So continuing on our week two discussion of adaptation and natural selection, there's also the, the topic of co-evolution and we're going to carry, cover a wide range of things here. We'll cover sexual selection because that's an very important method of changing phenotypes from one generation to the next, and then we'll talk about co-evolution which involves natural selection, and then we'll get into a brief discussion about what does it mean for things to be adapted, and how do we recognize adaptations. Now, we often associate sexual selection with sexual dimorphism. So, we, we see two birds of paradise. The brightly colored, gaudy male and the less gaudy female. And so females have a mating asymmetry, not all of them get males. And not all males get females. Now, there's, there's a lot of theory explaining this. And what we start with is the idea that the preferences of the female for male traits is heritable, in some manner. And the traits themselves is heritable in some manner. So Here's a curve for the female preference, and here's a curve here between this trait for the male and this trait for the male. So, some females have a preference more for trait one, some females have a preference for more than trait two. Now one of the keys of understanding this is, if the traits of the male, and if the preferences of the female are genetically linked, then they can evolve together. And over time, they can evolve stronger and stronger. So for instance, if we take, if a female, chooses trait one, then their offspring are going to be in this part of the curve. So they come down here. And if they, choose this part of, of, they, they have a preference but it's in this part, for trait one. Then they come down here. And if, if there's preferences for trait two here for instance. In this curve then those come down here. But most of them, if, if they have a preference for trait two, most of the population will have this very large offspring. And in doing so it, it, it canalizes, so to speak, the traits. So if trait one is a small tail, then these tend to have small tails for their sons. And if these traits for the males have long tails, these offspring tend to have traits with the long tails. So there's a correlation between the preference and the trait. On the one hand, for small tails and the preference and the trait on the other hand for, for large tails. In Africa there are a whole host of finches with long, long tails. And, so we can experiment, and they have experimented with it. So, they can cut the tails in some, and they can elongate the tails in others. So, here's a really clever experiment. On the left is the before treatment. They take some males and they shorten the tail. They take some males, they cut the tail, and then put it back, so that it's a, it's like a control. In others, they just leave it alone, and in others they elongate the tail by adding feathers to it. Now the question is, do long-tailed males get the girls? Well, indeed, they do. The number of offspring from these shortened birds are much, much less than if you elongate the tails. So there's, there's a real preference and a real drive on, on the part of selection to, for females, to choose the long-tailed males. Now, lots and lots of birds, lots of mammals, lots of different kinds of insects have very exaggerated ornaments. And there's a reason why some of these may evolve. And you need to do a lot of experimental work to sometimes sort that out. So for instance, there could be pleiotropic gene effects. And what that's saying is that there could be selection for something else, just let's say, body size. But, at the same time that you're choosing body size, you may be choosing long tail. So you're not actually choosing, selection is not choosing the long tail per se, it's choosing body size. And pleiotropic gene effect means that different kinds of, of traits may hitchhike on other traits, selection for other traits, simply because there's gene effects affecting both of those kinds of traits. And we can select for ecological sex differences. In different situations, we can, one of the most important is male/male contests. So the males with the gaudiest plumage, let's say sometimes do better in male/male conflicts for females than, and the big, big elephant seals with their large, large bodies and big probosses and big bulk. They get more females in the harem then less aggressive, smaller, smaller males. Then there's this whole notion that females are actually choosing the males and there's a lot of experimental evidence that there's mating preferences on the part of females. And if we look at the, the number of kind of ornaments that have evolved, and the female preferences for them, we don't need to go into all of this, but in the, this one, this one and this one are most important. we went through the fisherian model where females select possibly one extreme or the other extreme, so there may be selection for instance in some, some species for long-tailed birds and short-tailed birds. I gave you an example of the long-tail bird. Then there's benefits. So females can sense by behavior or other things that certain males probably have better fitness. And that the young from her are going to better fit in the environment. And, most able to pass her genes on into the next generation. And, then there's things called sect sensory bias. And, and so there's, there's ways in which, through pheromones and other mechanisms, that females can sense males that might be, might be very good. Now let's turn to co-evolution. Co-evolution is a term that applies to lots and lots of things. So there's cooperation amongst within populations. especially for instance if, if you, if you're related to another individual. You may cooperate with them more if you, than you would with an unrelated individual. Then we'll also look at co-speciation and co-phylogeny. And in the instance of co-adaptation. And then there's symba, things like symbiosis and mimicry. Which we'll, we'll also look at. But this is a really cool example of co-evolution with a weevil, on north islands and south islands, and here is a fruit that they feed on. So on the north island, there's a very small bill. And a small fruit. But on the south island the, the fruits have gotten bigger and bigger and, and at the same time the bills have gotten bigger and bigger. And what's going on here is that the fruits are responding to this predation by getting bigger and bigger, and, in the hopes, so to speak, of having less and less predation. But there's a evolutionary gain going on between these two. So here's rostrum length, and here's the thickness of the fruit. And if you're in the green zone, there's a defense success, and if you're in the blue zone there's a boring success. So, what happens is they reach some sort of equilibrium along this line here, between the advantage for the weevil and the advantage for the plant. And so there's a co-evolution going on. So, this line represents the 50% sucess line for weevils, and at some point, you can only have it obossus so long or you can only have a, a fruit so big. And so there's an evolutionary kind of arms race going on here. Here's a case of mimicry. And there's lots and lots of examples of co-evolution within mimics and models. So the models usually are unpalatable to birds. They, they carry toxins, and the mimics evolve toward having the same type of pattern. So here, we have mimics, and they have evolved very, very quickly, actually, to evolve exactly like their models. These are all unpalatable. These aren't. And so by being like an unpalatable, the the mimics have a greater chance of survival. And then there are actually some species that are both mimics and models. They, they both have some degree but unpalatability, but then they both mimic one another. Now here is an example of co-evolution and here is an example where understanding co-evolution has a great deal to do with human health. Some years ago, out in the southwestern United States, 15 to 20 people died within a very short period of time. And so they suspected it was a virus. And they identified this virus and went into the collections that the tissue, frozen tissue collections at the University of New Mexico. And what they found out was that this was a hantavirus. They, they quickly identified that. And so it looked like there was an emerging new hantavirus in the United States that was killing people. Now hantaviruses are spread by wild mice hosts who, when they defecate, that, that defecation becomes very dry and it becomes aerosol. And that's how humans get it. They breathe in the aerosol and these viruses are in all of this aerosol. This took place on Indian lands where people were still living in and many people were living in very poor conditions with dirt floors and mice were running in and out. And they discovered that this was the cause, this, these mice were defecating in these buildings, and, and releasing the virus into the air as, as the poop dried. Right? But what they did at the University of New Mexico, they said, well let's look at other species of mice. And then look at the viruses that might be in these other species of mice. And what happened was just utterly remarkable. They found that we have the strains, the, the places where all these are found. In red are the Hantavirus phylogeny. In black is the host rodent phylogeny. And you can see that there's a lot of co-evolution in these viruses and their host. It's not perfect, but it's very, very tight. Now, the interesting thing about it is that they, in doing these studies, they found that Hantavirus had been in the United States, in North America, and in South America even for over about 20 million years. When rodents came in from Asia into North America about 15-20 million years ago. Hantavirus came with them, so it wasn't a new emerging virus.
