>> So the lecture -- little general lecture -- today is on natural selection and then what you might think of as the adaptationist program. Or in other words, how after natural selection and whatever's happened throughout history -- for million of years as species rose and fell -- how after that's all, you know, happened and we have a snapshot in time, the present time, how can we scientifically kind of infer back interpretations about something about history and, you know, like, how things came to be the way they are. So you might think of adaptation by natural selection as being like a kind of algorithm. So just like a computer algorithm. And every generation it loops through, it loops through the algorithm. And things can happen. They don't always happen, but they can happen. And so there's kind of, like, conditions of that algorithm. And then there are outcomes of the algorithm. So you could imagine an algorithm that's like a very simple video game, right, where every generation it loops through something and it moves the organism around on the screen. Right? And then sometimes the organism ends up in one part of the screen, and sometimes the organism ends up in another part of the screen. Similarly, as organisms have been kind of looping through their life cycles every generation for millions of generations, then things have been happening to them. And what happens is that if we consider a population of organisms, as that population changes through time -- not just the individuals but the population, if we consider it at the population level -- then what happens is good genes accumulate. And so the organism comes to be this amalgam of many, many good genes. Let's think about the algorithm a little bit. So a first condition for adaptation by natural selection is that individuals vary within a population. And, you know, if you go out and look at all of those mallard ducks, then, you know, some of the mallard ducks are a little bit flashier than other mallard ducks. And even, you know, of the males. And so that's what I mean by variation within a population. If you went out and looked at the fox squirrels, there would be some fox squirrels that would have bushier tails than other fox squirrels. And then another condition of adaptation by natural selection is that some of this variation is heritable, that to some extent -- it doesn't have to be a lot -- but to some extent baby squirrels look like their mamas and papas. If you have a mama and papa squirrel, and they both have tails that are fluffier than average, then you'll end up with baby squirrels that have tails that are fluffier than average. So that's what I mean by some of the variants as heritable. And then when the environment is good -- so sometimes during the course of history this population has experienced good times. And when those times were good, the population had the potential to grow. And I think that that's kind of generally true of organisms. Like, if organisms -- if their populations couldn't grow, then they'd probably be crashing. And if they were crashing, they went extinct. And so we don't have them with us anymore. Whereas the species that we have with us are ones that, during the course of history, there were times when things were so good that they could make more offspring than there were parents; population could be growing. But this growth, it can't go on for very long. Because if you think about it, like, two squirrels -- if two squirrels make four squirrels, and four squirrels make eight squirrels, and eight squirrels make sixteen squirrels, and pretty soon you'd have the earth would be overrun with squirrels. So in other words, there's during these times when the environment's good more squirrels are produced than can possibly survive, which means some of them are going to have to die. Right? And then the ones that fail to survive or fail to reproduce -- a condition for adaptation by natural selection is that there be this relationship such that the ones that fail to survive, it depends on part on the traits. So for instance, if having a fluffier tail meant that those squirrels did just a little bit better than squirrels with less fluffy tails at jumping between trees, and gathering food, and surviving, and making more baby squirrels -- that's what I'm talking about in this condition -- that there's a relationship between survival or reproduction and the traits or dependence of the traits, the dependence of survival and reproduction on the state of the traits. So if these things are true, then that allows for this process that we call adaptation by natural selection. And what that would be is that across generations there would be an increase in the frequency of genes that affect traits that are apt. So if it turns out that having a fluffy tail is apt, then we start out at, say, some cut in time. And let's say this cut in time we have squirrels that look basically like ground squirrels, you know, the tails aren't very fluffy. And these organisms now are living where there's trees and there's food up in the trees. Okay? So these quasi-ground squirrels, kind of pre-tree squirrel things, they're crawling up in the trees, you know, and getting food and stuff like that. And of course, as they jump from limb to limb, occasionally they fall down and hit the ground and, you know, they die [inaudible]. So there's some of these squirrels that are either dying or failing to reproduce in part because of their traits. And more squirrels are being produced than can possibly survive. So there's this kind of relentless force of intraspecific competition where the best of all possible squirrels are doing better than, you know, merely perfect squirrels. Okay. And then let's say that having a fluffy tail, it confers better ability to jump between tree and tree, you know, not fall so much. Then what would happen, it's not that individual squirrels would get tails that are fluffier and fluffier, and then they would pass on that, you know, well-developed tail to their off string. That might or might not happen. But what would really cause this adaptation is that if there's variation among the squirrels in the ability to make fluffy tails, then the ones that make fluffy tails, they will pass on more of those genes for fluffy-tailedness to the next generation than the ones that had naked tails. You know? The sparsely-furred tails, that would be partially caused by genes. And those genes would just, you know, not show up very much in the next generation because most of those squirrels, you know, didn't do as well as the squirrels that were able to jump between the trees; and be very confident; and gather lots of nuts; and make lots of babies; and stuff like that. And so anyway, as this process goes on from time to time, then there's new genetic variation that arises. And this new genetic variation that arises ultimately comes from mutations. Like, there's a whole bunch of genes that affect squirrel tail fluffiness. And from time to time mutations happen. And some of these mutations probably make the tails less fluffy and some of them don't affect -- some of these mutations don't affect tail fluffiness. And a few of the mutations make the tails more fluffy. Well, the ones that make the tails less fluffy, those mutations aren't going to stick around for very long because they confer on the squirrels a kind of inferiority. And those squirrels then don't have very many babies, maybe because they die or maybe because they don't get that much food or whatever. Whereas the mutations that confer greater fluffiness on the tail, they would end up in a whole bunch more squirrels because the next generation of squirrels is going to be descended from squirrels that have pretty fluffy tails. Now, another thing about this whole process is it turns out it doesn't have to be anything like 100 percent. You know, you can have a tiny, tiny amount of selection -- so small that you can even not measure it. Like, a one percent difference in the number of offspring that are produced. You know, it's nothing. You know, you hardly ever notice it. But if you have a one percent difference for five thousand generations, and even a one percent difference in selection for five thousand generations with a tiny amount of heritability, you're going to go from an essentiality naked tail to something that's just, you know, extravagant. You know, like the tail that you can fly between trees on. It will go to the as fluffy as -- it basically go as fluffy as is valuable to have. Because it's not that hard to add fur to a tail. Now, it can't break the laws of physics, right? You can't have squirrels that are like, you know, loaded up with helium, floating around, anything like that -- balloon squirrels. But within the constraints of what's possible, it's very easy after not that many generations to get very close to the optimal fluffiness. And that's true of pretty much any dimension of an organism. You know, like leg length, or height, or, you know, how hairy you are -- all that type of stuff. It's, you know, pretty heritable. There's not a lot of physical constraint; it just sort of optimizes within a thousand generations or something. And if you can do [inaudible] mathematical models, it's amazing. It doesn't take the amount of time that exists -- that we know exists -- in the fossil record. It's not like, you know, it takes ever since the dinosaurs disappeared in order to make a squirrel's tail fluffy. No, it's, like, since the glaciers were here. It's hardly anything. Okay. So in the course of writing paper number one -- which I think you started by now, I hope -- it may come upon you that you want to tell some story about how when the world was young, squirrels got fluffy tails or something. Whatever, you know? You might end up giving some interpretation that's adaptive. And so I wanted to give you a little guide to things that I gag at. Like, so that when you're revising your paper, you can say, "Oh, I think that's the type of thing he doesn't like," and then you can write it, you know, in the type of language that I would find more reflective of this process. Okay. So I put little thing here. Click here for the do's and don'ts of posing historical scenarios that involve natural selection. I'll click there. And you can go read this later; you don't have to take notes or anything. It's just up here. But I thought I'd read you some of these things. And just to illustrate some things that I do and don't like. All right. Well, that's all I have to say about that.