait. Then there has to be heritability for the trait. The heritability of height in humans is about 80%. So all of the conditions for natural selection on height are present in this room. All you have to do is go out and have kids and it will happen.

So if you’re ever in doubt about whether evolution is operating in a population, go back to these basic conditions. You can always decide whether it’s likely to be operating or not. We can turn natural selection off by violating any of these four points. If there’s no variation in reproductive success–for example, if there is lifetime monogamy and a one-child policy, there will be zero-variation in reproductive success if everybody just has one child; of course some people will still have zero, but that’s about as close as you can get.

If there’s no variation in the trait–if the trait is like five fingers; there are very few people with six fingers; there are some, but very few. If there’s a non-zero correlation between reproductive success and the trait; if there is a zero correlation between reproductive success and the trait. We’ll go into all the conditions for that. That results in neutral evolution. Okay? Then things just drift. Well have a whole lecture on that. Or if the trait is not heritable, if there’s no genetic component to it, then it won’t evolve.

So Natural Selection-I wonder why it’s doing that? Sorry- Natural Selection does not necessarily happen. It only happens under certain conditions. Essentially in this picture, this is what I’ve just told you about Natural Selection. If there’s variation in the trait, represented on the X-axis, and there’s variation in reproductive success, based on the Y-axis, and there is a correlation between the two, represented by the fact that I can just about draw a straight line between these points, Natural Selection will occur and it will push the trait to the right.

If all of these conditions, except the correlation, occur–so you have variation in the trait, variation in reproductive success but no correlation–then you get random drift. And these two situations result in radically different things. This situation produces adaptation, it produces all of the fantastic biology that you’re familiar with. It’s produced meiosis; it’s produced your eye; it’s produced your brain. It’s extremely powerful.

This situation on the right, the random drift situation, is what connects microevolution to phylogenetics, and it’s what allows us to use variation in DNA sequences to infer history. And I’ll get to that. That statement right now is opaque. Don’t expect that one to be transparent at this point. But two or three lectures from now I will go into that in detail and you will see that we need to have a process of drift in order to generate a kind of large-scale regularity that gives us timing and relationship in macroevolution.

So both are driven by variation in reproductive success. The difference is in whether there’s a correlation between the variation of the gene or the trait and the variation in reproductive success.

The Power of Selection and Adaptation

Azure kingfisher (Ceyx azureus) / Wikimedia Commons

If we have strong selection, we can get pretty amazing things. I could illustrate adaptation a lot of different ways. I could do it say with the leaf cutting ants that were the first farmers; they domesticated a fungus 50 million years ago and have been cultivating it ever since. That would be one way I could do it.

I could do it with the exquisite morphology of the deep sea glass sponges and how efficient they are at filtering stuff out of the water. I could do it with the design of a shark’s body. Lots of stuff.

I’ll do it with bats, in part because when I was a Yale undergrad I worked on bats in this building. We had a guy that did research on bats at that time. Now a lot of bats are insectivores, and they will hunt moths at night, in complete darkness. They do it with sonar.

The bat only weighs about say 50 to 100 grams, and it is making a sound that is as loud as a Metallica concert when you’re standing right next to the lead guitar’s speaker system. Okay? Or it’s as loud, if you like, as a Boeing 747 taking off from a runway. It’s this tiny little creature. It’s making an incredibly loud sound. It’s 130 decibels.

It does that because the intensity of sound, the amplitude of sound, decreases with the square of distance, and it needs to detect an echo coming back from the moth. The echo coming back from the moth–which by the way it can pick up at a distance of about 20 feet–is about a million times less loud, and it’s only coming in about one to two milliseconds later. So imagine, there you are, you’ve gone “woo”–except a lot louder than that–and milliseconds later you hear “click”, and you haven’t deafened yourself.

That’s exquisite. It has all kinds of physiology in its ear to hear the returning echo, and it can actually discern whether or not it’s looking at a kind of a fuzzy moth or a smooth beetle. The moth has all kinds of adaptations to try to get away from the bat. It hears the bat. The bat’s cruising around, the moth hears the bat. The moth goes into a desperate spiral, diving towards the ground–okay–the bat starts to swoop in. There is a mite that lives in the ear of moths. I think you begin to understand the problem that this mite has. If the moth gets caught, the mite will be eaten.

The mite’s solution? It only lives in one ear. If you collect moths and you look for mites in their ears, you will find that they are always only on one side. So the moth always has a clear ear so it can hear the bat. There’s stuff like this all through biology.

There’s another kind of a bat, called a Noctilio, hunts fish. A Noctilio basically detects ripples in the water surface, and then it swoops down and it gaffs the fish with its hind legs. It can detect a wire 1/10th of a millimeter in diameter, sticking 1/10th of a millimeter above the water surface. When I was taking care of bats, I’d never seen a Noctilio. I thought, “God, this must be the greatest bat in the world.”

About four years ago, on the Amazon, my wife and I went out in a canoe, at sunset, on a lake, just off the Amazon River. It was starting to get dark. All day long the kingfishers had been fishing on that lake, and during the day the lake had gotten covered with a lot of food that the fish wanted, but they were afraid of the kingfishers. As it got darker the kingfishers couldn’t hunt anymore and the whole surface of the lake dimpled with the fish coming up to eat the food.

So their timing was exquisite. They knew exactly how dark it had to get before they were safe. The fish came up and started to eat the food. At that point–it was just shortly after sunset–the bat falcons were still stationed around the lake. You could see, up on the trees, falcons sitting up on the limbs and making flights off of the limbs. About 15 minutes after the fish started to eat, it got dark enough so that the bat falcons couldn’t hunt anymore, and at that point Noctilio came out, and the water was covered with hundreds of bats that were catching the fish. They were catching the fish within a meter of us.

Now there are a couple of things about that story that I think, uh, I’d like to underline. One is that that entire community is exquisitely adapted. Every element in it knows when everything is going on and what the risks are, and what the costs and the benefits are. The other thing is that I had benefited from a liberal education, and when that bat came out, and was flying around a meter away from my canoe in the Amazon, my life was so much richer because I had been waiting to see it for 40 years. I had heard about it in a course at Yale. I knew where it fit in. I knew what kinds of adaptations it had, and boy was I happy to see it.


Simulation of a common example used describing the effect random sampling has in genetic drift. Dots indicate samples from each generation that are transferred to the next generation. In this population of 20, there is a shift from an allele frequency of 50% for the blue allele to 100% for the blue allele in just 5 generations.

So adaptation can be impressive. Drift is something that actually appeals to the geeks among us. I have a geeky side too, okay? Drift isn’t such a morphologically or artistically beautiful thing. It’s a mathematically beautiful thing. Drift happens whenever there is no correlation between reproductive success and variation in a trait, and it produces patterns like this.

So here we start off with 20 populations, and we start them all with a gene frequency of 0.5, and we let meiosis–which is like flipping a fair coin–and we let variation and reproductive success take their course, and we just run these populations for 20 generations, and you can see that there’s just about an equally likely distribution of end-states out here. So we all start off at 0.5, and it gets noisy as we go along.

So this is an image of the process of drift, and if any of these populations happens to get up to 1, or down to 0, in terms of gene frequency, the process will stop, because those are absorbing states. If the frequency becomes 1, then everybody’s got it and there can’t be any change, and if the frequency becomes 0, then nobody’s got it and there can’t be any change. So that’s what’s meant by absorbing state.

Now to a first approximation, whole organism traits are the products of Natural Selection. Maybe not in the immediate past, but usually at some point in the history of life, a whole organism trait will have been under Natural Selection. So it will have been shaped and designed by this process. And to a first approximation, a lot of DNA sequences have been shaped by drift. So we see design in the whole organism and we see noise in the genome–to a rough cut; lots of exceptions.

There are DNA sequences that have clear selective value; in fact, there’s a whole literature on this now. If any of you want to write an essay on signatures of selection in the genome, you can find lots of stuff on that now, on how to recognize that a chunk of genome has recently been under selection. There are whole organism traits that have no apparent selective value; for example, the chin.

The chin actually is the result of evolution, operating on development, to take a face, which is like that of a gorilla or a chimpanzee, which bulged out like this and essentially flattened it out; so that we are vertically much flatter than a chimp or a gorilla, and as a result of this being pushed back, something that was there, but kind of covered up, stuck out.

So that’s where the chin came from. That doesn’t mean chins were selective. Now it may be that after they originated, that there could’ve been a little bit of sexual selection operating on them. But certainly the developmental process that originally produced them didn’t have to be adaptive. It could just be a byproduct of something that was going on, basically from the mouth up.

So the themes of microevolution are selection and drift. Natural selection is driven by variation in reproductive success. The strength of selection is measured by the correlation of variation in a trait with reproductive success. When there’s no correlation, there’s no systematic change, and then things just drift, okay?

History of Life

Artist sketch shows the crater formed by an asteroid impact in the Yucatan Peninsula in Mexico resulting in the extinction of dinosaurs. / Wikimedia Commons

Now macroevolution; the big scale process, the big picture. Well here are sort of the basic statements about macroevolution. If anybody asks you, “What does this fancy word macroevolution mean?”, tell them basically this is it. There’s one tree of life. Everything on the planet had a common origin. Everything is related to everything else, with the possible exception of the viruses, which are too small for us to decide; their genomes are too small. The branch points in the tree, speciation events–that’s when new species were formed.

This history is marked by striking major events. There have been mass extinctions. There have been meteorite impacts. There have been major changes in the organization of the information structure of life. And the biological disciplines that you may encounter map onto this timeline. So actually different parts of biology study different parts of this process.

The tree looks like this. This is the large-scale tree. So at this scale, what you see here are the three kingdoms of life, which are the bacteria, the archaea, and the eukaryotes, up here; the root’s at about 3.7 billion years, not million years. And at one point a purple bacterium got into the eukaryotes and became a mitochondrion, and at another point a cyanobacterium got into various plant lineages, three times, and became a chloroplast.

So that’s the large scale. And you’re probably searching around on that to find out where you, the most important thing in the universe are, and you’re way up here, on a little twig. Okay? Now if we blow that up and just look at the multi-cellular organisms, multi-cellularity looks like it originated around 800 million to a billion years ago. And these are the fungi, these are the things we call the plants, multi-cellular plants, and then off in this direction we have got a fairly complicated series of branches that end up with us being up here. Okay?

The things that are–this was done by Tom Pollard, at MCDB, about five years ago, and at that point the things in yellow had genomes that had been completely sequenced. Now there are hundreds of completely sequenced genomes. So for the first two billion years of life most of the action is down in the basal radiation. So going on with bacteria, archaea and eukaryote ancestor; single-celled things. At that scale–we’re just way up at a small twig on the tip–and symbiotic events brought mitochondria and chloroplasts into eukaryotic cells.

Already this is telling you something interesting about yourself. You are a community of genomes. You are not a unitary genome. You’ve got that mitochondria in you. The main themes are basically that the speciation events that have occurred, particularly over the last billion years or so, have created a tree of life that describes the relationships of everything on the planet.

Systematic biology, phylogenetics, tries to infer the history of life by studying those relationships. And there’s a real deep issue here of how do we infer the tree? The tree–organisms don’t come with a barcode on their foreheads telling us who they are related to. We have to try to figure out who they’re related to, and when we understand the relationships, then we know the history, because the relationships define the history.

So we work with hypotheses about history, and we test these hypotheses against each other and try to come up with the one that’s most consistent with the data that we’ve got. And they give us a historical framework within which we can then interpret what’s happened. There are major events that have happened. Briefly these are they.

Life originates about 3.6 to 3.9 billion years ago. And, by the way, it seems to have originated fairly quickly. Within probably about 100 million years–see I’m being an evolutionary biologist again–within just a hundred million years, uh, after water could exist on the surface of the planet in liquid form–so following the meteorite bombardment, when the surface of the planet cools down enough for water to be liquid–life probably originates pretty quickly. And arguably, within the first hundred generations, the first parasites were around. So those things happened pretty quickly.

Eukaryotes and some examples of their diversity / Wikimedia Commons

Then eukaryotes and meiosis, which is how a biologist refers to organized sex, happened about 1.5 to 2.5 billion years ago; multi-cellularity, which gives us developmental biology, about a billion years ago. All the major body plans for animals appear to have, with the exception perhaps of the, uh, jellyfish and a few of their relatives, they all seem to have originated about 550 million years ago.

There was a near loss of life on the planet in the Permian mass extinction. We will study that later in the course. You’re welcome to write an essay on mass extinctions if you want to; you know, big death is kind of exciting. It seems to have occurred basically by a process of poisoning of the oceans. The flowers radiate about between 65 and 135 million years ago.

Language is important because once language occurs, then we have an independent kind of information transmission from generation to generation; we get cultural transmission. That’s probably about 60-100,000 years old; at least with syntax and complicated information storage. Writing is only about 6000 years old. And of course the important stuff is quite recent.

So this is a view of life that goes from bacteria to dinosaurs to rock and roll; and that all can be studied with evolutionary principles. How do the biological disciplines map onto this? Well microbiology and biochemistry try to study things that are common to all life. That means that the same chemical reactions that go on in bacteria go on in the human liver, and that’s about one-and-a-half to four billion years old. Okay?

Genetics and cell biology study stuff that follows the evolutionary invention of meiosis; to a large degree. There is bacterial genetics, but eukaryotic genetics is something which is studying things that are about 1.5 billion years old. Developmental biology and general physiology, those are multi-cellular disciplines; they depend upon the existence of a multi-cellular organism. That thing didn’t come along until about a billion years ago. Neurobiology, you need a complex–you need cephalization–you need to have a complex nervous system. That studies phenomena that are probably about 500 to 600 million years old. Same for behavior.

There are several anthropologists in the class. You guys are studying things that probably originated along our branch of the tree, within the last 15 to 20 million years. So there is a temporal assembly of biology, as a discipline, as well as there is of life, on the planet.


Generations / Wikimedia Commons

So the key concepts are that there are two kinds of explanation in biology. One is the proximate or mechanical question, which is answered by studying how molecules and larger structures work. Those are basically physical and chemical explanations. And then there are the evolutionary questions, which is why does the thing exist; why did it get designed this way? And that could be answered either through selection or through history; or the best way to do it is to use both and combine those explanations.

The thing that distinguishes biology from physics and chemistry is Natural Selection. This is not a principle that you can find in a physics textbook or in a chemistry textbook. This is something that is a general principle that actually applies to lots of things besides biology, but it’s not contained within physics and chemistry. And there is a pattern in biology that unites biology with geology and astronomy, and that’s history. So there is an important element of historical thought in evolutionary biology, as well as the more abstract action of natural selection on designing organisms for reproductive success and shaping changes and gene frequencies.

Now I want to end the lecture by telling you something astonishing. I won’t always be able to tell you something astonishing in every lecture. We are continuous with non-life.

Here’s how I’m going to convince you of that. Think of your mother. Now think of her mother. Now think of your mother’s mother’s mother. Now I want you to go through a process like you’ve done in math where you do an inductive proof; you just go back. Just let that process go. Okay? Back you go in time. Speed it up now. Okay? We’re back at ten million. Now we’re at a hundred million. Now we’re at a billion years. Now we’re at 3.9 billion years. Every step of the way there has been a parent. 3.9 billion years ago something extremely interesting happens. You pass through the origin of life, and there’s no parent anymore. At that point you are connected to abiotic matter.

Now this means that not only does the tree of life connect you to all the living things on the planet, but the origin of life connects you to the entire universe. That’s a deep thought. Every element in your body, which is heavier than iron, and you need a number of them, was synthesized in a nova, uh, supernova. The planet that you’re sitting on is a secondary recycling of supernova material, and your bodies are constructed of that stuff and they use it in some of their most important processes.

So the vision that evolutionary biology gives you is not only the practical one of how to think about and analyze how and why questions in biology, it’s also a more general statement about the human condition, and I hope it’s one that you’ll have time to reflect on.