Melina Hale (U. Chicago) 1: The Evolution of Neural Circuits & Behaviors: Introduction to Evolution

Melina Hale (U. Chicago) 1: The Evolution of Neural Circuits & Behaviors: Introduction to Evolution


Hi. My name is Melina Hale. I’m a professor at the University of Chicago. In my lab, we study neurobiology, biomechanics, and evolution. I’m going to present two different topics. The first is an introduction to evolution. Then we’ll go on to talk about a specific example from my lab of how we map the nervous system and aspects of the nervous system onto the evolution of animals. We work in my lab, specifically, on vertebrate animals, things like fish and tetrapods, mammals, and reptiles, and so I’m going to focus on that part of biodiversity in my talks. There’s a lot of other organisms out there, of course, invertebrates, and insects, and plants, and microbes, that we won’t touch on in these lectures. So, we’ll start with this introduction into evolution. What is evolution? Now, Charles Darwin originally proposed the theory of evolution, which can be summarized in a very succinct phrase: descent with modification. Now, let’s break that down a little bit, though, to a broader definition, which is change in the heritable characteristics of organisms from generation to generation. We can break that down even further to look at the component parts of that sentence. First, if we think about this idea of generation to generation, that means that when we look at evolution, we’re really not talking about changes in individuals or over short time frames. Instead, we’re talking about changes that we see over a long history of the descent of an organism over time. What about heritable characteristics? Well, we all have lots of characteristics to our bodies. We may have big muscles if we exercise a lot, we may have had injuries in our lifetime that have given us scars. Those are not heritable characteristics. Heritable characteristics are the types of traits that we pass on to subsequent generations, or that we inherited from our parents and grandparents. Heritable characteristics are an important part of evolution, because it allows transmission from one generation to the next, and on and on through evolutionary history. Now, the last part of this is change, and change is also really important. There has to be the ability in evolution for these heritable characteristics to vary, to change in response to environmental factors that might favor one type of characteristic or another, and we’ll come back to that. And that’s what Darwin was getting at with this idea of modification, that there’s going to be change in how organisms are organized and how they look over time. So, here’s an example, a cute picture of a pair of dogs and their puppies, where you can really see the variation in characteristics, even in one generation. If you look at the parents and you look at the pups, you can see some of the puppies look like one parent, with, you know, pure light fur, others look like the other parent, with very dark fur around the face, but yet there are other puppies in the litter that look different yet again, that have a mix of the characteristics of those two adults. So, you can get a sense of the variation in this image that can be explored in evolution and capitalized upon through evolutionary time. One example of variation that’s been really important for us to understand how we can change the characteristics, the features of a species, over time, is the peppered moth. So, these two moths, that look very, very different — the light one on the left and the dark one on the right — are the same species. They can interbreed. Now, the dark one and the light one, as you might expect, do better in different types of environments. This color characteristic varies, of course, and in some environments it benefits the organisms to be light or to be dark. In other environments, that same characteristic may be detrimental to the animal. So, these peppered moths provided a classic example of how characteristics can vary with environment, and how populations of a particular species can vary. So, this was noted particularly in the industrial revolution. At that time, we went from manufacturing using people sewing or create objects to using a lot of machines to make products. With the use of machines came the use of coal, and with coal came soot, or pollution in the air. Now, with that soot and pollution, you could imagine that structures in the environment, like trees, would become darker, and the peppered moth populations changed in order to accommodate that. And the darker morph of the peppered moth survived better. Right? It was better camouflaged against potential predators in the environment. When the environment cleared up and pollution decreased, the tree barks became lighter and the lighter version of the moth actually survived better. So, we can see variation in the characteristics in a population, even over this short amount of time, and due to a human-induced artifact in the environment, this pollution from coal. Now, just to show you how striking this difference can be in the camouflage of these moths on trees, we can see some here. So, here’s our dark morph and our light morph, and if we look at this tree, we can see both the dark morph and the light morph. Here’s the light one right down here, and you can see it better camouflages against the light bark in this healthy tree. The dark morph stands out against that light tree, expect in this area over here, where it’s against this injury to the tree, which shows up darker. Another example in variation in populations that we’ve probably all had experience with is in bacteria and the treatment of bacteria with antibiotics. So, when we go to our doctor’s office with a bacterial infection, we’re prescribed antibiotics, medicine to kill those bacteria, and doctors are often very specific about the need to take that medicine over a precise time course, and in particularly they say, “Don’t stop the medicine early. You have to take the full course of medicine. Even if you’re feeling better, take the full course of medicine.” It’s important to do that. Why is that? It’s because of the selection that’s acting on the variation in the population. So, when we have a bacterial infection, the species of bacteria that’s in our bodies may have lots of variants to it, and this is shown in number 1 on the left. They might vary in aspects of their biology, including how strong they are, how resistant they are to antibiotic medicines. If we treat them, shown in point 2 over here, but we don’t treat them long enough, which are the bacteria that are going to survive? It’s going to be the ones that are the strongest, that are the most resistant to the medication. So, if we don’t kill them and we stop taking the medicine, they’ll be able to multiply and will take on a larger part of the population of the bacteria. It’s not unless we kill them all that we can prevent those resistant bacteria from then multiplying and becoming a problem for our antibiotic medications down the road. So, I’ve shown you several examples of how populations of a species can vary, whether it’s peppered moths or bacteria, but how do we go from that population-level variation to the evolution of new species? This is called speciation, and in general what happens is that populations of a species will be separated and unable to interbreed, and if they’re separated for a long enough period of time, when they come back together they may not be able to interbreed, and then we would call them different species. One of the ways that interbreeding is prevented is through geographic isolation. One of the students in my lab, Andrew Trandai, actually helped me out by developing this hypothetical example that I’m going to show you on what a speciation event might look like, so I have to thank Andrew for all of the images that are coming up in the next series. Okay, so in our hypothetical example, what we’re looking at is some rodent squirrel-like animal in an environment — one species — all together as one population. So, how do we separate them and get new populations to evolve? Well, in Andrew’s example, here, we have flooding and an aquatic barrier that these animals cannot cross, so effectively the population in the trees and the population in the sand are separated now and will be evolving independently. Over time, if we look at each of them, we may see differences being incorporated into their biology. Just superficially, we might see the animals that are in the forest turning a different color, other aspects of their anatomy might change to live in the trees. On the opposite side of our river, we may see the populations that are in more of a sandy desert environment change coat color to match that environment, or change size to better adjust physiologically to this drier environment. Then ultimately, once these differences have occurred over, again, a very, very long period of time, through evolution, what would happen if the river dried up and these animals were able to come back together? Well, they might come back together and be able to interbreed, but they may come back together and not recognize each other as the same species, and therefore, even though they’re together in this environment, they would not interbreed and their independent characteristics would be carried on from generation to generation in those species. So, that was an example of geographic isolation, and the biggest example of geographic isolation happened about 200 million years ago, when Pangaea, which was this big super continental landmass, broke apart to give us the different continents that we know today. So, South America and Africa broke apart from North America and Europe, and those continents moved and separated around the globe. With that separation, the species that were together prior to this breakup then became separated, and so if we look at species that are in Africa versus South America, for example, we can see animals that perhaps came from the same lineage, but now are very, very different, and are in fact different species. Okay, so we’ve talked about this process of evolution and how it can occur. What if we want to understand the evolutionary history of the animals that are alive on earth today? Well, we have to use a different set of techniques to do that. Here’s just some of vertebrate diversity and, as I said at the beginning of the lecture, we also have lots of plants and invertebrates and insects. So I’m just showing you a very small part of biodiversity here. How do we figure out, with animals so diverse as these, how they’re related to one another? And how they evolved through time? Well, we can take a very simple example of how we construct our own family trees over very short time periods, over several generations, say. We research our genealogy, we use birth notices and death notices, and we recalled history from our parents or grandparents, and we can use that to construct relationships among our relatives and ourselves. This is a really interesting family tree that’s on the wall of a Czech castle, actually, and shows the relatedness of this family, going from a founder down at the base of the tree, in the trunk, up to the descendants at the top of the tree. So, if we take a hypothetical example, again, of building a family tree, and we start with this family of green-ish and blue-ish, big-eared and small-eared organisms, and try to construct how they’re related, we can just look and see how family trees are organized. So, here I’ve taken that population and put them onto their tree — that I made up — and we can see that they’re related to one another. So, the individuals that are connected at the first branch are siblings. They have the same parents. If we move back in the tree, we’re looking at the different common ancestors of these individuals. So, if we go back, these groups that are bracketed in the orange boxes are shared pairs of grandparents, so they’d be cousins. And if we look down near the base, we can see that all of these organisms share a pair of grandparents. Now, because we’re in recent history and we have all sorts of ways to record our history, we may even know what these grandparents look like, what our common ancestors of us, and our sibling, and cousins, look like, and I’ve reconstructed them this way. If we look at at a group of animals that’s as broad as fish and mammals and amphibians and reptiles, though, we don’t have that record, to know what those common ancestors are or what they looked like. We have to use other types of approaches, called phylogenetic approaches, to basically try to reconstruct the common ancestor and how those species are related. So, if we take this set of vertebrates, this small number of animals, and try to put them on a tree, this is what it would look like, and this is based on lots of peoples’ research over many, many years, and I’ll run you through it quickly. On the far left, we have the base of the vertebrate tree, and these are lampreys, these are animals that don’t even have, really, jaws. They have these suction discs that rasp and grip onto other species. As we move up the tree, we get into things like sharks, and skates, and rays, that have jaws, but they have a cartilaginous skeleton. When we move up yet again, we get to the bony organisms that include the fishes, shown with these anemone fish, the third image from the left, and then we get up into the tetrapods, that include amphibians, reptiles, birds, and mammals. Now, how do we construct this kind of tree when we don’t have these detailed records that we have of families? Well, we do it by looking at what characteristics these organisms share and what characteristics vary between them. There are lots of different types of characteristics that we can use. So, one of the features that we look for when we’re looking at shared characteristics, or similarities and differences among organisms, are anatomical features, things like the shape of bones or where sutures — where bones connect to one another — or where we see holes through our skull or other parts of our anatomy. Bone and other structures from the body provide really nice characters that we can use to try to figure the relatedness of organisms. In addition to using anatomical features to try to understand the evolutionary history of organisms and their relatedness, DNA is now also providing a really powerful way of generating characters to try to understand how organisms have evolved. In particular, we can compare a single gene among different organisms, different animals and species, and see how it varies and how it’s similar, and look for changes in that organization of the gene itself that might give us signals about how close a species is to another species and the relationship among them and to different species. Now, another set of data that’s been useful in understanding evolutionary history, of course, is fossils. They’re really important. Now, fossils provide information about when and how features arose. They won’t, though, provide the common ancestor. It would be very unlikely to actually dig up a fossil that gives you the exact common ancestor of a species but, nevertheless, what they can provide us, how they can ground our understanding of when an organism or particular elements and characteristics of an organism arose, is incredibly important. So, to summarize our introduction to evolution and some of the major points we’ve talked about… first, evolution is change in the heritable characteristics of organisms from generation to generation, descent with modification as proposed by Darwin. Variation in characteristics allows some subsets of populations to be selected for or against. And selection can cause change in the characteristics that persist in a population, and this can allow for populations to diverge. Reconstructing how the diversity of organisms evolved involves making trees, or these phylogenies that I talked about, that show different organisms are related to one another. And phylogenies, though, depend on identifying characteristics that are shared between organisms and that can suggest their common ancestry. And, again, we can get those characteristics from morphology, from genes, from all sorts of different sources. Thank you.

2 thoughts on “Melina Hale (U. Chicago) 1: The Evolution of Neural Circuits & Behaviors: Introduction to Evolution

  1. Thank you Dr. Hale for this clear explanation. I have one remark:

    At 5:00 you said: "peppered moths' populations changed to accommodate that" which might give the impression that they had the "intention" to change in response to the environmental changes.
    I know that it is common to use such phrases when explaining evolution, but I'm afraid that some people (especially the beginners) might refuse evolution because they misunderstand the meaning.

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