 All right, good morning. Thanks. First person to actually say good morning back. Are there any questions about previous lecture? So we're down to four more lectures on evolution. And today what I want to talk about is speciation. So let me just start off the lecture with a couple observations. So the goal, like I said, is to talk about speciation and specifically what the model is for how species form. How do we actually think species form today? And the idea is actually a fairly old one. So Darwin had some ideas on how species form. In fact, his book is titled On the Origin of Species. And it's a remarkable fact that even though the book is called On the Origin of Species, most of the time he talks about natural selection, this process that leads to organisms becoming adapted to their environment. He actually doesn't talk a lot about how species form. And what he does talk about is part of the model we now believe today. That is to say, Darwin's belief was that species form when groups become isolated one from the other. And his view of speciation was purely morphological. So who is interested in how these morphological differences, how differences in traits evolve in different groups? Now the modern view, remember we talked about the biological species concept, which is that organisms are reproductively isolated one from the other. So the modern view also has to account for how organisms become reproductively isolated one from the other. Now here's an example where reproductive isolation isn't perfect. So I'm going to start off with the cases where we know something went very, very wrong. And so here's an example of something going very, very wrong. You had a male lion mating with a female tiger, and you produced what's called a liger. There is an example. Here's another example you might be familiar with, mules. These are offspring of a male donkey and a female horse. They're infertile, so they can't produce young, but they're very robust beasts. In fact, they're deliberately bred because they're good pack animals. And I don't know if you know this, but the henny is just the opposite cross. So you take a female donkey with a male horse, and you get a henny out. I think a tie-on is a female lion with a male tiger. And here's an example of a zebraoid where you take a female horse and a male zebra, and you get a hybrid form. So these are just examples I wanted to start off with just to get you a little bit interested in formation of species. Whenever we see these instances in nature, we realize, like I said, something didn't go right, that reproductive isolation wasn't complete and that some organisms made a mistake. And it's actually quite a severe mistake in these cases because if you're a female donkey and a male horse and you produce an offspring, your offspring are infertile. So basically, your fitness is quite low in that case. So natural selection should favor organisms choosing the right mates, mates of the right species at least. Let me blank the screen, the screen up. Like I promised, everything, things are gonna be more on the board now. So let's go back to the main observation, which is today we have many millions of species alive. All which are distinct one from the other, reproductively isolated one from the other for the most part. And we think that those species are related to one another through some, or we know that they're related to one another through some unknown evolutionary tree. So here's an example of an evolutionary tree. And the similarities in how organisms, in similarities in their features among living organisms leads us to believe that there was a single common ancestor that lived many billions of years ago, maybe three billions of years ago. So for instance, all organisms on Earth share the genetic code. They have a common way of coding for DNA, coding for amino acids and proteins. They also share a common mechanism for translating the DNA into proteins. These are things that all organisms on Earth share that leads us to believe that there is a common ancestor of all life. Now the point being if you start off with one species three billion years ago and now you have millions, the main process that generates that diversity is speciation. That's what we call it. We call this process of splitting speciation and this process again is what I wanna talk about today. Now if you blow up what happens here, so this is time with this being the most recent, this being distant in the past, we imagine we have some population that diverges. So you have some individuals going off into one population and the other individuals going off into another population and that there's some barrier to gene flow between those two populations. When we talked about population genetics, we talked about the four different mechanisms that change allele frequencies, but gene flow was the process in which individuals migrate between populations. So you need to have some process whereby you have two populations and you don't have gene flow occurring anymore. So you turn off gene flow. And this mechanism in which we turn off gene flow, so we might imagine there's some barrier to gene flow that occurs right here. This is the barrier to gene flow and there'll be two different mechanisms that cause barriers to gene flow that I will discuss. One are called allopatrix speciation and the other one will discuss a simpatrix speciation. So let's talk about a barrier to gene flow that's an allopatric mechanism first. So we're gonna imagine we have some population. So here's our population and I guess the axes here, there's north, so we're looking at the map. We're looking at the distribution of the species on a map. And one allopatric mechanism that we'll discuss is called a vicariant event. So what is a vicariant event? That is a barrier that occurs dividing a population into two. So you have a climatic change or you have a river forms perhaps that divides up a population. There's many different ways you can physically separate a population into two. But the point here is in a vicariant type of process, you divide this population into two and it's important that this barrier actually turns off gene flow. So not only does it divide the population into two, but it prevents individuals from this side to getting to this side and vice versa. You're turning this, this vicariant event is turning off gene flow. And there's a number of ways in which this could occur. So for instance, well here's an example, let me see if I can, here's California, there's the Gulf of Mexico. You come down here, there's South America, and South America comes up here. You have some complicated thing here. There's my map of the world at least the Western Hemisphere. So there's North America, there's South America, it's about as well as I can draw that. But you know that the Panama Canal is about here and this area here is called the Isthmus of Panama. Well until quite recently, at least geologically speaking, there was a strait here. You can actually, the water flew, there was a water barrier between North and South America. And when the oceans lowered, this Isthmus popped up above the ocean. And this Isthmus of Panama, before it was a barrier to gene flow between North and South America when it was there. But once it popped up, it was actually a barrier to gene flow between organisms that lived in the Caribbean and those that lived in the Pacific. And I will give an example of pairs of species on either side of, there's many pairs of marine organisms on either side of this barrier which are what are called sister species. They're each other's closest relatives. With the idea being that this barrier caused a division in the population that separated populations one from the other and caused them to speciate over time. So that would be an example of a vicariant event. Yes, anything that would divide a population into two, I would say would be a fair game. Most people think about, you know, abiotic events being the cause but you can probably imagine other scenarios. Just as long as you're separating the population geographically. Any other questions? Other type of allopatric event I want to talk about. We can term a founder event. Now we spoke about founder events when we spoke about, when I talked about genetic drift, right? That's the idea that you have some big population, so big population, and then you form a new population from just a few individuals. And the point that I made when we were talking about genetic drift is that the individuals that form the small population are a random sample of individuals from the big population. So there's a good chance that they'll just happen to carry some rare unusual combination of alleles. And I gave an example in lecture of an elevated rate of Huntington's disease in a population in Argentina. But let's imagine that this big population is actually the mainland. Maybe it's a continent such as South America and out here we have an island that had just recently poked its head above the ocean. So a lot of these islands in the Pacific or Atlantic, for instance, are caused by volcanoes that are on the bottom of the ocean. And as the volcanoes grow and grow, eventually they pop up above the surface of the water. And so when they pop up above the surface of the water, what that means is you have new land, a new environment for organisms to colonize. So if you have a fortuitous colonization event by, say, a bird or a turtle or a lizard, they can found a new population on this island. And the idea here is that, first of all, as you know, genetic drift will be a factor causing this population to diverge from its founding population. The second thing you should note is that if this distance is large enough that the population can be effectively isolated from the mainland population. Yes, of course, it was founded by a founding event, colonization event from a few individuals, but these can be very rare. In terms of once every 1,000 years, you might have a successful colonization event. But given enough time, of course, it's almost certain to occur. But once these small populations are founded, they can become essentially, they're essentially reproductively isolated from the mainland population. So this would be an allopatric event in that sense that gene flow is no longer occurring between the two. And secondly, it's likely that these organisms are going to find themselves in a completely new habitat where things like the temperature, the other organisms they're competing against, the food sources, whatever, are likely to differ quite dramatically from what the conditions were on the mainland. And so natural selection will also play a role in causing this population to diverge from its parental population. Once again, this is just talking about the barriers that can disrupt populations and turn off gene flow. And so in an allopatric model, you can think of vicarians dividing some factor, dividing the population into two, or you can imagine new organisms being founded, new populations being founded by just a few colonizers. And this is the type of mechanism we think occurs in places like the Hawaiian islands or the Galapagos Islands, where, yes, go ahead. There is a geographical barrier that's the ocean that separates them. It's a form of allopatric speciation. So I'm just trying to give you some different flavors of how things can be isolated from one another. And also, how is it? Well, vicarians usually think of the population sitting there and then something dividing it, whereas this is part of the population moves to the new place as a founder event. Just to talk a little bit about these islands, I mentioned, I believe, when we're talking about the Galapagos, that things like the organisms like the turtles, the Galapagos tortoises that occur on these islands, their closest relatives are found in South America. So remember, this is Ecuador. There's the Galapagos Islands. The closest relatives of the Galapagos tortoises are found on the mainland. The closest relatives of the birds, the finches, the Darwin finches that live on the Galapagos are also found on the mainland. So the evolutionary tree of the organisms that you find on these islands often supports that the colonization was from the nearby mainland and that you had some sort of radiation of new species on those islands. And there's nothing to say that when you have multiple islands, like you do in the Hawaiian islands or the Galapagos Islands, you can have a colonization event on one of them and then you can have individuals hopping from one island to another. So you can actually repeat this process on a much smaller scale and get even more species as you do on the Galapagos or the Hawaiian islands forming in these small isolated archipelagos. Is there anything else I want to talk about? Oh, there are some interesting observations that people have made, for instance, that support the idea that these types of colonization events can occur. So for instance, when you have storms, everybody here know what a mangrove forest is? OK, so the mangrove forest you find in the South, closer to the equator. You don't find them around here. You basically form plants, mangroves that form right along the coastal regions. When you have these big storms, what you can actually do is they can actually rip up portions of these mangrove forests. And you get these floating rafts of a mat of mangrove trees. And often people, when they find these out in the ocean, they'll actually find some poor lizard. There's a poor lizard hanging out on the mangrove raft or other organisms as well. But lizards are a common thing to find. It's just an unfortunate lizard that happened to be on a mangrove that was swept out to sea. Now, most of these rafts and the lizards on them probably aren't so lucky as to find a new island. But occasionally, the idea is that you can actually have some lucky lizard land on an island and can, in principle, form a new population. So there are some observations that support this. And of course, Darwin was also quite interested in whether or not you could have long-distance dispersal of seeds and plants in salt water. So he was doing these experiments where he dropped seeds of various types into salt water and asked if they'd actually germinate after a month or two months or three months in salt water. With the idea being, once again, that you can have plants also colonize in these islands just by their seeds floating around in the ocean water and eventually washing up on the shores. OK. Is there anything else I wanted to say about allopatric speciation? I don't believe so. Oh, and just a little bit more about the nature of these barriers. So as humans, we think of barriers as having to be pretty dramatic before they could actually cause the disruption to the gene flow. But it really depends on the biology of the organism. What might seem like a trivial barrier to us, say a small stream that just happened to have formed a creek even, can be a large barrier to another organism, such as a snake or some other organism that can't swim. So there's some organisms see what we might think of as trivial barriers to dispersal as insurmountable barriers. So keep in mind that the habitat can be divided in ways that we can't necessarily recognize easily, but the organisms themselves recognize as being quite substantial barriers to gene flow or dispersal. So the nature of the vicariant event or the founder event can vary depending on the organisms. And whether that's a significant event or not really depends on the biology. Birds, for instance, tend to see geographic barriers as being much less consequential than insects would say, small insects. OK. So that's what I wanted to say about allopatric speciation. Before I get to sympatric speciation, I want to talk a little bit about what prevents species from coming back together again. So remember, just to remind you, we're going with the biological species concept, which says that individuals from different populations or individuals from different species, if they can't interbreed and form fertile offspring. So you have this bit about how do you prevent offspring from being prevented, being formed in the first place. And there's two different mechanisms that we'll discuss. Some sorts of barriers we're going to call it, we're going to term pre-zygotic barriers. These are barriers that prevent zygotes, the fusion of egg and sperm from being formed in the first place. So what types of barriers might we term pre-zygotic? Well, behavior is one pre-zygotic isolating mechanism. You can imagine that natural selection can operate quite strongly on individuals from making the wrong choice in terms of mating. If you make the wrong choice, that's a very deadly mistake in terms of your reproduction, because you're forming offspring that themselves can't reproduce, perhaps. So natural selection often will favor differences in mating behaviors that would make it so such that organisms won't recognize each other as being from the same species. If you look at birds, they often have very elaborate courtship dances. And those courtship dances are specific to individual species. And the same goes for insects and other organisms as well. There's behavioral differences that isolate one group from another. And natural selection probably plays a strong role in causing those isolating mechanisms. You can also have habitat isolation or temporal isolation. So maybe, for instance, the mating season is different in the two different populations or two different species. So even though they might, in theory, might have found each other attractive, that they never actually bump into each other because their mating seasons are at different times. Or once again, maybe they're mating in different habitats or maybe different fruits. We'll give examples of that. But there's all sorts of isolating mechanisms, behavioral and otherwise, that can cause organisms to not form zygots or not mate in the first place. Oh, and one last thing I should mention is kind of an obvious one, mechanical. I'll give an example in a bit when I bring the board down or the screen down. There's often cases where there's sort of a lock and key mechanism that prevents copulation from occurring in the first place. Some of the most rapidly evolving traits in many organisms are the genitalia. So for instance, in flies, I mentioned last lecture that most aspects of the fly from different species, amateurs or even experts, can't use most features of flies to distinguish species. What they end up doing is they turn the males over and they look at their genitalia. And that's the surefire way of identifying one species versus another in many species of flies. So there's also mechanical isolation as well. Now, I actually got the same question independently twice last time after the lecture, which is, well, how about Chihuahuas and Great Danes? And clearly, I think people would consider Chihuahuas and Great Danes as the same species, even though they're probably mechanically isolated one from the other. But the idea here is that you would still have gene flow between Chihuahuas, the breed of Chihuahuas, and Great Danes, but it would have to be, I imagine, through intermediate breeds of intermediate size. But if that's something you're thinking about, we're thinking about, it's a mechanism that's not necessarily going to be an isolated mechanism, but one that's often associated with an isolated mechanism. But anyways, here are some examples of pre-zygotic isolating mechanisms. Now, you might imagine everything comes in twos or threes in the world, it seems. So in this case, you have a pre-zygotic. You can probably guess that there's also a post-zygotic isolating mechanism. And these are mechanisms that cause the, so now that the act has occurred, you've had fertilization of the egg by the sperm. And so the question is, what happens next? Do we have the formation of an individual that's fertile? That is to say, even though you've made the mistake, the offspring is fertile. If that's the case, if you can actually form fertile offspring from a mating from two different populations or two different species, then the argument here would be that the isolated mechanisms aren't very strong and that maybe those two species, those two populations can come back together sometime in the future. Because the isolated mechanisms, if you can actually form fertile offspring, then the offspring, the isolated mechanisms aren't very strong and that you can later have these populations merge into one another and you won't have the formation of species. So these post-zygotic mechanisms often involve genetic mechanisms that cause the offspring either to die, that is to say that they never actually, you have partial development, but then the embryo dies or that you have infertile offspring being produced. So the hybrids are infertile. And I'm not gonna go into great detail about post-zygotic isolated mechanisms. I'm gonna talk about one genetic model for how post-zygotic isolation can occur. And the model I'm gonna talk about is the Dubjonsky-Muller model. It's a Dubjonsky-Muller model of incompatibilities. And to introduce this model, I'm gonna first show you a wrong model. So I don't want you to remember the wrong model, but I wanna see you to get the idea. So the idea here is we have two different species and we're gonna think about one locus. We're gonna think about one genetic locus which we'll call the big A locus. So we have big A, big A here. And let's imagine that along one of these species or one of these populations, we still have big A, big A, that there's no mutations. All the individuals are homozygous for the big A allele. Let's imagine somewhere along this lineage we have the origination of a little allele and that that little allele later becomes fixed. So one mechanism, a wrong mechanism, is that when you think about the hybrids being formed between these two different species, of course they all have to be heterozygous. Cause all these guys are homozygous big A, all these guys are homozygous little A. When you have a hybrid form, they have to be heterozygous for the big A little A alleles. True? So the wrong model states that there's something wrong with these heterozygous individuals that for whatever reasons the heterozygous individuals have a lower fitness than either the homozygous states. What's wrong with that model? Did anybody see the flaw? Yes, you couldn't get this state in the first place. Very good. So what it means is that natural selection would have had to go down an adaptive valley. In order to get the, if these guys are having a lower fitness, the hybrids, then these guys, this intermediate form in this population also must have had an intermediate lower fitness as well. And so it's very difficult then to explain how this population on the right could have even gone through this heterozygous state if the fitnesses were lower. Cause natural selection wouldn't allow that, would not favor that. It's the only reason, that's why this model's wrong. But the reason I'm showing you this model is because we're starting to think about alleles in loci. So the Dubjonsky-Muller model, the one we think is a correct model and there's some evidence that's starting to accumulate of examples of Dubjonsky-Muller genes, it's a two locus model. You think about two different loci that might be coding for two different proteins. So we'll make our population again, we have, let me see. I'm gonna erase the top part here so I can make the tree a little bit taller. So let's put this right here. We have two loci, the A locus and the B locus. And in our example, the ancestral B homozygous for the big A allele and also homozygous for the big B allele. Now these two, the proteins that are coded for these two different genes, they might interact one with the other. So you may be familiar with this, but proteins often interact, actually physically contact one another to either perform important enzymatic functions or to make structures in the cell. So that's what proteins do, they do things with other proteins. So we're imagining perhaps that A and B, these two loci, the gene products of the proteins produced by A and B, actually interact. So let's imagine that on the left side we have, let's see, I don't wanna do this. We have a change in this locus here, in the A locus, so that we have little A and big B over here. Now this can be a perfectly fine transition. So you have this little A allele arise, and the only thing that's asked of this little A allele is that it works with the big B allele. If it does, it's fine. Now on this side we're gonna have the opposite occur, so we're gonna have big A big A, but we're gonna have a mutation that causes a big B allele to become a little B allele, and then later we're gonna have big A big A, little B, little B over here. Once again, we're not asking anything too difficult of the little B allele. All it ever has to do is work with the big A allele. Now what happens when the hybrids form here? What are they gonna be? Well by necessity, hybrids are gonna be big A, little A, big B, little B. The point here is that there's some allele combinations, for instance, the big A, the little A allele, has it ever seen the little B allele in its entire evolutionary history? No, it's never seen each other. What a geneticist would say is that little A and little B have never been tested with each other. There's no guarantee that the little A and the little B alleles will work with one another. That is to say, if they're performing some enzymatic activity, there's no guarantee that they'll continue to carry on an enzymatic activity, or if they form some structure in a cell, there's no guarantee that the structure will be the same. Now you're probably saying to yourself, well how about big A and big B? They work down here, but the point is that you're gonna have some protein interactions that are normal and some that aren't. And so the Dubjonsky-Moller model of genetic incompatibility says that the incompatibility is between these mutations that have never, these alleles that have never been tested and they break something that was formerly working. Okay, and so the hybrids, when you break something that was formerly working, the hybrid organism is less fit. It's either dead, that is it's a lethal hybrid, and because of these incompatibilities, normal development can't occur, or for whatever reasons it has a lower fitness, or it can't produce fertile offspring. So that's the Dubjonsky-Moller model. I'm not gonna give you any specific examples, but there's probably about a dozen of these examples now out in the literature. So they're usually involving fruit flies, and they're actually pretty complicated experiments to actually show these Dubjonsky-Moller incompatibility in real life, but this is the leading model for how, of a post-cygotic mechanism that keeps species from forming hybrids, okay? It's the hybrids that are less fit, or they don't work for some reason, and that keeps populations, species from coming back together. Are there any questions about that? So when you add third, or more low side, I mean the population genetics theory that people develop that goes along with this model tends to have what's called a snowball effect. And so that these Dubjonsky-Moller incompatibilities is one arises and the second arises more rapidly in the third, so you really get a very rapid evolution of more and more incompatibilities that causes the species to remain separate. So that's the, I mean, you can add in a third or fourth locus, but it's not like you're gonna help things out, you're making it only worse. Any other questions? That was a good question. Yes. So that sometimes, and so what I've avoided in this lecture and what I'm going to avoid is speciation in plants, where there's a mechanism of speciation called polyploid speciation where in plants you can have individuals from different species forming hybrids that are then themselves are perfectly fine and are genetically incompatible with the two parental species. So it's almost like instant speciation. It's quite complicated and I decided last year not to talk about polyploid speciation because I spent hours and hours going over it, but it's actually quite fascinating and worth the mac and give you the literature if you're interested in finding out more about it. Your book discusses polyploid speciation as well. But yes, sometimes that can happen. The hybrids can actually be new species themselves and that's a common mechanism of species formation in plants. Are there good questions? I mean, these are good questions. Any others? I think what I wanna do is go back to the screen and give some examples while I don't forget. So here's the Pacific Ocean, big place as you know, and I'm just focusing my attention on the Hawaiian islands and I'll have a more detailed map, but you may be aware that the Hawaiian islands are a series of islands with the biggest being the youngest down here, which is Hawaii. And then as you go along this chain of islands, the islands tend to become older and older. Kauai, for instance, is quite a bit older than Hawaii. And the idea here is that the continental crust is moving over a hotspot. So you always have the southernmost island is the one that's been actively formed and in fact further off, further south of Hawaii is another mound that hasn't broken the surface, but is the now more active and growing future part of that archipelago. But here's an example that there's lots of examples on Hawaiian other isolated islands like that of radiations of a similar type of organism. So here's honey creepers, some of which are already extinct, but these are honey creepers, a type of bird that occurs on the Hawaiian islands. You've had a large number of these things. All these birds are each other's closest relatives, which suggests that they're all formed from a single ancestral species that somehow made it to the island. Okay, and then radiated. And you've got a radiation in form. So if you have some of these honey creepers specialized on breaking seeds, typically seed eaters have these thick and short beaks versus others that are nectar feeders, I believe, or insectivores. But you've had this radiation of honey creepers on the Galapagos islands or on the Hawaiian islands. This is one example of a radiation on an island. There's also been radiations of major radiation fruit flies on the Hawaiian islands. And here's another example of a radiation of organisms. This is an example of these rift lakes in East Africa. Here's the Lake Malawi, for instance. And in these lakes, you've had a radiation of fish called cichlid fish, okay? And these lakes can actually contain a hundred or more species of cichlids. And the idea here is that these lakes, although looking at a map, of course, they look like they're a uniform habitat, but they're actually quite heterogeneous. Not only do you have heterogeneity along the shores, but the environment can be partitioned by depth. So you have some cichlids that specialize to live in the water column versus others that are along the shores or in the deeper parts of the water, of the lake. So you've had this radiation of cichlid fish in these lakes that's probably driven partially, at least by allopatric speciation, okay? Divisions of populations up into smaller populations. And this is a new word, actually. I've used this word a couple times now, adaptive radiation. The biologists often use the word adaptive radiation when they're speaking about an especially diverse or speciose part of the tree, okay? So there are some parts of the tree of life where you see more species that seem to have occurred at a higher rate. Species formation seems to have occurred at a higher rate. Cichlid fish are the classic example of an adaptive radiation where, in a relatively short amount of time, you've had many, many species formed. It's often the case that these adaptive radiations appear to be linked to some trait. And the question then becomes, is that did the evolution of that trait somehow help or cause the adaptive radiation? And often too, like in the cichlid fish, there's other factors that are probably contributing to the adaptive radiation such as sexual selection. So female choice might also be playing a large role in causing that adaptive radiation. Okay, let's go on. Here's an example of an allopatric speciation event of what are called stomatopods across the isthmus of Panama. So there's lots of examples, including these guys of species pairs, sister species, where here's your isthmus of Panama, there's the Panama Canal, but you have one species here and another species here, and these two species are each other's closest relatives. So you have lots of pairs of species just like that. And here's an example from stomatopods that was described by Nolton in 1993. And here's an example of variation of the genitals of Drosophila, once again sort of referring to this mechanical isolation that can occur in many species. That's one reproductive isolating mechanism. They can't do it, so to speak. Now there's some experimental examples of how isolation can occur just based on natural selection. So this is a classic experiment by Diane Dodd that was performed or published rather in 1989. And the basic experiment was this. You probably know that you can take bottles, you stopper them, and you can put some fruit flies in there. There's the fruit flies, right? And typically what they do is at the bottom of these bottles they put some sort of food source, okay? Sometimes I mean I'm not a Drosophila geneticist, but remember you go into these labs they smell of bananas because bananas is one of the food sources. But what Diane Dodd did is she basically had some food sources had a starch base for the sugars. And then she also had bottles of flies where the food source was maltose. Two different sugar sources. And she basically passages flies from one bottle to another for eight generations on the starch base medium. And then she also did four replicate experiments over here where she passage for eight generations flies from one fly bottle to the next on the maltose base medium. So she let these guys go independently for eight generations. Now the first thing she could do is she could ask, well this is only eight generations, but at the end of this experiment, you have to remember the flies were all from the same bottle when she started these off. So she just took one bottle of flies and split them into eight bottles. But she asked after eight generations, could she detect any genetic differences between these different fly lines, these different eight bottles? And she did. So she's using a technique that nobody really uses anymore called starch gel electrophoresis. She was able to detect differences in proteins in these different flies. Probably caused by natural selection or selection operating differently in these different flies, selecting for flies that grew well on starch versus maltose. But interestingly, she also found differences in the female choice after eight generations. So what she did is she took females that were grown say on the starch in the starch bottles and took them from a male that was on a starch or maltose. And she asked, how often does the female choose males that were grown on the same medium versus different? And so this is the experimental outcome. She found that the females actually were choosy. They preferred the males that lived on, that were grown on the same medium. And so you can also do the reciprocal experiment. You take the maltose females and ask, does she do these females that were grown in the maltose bottles? Do they prefer males from the starch bottles or the maltose bottles? And they preferred the maltose bottles. So what you're seeing is the evolution of choosiness in the females as well, to prefer males that were grown in a similar habitat. She also did experiments, the control experiment, which is you take females from the same starch bottle and made them with males from the same starch bottle or try to make females that came from one starch bottle and see if they preferred or didn't prefer males from a different starch bottle. And the control experiment wasn't conclusive or there were no significant differences. So the conclusion is that there was a significant difference here with the only factor being the sugar source in the bottle. So this is an experiment that's been repeated multiple times in an even different species. Yes, probably behavioral differences is the idea. But the female, I mean, it's hard for us to tell but the flies can tell. So for whatever the females are picking up on, that it's significant to them, okay? So it's the classic thing where in the biological species concept in a century, you're letting the individuals themselves determine whether they're different species. So sometimes we can't really determine any traits or see behavioral differences ourselves but the organisms themselves certainly are picking up on something significant. In this case, that's the case as well. So now I wanna turn to the last thing I haven't talked about which is sympatrix b-siation. So sympatrix b-siation is b-siation that occurs within the range of the population. So it's like you have overlapping populations and yet you still have divergence in those two populations. So they're not geographically separated but they nonetheless undergo speciation. And the example I'm gonna give is with the ragelitas, a type of fly. It's an agricultural pest. And this is a case, many of these examples of sympatrix b-siation occur when the organisms have a lot of fidelity to a particular, say, food source. And so in this case with ragelitas is what you have is you have, well, ancestral you have a hawthorn fruit. And what happens is these males, well, here's a male, the males hang out around these fruit and they wait for a female to come. The females are gonna lay their eggs over posit on these fruits. So the female comes along, here's the female. I guess they have eyelashes. There's the female. The female's coming here, she's gonna mate with the males. Remember the males are hanging out around these hawthorn fruits. The female mates with the male and then she lays her eggs. There's the eggs on the hawthorn fruit. And then of course what happens is these eggs hatch and they spoil the fruit, which if it's a hawthorn tree I guess you don't really care, but they spoil the fruit. The larvae grow and then they have fidelity to the same host. So they remember that they're on a hawthorn and that the males, for instance, will return to the hawthorn to hang out, buzz around the hawthorn fruit and the females will remember to oviposit. Now historically about 100 years ago, at least some of these ragalitas made a mistake. And they started to hang out, not around hawthorn fruits, but around things like apples. This is when they became an agricultural pest because when the females oviposited their eggs on the apples, they spoiled the apples. And of course the people that raised apples were not happy about that. So all of a sudden the ragalitas which weren't on anybody's radar were suddenly classified as a pest, okay? But the point is you had at least some individuals change their preference. Now the point is that the individuals, so here you have another group of flies, here's the apple. You have another group of flies buzzing around the apples, ragalitas buzzing around the apples, mating the males hanging around the apples, the females laying their eggs in the apples. These individuals can, in terms of the ragalitas, they might pass each other in the same orchard, right? You can imagine this guy's name is Sam, here's Ralph. They can be passing each other, say hi Sam, and the other guy says hi Ralph. This guy goes to apple, this guy goes and mates with the females on the hawthorn. They never actually gene flow between these populations of the guys that live on the apples, mate around the apples, and the ones that mate overposit on the hawthorn fruit, they're reproductively isolated, even though they might buzz by one another in the same orchard, okay? So this is an example of simpatrix p-station because they buzz by one another. There's nothing in principle keeping these guys from mating with these guys except for the fact that they're behaviorally isolated. So that's the story with the ragalitas. It turns out that like I mentioned, this switch to apples and some other agricultural things like cherries and pears occurred about 100 years ago. It's a big pest now. You do have what's called assortative mating. So this is an example of assortative mating where the apple males and females only mate with one another and the hawthorn males and females only mate with one another. So this assortative mating is causing gene flow to be turned off. And today, after only 100 years, you have two races. The apple hawthorns are genetically different. One from the other using sort of a method that can detect genetic differences. And they've also been able to now document other differences between the hawthorn and the apple populations. For instance, the developmental time is different. 40 versus 60 days for the hawthorn maggots. So this is an example of perhaps incipient speciation occurring in a simpatric way. All right, so I think that is where, well, I know that's where I'm gonna stop for today. We'll pick up with phylogenetic methods next time.