In this lecture, we're going to get deeper into evolutionary biology. And were going to talk about species as being discrete real things in the world. We're going to talk about the origin of species, what are the mechanisms by which species arise? And then we'll start discussing about how to build phylogenetic trees, building the tree of life. Now again, Darwin's first idea was descent with modification. And from that came the notions of phylogenetic trees. But first you got to understand what are we putting on these trees? Well, we put species, and groups of species. And understanding species is a really important concept in evolutionary biology, but it's also a very difficult concept. But in essence, species are real things out in nature. We, we have to think as biologists and as scientists that the things that we're studying there are real discrete entities. And they represent products of an evolutionary history. and that helps us understand the process itself by which species arise. But also a very, very imprtant, it helps us classify organisms. And classifying an organism is extremely important for all kinds of solving biological problems. Understanding what species are is very important in biological science and applied science. We, we discuss conservation units. People want to conserve species. They want to conserve groups of species. They want to go out and describe new species, so new species are being described all the time. But you gotta know what a species is in order to say its a new species. Invasive species, the United States spends billions and billions of dollars mitigating against invasive species. So, we need to know what they are, where they come from, what they're related to. And there are many pathogens and hosts that we are very concerned about discovering. In the last lecture we, we talked about West Nile Virus. And there are many other things that we use the notion of species in to, to find out what they are and to to describe them. And, then there's agricultural wild relatives. We use a lot of wild relatives of a plan, of food crops, to help improve food crops. A very good example is that the wild relatives of corn live in the mountains of Mexico. And they have genes that when planted in, that when bred into, the cultivated corn in, in Mexico, prevented disease. And, and therefore enhanced their crops. So what is a species? This is where biologists have debates. And there are a multiple of definitions of species out there. But there's two real ideas that go across almost all of those definitions. One, is that these populations or groups of populations can be told apart by some feature. So they have a red wing, or a white wing, for instance. So, these are phenotypic characters, but also genetics is in, increasingly being used to help individuate new, new kinds of species, and in, individu, individuate populations into species. Then another big idea that is, constantly talked about, is that well populations, are different species if they don't interbreed with other populations that look sort of like them. So, non-interbreeding is also a very big concept. But there's a long, long debate and we argue over how much difference is necessary, to call this a different species. And we know that very distantly related, species can interbreed. Yet no one would call them different the same species, simply because they interbreed. And we'll have examples of that as we go along. Now, this is a great example of, of the problem of species, and it's very important for conservation biology. My colleague, George Bericlow, here at the American Museum of Natural History, studies spotted owls. And so he's done genetic analysis, all up and down the west coast to do genetic analysis. And he found that there were actually three different kinds of spotted owl. And each of those different kinds of spotted owl is actually different in the phenotype. In the plumage of the bird, they are a little bit darker or a little bit lighter they have a little bit different spotting or not. But the genetic says there's three different kinds. And all of those have been thrown into one species by a lot of people, but you want to manage them as if there are three species there. Now, how do species arise? This is one one thing that most evolutionary biologists totally agree on. That the, the, the most common mode of speciation, origin of species, is called allopatric speciation. And it, it starts with a very large widely distributed population, sets of populations. And a barrier arises dividing these populations into two, making them allopatric, that is, they are apart from one another. And over time, they differentiate. So they, so this one differentiates into something that's different from it's sister species, as we will talk about in a minute. And so there's genetic differention, there may be adaptation and natural selection going on to make them different. But we start with one at time 1, we, we subdivide the populations at time 2. And at time 3 we have two species where once there was one. Now if we, there's another way of looking at allopatric, that's also common. And, we start at time 1 with a population, and a barrier already there. But then at time 2, there's a long-distance dispersal event across the barrier into another area that it wasn't once there, it wasn't there once before, and it becomes isolated. And it then differentiates into in allopatry, all by itself. And so over time it becomes different. Now the important point here is that differentiation can take place, only in the presence of a barrier. In other words, there has to be isolation. We have to prevent gene flow. Now, some gene flow is okay. But over time, one needs more or less, almost no gene flow for these things to differentiate. And this is a big area of study in evolutionary biology right now. How much gene flow is is, necessary? Or the lack-thereof as necessary for speciation. Now in allopatry, there are 3 main mechanisms that can lead to differentiation, in other words phenotypic change, and, and genetic change. The 1st is genetic drift, and were not going to talk a lot about genetic drift but it is simply, that randomly, things change over time. Characters become fixed. A mutation in DNA could become fixed, It, in small populations. And that might have a phenotypic effect. The most important two processes that are fixing genetic and phenotypic differences are natural selection, and sexual selection. So natural selection is all about that genetic changes and phenotypic changes occur because they, they confer a benefit on reproduction of, of individuals and populations. Sexual selection has to do with mate choice.
