 All right, good morning. Are there any questions on the last lectures? Anybody have any comments or questions? I believe, has anybody had difficulty finding the course materials? So I should have all the lectures, notes, except for perhaps the last one, which I just sent in to have posted. They should be up on the web. Everybody's found where those are. I've got a number of comments, too, about the slides. So I'll comment again that the population genetics lectures did not have any PowerPoint, or I use keynote slides that were posted, so there should only be lecture notes there. And some people have complained once again about the quality of the slides, especially in the webcast. And I'm sorry about that, but there are some advantages to showing up live to the lectures. I can't make it too easy for you. Well, without any questions, I want to move on to a lecture today that I suspect none of you will really be interested in. I mean, you'll probably just want to sleep through this. It's about sex. And many of you probably already thought, well, I've already kind of had this lecture. Haven't we already done this? You may remember that very, very awkward conversation or speech your parents gave you probably about a decade ago. They always start off when a man and a woman love each other very much that starts off like that. This is not that lecture. After I'm done with you, you'll be so uninterested in sex that there'll be no more parties or anything at the campus for a while. So what do I want to talk about? I want to talk about the evolutionary explanation for sex. And it's important that we go over, first of all, the evolutionary explanation of sex I think is an interesting one. As you'll see, there are some costs associated with sex. So it has been a real enigma of why so many organisms actually undergo sexual reproduction. So I'll try to give the evolutionary explanation for that. And this is also going to be a building lecture for the next one, in which we'll talk about sexual selection. So this sort of explanation for why you see traits out there, especially traits in males that seem so maladaptive. Long tail feathers on male birds, for instance. Why do these animals have traits that seem obviously maladaptive that it's not good to have in terms of survival and reproduction? So this lecture is going to have a couple parts. I'm going to talk about what it is and not the mechanics. There's nothing lurid here. It's just going to be, from our perspective, as evolutionary biologists, let's just dissect sex away and go right to the point. What is sex accomplished? I want to talk about some of the variety of ways in which organisms accomplish sexual reproduction. It's an evolved trait. It's a trait that continues to evolve, and so there's a lot of variation in how organisms do it, so to speak. I want to talk about the cost, and then I want to talk about the evolutionary explanation. So that's my goal for this lecture, and hopefully I'll be able to go through it all. OK, so let's start off with what it is. And so once again, if you're interested in the details of meiosis and gamete formation, we're not going to be going through that in this lecture. This is going to be kind of a high-end, high-level type of overview of what sex actually accomplishes. Your textbook actually has quite a bit of detail on how meiosis occurs and how gamete formation occurs. If you're really interested, you should read that part of your textbook, or you'll get it in Biology 1A, I imagine, as well. From our perspective, we're going to just look at a cell as a very simple thing. It's just a circle, all right? And this is the nucleus of a cell. I should mention that we're going to be dealing with diploid organisms, all right? That is to say organisms that have two pairs of chromosomes, each copy of the gene is doubled up, and they get one from mom, one from dad. We'll talk about diploid organisms. In a nucleus, of course, is interesting from our perspective because that's where the genetic material is. So what these squiggly lines represent are chromosomes. That's a pair of chromosomes. This would be the centromere that actually holds the two chromosomes together. Here's the squiggly pair of chromosomes, and here's another pair of chromosomes that's straight. Now, we imagine that we have the chromosomes or where all the genes are contained, obviously, and we're going to consider one locus on this chromosome and one locus on this chromosome. We'll call this the A locus, and we have a big A allele on one of the chromosomes and a little A allele on the other chromosome. And on this chromosome, this other chromosome pair will have another locus, which we'll call the B locus, and we imagine that one of the chromosomes, maybe from mom, had a big B allele, and the other chromosome from dad, perhaps had a little B chromosome. Now, the first important point about sexual reproduction is that during the formation of gametes, this is, there's a process in which gametes are formed, you have a reduction division in which the gametes get one of the two chromosomes, one from mom and one from dad, okay? And the probability that any one of these two chromosomes goes into any specific gamete is one half. It's a fair process, okay? Now, I say it's a fair process. There are examples out there in nature where it's not a fair process, where some gene tries to take over the formation of gametes and represent itself in the gametes at a higher frequency. We're not talking about these examples. For the most part, which of these two chromosomes goes into the gametes is fair. So what types of gametes can be formed? So here's an example of a gamete. A gamete, of course, just reminds you as an agarous berm. So we can get one copy of the squiggly chromosome and one copy of the straight chromosome in the gamete. Now, one of the types of chromosomes that can be formed is one that has a big A allele on this chromosome and a big B allele on this chromosome. So that's one of the types of gametes that can be formed, there's three other gametes that can be formed as well. Here's one of them. Let's see, here's a squiggly. So we can have the big A and the little B. We can have, here's the squiggly chromosome, this trait, we can have the little A and the big B. And the last guy that can be formed, I'm writing too big. I'll just write it here. It can be little A and little B. So this process of independent assortment, this process of independent assortment of the chromosomes into the gametes can create a different type of variety of different gametes. Note that if you have a union of two gametes of this sort, little A, little B, you're gonna have an individual that's homozygous, little A, little A at one locus and little B, little B at the other locus, right? That is now, that is potentially a genotype combination that's not found in the parents. Maybe it's two heterozygous, maybe it's two individuals like this that mate, okay? Two individuals that have big A that are heterozygous at both of these loci for these two chromosomes. Maybe you have two parents that mate that are this sort and yet they can form young offspring that are homozygous for little A or homozygous for big A. You can get combinations of genes in the offspring that are not present in the parents through this process of independent assortment. So this is one of the themes you'll see in this lecture is that sexual reproduction causes variation in the offspring. So you actually see more variation in the offspring than you might have seen in the parents in this case. So that is assortment, independent assortment. Once again, causing gametes in the, if you have the union of these gametes, you can cause individuals to be formed that are very quite a bit from the parents. The other process I wanna talk about occurs during meiosis during this reduction division, forming gametes called recombination. And in this case I want to talk about what happens on one chromosome. So here we have our chromosome pair and let's imagine we have on one of the chromosomes, maybe this is the chromosome from dad, we have a big A here and a little A over here. Over here we have a big B and little B. Now with just independent assortment alone, you cannot break up the big A, the association between the big A allele and the big B allele. Those two alleles without any other process going on are stuck together on the same chromosome as are the little A and the little B. So the gametes that would be formed using independent assortment alone would always have the big A with the big B. Now you might, if you have other chromosomes in the genome, if you have other loci on those you still would benefit from reassortment or independent assortment, but not if they're stuck on the same chromosome. So recombination is this process actually allows the chromosome arms to exchange. You have this process called crossing over and it really is crossing over. You have this chromosome, basically you have a break form and it basically reattaches to this chromosome over here. So the chromosomes that can be formed through recombination of these two chromosomes are the following. You can have big A with little B and you can have little A with big B. These are the two chromosomes that are created through recombination of these two chromosomes if the recombination event occurs between the two. There's of course two other chromosomes that can be formed. We can still have big A with big B and little A with little B. These are the two chromosomes that are formed if you do not have a crossing over event between the two chromosomes. Or if you have I guess an even number of crossing over events between the two chromosomes. Now are there any questions about recombination and independent assortment? Again, recombination you might see also can cause variation in the types of offspring that are created. If you have the fertilization of gametes, so now let's just put these into gametes. So this is one gamete, here's another gamete and so forth. You can actually have gametes that are formed that have now a little B associated with them. And so you can actually have individuals that would be perhaps heterozygous for big A and little B which could not have been created before. Again, more variation created in the offspring. The common theme you see was sexual reproduction. That's all I wanted to say about what it is. From our perspective as evolutionary biologists, all we care about is that sexual reproduction causes independent assortment and recombination which means that the offspring are potentially more variable genetically. You have more variation on which natural selection can operate. Now a little bit about the variety. So as I mentioned, sexual reproduction is an evolved trait and there's a wide variety of ways in which organisms reproduce out there. And that's what I want to go over now. I think I'll just label these points sequentially. So the first thing I want to point out is the difference between esogamous species and anisogamous species. So esogamy versus anisogamy. So esogamous species are ones in which the gametes that are formed are the same size. And as you might imagine, anisogamous species are ones in which you have a difference in the size of the gametes that different individuals form. So for instance in humans, males and females form gametes that are vastly different in size. I believe like over a thousand or a million times, the eggs, a thousand times at least bigger than this sperm. So you have big gametes and you have little small gametes. By definition, the female are the individuals that create the big gametes. So these are by definition females and these are by definition males, which kind of gives you another way of determining whether the person you're looking at is a male or female if you're ever in doubt. Just if you can get ahold of the gametes, you have a sure fire way of figuring out if they're male or female now. So it's a very useful class from that perspective too. Now there are some species, most species out there are anisogamous. We're an example, mammals, most species are. There are a few esogamous species, algae, produce gametes that are roughly the same size and a lot of different fungi also produce gametes that are about the same size as each other. Now this point, I mean this is maybe an obvious point too, but this point will become more important when we talk about sexual selection. It turns out that females, because they're the ones that produce the bigger gametes, invest a lot more in reproduction than males. And that sets up an asymmetry in which the females can be the choosy sex and the males are, well, they're not choosy. And we'll talk about that in the next lecture. So just remember isogamy and anisogamy, it seems like an obvious point, but it has important repercussions evolutionarily. So that's the first point I wanted to make. Okay, you also have, okay, so you also have cases in which species either have separate sexes, obviously were examples of that, or species that are hermaphroditic. That is to say that a single individual can produce both sperm and eggs. Examples of hermaphroditic species, things like peas, can perform both male and female gametes. Banana slugs, a lot of slugs and mollusks can also do the same thing. If you're familiar with UC Santa Cruz's mascot, the banana slug, that's a hermaphroditic species. Now there's also, you should also realize that with hermaphrodites, you can have, they can either, they can often hermaphroditic species, they can self-fertilize. That is to say a single individual can fertilize itself. It can take one of its eggs and one of its sperm and fertilize itself. Examples of self-fertilizing species are things like peas, which can be self-fertilized. Banana slugs and lots of others cannot self-fertilize. They have to still find another individual and then they can exchange eggs and sperm appropriately to make offspring. You should not confuse hermaphroditic species or this mode of reproduction with cloning. That is to say, we'll talk about this in a moment, but individuals that are asexual, asexually reproducing, often, or they'll produce offspring that are genetically identical to the mom, okay? Hermaphroditic species, because the eggs and sperm are still undergo independent assortment and recombination, the individuals that are formed in hermaphroditic species are not identical to one another. The offspring are not identical. So don't get this confused with cloning, okay? If you're wondering why there's a long silence while I'm writing things down, it turns out I can't write and speak at the same time, so this sort of by necessity. So the next thing I want to talk about briefly is genetic versus environmental sex determination. Now, the species you're familiar with and you've learned about, if you've learned about this in high school biology, for instance, you think about the genetic sex determining mechanisms. That is to say, humans, for instance, have 23 pairs of chromosomes. 22 of those chromosomes are what are called autosomal. And one of the pairs is called the sexual sex chromosomes. And in mammals, those sex chromosomes come in two flavors. So the autosomal chromosomes, they look identical to one another, okay? At least, if you were to look under a microscope, they look identical, the pairs do. The sex chromosomes differ in size. And one of them, the larger chromosome is called the X chromosome, and the smaller chromosome is called the Y chromosome. You should remember, of course, that these are just labels. If you look at the chromosomes through a microscope, they don't literally look like Xs and Ys. It's just a label that was applied to them. The big chromosome is the X, the little chromosome is the Y. And in humans, in lots of other mammals, the X individuals, they become females, and the XY individuals become males. This is all stuff you probably know. Just as an aside, just to throw another term at you, the males are said to be the heterogamedic sex. That is to say, the gametes of the males form can be potentially different, right? Because some of the sperm will have an X chromosome, and some of the sperm will have a Y chromosome. Those sperm that go on to fertilize an egg, the individual become male, obviously, because the X will pair with a Y. Now, not all species are like this. Birds, for instance, it's the females that are the heterogamedic sex. Butterflies, the females are the heterogamedic sex. So in these other types of species, once again, the sex chromosomes are labeled, but they get the labels Z and W. And it's the ZZ individuals that become males, and the ZW individuals become the females. As I said, the females are the type, the individuals that form two different types of gametes, that they are the heterogamedic sex. Sorry? This would be an example of birds and butterflies. So sex determining mechanisms can evolve, and there's a lot of interest, frankly, in the field right now about how sex chromosomes evolve, and it's really been only the last 10 years where people have been able to study how sex chromosomes evolve on sort of a molecular genetic basis. And there's a woman in our department, Dr. or Professor Doris Backtrog, who studies the evolution of sex chromosomes. It's a very interesting field. Anyways, this is just to point out that sex chromosomes can evolve, and that there are different mechanisms, different genetic sex determining mechanisms out there. Now, what about this environmental sex determination? Well, it turns out that some species don't have genetic sex determination. The sex that you turn into is determined by the temperature at which the egg is incubated. Now, in the last lecture, I believe I talked about the experimental evolution. I showed two pictures, one of Holly Wickman, who was from University of Idaho in Moscow, and the other photo was of Jim Bull, who's at the University of Texas at Austin. Jim Bull, Jim and Holly, Jim now studies experimental evolution in viruses, but his career started as a herpetologist. Herpetologists are people that study amphibians and reptiles and snakes, and he is the one that discovered that organisms like turtles and crocodiles that the sex is determined by the environment. So a pretty remarkable discovery. And in turtles, I always get this confused, but in turtles, I believe males are formed at low temperature, is that right? Yes. And in crocodiles and alligators, it's just the opposite. Males form at high temperature. And of course, the question comes up, if you have a cold season or a warm season, why aren't all the individuals formed of the same sex? And it turns out that the threshold at which the critical temperature above which you have males or females is basically around the nest temperature. So the nests are much more constant in temperature than the ambient temperature of the air. Justin, it's an ecological question, but it is the case that you still get a mixture of males and females in these cases. Okay, so that's another thing, point number three. Let's go on to the fourth variety. I believe the last. Okay, so there are some species that are capable of asexual reproduction and many that aren't. And some that can switch between asexual and sexual reproduction. So what is asexual reproduction? Well, this is the formation of an individual from an unfertilized egg. Unfertilized diploid egg, I should say. So you have mom. There's mom, that's a symbol for females. She produces a diploid egg that's genetically identical to her. It does not undergo meiosis. Okay, it does not undergo recombination or independent assortment. And that egg grows up to be a daughter that's genetically identical to the mom. Okay, so genetic, this is what we think about when you think about cloning. Okay, you make a bunch of individuals that are genetically identical to one another. Now, when I say genetically identical, I mean, except for any mutations that happen to have occurred in the formation of this diploid unfertilized egg, but for the most part, genetically identical. Now in plants, this process of asexual reproduction is called apomicsis. This is what we call it when plants do this. In animals, this is called parthenogenesis. You would think it would have been quite convenient from your perspective, especially if they could have just used the same name for the same process. But of course, plant biologists have to call this phenomenon one thing and vertebrate biologists have to call it something else. But it's the same basic idea. You're forming offspring that are genetically identical. This is something that can occur quite frequently. There's lizards, for instance, that are parthenogenic. Lots of plants that undergo apomicsis. Now, is there anything else I wanted to say here? Is there a question? Yes. I'm sorry. At the best of times, my writing is not very good. Apo, M, I, X, I, S. It's also in the notes, hopefully spelled correctly. Yes, they're female. You're producing offspring that are all female because males don't produce offspring. So the one thing that I'm pretty certain of is that males don't give birth or lay eggs. I'm not being a little facetious, but basically, they're defined to be females because they can produce offspring, eggs that undergo growth, right? So they're defined to be females, but they're clearly a little bit different than the normal sexually reproducing species. All the gametes are the same size, in a sense, so by my definition, that the females that always have the larger gametes, how does this work? They're just called females. Any other questions? Those were good questions. Yes, it's just how they, so it's not clear if it's beneficial or not. That's just the system that evolved. It's clearly not too detrimental. They seem to be able to, about equal numbers of males and females are being produced in turtles and in crocodiles and in alligators, so it's not, it doesn't seem to be a hindrance. It's just the mechanism by which they determine sex. I should say that I forgot to mention one thing when I was talking about hermaphroditic species, so I'm gonna erase this portion of the board. It's remarkable that I forgot this because it's the only thing I did as an undergraduate with research. And there are hermaphrodites that are sequential, so that is to say, you never find both sexes together in the same individual, but they undergo sex change during their life. So there are some species which are called protandrus hermaphrodites. These are male first. They go from male to female. And there's other species, they're called protognus. I think I've got this right. They go from female to male. So an example of a protandrus species is actually can be found in the intertidal area, in the Bay Area. So if you go to Half Moon Bay, for instance, and go out to maybe knee-deep water and scoop up the sand, you'll find these little tiny clams. Everybody know what a clam is? The bivalves, they have two shells. They live in the dirt. They filter water and eat the gunk in the water. And these clams, they're called transonella. That's the genus. They're very small. They're maybe five millimeters at the largest. So it's maybe a quarter-inch in size. And they go from male to female. They brood their young. So the clams that come out of the female are fully-formed clams. They're just little tiny guys. There's probably an ecological explanation for protandrus and protognus species. And the clams, the argument is that they're so small and because they're brooding that you can't hold that many clams in this very small size. So it's the females that have more volume. They're the bigger individuals in the population. So it might be advantageous to be a female because when you're large, because you can hold more offspring. And of course, sperm are very small. So even as a small clam, you can produce a real excess of sperm. So that's the explanation you often see for protandrus-type species. And protognus species and examples or many reef fish are protognus. And one of the arguments there is that some of these reef fish are territorial and where the males defend the territory. So perhaps it's advantageous to be bigger. You better defend your territory. So the males turn out to be the larger individuals. Anyways, there's often ecological explanations associated with protandry and protogyny. There are some just going back to the asexual and sexual capable of asexual reproduction. There are species too, just like there's species that can switch sexes halfway through their life. There are species that can go through asexual or sexual reproduction, often depending upon the environment. They'll often go through asexual reproduction when the environment is stable and it's advantageous to undergo very rapid population growth. And they'll often be sexual when things become more uncertain. Things like aphids, I believe, are capable of asexual or sexual reproduction. Water fleas, Daphnia, there's another example of that. Now, I believe that's all I wanted to say about the variety, the different types of variety of sexual reproduction you actually see out there. I want to go on to the cost of being a sexually reproducing species. And so the first thing we have to realize is that there is a cost undergoing sexual reproduction. And we might be able to list some of those costs. Erase this portion of the board. So what are the costs of sexual reproduction? Well, one cost is just you expend energy. So especially males and many species expend a large amount of energy just getting access to mates. So this idea of separating the sexes and undergoing sexual reproduction can be quite costly. There's also, you're probably familiar with, potential for disease, picking up parasites. So there's a lot of parasites out there that have exploited sexual reproduction. We call them sexually transmitted diseases, but this is a potential cost of sexual reproduction. And the last thing, and the thing I want to talk about the most is the so-called two-fold cost of sex. So what is the two-fold cost of sex? Well, to explain this, I want you to imagine two different populations of organisms. Go over here. One of the populations is asexual, and the other population undergoes normal sexual reproduction. And to simplify matters further, we're gonna assume that in both cases, the asexual individuals and the sexual individuals, in both cases, they produce two offspring. Now let's look at the sexual, rather the asexual species. So here we have our female, and in this example, this female can produce two offspring, both of which are female. So the population in that one generation is doubled. We're from one individual to two individuals. Now each of these offspring are female, and because they're asexual, they don't need a male. So they can also produce two daughters, and we now have four individuals. The population is doubled again. The point being that asexual species, the population can double every generation in this made-up example, because each individual produces two offspring. Now let's also imagine, so this is the asexual. Now let's look at the sexual species. So here's our female, and as many of you females out there probably know, males are pretty worthless as far as reproduction goes, except to provide fertilization for your eggs. So you need to find some male to reproduce with, and then this female, on average is gonna produce one female and one male offspring. Remember, we're keeping the game fairer. In both cases, they're producing two offspring. Now this male doesn't do anything in terms of reproduction. It just provides a sperm for some egg, perhaps. So we won't worry about him, but this female here finds some mate, some male out there she mates with. Shouldn't be a problem. And she's gonna produce, on average, one female, and she's gonna produce one male, which is worthless as far as the reproduction goes. So note that the population is not increased in size. It's gonna remain stable. You're gonna have two individuals, this female, on average, is gonna produce two offspring, one of which will produce two offspring, one of which will produce two offspring. The point here is that in asexual species, a population can grow quite rapidly. Whereas in a sexual species, because the sexual species are always producing these males, their population does not grow as quickly. Of course, the census, you go out there and count the population. Of course, you're gonna see males there, and they're gonna count towards the population size, but they're not the ones that are producing the young. So that is the two-fold cost of sex. So the question becomes, if sex is so bad, if it costs so much energy, you expose yourself to the risk of disease, and especially if you imagine a species that is asexual and compared to the sexual ones, how can sex ever evolve in the first place? And just to go back to this two-fold cost of sex, I want you to imagine, here's our population. Imagine that you have a sexually reproducing species. So here's each circle as a person or individual. So we have our sexually reproducing species that's composed of, say, randomly mating individuals. And imagine a mutation that causes some individual to all of a sudden not need to mate. So this individual now can reproduce asexually, whereas all the other individuals in the population are obligatory sexual. This individual's gonna produce females, both of which can reproduce. So this individual, just from what we know about population genetics, the mutation that causes asexuality when it appears in a sexual species, it should spread through the population, through natural selection. So why is it the case that we see so many sexually reproducing species? And it's a case that we don't just think that there's sort of an evenness between being asexual versus sexual. It's pretty clear that being sexual is better. Why do we think that? We think that because asexual species tend to be quite young. They don't persist very long. We can tell that through what's called a phylogenetic analysis, or a phylogenetic tree. You guys have gotten phylogenetic trees in lab? Yes, okay, so this shouldn't be too unfamiliar, but let's make a hypothetical tree. Maybe this is gonna be a tree of lizards and snakes. So here's our lizards and snakes, all of which are sexual at the tip, except what you tend to see are small parts of the tree that tend to be asexual. So maybe you'll find the whipped-tail lizards, and there's some other lizards, for instance, that are asexual. They're not closely related to one another, and they're basically one asexual species surrounded by a bunch of sexual species. What does this mean? It means that asexuality must have arisen along that one twig. And it must arise quite common, often, because you actually see multiple species of lizards that are asexual, but you don't see one large, old group of asexual lizards. This means that asexual species, when they're formed, don't last very long. They tend to go extinct at a higher rate than the sexual species do. There's one exception to this rule, I should mention, just to be fair, and that's the Deloitte rotifers. They're very small organisms that live in lakes. They appear to be anciently asexual. They're the only group that we can think of that's like that. All the other cases that are known, the asexual species tend to be quite young. So what is the advantage to being sexual? Because there must be some advantage, or we wouldn't see some asexual species. Everything should be asexual. Well, this is where I'm gonna actually go on to the evolutionary explanation. This is the last portion of this lecture. Evolutionary explanation for the advantage for sex has to do with these processes of recombination and independent assortments that I talked about. And there's two different models. One is called Fisher's model, the Fisher model, and that is a model in which you adapt more rapidly to your environment. So that's one explanation. The other, and so you might know Fisher because he's the population geneticist that I'm so enamored with and that I mention all the time. And the other fellow is Muller, and this is the explanation that sexual species can more easily purge deleterious mutations. So those are two different explanations and I'd like to spend a little bit of time to explain both of these mechanisms. I should mention too that Muller worked a lot in the 1930s. He's the fellow that discovered that radiation, for instance, causes mutations in cancer. They got the Nobel Prize for that. And people at university did all that research at the University of Texas at Austin, but then he was expelled or kicked out of the university because he was a communist, but then he got his Nobel Prize afterwards when he was at another university. So Texas always likes to think, try to claim this Nobel Prize as their own, but it really isn't because they kicked him out, but anyways. So let's talk about the Fisher model. Let's talk about the Fisher model first. And the way I want to explain this is by looking at a population as a bunch of chromosomes. So here's our population. It's a bunch of chromosomes. So there it is. And we all know what happens if you have a beneficial mutation land on one of these chromosomes. So star means it's a good mutation that the individuals that have that chromosome that happen to have at least one copy of it have a fitness advantage. They've survived better, they reproduce more offspring. What happens when we have a beneficial mutation? Natural selection tends to favor it. And you'll find more and more of the individuals in the population over time will tend to have that beneficial mutation. That's quite simple. You guys shouldn't be surprised by that result at all. Now, things become a little bit more complicated when we think about having beneficial mutations at a second locus. So let's imagine that there's a second locus where you can also have another gene perhaps where you can also have a beneficial mutation land on that. So this individual, individuals bearing this chromosome also have a fitness advantage. Now, let's make the example a little bit more interesting. If both of those mutations get onto the same chromosome, this individual's the best. So you have three different chromosomes or four different chromosomes. You have the chromosome that doesn't have either of the beneficial mutations. Those are the worst. These guys that have one beneficial mutation, they're better. And the guy that has the lucky chromosomes that have both mutations on them, they're the very best in terms of the fitness they confer on the individuals that bear them. The scenario clear, pretty simple. So best, good, worst. Now, in the absence of recombination, the only way to get both of those mutations together is to have this mutation land on a chromosome that already has that mutation. The only way to get this mutation with this one is that the mutation happens to occur right here with a beneficial mutation at the second position, the second locus. When you don't have that occur, you have the mutations occur on different chromosomes and what happens? Well, these two chromosomes compete one against the other in the population. And ultimately, the winner will be determined by whether this mutation is better or worse than that mutation. So in the absence of recombination, which is a process that exchanges bits of the chromosome and can bring different beneficial mutations together, in the absence of recombination, these mutations will either fight it out, compete one against the other in the population with the ultimate winner being determined by which of these mutations has the higher fitness or you have to wait till the second mutation arises in the same background. And eventually, this double mutation will win. So when is it likely that both of these mutations will occur on the same chromosome? Well, it's gonna occur when one of these two chromosomes, the chromosomes that have only one of them, one of the mutations, when lots of the individuals in the population are of that type, then when the second mutation occurs, then it's more likely to occur in the same background. So the general pattern in asexual species is that adaptation, when you consider multiple loci together, adaptation tends to occur sequentially. First one mutation occurs, one beneficial mutation occurs, it spreads to high frequency and when it's at high frequency, then it's more likely that a second beneficial mutation will land in the same background and then both mutations can spread to high frequency. So you can sort of see there would be a sequential fixation of the two mutations. First one gets to high frequency, then you get the second mutation landing in the same background and both can go to high frequency. Sexual species don't have to deal with that, right? They can bring through recombination these two beneficial mutations together. So they don't have to wait around for one of the mutations to get to high frequency. These two mutations can be at low frequency and if the two chromosomes happen to be in the same individual and that individual undergoes recombination between those two, then it can form gametes that have both of those beneficial mutations together in the same chromosome. Are there any questions about this explanation? It's basically an explanation for more rapid evolution in a new environment. That's Fisher's explanation. It's bringing together good mutations. Muller's model is a little bit different and it involves deleterious mutations. So once again I'm going to illustrate our population with just as a series of chromosomes. Here's our population of chromosomes and we can imagine that some of these chromosomes might have beneficial mutations somewhere. So those are the good mutations. But you can also have deleterious mutations. That's a sad face. So there's a deleterious mutation that occurs somewhere else. So that's to say this is the most, most mutations are deleterious, right? Only occasionally as a mutation actually make improved things. Most of the time when you have a mutation you break something that's already working just fine. So in the absence of recombination when this mutation occurs, well natural selection tends to remove it from the population. Remember we talked about purifying selection. When a deleterious mutation occurs by itself, this chromosome will have a lower fitness and natural selection tends to remove those types of mutations through purifying selection. One of the processes I talked about when we talked about natural selection. But you can also have situations like this where an otherwise perfectly good chromosome, or even a chromosome that would be quite good because it has a beneficial mutation also happens to be associated with a deleterious mutation. In asexual species there's no way of breaking up this association between a deleterious mutation and a beneficial mutation. It might be for instance that this is the worst chromosome because it has one deleterious mutation. This chromosome is intermediate in fitness. This one might be better than this one because it still has this beneficial mutation that outweighs this deleterious one. And of course the very best chromosome is the one that doesn't have any deleterious mutations at all. So we'll call this the worst, better, better still, and best. Just to think about assigning fitnesses to these different chromosomes. This would be the best scenario to be in. It might be a chromosome that doesn't have any deleterious mutations associated with it but has a beneficial one. Again, asexual species, because they don't undergo recombination have no way of breaking up the association between the beneficial and deleterious mutation. Sexual species can recreate kind of a pristine or a good chromosome by recombining this mutation with a chromosome that doesn't bear that mutation. So for instance a recombination event between these two chromosomes would be able to recreate this chromosome as well as that chromosome in the gametes. But at least it's reproducing at least one variant of the chromosome that doesn't have that deleterious mutation. So this is Muller's explanation that purifying selection or that sexual reproduction allows deleterious mutations to be more easily purged from a population. So that's the second explanation. And it's not clear which of these two factors, both of them are probably working in real life and out in nature, but it's not clear at least to me which of these two processes is more important in terms of the evolutionary explanation for sex. But these are our best explanations for why sexual reproduction is so widespread in nature, faster evolution and more easily purging deleterious mutations. So that's all I wanted to say. It's a lecture that's quite different than the one your parents gave you. See you next time.