Sexual Selection and Altruism Overview

We'll continue along our discussions from last lecture. Remember last lecture I gave you the speech your parents probably didn't give you about the birds and the bees. I mean, unless your parents talked about independent assortment and recombination and variability being produced, I suspect they didn't. So what I want to do today is talk about, as I sort of mentioned the last time, the canna worms sex sort of opens up and some of the interesting things that occur. Just as... Just to give you a few slides that I didn't show you last time, this is just an example of a parthenogenic species. This is a whiptail lizard. That is to say, these lizards of this species are all female. The young of these females are identical to them. And here's an example of an asexual species, Daphnia, which has a choice. Sometimes it undergoes rounds of asexual reproduction, sometimes it undergoes rounds of sexual reproduction. And organisms like Daphnia are important because it shows you that Many organisms do have a choice about how they do things, right? And so there must be some advantage to sexual reproduction, even though I gave you a long list of reasons why it shouldn't evolve. The so-called two-fold cost of sex is one example. Now, what I want to turn my attention to today, largely, is the evolution of traits like seen in these birds of paradise. These are all males, and many species exhibit what's called sexual dimorphism. The males and the females differ in form. It's true in humans, for instance. In the example of these birds of paradise, the males sport these very long tail feathers. They often are very brightly colored. They have interesting ornamentation on their heads and so forth. And the females typically are just bland. They look like what you'd expect a good bird to look like, somebody who wants to hide from predators. And so the question is, how can these characteristics, these elaborate characteristics, evolve when They're clearly maladaptive in at least some ways, right? If you have a very long tail feather, it's literally a drag. It slows you down, you can't fly as well. If you have brightly colored plumage like these males over here do, you're much easier to spot to predators. So there are these characteristics that are difficult to explain with the vanilla flavor of natural selection that Darwin came up with in 1859. So how do these characteristics evolve? Well, the explanation... It's called sexual selection, and this is an idea that Charles Darwin put forward, as well as Alfred Russell Wallace. He's the fellow that co-discovered natural selection. Charles Darwin explained this in a later book he published around 1871. And the idea is that these strange traits tend to be associated with the males of the species. And the idea is that animals don't only compete to survive, but they also compete to reproduce. and that somehow these secondary characteristics, these bright plumage colors, for instance, might have to do more with access to females than they do to survive. To get into the next generation, to get your genes to the next generation, not only have to survive, but you have to successfully reproduce, obviously. And just to go over some of these secondary characteristics, I mean, lots of males have these crazy characteristics, like the Irish elk, which is now extinct. Its antlers were over 12 feet in length. just tremendously huge antlers. Bighorn sheep, the males have these large horns, which are associated with fighting with one another. Here's an example from beetles. There's the female beetle, looks like a beetle. The male has these long extensions from its head. And of course, you might be familiar with elephants, where the males sport long tusks, and the females have shorter tusks. Here's an example from a lion, where the lion has this big mane, clearly different than the females of the species. Here's an example from a pet pheasant where not only the male's more brightly colored, but the males have these spurs along the side of their legs. And they use these spurs, these sort of sharp points here, to fight other males, to gouge them. And then there's some guys that have these little fake spurs. The females actually tend to prefer these nice spurs. And there's males out there who are horrible fighters, but they try to fake out the females with these sort of fake spurs. What types of sexual selection are we going to be discussing? Well, there's two flavors of sexual selection we talked about. One is male-male competition, that is to say competition among males for access to the females. And the other type we'll be discussing is mate choice or female choice because the female is almost always the choosy sex, and we'll describe why that is the case. So this would be intersexual selection, competition between males and females. So male-male competition. There's a variety of different kinds of male-male competition. You have competition within groups for dominance. I'll give you an example of that. You have examples where a male defends a group of females from access. from other males, that's called female defense polygyny, or they might defend a territory, and hopefully a good territory that females might want to hang out in. And then we'll describe Lex, or I'll describe Lex. So within group dominance, if you have more than one dog in your household, you're probably familiar with this, that the dogs set up a hierarchy pretty quickly. In our family, we have two dogs, and it's very clear which is the top dog in our family. This is an example of... within group dominance where some individuals are higher in the hierarchy than others and they have access to the mates. So an example of that would be, for instance, gray wolves. Here's an example of female defense polygyny. These are northern elephant seals. You can see these are the two males fighting with one another. They sort of have these tremendous fights, almost like sumo wrestlers, except a little bit more deadly, because they can actually get really gouged up. And what they're doing is they basically take over a part of a... beach where the females might be breeding. And if they can defend that patch of ground from other males, they can mate with the females on that patch of ground. Actually, I had a rough experience, I shouldn't say rough, a frightening experience when I was an undergraduate. I was doing a bird survey on San Nicolas Island as part of a research project, and I just had my head up and they were looking for birds, of course, and I got between an elephant seal and the ocean, which is a really bad idea. They're very, very large, scary animals. They don't look like it, but they are. Here's an example of a species that has territorial defense polygynies, an impala. Once again, a male will defend a particular territory from encroachment from other males, and they'll mate with the females that are in that territory. So far, we have nothing to do with females being choosy, but you often see these secondary characteristics in these species, such as horns or antlers in the males that they use to fight other males. Here's an example of water bucks head-butting, same type of thing. Remarkably, when you think about male-male competition as sort of ending after the copulation has occurred, but it turns out that's not the case. There's lots of species where the females can be mated multiple times by different males. And this happens, for instance, in Drosophila. If you put a bunch of Drosophila in a vial, the females will mate with multiple males. So after the act, the sperm from different males actually compete within the female's reproductive tract for access to the egg. And there's examples of flies, for instance, that after they mate with the female, they actually sort of insert a plug to keep other males from having access to that female. Or examples of males with their penis can sort of scoop out the plug and put in their own ejaculate. So there's lots of examples that are becoming more and more interesting as people discover that the sperm compete with one another as well for access to the eggs. So male-male competition can occur at several different levels. Now the next, so that's all I wanted to say about male-male competition. There's another example of sexual selection which is called mate choice. This is typically the cases that the female chooses which male she wants to mate with. There's two broad categories. The female bases her decision upon resources the male brings or some other indirect benefit that the male is going to bring. So I'll describe examples of both. Now let's sort of, first of all, claimed several times that in most cases it's the females of the choosy sex, but not always. Why is that the case? Well, this is a direct result of anisogamy. Remember I talked about isogamous species are those that have gametes that are produced that are about the same size. Most species are anisogamous, meaning that some gametes are much larger than the others, and by definition the females are the individuals that produce the larger gametes. Well, these larger gametes cost quite a bit. The reproduction for females is limited by the number of eggs she can produce. Males, on the other hand, are not limited by the number of sperm they can produce. They're a dime a dozen, so to speak. They're limited by access to fertilization to eggs. So this sets up an asymmetry where the female basically can guarantee to have her eggs fertilized, whereas not all males are guaranteed to be reproductively successful. In fact... When you go out into nature and you look at males and females and you ask the question, how successful are individuals at reproduction? Almost all females are successful. That is to say, there's very little variation among females in most species for how successful they are at breeding, producing young. However, males are much more variable in their success. Some males are fabulously successful and some are complete losers, meaning they don't have any young at all. So males typically have a much higher risk. type strategy in terms of how they live life. All right, so it is a case that eggs are more expensive, they cost more to produce. And also in species that have internal fertilization and that carry their young to term, that's quite costly. It's an expensive undertaking for the female, not so expensive for the male. So there are cases, however, when you have a sex role reversal where the males are the choosy sex. And that occurs in these rare cases where the male contribution to, say, raising the young exceeds the female cost of producing the eggs. And so there are some examples of that. For instance, the seahorse. The males will raise the young. The female basically lays the eggs and takes off, kind of what males typically do in most species. And the male is stuck raising the young. So in this case, the male can be quite choosy about which female he mates with because he is controlling the resources. He's the resource-limiting step. All right, so mate choice. So why might... Usually females choose in the first place. Well, there's one explanation for a lot of these elaborate characteristics you see in males is to ensure that the female mates with the correct species. Okay, so that often you have not only plumage or secondary characteristics or traits that males carry, but also often elaborate dances and whatnot in these courtship displays. And often these courtship displays are very species, they are species specific. And so one thought is that these types of displays and behaviors ensure, or better ensure that individuals mate with the correct species. You can imagine that if you make a mistake in terms of which species you mate with, that's a big mistake in terms of your reproductive potential. Another idea is that the mate you choose might be better able to fertilize your eggs or might give you a higher fecundity. Or maybe the male brings you food, or is a better parent, or brings to you a better breeding territory. That is to say, a territory with more resources. There's a number of others that you can imagine as well. So let's talk about, of all those reasons, you can break them into two broad categories. So the direct benefit, that is, what does the male bring that's going to help you out immediately in terms of resources? So these are direct or proximal benefits. And an example would be the bush crickets that give the female what are called nuptial gifts. So they produce these spermatophores, which are very rich in protein, often consisting up to 30% to 40% of the male's weight, and basically it's a large energy content. And the female will accept these gifts, and basically the more spermatophores allow the female to lay more eggs. She has more energy with which she can expend on reproduction. So this would be an example of basically giving food or resources to a female, and the quality of that gift being something the female can use to choose among different males. That's an example of a direct benefit. Now, indirect benefits, these are benefits that are a little bit more difficult to measure. And so the only indirect type of benefit I'm going to discuss in this class is the so-called good genes hypothesis. That is to say that the female chooses males based on the quality of the genetic constitution, so to speak. And the idea here is that the female wants to mate with as high a quality of male as possible because then her children will also be of high quality. So what are some of the components of the good genes hypothesis? So first of all, like I said, the females are going to choose a mate which offers high quality genes, influencing survival. Mate quality has to be indicated by some secondary characteristics. So, for instance, the brightness of the plumage. So, if the brightness of the plumage in some way indicates how good your genes are, then the female has some basis on which to choose. So, if I choose males with bright colors, in general, those are the individuals that are going to have better genes. The secondary trait has to be heritable, and there also has to be heritable variation in the male quality. quality, sort of the mate quality. That is to say that in order for this hypothesis to work, there does have to be variation in how good the males are out there in terms of if you mate with one male, your children will have lower fitness. If you happen to choose correctly, your children will have a higher fitness. And there has to be some cost to the males bearing the trait. So some of these, some variants of the model, there's no cost to the trait. And others, there is a high cost to the male bearing the trait, the so-called handicap model. That is to say, If you have very long tail feathers and you can survive with those very long tail feathers, then the female can choose that trait, saying that in order for that male to survive with those long tail feathers, he must be pretty good. kind of a handicap model. But in any case, there's variations in this model, some of which don't require the secondary trait born by the males to be costly and some where they must be costly. So let's give some examples of the good genes. These are very difficult experiments to perform because you have to measure variation in the males and the traits that the males have. and also in the quality of the offspring that are produced. So they're very tricky experiments. So one example is in Hyla versicolor. It's a gray tree frog. And these frogs have short or long calls. So the males go to a pond, and they have these calls, and they can be classified as short or long. And the females are attracted to the calls. And generally speaking, they choose, or they like the long calls. So the question is this, is if the female chooses the long call, males, do her children, do her offspring have some sort of benefit? So what we've got here is a table of things that these researchers were able to measure. So they were able to measure the larval growth, how rapidly the larvae grew, how long it took them to reach metamorphosis. So in case you don't remember, frogs lay eggs, the eggs hatch into tadpoles, and the tadpoles undergo metamorphosis into adults, right? Just keep that in mind. You can measure the time to metamorphosis, the mass at metamorphosis, how well the larvae survive, and how fast the adults grow. And whenever you don't see a LC in the box, that means that there is no significant difference in the offspring between long-call and short-call frogs. But you see here, when you raise the larvae in a high-food environment, the long-call frogs had an advantage in terms of their larval growth and their time to metamorphosis. And the low food, if you raise the larvae in a low food environment, then the long-call males also produce offspring that are better at growing. Not in all cases, not in all the characteristics that were measured, but at least in a few. So this has taken a support for the good genes hypothesis. There is some variation in the quality of the offspring produced based on whether a female chooses the long-call or the short-call males. Yes. Oh, yes. So if a trait has some sort of, I mean, what I'm saying is you can think about a trait evolving in two different scenarios. There's the good old natural selection, which always will work for having a trait spread through a population. So if that trait directly enhances the survival or reproduction of that individual horse, it will spread. But it's these characteristics that are difficult to explain any other way that natural selection would have a hard time explaining. So for instance, if you can imagine a scenario where you just... that the females all of a sudden didn't become choosy, say, in the birds of paradise, which is the first slide I showed with the birds with the very long feathers, then the argument would be that natural selection would not favor those long-tailed feathers. Natural selection alone would prefer to have males that are cryptic, right? That predators have a hard time seeing. So it's these traits that would be difficult to explain using good old natural selection that pose the problem, frankly. How do these characteristics evolve? How do seemingly maladaptive traits evolve? Well, the idea is, yeah, they're maladaptive in terms of your survival, but they grant you better access to females. And selection in general is all about leaving more offspring in the next generation. Is that clear? Okay, so the last thing I wanted to talk about with respect to sexual selection is a little bit about lecking and the evolution of lecks and the so-called sensory bias exploitation hypothesis for sexual selection. So a little bit about a lex. So lex are aggregates of males. So lots of, for instance, in bird species this happens quite often, where you'll have several males will hang out on a branch of a tree, say, and the only reason they're there is not for food, it's for, they basically are advertising for females. So females come by, so like on a shopping trip, and they say, you, and they go off with that male and he's the lucky guy, so to speak, right? Now the female doesn't come to these places, to these lecking sites, for anything other than to choose a mate. Okay, so they're not there for the food or anything. They're just there to choose a mate. So it's often the case that they don't have these resources. I think I described everything I wanted to talk about there. And it's also the case that in these lecking species, the male does nothing but mate. So it's typical in lots of species. The male provides the sperm and then he's gone, and the female's left with the chore of raising the young. So there's a number of models for why lex might evolve. One is that by aggregating like this, sort of having a one-stop shop experience for the females, that there might be a lower predation risk for the males and females. It might be that there's some sort of passive attraction, that is to say, if you're a bunch of males hanging out together, that maybe you're more likely to draw females and increase your chance of reproducing as well. There's this black hole model, which is the females aren't particularly choosy, but they want to avoid Danger is associated with mating. Perhaps going there, picking a male and leaving is much easier than going through the forest looking for a mate. Or the hot shot model where basically the females are going to choose one of the birds and the other birds are hanging out around this really hot looking male just with the hopes that they'll, you know, get somebody, which is kind of the pathetic model for this entire thing. So what's an example of a lecking species? So this is a, bower birds are a bird species that live in Australia and New Guinea and they produce what are called bowers, that's why they're called bowerbirds, which are basically leks. You have a part of the forest where you have a bunch of these different bowers, different males producing bowers, and the female comes to these sites in the forest and looks at the bowers and chooses among the males based on the quality of the bowers. So this is an example of a bower. I'll show you another one. Here's the male, here's his bower. Basically what he does is he sort of stamps down grass and has twigs in such a way that there's like a little courtyard here. And often they'll bring brightly colored objects to these bower sites. So they'll decorate their bower with, some of them like really, they like blue objects. So they'll like take parakeet feathers if they can find them. But nowadays it's more like blue bottle tops and things like that they'll bring to these bower sites. So here's an example of a bower. Here's the female checking out the bower. Notice she's kind of drab colored. The male's this beautiful black satiny bird. She's checking it out saying, hmm, nice bower. Nice structure, straight leaves. He's sort of doing a dance and, hey babe, you know, I'm over here, and I don't know what happens. I mean, either the bower wasn't very good and she took off and looked at another bower, or she said, this is great, come on over. So here's some examples of bowers and different bower bird species. Some of them can be quite elaborate structures. I mean, look at these things. They're just amazing. And remember, these aren't nests. It's not like... These are nests where the female will eventually lay her eggs. These are just there as courtship displays, like courts, essentially. So that's an example of a lack. Now, the following thing I just can't help but talk about. There's a lot of interest among evolutionary biologists and ecologists in sexual selection, especially. And there's a fellow, Jerry Borgia, who's at the University of Maryland, whose students and postdocs have done a lot of work. Studying bowerbirds essentially. And so they did this experiment where they're looking at, they're trying to test, like I said you don't have to know this part, I think it's such a cool experiment. They were trying to test the visual cues that the females were giving to the males about whether she was receptive to mating or whether she wasn't. And so they wanted to be able to control this very finely. So what they did is they made robotic birds, okay. They called them fembots. If you've seen Austin Powers you know what what we're referring to, the fembots. And so here's a robotic Female bowerbird. Here's the sort of control apparatus. They had these little remote controls they could sort of hide behind a tree and say, okay, let's see if the male likes this or, you know, what the male will do if I do this signal or that signal. So here's an example of what they did is they sort of, when the male was off, they'd kind of sneak into his bower and put the female there and the male would come back and say, woohoo, and you know, do his courtship dance and then they would sort of manipulate the female fembot. With the idea being, well, if she dips her head, what does the male do? If she sort of does this, what does he do? And so forth. So here's an example. Here's the fembot right there. The male's pretty excited. This is great. He's going to do his dance. Kind of realistic looking bird, but maybe he's even got sound. So he's looking around. Apparently they've done something to the remote control, which is getting them very convinced that this is it. This is so... This is... This is so embarrassing. This is so embarrassing. Okay. Anyways, that was a fembot. So mostly just to prove to you that scientists can have fun too. Fooling, I mean, it's a weird sense of fun to fool birds into having sex with, you know, robots, but fine. Anyways. back to serious science. I want to talk a little bit about one last hypothesis for the evolution of male characteristics. And this is the so-called sensory bias or exploitation hypothesis. And the idea is that for some reason, it doesn't have to be related to sexual selection. ...section at all, but some trait evolves that allows females to be more receptive to some sense. So, for instance, maybe something in the ear changes so that they hear low vibrations better, or maybe something in the eyes changes in such a way that they're more sensitive to bright colors in some spectrum. Whatever it is, it doesn't matter, but the idea is that the male's secondary characteristics, the trait that you think of as being sexually selected, evolves... to exploit that pre-existing sensory bias. So I'll give you a couple of examples. One is in Physolemus coloratum and pustulosum. These are, once again, frogs, where the males... I used to have a recording of this, and I lost it, and I can't recover it. But the males have a call which has a series of whines and some chucks in it. Okay? And I can't... reproduce. I know there's a fellow, Mike Ryan, at the University of Texas who's great at frog calls. So in the middle of his lectures, he would give frog calls. I'm not Mike Ryan. I can't do it. But anyways, the males will attract, the males of the Physolemus postulosum, they will attract maids by calling using wines and chucks, whereas the Coloradum, their male calls only have the wines. And if you look at the phylogenetic tree, you can actually show where the chuck evolves, so wines are down here, but the chucks evolve only in the males of Postulosum. Now what they did is a series of experiments where they would record the sound of frogs with wines and chucks or just wines or just chucks and go to these ponds at night and play these recordings and ask whether the females of Coloradum actually prefer the calls of their species, just the wines, or do they really like chucks as well? So that is to say if you Play a recording of a mate call that normally they wouldn't hear, one that has some chucks put in the end. Will they be more attracted to that or would they be more attracted to just the calls with the whines? It turns out that these females of this species actually prefer the chucks. Even though their males don't have chucks, they prefer them. So you can actually plot the trait for the preference in the females right here. So the idea here is that the preference for these traits under the sensory exploitation hypothesis has to predate the evolution of the male characteristic, right? Otherwise this hypothesis doesn't work. So here's an example where the chuck preference evolves in the females and later in postulosum, viscidum postulosum, the chucks evolve in the males. There's another, I think I just told you that, another experiment with swordtail fish. This is actually was a study done, gosh, almost 20 years ago now, 15, 20 years ago, by Alexandra Basolo. She was also at the University of Texas working with Mike Ryan. And what she did is she looked in swordtail fish, and their closest relatives are these so-called platyfish. Now, these swordtail fish, they're called swordtails because the males have a little extension on their tail called a sword, and often there's little bright colors or stripes along the swordtail. What she did is she manipulated the tails of platyfish in such a way that... Sometimes she would glue on an extension to the tail that looked a lot like a sword from one of the swordtail fish, or sometimes she'd glue on a clear extension, just a little bit of plastic that was acting as a control, and ask the question, in these platyfish, do the females prefer males that have a sword, even though the males of that species really don't, or do they prefer males without swords? And she found through these mate choice experiments that the males in these platyfish really actually would... The females would actually prefer a male that had a sword. They don't, but they would actually prefer one. And she also did the preference experiments here where you can actually chop off a tail and then reattach a long sword or a longer-than-normal sword or, once again, the control where you chop off the tail and then put on a clear bit of plastic that's transparent. And once again, as you might expect, the sword-tail females prefer males with swords. In fact, they prefer swords that are even longer than you can find in many of the males. So the idea here is that the sword preference evolved here in the phylogenetic tree, and the actual swords in the males evolved after. Once again, this is an observation that's consistent with the sensory exploitation hypothesis, that for some reason, visually, females prefer these long swords, and that later on you have the male trait evolve to exploit that pre-existing bias. Okay. Now, are there any questions up to this point in the lecture? Yes. Several questions. Okay, I'll start with you, then I'll go back there. So, we'll get to speciation later, and one of the definitions of species is that... Is that they can't produce fertile offspring? So no, they typically can't. Okay, so they would actually, but you can do the experiments in the platy fish by, you know, like I said, manipulating the tails and saying, let's put on a long tail here, even though the males don't have it, do the females prefer it? And they do. So the preference for these swords exists there, but that doesn't mean that, you know, they might actually think that that other species males looks pretty good, but they don't make the mistake, or if they did, it's a consequential one. Yeah, that is, like I said in the very beginning, for whatever reasons, there's some, you know, so for instance, organisms, you know, our senses, my hearing, our hearing is attuned to different ranges than other species. So for whatever reason, you know, you have the senses you have, okay? They've evolved under maybe a good old natural selection. And the point here is that whatever preferences or whatever... Sensory biases you tend to have, the males are trying to exploit it. So it doesn't say anything about how the preference evolved at all. It just says it's there. And you kind of know about these cell phones, rings that adults like me can't hear because my ears have gotten worse, but teenagers can. Well, it's like that, but even more so when you go to different species. Like dogs, for instance, can hear what higher-pitched sounds than humans can, right? There's a... You could imagine a, we don't have this type of thing, but a mate calling humans that was in the dog range, it would be completely ineffective, right? We can't hear it, the females can't hear that. Any other questions? Those were good questions. Hopefully that clarified it. Yes? Yeah, so the idea is, so one thing, maybe I didn't emphasize this enough, but natural selection always operates on the variation that exists in the population at that time, right? So if you have a population where there's no variation in, you know, tail length, for instance, natural selection, or sexual selection for that matter, can't grab a hold of it and change it as a trait. So you can't have a trait evolve until you have variation for that trait in the first place. So the idea is at some point along that lineage, I mean, you give enough time and perhaps mutations... Mutations will occur that will give you some variation in tail length. Natural selection was able to grab a hold of that, or sexual selection in this case, and lengthen the tails. But you can't have evolution without variation. It would be very difficult to select for luck, for instance. There is a great science fiction book, I forget which one it is, but it's Terry Pratchett, The Ring World, but basically these outer space people select humans for luck by basically... Always allowing people that won this reproduction lottery, that they were able to have kids only if they won a lottery, with the idea being these people that are reproducing must be somehow lucky. Of course, it's just chance. There's no variation in your luck. It's kind of a silly example of evolution which could never occur, but it was kind of a fun one in a science fiction book. You have to have variation. There can't be variation for luck, and yet in this book, luck evolved. Remarkable things happened to this person in the book because she was so lucky. Okay. So I want to turn my attention to one subject that kind of got lost in the mix when we had these RRR days at the end of the semester. You know, I lost a lecture, but I think it's important to include this material, so I'm sort of stuffing an extra lecture into this one. So I apologize if there's a lot of material today. But what I want to do is discuss the evolution of altruistic behavior. Now there's lots of behaviors and traits we can't explain. So for instance, we just discussed sexual selection, which can... can explain these seemingly maladaptive secondary characteristics in males. And this is always easy to understand, right? So when you have predation, like the winner and the loser here can't be clear. This guy's the winner if you need help, and that's the loser, okay? Things like that are quite easy to explain. But there are behaviors that would be difficult to explain via just natural selection or even sexual selection. And these are seemingly helpful or altruistic behaviors. And so, for instance, there's in the building grounds world, a very famous example. There's examples of alarm calls. So you're some squirrel and you see a hawk, you have a choice. You can, you can, and there's other squirrels around you, so other guys around you, can either just duck out of it, say I see that hawk, I'm out of here, or what they do is they give an alarm call before they, they scurry away, basically alerting the other ground squirrels in the area that there's a predator. Now the problem with that behavior is, it sounds, sounds inconsequential, but the problem is when you make that call, when you When you raise that alarm call, you make yourself more conspicuous, and you're more likely to die and get swept down upon by that hawk and eaten. So there is a cost to that helpful behavior. It's an altruistic, a seemingly altruistic behavior. So there's examples of alarm calls or sentinel behavior where some individuals will have a lookout while others scurry around eating, such as the meerkats, very cute little animals you see at the zoo, or nest helping. where some individuals might forgo reproduction to help out raising young, or use social behavior, where there are certain individuals that completely forgo all reproduction to help raise other individuals. These are all examples that would be very difficult to explain on the surface of things via natural selection. How can those helpful behaviors evolve? So all these are examples where the actor, some individual performs an action that benefits some other recipient. And so it might be useful to think about And so I'm going to be talking about several different examples of helpful behaviors and how they might evolve. One is mutualism, which is an easy one to explain. That's the actor directly benefits from the behavior. Reciprocal altruism is where the actor, the one that's providing the helpful service, actually eventually will benefit. Not immediately, but eventually. And then kin selection, where the individuals you're helping out are related to you, closely related. This is so-called indirect selection. We'll talk about that as well. So it's worthwhile sort of trying to classify the different types of, you know, behaviors or interactions you can have. So mutualism, the donor and the recipient both experience some sort of benefit. Easy to explain, right? If you're a trait that helps yourself out, you know, your survival or reproduction, that's an easy trait to evolve under natural selection. At reciprocity, once again, the... The benefit to you might be delayed, but you eventually benefit. Once again, easy to explain. Altruism, altruistic behavior is difficult to explain because you suffer some sort of cost where some other individual gains. We'll explain how that can evolve. Selfishness, that's easy to explain. You benefit, the other guy doesn't. Predation is a good example of that, right? You definitely benefit, the other guy doesn't. And spitefulness is something... we don't see out in nature, but basically you and the other, the recipient, both suffer some costs. My kids have that, but you don't see that out in nature. So here's an example of mutualism where, for instance, groups of lions, they often will cooperate, and by cooperating they can bring down larger prey and they have a higher success rate. And they can better defend their kills from other lions and hyenas if they operate as a group. So everybody gains, everybody benefits. Blue gill males, excuse me, off in the bottoms of lakes, they'll form nesting sites where you have 50 to 100 males in nests. And this can be thought of as mutualistic because by aggregating, you can actually have a lower predation risk. So basically, you can better defend that nesting site from other predators, and you can think of this type of behavior as a mutualistic type of one. Often you'll have male lions cooperating with one another to oust the resident top lion in a pride. Often these males that help oust the top lion are closely related, like brothers. But by cooperating, they can actually have a better chance of ousting the current top dog or top cat, I guess. Okay, in a little bit, those are some examples of mutualism. Those are easy to explain. Reciprocal altruism is a little bit more difficult because you need to show that eventually the actor is going to benefit. So you do something helpful to some other individual, you experience some sort of cost, but then you have to show that eventually that act will be reciprocated. this type of behavior is going to be more common when you actually have repeated interactions between individuals. If this is one of these things where you come across some other individual, you help them out, and then you never see them again, how can the act ever be reciprocated, right? So you have to have sort of repeated interactions among individuals. That is, you have to say you have many opportunities for altruism and return on that altruistic behavior. You have to be, at least, have a reasonable enough memory that you can remember who to help out and who not, so you can't be too dumb. And, um... And the potential altruists have to interact in some sort of symmetrical situation. That is to say, you're not being nice to me because I made you be nice. You were nice to me because you had a choice. You gained something else besides just being forced to be nice. So some examples of reciprocal altruism are examples of grooming behavior in many primates. Basically, the groomer is helping the recipient by removing parasites and debris from the fur, but the favor is returned usually quite later. You groom me, I'll groom you later. And here's a really cool example in bats. So these are vampire bats. Actually, they hang out in little nests at night where you have a group of maybe almost a dozen individuals hanging out in a tree trunk, say. They're often closely related, but sometimes you have fairly unrelated individuals there as well. And these guys go out at night and they feed on blood, which is what their vampire black, that's what they do. And sometimes they go out at night and they fail to get a blood meal. And that's costly. I mean, bats have a very high metabolism, they need lots of energy. And so what they'll often do is they'll often share blood meals by regurgitating to the others in that nest. Now they'll share more frequently with relatives and nest mates, and they especially will share more frequently... with those that shared with them earlier. So that is to say they can remember that, you know, Joe over there gave me the meal two nights ago when I had bad luck, and so Joe's having a rough time of it today, and he'll share with Joe, okay? I mean, I don't know if they remember their names, but that's the idea. Now the last thing I want to talk about is this idea of kin selection, okay? This guy, J.B.S. Haldane, is a very famous population geneticist, one of my three heroes in population genetics, along with R.A. Fisher, as you know. And J.B.S. Haldane did a lot of the earliest work on how natural selection changes allele frequencies in populations, and he's also one of these people that you come across occasionally that always has the right thing to say at the right moment. I don't know if you're like me, but I usually think of the perfect comeback like a week later. I say, oh, I should have said that. And this is the guy who had the perfect quip right at the right time. And so this is... Somebody said, well, would you lay down your life for your brother? He said, no, but I would lay down my life to save my brother? No, but I would to save two brothers or eight cousins. It's just so perfect. The guy's just brilliant. And here's some of his other quotes, not that you have to remember any of these, of course, but he said, he was known to say, the Creator, if he exists, has a special preference for beetles, because there's so many beetle species out there. So the four stages of acceptance of an idea. This is a worthless nonsense. Two, this is interesting, but perverse point of view. this is true but quite unimportant, and then the last one is I always thought so. And then finally is this other one, now my own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose. But the guy, he just had the right, could say the right thing at the right time. And so that idea, which was encapsulated with that first quote about laying down your life for brothers or cousins, was formalized by this fellow, Bill Hamilton, who, they don't give out Nobel Prizes in evolutionary biology, but if they did, this guy would be a shoo-in. He died in 2000, kind of young, from malaria. He did a lot of work in the tropics. And he came up with this formalism called kin selection to explain altruistic behavior. So what is kin selection? So this is kin selection in words. Don't get freaked out by this, but this is Hamilton's rule. So a gene for altruistic self-sacrifice will spread through a population if the cost to the altruist is outweighed by the benefit to the recipient devalued by a fraction representing the genetic relatedness between the two. So what does that mean? So the key thing here is that you have a cost. So you experience some cost by your altruistic behavior. There's some benefit, obviously, to the guy that receives that act. And the important point here is that individual that receives the altruistic act, the recipient, is related to you in some way. So the idea here is that if... if the individual is related to you closely enough, that that gene, even though it's disadvantageous to you, can spread through the population because you're helping out related genes spread through the population. So this brings up this idea of what's called inclusive fitness. So fitness that's not only, so you can think of your fitness being divided into two parts, so to speak. You have the fitness as we've always described it, your survival and reproduction, number of offspring you put into the next generation. But then you have this other part of your fitness, which is the indirect part, which is to say, how many closely related individuals are also going into the next generation? So this inclusive fitness, the fact that my sister has a couple of kids, they count as sort of a devalued fraction of what my kids count towards my fitness. My kids count wholly towards my fitness in the next generation. My sister's kids, they're closely related. They're related by a factor of one half to me. They count as like half a kid as far as I'm concerned, in terms of if I were tallying up how many kids I have. I have three kids, it turns out. I didn't, you know, it's one way of thinking about it. I have two kids at home and then my sister has a couple down south. So that's one way of thinking about this. And then kids that are like my cousins and so forth, they're devalued by a smaller fraction because they're less and less closely related to me. And eventually, the relatedness of any two individuals in this room is quite low. If you randomly choose two individuals, and so, sure, you might have some kids, but they hardly count at all in terms of the number of kids I can count as being my own. So this is Hamilton's rule in sort of an equation form. C is the cost you experience, B is the benefit the recipient gains, and R is the relatedness. So if B, the benefit times the relatedness... that product minus the cost is greater than zero, then a gene for altruistic behavior can spread through a population because you're helping out closely related individuals. And I'm going to delete this part from the slide, so don't worry about it, okay? It's just another way of stating the same thing. Now, there's a little bit about relatedness. How do you actually calculate relatedness? I'm not going to go through this in great detail or expect you to know very much about it, but... In, for instance, full siblings, that is to say, siblings that share the same mother and father, the relatedness is one half. And the idea here is you do the following thing. You go to some locus in the genome, and you pick one of the two alleles at random from mom or dad in one individual, and you do the same thing in the other, and you ask, what is the probability that those two genes are identical by descent? That is to say, that they come down and they can be traced to mom or dad directly. So this is one way you think about it. If you take two genes from brother and sister, say, those two genes could either be traced through mom or they could be traced through dad. And each time you go down this path, it's a one-half times a one-half. That's a one-quarter plus a one-half times a one-half, a one-quarter. The one-half you can think about as being the probability you trace it through mom. The other one-half is the probability you trace it through dad. But the overall relatedness between full sibs is one half. In half sibs, they only share one parent, and the individuals can only trace that gene potentially through the shared parents, so it's a one half times a one half. Or half sibs are related to one another by a coefficient of one quarter. So trying to put, you know, get you thinking about how you're related to other individuals. You're related to your parents by a factor of one half. Related to your siblings by a factor of one half. Related to your grandparents by one quarter. Each generation you go up you lose one half of the relatedness. And the further, more distantly related to individuals are, the smaller this R is. I think to explain this next part is going to take more than three minutes. So what I'm going to do is finish up with with kin selection in the next lecture, and then we will continue with, I think, biogenetics.

Just to give you a few slides that I didn't show you last time, this is just an example of a parthenogenic species. This is a whiptail lizard. That is to say, these lizards of this species are all female. The young of these females are identical to them.

And here's an example of an asexual species, Daphnia, which has a choice. Sometimes it undergoes rounds of asexual reproduction, sometimes it undergoes rounds of sexual reproduction. And organisms like Daphnia are important because it shows you that Many organisms do have a choice about how they do things, right?

And so there must be some advantage to sexual reproduction, even though I gave you a long list of reasons why it shouldn't evolve. The so-called two-fold cost of sex is one example. Now, what I want to turn my attention to today, largely, is the evolution of traits like seen in these birds of paradise.

These are all males, and many species exhibit what's called sexual dimorphism. The males and the females differ in form. It's true in humans, for instance. In the example of these birds of paradise, the males sport these very long tail feathers. They often are very brightly colored.

They have interesting ornamentation on their heads and so forth. And the females typically are just bland. They look like what you'd expect a good bird to look like, somebody who wants to hide from predators.

And so the question is, how can these characteristics, these elaborate characteristics, evolve when They're clearly maladaptive in at least some ways, right? If you have a very long tail feather, it's literally a drag. It slows you down, you can't fly as well. If you have brightly colored plumage like these males over here do, you're much easier to spot to predators.

So there are these characteristics that are difficult to explain with the vanilla flavor of natural selection that Darwin came up with in 1859. So how do these characteristics evolve? Well, the explanation... It's called sexual selection, and this is an idea that Charles Darwin put forward, as well as Alfred Russell Wallace.

He's the fellow that co-discovered natural selection. Charles Darwin explained this in a later book he published around 1871. And the idea is that these strange traits tend to be associated with the males of the species. And the idea is that animals don't only compete to survive, but they also compete to reproduce.

and that somehow these secondary characteristics, these bright plumage colors, for instance, might have to do more with access to females than they do to survive. To get into the next generation, to get your genes to the next generation, not only have to survive, but you have to successfully reproduce, obviously. And just to go over some of these secondary characteristics, I mean, lots of males have these crazy characteristics, like the Irish elk, which is now extinct. Its antlers were over 12 feet in length.

just tremendously huge antlers. Bighorn sheep, the males have these large horns, which are associated with fighting with one another. Here's an example from beetles. There's the female beetle, looks like a beetle. The male has these long extensions from its head.

And of course, you might be familiar with elephants, where the males sport long tusks, and the females have shorter tusks. Here's an example from a lion, where the lion has this big mane, clearly different than the females of the species. Here's an example from a pet pheasant where not only the male's more brightly colored, but the males have these spurs along the side of their legs.

And they use these spurs, these sort of sharp points here, to fight other males, to gouge them. And then there's some guys that have these little fake spurs. The females actually tend to prefer these nice spurs. And there's males out there who are horrible fighters, but they try to fake out the females with these sort of fake spurs. What types of sexual selection are we going to be discussing?

Well, there's two flavors of sexual selection we talked about. One is male-male competition, that is to say competition among males for access to the females. And the other type we'll be discussing is mate choice or female choice because the female is almost always the choosy sex, and we'll describe why that is the case.

So this would be intersexual selection, competition between males and females. So male-male competition. There's a variety of different kinds of male-male competition.

You have competition within groups for dominance. I'll give you an example of that. You have examples where a male defends a group of females from access.

from other males, that's called female defense polygyny, or they might defend a territory, and hopefully a good territory that females might want to hang out in. And then we'll describe Lex, or I'll describe Lex. So within group dominance, if you have more than one dog in your household, you're probably familiar with this, that the dogs set up a hierarchy pretty quickly.

In our family, we have two dogs, and it's very clear which is the top dog in our family. This is an example of... within group dominance where some individuals are higher in the hierarchy than others and they have access to the mates.

So an example of that would be, for instance, gray wolves. Here's an example of female defense polygyny. These are northern elephant seals. You can see these are the two males fighting with one another. They sort of have these tremendous fights, almost like sumo wrestlers, except a little bit more deadly, because they can actually get really gouged up.

And what they're doing is they basically take over a part of a... beach where the females might be breeding. And if they can defend that patch of ground from other males, they can mate with the females on that patch of ground. Actually, I had a rough experience, I shouldn't say rough, a frightening experience when I was an undergraduate.

I was doing a bird survey on San Nicolas Island as part of a research project, and I just had my head up and they were looking for birds, of course, and I got between an elephant seal and the ocean, which is a really bad idea. They're very, very large, scary animals. They don't look like it, but they are.

Here's an example of a species that has territorial defense polygynies, an impala. Once again, a male will defend a particular territory from encroachment from other males, and they'll mate with the females that are in that territory. So far, we have nothing to do with females being choosy, but you often see these secondary characteristics in these species, such as horns or antlers in the males that they use to fight other males. Here's an example of water bucks head-butting, same type of thing.

Remarkably, when you think about male-male competition as sort of ending after the copulation has occurred, but it turns out that's not the case. There's lots of species where the females can be mated multiple times by different males. And this happens, for instance, in Drosophila.

If you put a bunch of Drosophila in a vial, the females will mate with multiple males. So after the act, the sperm from different males actually compete within the female's reproductive tract for access to the egg. And there's examples of flies, for instance, that after they mate with the female, they actually sort of insert a plug to keep other males from having access to that female.

Or examples of males with their penis can sort of scoop out the plug and put in their own ejaculate. So there's lots of examples that are becoming more and more interesting as people discover that the sperm compete with one another as well for access to the eggs. So male-male competition can occur at several different levels. Now the next, so that's all I wanted to say about male-male competition.

There's another example of sexual selection which is called mate choice. This is typically the cases that the female chooses which male she wants to mate with. There's two broad categories.

The female bases her decision upon resources the male brings or some other indirect benefit that the male is going to bring. So I'll describe examples of both. Now let's sort of, first of all, claimed several times that in most cases it's the females of the choosy sex, but not always. Why is that the case?

Well, this is a direct result of anisogamy. Remember I talked about isogamous species are those that have gametes that are produced that are about the same size. Most species are anisogamous, meaning that some gametes are much larger than the others, and by definition the females are the individuals that produce the larger gametes. Well, these larger gametes cost quite a bit.

The reproduction for females is limited by the number of eggs she can produce. Males, on the other hand, are not limited by the number of sperm they can produce. They're a dime a dozen, so to speak. They're limited by access to fertilization to eggs.

So this sets up an asymmetry where the female basically can guarantee to have her eggs fertilized, whereas not all males are guaranteed to be reproductively successful. In fact... When you go out into nature and you look at males and females and you ask the question, how successful are individuals at reproduction? Almost all females are successful. That is to say, there's very little variation among females in most species for how successful they are at breeding, producing young.

However, males are much more variable in their success. Some males are fabulously successful and some are complete losers, meaning they don't have any young at all. So males typically have a much higher risk. type strategy in terms of how they live life. All right, so it is a case that eggs are more expensive, they cost more to produce.

And also in species that have internal fertilization and that carry their young to term, that's quite costly. It's an expensive undertaking for the female, not so expensive for the male. So there are cases, however, when you have a sex role reversal where the males are the choosy sex. And that occurs in these rare cases where the male contribution to, say, raising the young exceeds the female cost of producing the eggs. And so there are some examples of that.

For instance, the seahorse. The males will raise the young. The female basically lays the eggs and takes off, kind of what males typically do in most species.

And the male is stuck raising the young. So in this case, the male can be quite choosy about which female he mates with because he is controlling the resources. He's the resource-limiting step.

All right, so mate choice. So why might... Usually females choose in the first place. Well, there's one explanation for a lot of these elaborate characteristics you see in males is to ensure that the female mates with the correct species. Okay, so that often you have not only plumage or secondary characteristics or traits that males carry, but also often elaborate dances and whatnot in these courtship displays.

And often these courtship displays are very species, they are species specific. And so one thought is that these types of displays and behaviors ensure, or better ensure that individuals mate with the correct species. You can imagine that if you make a mistake in terms of which species you mate with, that's a big mistake in terms of your reproductive potential.

Another idea is that the mate you choose might be better able to fertilize your eggs or might give you a higher fecundity. Or maybe the male brings you food, or is a better parent, or brings to you a better breeding territory. That is to say, a territory with more resources. There's a number of others that you can imagine as well. So let's talk about, of all those reasons, you can break them into two broad categories.

So the direct benefit, that is, what does the male bring that's going to help you out immediately in terms of resources? So these are direct or proximal benefits. And an example would be the bush crickets that give the female what are called nuptial gifts.

So they produce these spermatophores, which are very rich in protein, often consisting up to 30% to 40% of the male's weight, and basically it's a large energy content. And the female will accept these gifts, and basically the more spermatophores allow the female to lay more eggs. She has more energy with which she can expend on reproduction.

So this would be an example of basically giving food or resources to a female, and the quality of that gift being something the female can use to choose among different males. That's an example of a direct benefit. Now, indirect benefits, these are benefits that are a little bit more difficult to measure.

And so the only indirect type of benefit I'm going to discuss in this class is the so-called good genes hypothesis. That is to say that the female chooses males based on the quality of the genetic constitution, so to speak. And the idea here is that the female wants to mate with as high a quality of male as possible because then her children will also be of high quality.

So what are some of the components of the good genes hypothesis? So first of all, like I said, the females are going to choose a mate which offers high quality genes, influencing survival. Mate quality has to be indicated by some secondary characteristics.

So, for instance, the brightness of the plumage. So, if the brightness of the plumage in some way indicates how good your genes are, then the female has some basis on which to choose. So, if I choose males with bright colors, in general, those are the individuals that are going to have better genes. The secondary trait has to be heritable, and there also has to be heritable variation in the male quality. quality, sort of the mate quality.

That is to say that in order for this hypothesis to work, there does have to be variation in how good the males are out there in terms of if you mate with one male, your children will have lower fitness. If you happen to choose correctly, your children will have a higher fitness. And there has to be some cost to the males bearing the trait.

So some of these, some variants of the model, there's no cost to the trait. And others, there is a high cost to the male bearing the trait, the so-called handicap model. That is to say, If you have very long tail feathers and you can survive with those very long tail feathers, then the female can choose that trait, saying that in order for that male to survive with those long tail feathers, he must be pretty good. kind of a handicap model. But in any case, there's variations in this model, some of which don't require the secondary trait born by the males to be costly and some where they must be costly.

So let's give some examples of the good genes. These are very difficult experiments to perform because you have to measure variation in the males and the traits that the males have. and also in the quality of the offspring that are produced.

So they're very tricky experiments. So one example is in Hyla versicolor. It's a gray tree frog.

And these frogs have short or long calls. So the males go to a pond, and they have these calls, and they can be classified as short or long. And the females are attracted to the calls. And generally speaking, they choose, or they like the long calls. So the question is this, is if the female chooses the long call, males, do her children, do her offspring have some sort of benefit?

So what we've got here is a table of things that these researchers were able to measure. So they were able to measure the larval growth, how rapidly the larvae grew, how long it took them to reach metamorphosis. So in case you don't remember, frogs lay eggs, the eggs hatch into tadpoles, and the tadpoles undergo metamorphosis into adults, right? Just keep that in mind.

You can measure the time to metamorphosis, the mass at metamorphosis, how well the larvae survive, and how fast the adults grow. And whenever you don't see a LC in the box, that means that there is no significant difference in the offspring between long-call and short-call frogs. But you see here, when you raise the larvae in a high-food environment, the long-call frogs had an advantage in terms of their larval growth and their time to metamorphosis.

And the low food, if you raise the larvae in a low food environment, then the long-call males also produce offspring that are better at growing. Not in all cases, not in all the characteristics that were measured, but at least in a few. So this has taken a support for the good genes hypothesis. There is some variation in the quality of the offspring produced based on whether a female chooses the long-call or the short-call males.

Yes. Oh, yes. So if a trait has some sort of, I mean, what I'm saying is you can think about a trait evolving in two different scenarios. There's the good old natural selection, which always will work for having a trait spread through a population. So if that trait directly enhances the survival or reproduction of that individual horse, it will spread.

But it's these characteristics that are difficult to explain any other way that natural selection would have a hard time explaining. So for instance, if you can imagine a scenario where you just... that the females all of a sudden didn't become choosy, say, in the birds of paradise, which is the first slide I showed with the birds with the very long feathers, then the argument would be that natural selection would not favor those long-tailed feathers. Natural selection alone would prefer to have males that are cryptic, right?

That predators have a hard time seeing. So it's these traits that would be difficult to explain using good old natural selection that pose the problem, frankly. How do these characteristics evolve?

How do seemingly maladaptive traits evolve? Well, the idea is, yeah, they're maladaptive in terms of your survival, but they grant you better access to females. And selection in general is all about leaving more offspring in the next generation. Is that clear?

Okay, so the last thing I wanted to talk about with respect to sexual selection is a little bit about lecking and the evolution of lecks and the so-called sensory bias exploitation hypothesis for sexual selection. So a little bit about a lex. So lex are aggregates of males.

So lots of, for instance, in bird species this happens quite often, where you'll have several males will hang out on a branch of a tree, say, and the only reason they're there is not for food, it's for, they basically are advertising for females. So females come by, so like on a shopping trip, and they say, you, and they go off with that male and he's the lucky guy, so to speak, right? Now the female doesn't come to these places, to these lecking sites, for anything other than to choose a mate.

Okay, so they're not there for the food or anything. They're just there to choose a mate. So it's often the case that they don't have these resources.

I think I described everything I wanted to talk about there. And it's also the case that in these lecking species, the male does nothing but mate. So it's typical in lots of species. The male provides the sperm and then he's gone, and the female's left with the chore of raising the young. So there's a number of models for why lex might evolve.

One is that by aggregating like this, sort of having a one-stop shop experience for the females, that there might be a lower predation risk for the males and females. It might be that there's some sort of passive attraction, that is to say, if you're a bunch of males hanging out together, that maybe you're more likely to draw females and increase your chance of reproducing as well. There's this black hole model, which is the females aren't particularly choosy, but they want to avoid Danger is associated with mating.

Perhaps going there, picking a male and leaving is much easier than going through the forest looking for a mate. Or the hot shot model where basically the females are going to choose one of the birds and the other birds are hanging out around this really hot looking male just with the hopes that they'll, you know, get somebody, which is kind of the pathetic model for this entire thing. So what's an example of a lecking species? So this is a, bower birds are a bird species that live in Australia and New Guinea and they produce what are called bowers, that's why they're called bowerbirds, which are basically leks. You have a part of the forest where you have a bunch of these different bowers, different males producing bowers, and the female comes to these sites in the forest and looks at the bowers and chooses among the males based on the quality of the bowers.

So this is an example of a bower. I'll show you another one. Here's the male, here's his bower. Basically what he does is he sort of stamps down grass and has twigs in such a way that there's like a little courtyard here. And often they'll bring brightly colored objects to these bower sites.

So they'll decorate their bower with, some of them like really, they like blue objects. So they'll like take parakeet feathers if they can find them. But nowadays it's more like blue bottle tops and things like that they'll bring to these bower sites. So here's an example of a bower. Here's the female checking out the bower.

Notice she's kind of drab colored. The male's this beautiful black satiny bird. She's checking it out saying, hmm, nice bower. Nice structure, straight leaves.

He's sort of doing a dance and, hey babe, you know, I'm over here, and I don't know what happens. I mean, either the bower wasn't very good and she took off and looked at another bower, or she said, this is great, come on over. So here's some examples of bowers and different bower bird species. Some of them can be quite elaborate structures.

I mean, look at these things. They're just amazing. And remember, these aren't nests. It's not like...

These are nests where the female will eventually lay her eggs. These are just there as courtship displays, like courts, essentially. So that's an example of a lack.

Now, the following thing I just can't help but talk about. There's a lot of interest among evolutionary biologists and ecologists in sexual selection, especially. And there's a fellow, Jerry Borgia, who's at the University of Maryland, whose students and postdocs have done a lot of work.

Studying bowerbirds essentially. And so they did this experiment where they're looking at, they're trying to test, like I said you don't have to know this part, I think it's such a cool experiment. They were trying to test the visual cues that the females were giving to the males about whether she was receptive to mating or whether she wasn't. And so they wanted to be able to control this very finely. So what they did is they made robotic birds, okay.

They called them fembots. If you've seen Austin Powers you know what what we're referring to, the fembots. And so here's a robotic Female bowerbird.

Here's the sort of control apparatus. They had these little remote controls they could sort of hide behind a tree and say, okay, let's see if the male likes this or, you know, what the male will do if I do this signal or that signal. So here's an example of what they did is they sort of, when the male was off, they'd kind of sneak into his bower and put the female there and the male would come back and say, woohoo, and you know, do his courtship dance and then they would sort of manipulate the female fembot.

With the idea being, well, if she dips her head, what does the male do? If she sort of does this, what does he do? And so forth.

So here's an example. Here's the fembot right there. The male's pretty excited. This is great.

He's going to do his dance. Kind of realistic looking bird, but maybe he's even got sound. So he's looking around.

Apparently they've done something to the remote control, which is getting them very convinced that this is it. This is so... This is... This is so embarrassing.

This is so embarrassing. Okay. Anyways, that was a fembot. So mostly just to prove to you that scientists can have fun too.

Fooling, I mean, it's a weird sense of fun to fool birds into having sex with, you know, robots, but fine. Anyways. back to serious science. I want to talk a little bit about one last hypothesis for the evolution of male characteristics. And this is the so-called sensory bias or exploitation hypothesis.

And the idea is that for some reason, it doesn't have to be related to sexual selection. ...section at all, but some trait evolves that allows females to be more receptive to some sense. So, for instance, maybe something in the ear changes so that they hear low vibrations better, or maybe something in the eyes changes in such a way that they're more sensitive to bright colors in some spectrum.

Whatever it is, it doesn't matter, but the idea is that the male's secondary characteristics, the trait that you think of as being sexually selected, evolves... to exploit that pre-existing sensory bias. So I'll give you a couple of examples. One is in Physolemus coloratum and pustulosum.

These are, once again, frogs, where the males... I used to have a recording of this, and I lost it, and I can't recover it. But the males have a call which has a series of whines and some chucks in it.

Okay? And I can't... reproduce.

I know there's a fellow, Mike Ryan, at the University of Texas who's great at frog calls. So in the middle of his lectures, he would give frog calls. I'm not Mike Ryan. I can't do it. But anyways, the males will attract, the males of the Physolemus postulosum, they will attract maids by calling using wines and chucks, whereas the Coloradum, their male calls only have the wines.

And if you look at the phylogenetic tree, you can actually show where the chuck evolves, so wines are down here, but the chucks evolve only in the males of Postulosum. Now what they did is a series of experiments where they would record the sound of frogs with wines and chucks or just wines or just chucks and go to these ponds at night and play these recordings and ask whether the females of Coloradum actually prefer the calls of their species, just the wines, or do they really like chucks as well? So that is to say if you Play a recording of a mate call that normally they wouldn't hear, one that has some chucks put in the end. Will they be more attracted to that or would they be more attracted to just the calls with the whines? It turns out that these females of this species actually prefer the chucks.

Even though their males don't have chucks, they prefer them. So you can actually plot the trait for the preference in the females right here. So the idea here is that the preference for these traits under the sensory exploitation hypothesis has to predate the evolution of the male characteristic, right?

Otherwise this hypothesis doesn't work. So here's an example where the chuck preference evolves in the females and later in postulosum, viscidum postulosum, the chucks evolve in the males. There's another, I think I just told you that, another experiment with swordtail fish.

This is actually was a study done, gosh, almost 20 years ago now, 15, 20 years ago, by Alexandra Basolo. She was also at the University of Texas working with Mike Ryan. And what she did is she looked in swordtail fish, and their closest relatives are these so-called platyfish.

Now, these swordtail fish, they're called swordtails because the males have a little extension on their tail called a sword, and often there's little bright colors or stripes along the swordtail. What she did is she manipulated the tails of platyfish in such a way that... Sometimes she would glue on an extension to the tail that looked a lot like a sword from one of the swordtail fish, or sometimes she'd glue on a clear extension, just a little bit of plastic that was acting as a control, and ask the question, in these platyfish, do the females prefer males that have a sword, even though the males of that species really don't, or do they prefer males without swords?

And she found through these mate choice experiments that the males in these platyfish really actually would... The females would actually prefer a male that had a sword. They don't, but they would actually prefer one.

And she also did the preference experiments here where you can actually chop off a tail and then reattach a long sword or a longer-than-normal sword or, once again, the control where you chop off the tail and then put on a clear bit of plastic that's transparent. And once again, as you might expect, the sword-tail females prefer males with swords. In fact, they prefer swords that are even longer than you can find in many of the males.

So the idea here is that the sword preference evolved here in the phylogenetic tree, and the actual swords in the males evolved after. Once again, this is an observation that's consistent with the sensory exploitation hypothesis, that for some reason, visually, females prefer these long swords, and that later on you have the male trait evolve to exploit that pre-existing bias. Okay. Now, are there any questions up to this point in the lecture? Yes.

Several questions. Okay, I'll start with you, then I'll go back there. So, we'll get to speciation later, and one of the definitions of species is that...

Is that they can't produce fertile offspring? So no, they typically can't. Okay, so they would actually, but you can do the experiments in the platy fish by, you know, like I said, manipulating the tails and saying, let's put on a long tail here, even though the males don't have it, do the females prefer it?

And they do. So the preference for these swords exists there, but that doesn't mean that, you know, they might actually think that that other species males looks pretty good, but they don't make the mistake, or if they did, it's a consequential one. Yeah, that is, like I said in the very beginning, for whatever reasons, there's some, you know, so for instance, organisms, you know, our senses, my hearing, our hearing is attuned to different ranges than other species. So for whatever reason, you know, you have the senses you have, okay? They've evolved under maybe a good old natural selection.

And the point here is that whatever preferences or whatever... Sensory biases you tend to have, the males are trying to exploit it. So it doesn't say anything about how the preference evolved at all.

It just says it's there. And you kind of know about these cell phones, rings that adults like me can't hear because my ears have gotten worse, but teenagers can. Well, it's like that, but even more so when you go to different species.

Like dogs, for instance, can hear what higher-pitched sounds than humans can, right? There's a... You could imagine a, we don't have this type of thing, but a mate calling humans that was in the dog range, it would be completely ineffective, right?

We can't hear it, the females can't hear that. Any other questions? Those were good questions. Hopefully that clarified it. Yes?

Yeah, so the idea is, so one thing, maybe I didn't emphasize this enough, but natural selection always operates on the variation that exists in the population at that time, right? So if you have a population where there's no variation in, you know, tail length, for instance, natural selection, or sexual selection for that matter, can't grab a hold of it and change it as a trait. So you can't have a trait evolve until you have variation for that trait in the first place. So the idea is at some point along that lineage, I mean, you give enough time and perhaps mutations...

Mutations will occur that will give you some variation in tail length. Natural selection was able to grab a hold of that, or sexual selection in this case, and lengthen the tails. But you can't have evolution without variation. It would be very difficult to select for luck, for instance.

There is a great science fiction book, I forget which one it is, but it's Terry Pratchett, The Ring World, but basically these outer space people select humans for luck by basically... Always allowing people that won this reproduction lottery, that they were able to have kids only if they won a lottery, with the idea being these people that are reproducing must be somehow lucky. Of course, it's just chance. There's no variation in your luck.

It's kind of a silly example of evolution which could never occur, but it was kind of a fun one in a science fiction book. You have to have variation. There can't be variation for luck, and yet in this book, luck evolved. Remarkable things happened to this person in the book because she was so lucky.

Okay. So I want to turn my attention to one subject that kind of got lost in the mix when we had these RRR days at the end of the semester. You know, I lost a lecture, but I think it's important to include this material, so I'm sort of stuffing an extra lecture into this one. So I apologize if there's a lot of material today. But what I want to do is discuss the evolution of altruistic behavior.

Now there's lots of behaviors and traits we can't explain. So for instance, we just discussed sexual selection, which can... can explain these seemingly maladaptive secondary characteristics in males. And this is always easy to understand, right? So when you have predation, like the winner and the loser here can't be clear.

This guy's the winner if you need help, and that's the loser, okay? Things like that are quite easy to explain. But there are behaviors that would be difficult to explain via just natural selection or even sexual selection. And these are seemingly helpful or altruistic behaviors.

And so, for instance, there's in the building grounds world, a very famous example. There's examples of alarm calls. So you're some squirrel and you see a hawk, you have a choice. You can, you can, and there's other squirrels around you, so other guys around you, can either just duck out of it, say I see that hawk, I'm out of here, or what they do is they give an alarm call before they, they scurry away, basically alerting the other ground squirrels in the area that there's a predator.

Now the problem with that behavior is, it sounds, sounds inconsequential, but the problem is when you make that call, when you When you raise that alarm call, you make yourself more conspicuous, and you're more likely to die and get swept down upon by that hawk and eaten. So there is a cost to that helpful behavior. It's an altruistic, a seemingly altruistic behavior.

So there's examples of alarm calls or sentinel behavior where some individuals will have a lookout while others scurry around eating, such as the meerkats, very cute little animals you see at the zoo, or nest helping. where some individuals might forgo reproduction to help out raising young, or use social behavior, where there are certain individuals that completely forgo all reproduction to help raise other individuals. These are all examples that would be very difficult to explain on the surface of things via natural selection.

How can those helpful behaviors evolve? So all these are examples where the actor, some individual performs an action that benefits some other recipient. And so it might be useful to think about And so I'm going to be talking about several different examples of helpful behaviors and how they might evolve. One is mutualism, which is an easy one to explain. That's the actor directly benefits from the behavior.

Reciprocal altruism is where the actor, the one that's providing the helpful service, actually eventually will benefit. Not immediately, but eventually. And then kin selection, where the individuals you're helping out are related to you, closely related. This is so-called indirect selection. We'll talk about that as well.

So it's worthwhile sort of trying to classify the different types of, you know, behaviors or interactions you can have. So mutualism, the donor and the recipient both experience some sort of benefit. Easy to explain, right? If you're a trait that helps yourself out, you know, your survival or reproduction, that's an easy trait to evolve under natural selection. At reciprocity, once again, the...

The benefit to you might be delayed, but you eventually benefit. Once again, easy to explain. Altruism, altruistic behavior is difficult to explain because you suffer some sort of cost where some other individual gains.

We'll explain how that can evolve. Selfishness, that's easy to explain. You benefit, the other guy doesn't. Predation is a good example of that, right?

You definitely benefit, the other guy doesn't. And spitefulness is something... we don't see out in nature, but basically you and the other, the recipient, both suffer some costs.

My kids have that, but you don't see that out in nature. So here's an example of mutualism where, for instance, groups of lions, they often will cooperate, and by cooperating they can bring down larger prey and they have a higher success rate. And they can better defend their kills from other lions and hyenas if they operate as a group.

So everybody gains, everybody benefits. Blue gill males, excuse me, off in the bottoms of lakes, they'll form nesting sites where you have 50 to 100 males in nests. And this can be thought of as mutualistic because by aggregating, you can actually have a lower predation risk.

So basically, you can better defend that nesting site from other predators, and you can think of this type of behavior as a mutualistic type of one. Often you'll have male lions cooperating with one another to oust the resident top lion in a pride. Often these males that help oust the top lion are closely related, like brothers. But by cooperating, they can actually have a better chance of ousting the current top dog or top cat, I guess.

Okay, in a little bit, those are some examples of mutualism. Those are easy to explain. Reciprocal altruism is a little bit more difficult because you need to show that eventually the actor is going to benefit. So you do something helpful to some other individual, you experience some sort of cost, but then you have to show that eventually that act will be reciprocated.

this type of behavior is going to be more common when you actually have repeated interactions between individuals. If this is one of these things where you come across some other individual, you help them out, and then you never see them again, how can the act ever be reciprocated, right? So you have to have sort of repeated interactions among individuals.

That is, you have to say you have many opportunities for altruism and return on that altruistic behavior. You have to be, at least, have a reasonable enough memory that you can remember who to help out and who not, so you can't be too dumb. And, um...

And the potential altruists have to interact in some sort of symmetrical situation. That is to say, you're not being nice to me because I made you be nice. You were nice to me because you had a choice.

You gained something else besides just being forced to be nice. So some examples of reciprocal altruism are examples of grooming behavior in many primates. Basically, the groomer is helping the recipient by removing parasites and debris from the fur, but the favor is returned usually quite later. You groom me, I'll groom you later.

And here's a really cool example in bats. So these are vampire bats. Actually, they hang out in little nests at night where you have a group of maybe almost a dozen individuals hanging out in a tree trunk, say.

They're often closely related, but sometimes you have fairly unrelated individuals there as well. And these guys go out at night and they feed on blood, which is what their vampire black, that's what they do. And sometimes they go out at night and they fail to get a blood meal. And that's costly. I mean, bats have a very high metabolism, they need lots of energy.

And so what they'll often do is they'll often share blood meals by regurgitating to the others in that nest. Now they'll share more frequently with relatives and nest mates, and they especially will share more frequently... with those that shared with them earlier.

So that is to say they can remember that, you know, Joe over there gave me the meal two nights ago when I had bad luck, and so Joe's having a rough time of it today, and he'll share with Joe, okay? I mean, I don't know if they remember their names, but that's the idea. Now the last thing I want to talk about is this idea of kin selection, okay?

This guy, J.B.S. Haldane, is a very famous population geneticist, one of my three heroes in population genetics, along with R.A. Fisher, as you know. And J.B.S. Haldane did a lot of the earliest work on how natural selection changes allele frequencies in populations, and he's also one of these people that you come across occasionally that always has the right thing to say at the right moment. I don't know if you're like me, but I usually think of the perfect comeback like a week later. I say, oh, I should have said that.

And this is the guy who had the perfect quip right at the right time. And so this is... Somebody said, well, would you lay down your life for your brother?

He said, no, but I would lay down my life to save my brother? No, but I would to save two brothers or eight cousins. It's just so perfect. The guy's just brilliant.

And here's some of his other quotes, not that you have to remember any of these, of course, but he said, he was known to say, the Creator, if he exists, has a special preference for beetles, because there's so many beetle species out there. So the four stages of acceptance of an idea. This is a worthless nonsense. Two, this is interesting, but perverse point of view.

this is true but quite unimportant, and then the last one is I always thought so. And then finally is this other one, now my own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose. But the guy, he just had the right, could say the right thing at the right time.

And so that idea, which was encapsulated with that first quote about laying down your life for brothers or cousins, was formalized by this fellow, Bill Hamilton, who, they don't give out Nobel Prizes in evolutionary biology, but if they did, this guy would be a shoo-in. He died in 2000, kind of young, from malaria. He did a lot of work in the tropics.

And he came up with this formalism called kin selection to explain altruistic behavior. So what is kin selection? So this is kin selection in words.

Don't get freaked out by this, but this is Hamilton's rule. So a gene for altruistic self-sacrifice will spread through a population if the cost to the altruist is outweighed by the benefit to the recipient devalued by a fraction representing the genetic relatedness between the two. So what does that mean?

So the key thing here is that you have a cost. So you experience some cost by your altruistic behavior. There's some benefit, obviously, to the guy that receives that act.

And the important point here is that individual that receives the altruistic act, the recipient, is related to you in some way. So the idea here is that if... if the individual is related to you closely enough, that that gene, even though it's disadvantageous to you, can spread through the population because you're helping out related genes spread through the population.

So this brings up this idea of what's called inclusive fitness. So fitness that's not only, so you can think of your fitness being divided into two parts, so to speak. You have the fitness as we've always described it, your survival and reproduction, number of offspring you put into the next generation. But then you have this other part of your fitness, which is the indirect part, which is to say, how many closely related individuals are also going into the next generation? So this inclusive fitness, the fact that my sister has a couple of kids, they count as sort of a devalued fraction of what my kids count towards my fitness.

My kids count wholly towards my fitness in the next generation. My sister's kids, they're closely related. They're related by a factor of one half to me.

They count as like half a kid as far as I'm concerned, in terms of if I were tallying up how many kids I have. I have three kids, it turns out. I didn't, you know, it's one way of thinking about it. I have two kids at home and then my sister has a couple down south.

So that's one way of thinking about this. And then kids that are like my cousins and so forth, they're devalued by a smaller fraction because they're less and less closely related to me. And eventually, the relatedness of any two individuals in this room is quite low. If you randomly choose two individuals, and so, sure, you might have some kids, but they hardly count at all in terms of the number of kids I can count as being my own.

So this is Hamilton's rule in sort of an equation form. C is the cost you experience, B is the benefit the recipient gains, and R is the relatedness. So if B, the benefit times the relatedness... that product minus the cost is greater than zero, then a gene for altruistic behavior can spread through a population because you're helping out closely related individuals.

And I'm going to delete this part from the slide, so don't worry about it, okay? It's just another way of stating the same thing. Now, there's a little bit about relatedness.

How do you actually calculate relatedness? I'm not going to go through this in great detail or expect you to know very much about it, but... In, for instance, full siblings, that is to say, siblings that share the same mother and father, the relatedness is one half.

And the idea here is you do the following thing. You go to some locus in the genome, and you pick one of the two alleles at random from mom or dad in one individual, and you do the same thing in the other, and you ask, what is the probability that those two genes are identical by descent? That is to say, that they come down and they can be traced to mom or dad directly.

So this is one way you think about it. If you take two genes from brother and sister, say, those two genes could either be traced through mom or they could be traced through dad. And each time you go down this path, it's a one-half times a one-half.

That's a one-quarter plus a one-half times a one-half, a one-quarter. The one-half you can think about as being the probability you trace it through mom. The other one-half is the probability you trace it through dad.

But the overall relatedness between full sibs is one half. In half sibs, they only share one parent, and the individuals can only trace that gene potentially through the shared parents, so it's a one half times a one half. Or half sibs are related to one another by a coefficient of one quarter. So trying to put, you know, get you thinking about how you're related to other individuals. You're related to your parents by a factor of one half.

Related to your siblings by a factor of one half. Related to your grandparents by one quarter. Each generation you go up you lose one half of the relatedness. And the further, more distantly related to individuals are, the smaller this R is. I think to explain this next part is going to take more than three minutes.

So what I'm going to do is finish up with with kin selection in the next lecture, and then we will continue with, I think, biogenetics.

Transcript for:Sexual Selection and Altruism Overview

Transcript for:
Sexual Selection and Altruism Overview