Session 4: How is Evolution Measured
Transcript of Part 2: Introduction to Evolution
00:00:07.24 Hi. My name is Melina Hale. 00:00:09.08 I'm a professor at the University of Chicago. 00:00:11.14 In my lab, we study neurobiology, 00:00:13.10 biomechanics, 00:00:14.22 and evolution. 00:00:16.09 I'm going to present two different topics. 00:00:18.12 The first is an introduction to evolution. 00:00:22.12 Then we'll go on to talk about 00:00:24.10 a specific example from my lab 00:00:26.26 of how we map the nervous system 00:00:29.20 and aspects of the nervous system 00:00:31.12 onto the evolution of animals. 00:00:33.18 We work in my lab, specifically, 00:00:35.06 on vertebrate animals, 00:00:36.18 things like fish and tetrapods, 00:00:38.17 mammals, and reptiles, 00:00:40.27 and so I'm going to focus on that 00:00:43.07 part of biodiversity 00:00:45.07 in my talks. 00:00:46.22 There's a lot of other organisms out there, of course, 00:00:49.08 invertebrates, and insects, and plants, 00:00:51.18 and microbes, 00:00:52.26 that we won't touch on in these lectures. 00:00:55.26 So, we'll start with this introduction into evolution. 00:00:58.10 What is evolution? 00:01:00.05 Now, Charles Darwin 00:01:02.13 originally proposed the theory of evolution, 00:01:05.00 which can be summarized 00:01:06.27 in a very succinct phrase: 00:01:08.14 descent with modification. 00:01:10.05 Now, let's break that down a little bit, though, 00:01:12.17 to a broader definition, 00:01:14.22 which is change in the heritable characteristics 00:01:17.10 of organisms 00:01:18.29 from generation to generation. 00:01:20.29 We can break that down 00:01:22.13 even further to look at 00:01:24.11 the component parts of that sentence. 00:01:25.18 First, if we think about 00:01:27.03 this idea of generation to generation, 00:01:29.12 that means that when we look at evolution, 00:01:31.05 we're really not talking about changes 00:01:33.00 in individuals 00:01:34.17 or over short time frames. 00:01:36.01 Instead, we're talking about 00:01:38.04 changes that we see 00:01:39.28 over a long history 00:01:41.23 of the descent of an organism over time. 00:01:45.11 What about heritable characteristics? 00:01:47.04 Well, we all have lots of characteristics 00:01:49.01 to our bodies. 00:01:51.02 We may have big muscles 00:01:52.24 if we exercise a lot, 00:01:54.09 we may have had injuries in our lifetime 00:01:56.11 that have given us scars. 00:01:57.21 Those are not heritable characteristics. 00:02:00.10 Heritable characteristics 00:02:02.02 are the types of traits 00:02:03.19 that we pass on to subsequent generations, 00:02:06.11 or that we inherited from our parents 00:02:08.29 and grandparents. 00:02:10.18 Heritable characteristics 00:02:12.15 are an important part of evolution, 00:02:14.11 because it allows transmission 00:02:17.12 from one generation to the next, 00:02:18.24 and on and on through evolutionary history. 00:02:21.23 Now, the last part of this is change, 00:02:23.29 and change is also really important. 00:02:26.08 There has to be the ability in evolution 00:02:29.05 for these heritable characteristics 00:02:30.29 to vary, 00:02:32.21 to change in response to environmental factors 00:02:35.13 that might favor one type of characteristic 00:02:38.09 or another, 00:02:39.16 and we'll come back to that. 00:02:40.25 And that's what Darwin was getting at 00:02:42.21 with this idea of modification, 00:02:44.13 that there's going to be change 00:02:46.01 in how organisms are organized 00:02:47.24 and how they look over time. 00:02:52.03 So, here's an example, 00:02:53.18 a cute picture of a pair of dogs 00:02:56.07 and their puppies, 00:02:57.19 where you can really see 00:02:59.07 the variation in characteristics, 00:03:00.25 even in one generation. 00:03:02.23 If you look at the parents 00:03:04.11 and you look at the pups, 00:03:05.21 you can see some of the puppies 00:03:07.03 look like one parent, 00:03:08.21 with, you know, pure light fur, 00:03:11.03 others look like the other parent, 00:03:12.27 with very dark fur around the face, 00:03:15.03 but yet there are other puppies in the litter 00:03:17.13 that look different yet again, 00:03:19.04 that have a mix of the characteristics 00:03:21.29 of those two adults. 00:03:24.05 So, you can get a sense 00:03:25.29 of the variation in this image 00:03:27.26 that can be explored in evolution 00:03:30.29 and capitalized upon 00:03:33.06 through evolutionary time. 00:03:35.29 One example of variation 00:03:38.05 that's been really important for us 00:03:40.11 to understand how we can 00:03:43.17 change the characteristics, 00:03:45.07 the features of a species, 00:03:47.14 over time, 00:03:48.28 is the peppered moth. 00:03:50.12 So, these two moths, 00:03:51.23 that look very, very different 00:03:53.06 -- the light one on the left 00:03:54.24 and the dark one on the right -- 00:03:56.01 are the same species. 00:03:57.12 They can interbreed. 00:03:58.23 Now, the dark one and the light one, 00:04:00.04 as you might expect, 00:04:02.07 do better in different types of environments. 00:04:06.03 This color characteristic 00:04:08.03 varies, of course, 00:04:10.00 and in some environments 00:04:11.29 it benefits the organisms 00:04:13.21 to be light or to be dark. 00:04:15.02 In other environments, 00:04:16.21 that same characteristic 00:04:18.16 may be detrimental to the animal. 00:04:20.14 So, these peppered moths 00:04:22.10 provided a classic example 00:04:24.01 of how characteristics can vary 00:04:27.07 with environment, 00:04:28.12 and how populations of a particular species 00:04:30.28 can vary. 00:04:32.22 So, this was noted 00:04:34.07 particularly in the industrial revolution. 00:04:36.19 At that time, 00:04:38.00 we went from manufacturing 00:04:39.22 using people 00:04:41.21 sewing or create objects 00:04:43.08 to using a lot of machines 00:04:44.26 to make products. 00:04:47.01 With the use of machines 00:04:48.21 came the use of coal, 00:04:50.25 and with coal came soot, 00:04:52.19 or pollution in the air. 00:04:54.06 Now, with that soot and pollution, 00:04:55.24 you could imagine that structures in the environment, 00:04:59.03 like trees, 00:05:00.19 would become darker, 00:05:01.28 and the peppered moth populations 00:05:04.05 changed in order to accommodate that. 00:05:06.27 And the darker morph 00:05:09.05 of the peppered moth 00:05:11.14 survived better. Right? 00:05:12.26 It was better camouflaged 00:05:14.23 against potential predators in the environment. 00:05:17.08 When the environment cleared up 00:05:19.21 and pollution decreased, 00:05:21.08 the tree barks became lighter 00:05:23.26 and the lighter version of the moth 00:05:25.25 actually survived better. 00:05:27.17 So, we can see variation 00:05:29.00 in the characteristics in a population, 00:05:31.19 even over this short amount of time, 00:05:34.20 and due to a human-induced 00:05:37.07 artifact in the environment, 00:05:38.09 this pollution from coal. 00:05:41.01 Now, just to show you how striking 00:05:43.05 this difference can be in the camouflage 00:05:45.05 of these moths on trees, 00:05:46.29 we can see some here. 00:05:48.19 So, here's our dark morph and our light morph, 00:05:50.17 and if we look at this tree, 00:05:51.28 we can see both the dark morph 00:05:53.15 and the light morph. 00:05:54.19 Here's the light one right down here, 00:05:56.20 and you can see it better camouflages 00:05:58.00 against the light bark 00:05:59.17 in this healthy tree. 00:06:01.05 The dark morph stands out against that light tree, 00:06:03.27 expect in this area over here, 00:06:05.22 where it's against this injury to the tree, 00:06:08.09 which shows up darker. 00:06:10.03 Another example in variation in populations 00:06:14.08 that we've probably all had experience with 00:06:16.14 is in bacteria 00:06:18.19 and the treatment of bacteria with antibiotics. 00:06:21.06 So, when we go to our doctor's office 00:06:22.26 with a bacterial infection, 00:06:24.08 we're prescribed antibiotics, 00:06:26.06 medicine to kill those bacteria, 00:06:28.19 and doctors are often very specific 00:06:31.09 about the need to take that medicine 00:06:34.11 over a precise time course, 00:06:36.11 and in particularly they say, 00:06:37.27 "Don't stop the medicine early. 00:06:40.12 You have to take the full course of medicine. 00:06:42.19 Even if you're feeling better, 00:06:44.21 take the full course of medicine." 00:06:46.07 It's important to do that. 00:06:47.25 Why is that? 00:06:49.02 It's because of the selection 00:06:51.00 that's acting on the variation in the population. 00:06:54.13 So, when we have a bacterial infection, 00:06:57.00 the species of bacteria 00:06:58.27 that's in our bodies 00:07:00.07 may have lots of variants to it, 00:07:02.04 and this is shown in number 1 on the left. 00:07:04.06 They might vary in aspects of their biology, 00:07:07.16 including how strong they are, 00:07:09.01 how resistant they are 00:07:11.01 to antibiotic medicines. 00:07:12.27 If we treat them, 00:07:15.06 shown in point 2 over here, 00:07:17.05 but we don't treat them long enough, 00:07:19.14 which are the bacteria 00:07:21.03 that are going to survive? 00:07:22.12 It's going to be the ones that are the strongest, 00:07:23.28 that are the most resistant 00:07:25.25 to the medication. 00:07:27.06 So, if we don't kill them 00:07:29.03 and we stop taking the medicine, 00:07:30.26 they'll be able to multiply 00:07:32.24 and will take on a larger part 00:07:34.28 of the population 00:07:36.20 of the bacteria. 00:07:37.25 It's not unless we kill them all 00:07:39.17 that we can prevent those resistant bacteria 00:07:42.06 from then multiplying 00:07:43.28 and becoming a problem 00:07:45.15 for our antibiotic medications 00:07:47.16 down the road. 00:07:49.07 So, I've shown you several examples 00:07:51.08 of how populations of a species can vary, 00:07:55.07 whether it's peppered moths or bacteria, 00:07:58.05 but how do we go from that 00:07:59.23 population-level variation 00:08:01.26 to the evolution of new species? 00:08:05.11 This is called speciation, 00:08:07.13 and in general 00:08:09.15 what happens is that populations 00:08:11.08 of a species 00:08:12.28 will be separated 00:08:14.13 and unable to interbreed, 00:08:16.03 and if they're separated 00:08:18.01 for a long enough period of time, 00:08:19.17 when they come back together 00:08:21.02 they may not be able to interbreed, 00:08:23.27 and then we would call them 00:08:25.25 different species. 00:08:27.02 One of the ways 00:08:28.25 that interbreeding is prevented 00:08:30.16 is through geographic isolation. 00:08:35.06 One of the students in my lab, 00:08:36.18 Andrew Trandai, 00:08:38.07 actually helped me out 00:08:40.13 by developing this hypothetical example 00:08:42.14 that I'm going to show you 00:08:44.23 on what a speciation event 00:08:46.13 might look like, 00:08:47.23 so I have to thank Andrew 00:08:49.19 for all of the images 00:08:50.28 that are coming up in the next series. 00:08:54.06 Okay, so in our hypothetical example, 00:08:56.29 what we're looking at is 00:08:58.29 some rodent squirrel-like animal 00:09:01.00 in an environment 00:09:02.19 -- one species -- 00:09:04.06 all together as one population. 00:09:07.20 So, how do we separate them 00:09:09.16 and get new populations to evolve? 00:09:11.17 Well, in Andrew's example, here, 00:09:13.23 we have flooding 00:09:15.29 and an aquatic barrier 00:09:17.27 that these animals cannot cross, 00:09:20.01 so effectively 00:09:21.29 the population in the trees 00:09:23.12 and the population in the sand 00:09:25.19 are separated now 00:09:27.16 and will be evolving independently. 00:09:30.21 Over time, if we look at each of them, 00:09:32.23 we may see differences 00:09:34.12 being incorporated 00:09:37.17 into their biology. 00:09:38.25 Just superficially, 00:09:40.06 we might see the animals 00:09:41.29 that are in the forest 00:09:43.23 turning a different color, 00:09:45.19 other aspects of their anatomy 00:09:47.15 might change 00:09:49.17 to live in the trees. 00:09:51.04 On the opposite side of our river, 00:09:54.18 we may see the populations 00:09:56.07 that are in more of a sandy desert environment 00:09:59.18 change coat color 00:10:01.09 to match that environment, 00:10:02.20 or change size 00:10:04.15 to better adjust physiologically 00:10:06.10 to this drier environment. 00:10:08.13 Then ultimately, 00:10:10.00 once these differences have occurred 00:10:12.06 over, again, a very, very long period of time, 00:10:15.01 through evolution, 00:10:16.08 what would happen if the river dried up 00:10:18.16 and these animals 00:10:20.24 were able to come back together? 00:10:22.27 Well, they might come back together 00:10:25.07 and be able to interbreed, 00:10:27.10 but they may come back together 00:10:28.29 and not recognize each other 00:10:30.18 as the same species, 00:10:32.00 and therefore, 00:10:33.16 even though they're together 00:10:34.25 in this environment, 00:10:35.29 they would not interbreed 00:10:37.18 and their independent characteristics 00:10:39.04 would be carried on 00:10:40.25 from generation to generation 00:10:42.12 in those species. 00:10:47.14 So, that was an example 00:10:49.04 of geographic isolation, 00:10:51.04 and the biggest example of geographic isolation 00:10:53.22 happened about 200 million years ago, 00:10:56.18 when Pangaea, 00:10:58.06 which was this big super continental landmass, 00:11:00.25 broke apart to give us 00:11:03.16 the different continents that we know today. 00:11:05.28 So, South America and Africa 00:11:09.16 broke apart from North America and Europe, 00:11:13.08 and those continents 00:11:15.03 moved and separated around the globe. 00:11:17.22 With that separation, 00:11:20.00 the species that were together 00:11:22.08 prior to this breakup 00:11:23.25 then became separated, 00:11:25.16 and so if we look at species 00:11:27.10 that are in Africa versus South America, 00:11:29.20 for example, 00:11:30.28 we can see animals that 00:11:32.27 perhaps came from the same lineage, 00:11:34.14 but now are very, very different, 00:11:37.06 and are in fact different species. 00:11:43.20 Okay, so we've talked about this 00:11:46.04 process of evolution 00:11:47.16 and how it can occur. 00:11:49.28 What if we want to understand 00:11:51.16 the evolutionary history 00:11:53.11 of the animals that are 00:11:55.22 alive on earth today? 00:11:58.09 Well, we have to use a different set of techniques 00:12:00.28 to do that. 00:12:02.14 Here's just some of vertebrate diversity 00:12:04.16 and, as I said at the beginning of the lecture, 00:12:07.11 we also have lots of plants 00:12:09.16 and invertebrates and insects. 00:12:11.10 So I'm just showing you a very small part 00:12:12.14 of biodiversity here. 00:12:14.14 How do we figure out, 00:12:16.06 with animals so diverse as these, 00:12:18.17 how they're related to one another? 00:12:20.16 And how they evolved through time? 00:12:23.10 Well, we can take 00:12:25.04 a very simple example 00:12:27.03 of how we construct our own family trees 00:12:29.24 over very short time periods, 00:12:31.23 over several generations, say. 00:12:33.21 We research our genealogy, 00:12:35.11 we use birth notices and death notices, 00:12:38.27 and we recalled history 00:12:40.27 from our parents or grandparents, 00:12:42.29 and we can use that 00:12:45.01 to construct relationships 00:12:46.21 among our relatives and ourselves. 00:12:49.13 This is a really interesting family tree 00:12:51.25 that's on the wall of a Czech castle, actually, 00:12:55.01 and shows the relatedness 00:12:56.19 of this family, 00:12:58.04 going from a founder 00:12:59.17 down at the base of the tree, in the trunk, 00:13:01.20 up to the descendants at the top of the tree. 00:13:05.24 So, if we take a hypothetical example, 00:13:07.25 again, 00:13:09.01 of building a family tree, 00:13:11.05 and we start with 00:13:13.06 this family of green-ish and blue-ish, 00:13:15.13 big-eared and small-eared organisms, 00:13:18.04 and try to construct how they're related, 00:13:20.21 we can just look and see how family trees 00:13:23.05 are organized. 00:13:26.01 So, here I've taken that population 00:13:27.29 and put them onto their tree 00:13:29.25 -- that I made up -- 00:13:32.01 and we can see that they're related to one another. 00:13:36.10 So, the individuals 00:13:39.12 that are connected at the first branch 00:13:41.21 are siblings. 00:13:43.11 They have the same parents. 00:13:45.21 If we move back in the tree, 00:13:48.01 we're looking at the different common ancestors 00:13:51.23 of these individuals. 00:13:54.17 So, if we go back, 00:13:56.21 these groups that are bracketed 00:13:58.23 in the orange boxes 00:14:00.14 are shared pairs of grandparents, 00:14:03.20 so they'd be cousins. 00:14:06.19 And if we look down near the base, 00:14:08.28 we can see that all of these organisms 00:14:11.07 share a pair of grandparents. 00:14:13.28 Now, because we're in recent history 00:14:16.25 and we have all sorts of ways 00:14:18.13 to record our history, 00:14:19.25 we may even know what these grandparents look like, 00:14:22.19 what our common ancestors of us, 00:14:24.15 and our sibling, and cousins, look like, 00:14:28.04 and I've reconstructed them this way. 00:14:30.01 If we look at at a group of animals 00:14:31.27 that's as broad as fish and mammals 00:14:33.27 and amphibians and reptiles, though, 00:14:36.04 we don't have that record, 00:14:38.18 to know what those common ancestors are 00:14:41.17 or what they looked like. 00:14:43.03 We have to use other types of approaches, 00:14:44.29 called phylogenetic approaches, 00:14:46.16 to basically try 00:14:48.26 to reconstruct the common ancestor 00:14:51.06 and how those species are related. 00:14:53.22 So, if we take this set of vertebrates, 00:14:56.22 this small number of animals, 00:14:58.20 and try to put them on a tree, 00:15:00.21 this is what it would look like, 00:15:02.04 and this is based on lots of peoples' research 00:15:04.08 over many, many years, 00:15:06.06 and I'll run you through it quickly. 00:15:08.20 On the far left, 00:15:10.18 we have the base of the vertebrate tree, 00:15:13.12 and these are lampreys, 00:15:14.28 these are animals that don't even have, really, 00:15:17.16 jaws. 00:15:18.20 They have these suction discs 00:15:20.02 that rasp and grip onto other species. 00:15:22.26 As we move up the tree, 00:15:24.13 we get into things like sharks, 00:15:26.00 and skates, and rays, 00:15:27.16 that have jaws, 00:15:30.02 but they have a cartilaginous skeleton. 00:15:31.29 When we move up yet again, 00:15:33.14 we get to the bony organisms 00:15:35.03 that include the fishes, 00:15:36.18 shown with these anemone fish, 00:15:38.14 the third image from the left, 00:15:40.08 and then we get up into the tetrapods, 00:15:42.26 that include amphibians, 00:15:45.17 reptiles, birds, and mammals. 00:15:48.14 Now, how do we construct 00:15:50.10 this kind of tree when 00:15:52.14 we don't have these detailed records 00:15:53.29 that we have of families? 00:15:55.12 Well, we do it by looking at 00:15:57.23 what characteristics these organisms share 00:16:00.14 and what characteristics vary between them. 00:16:02.26 There are lots of different types of characteristics 00:16:04.15 that we can use. 00:16:08.04 So, one of the features that we look for 00:16:10.20 when we're looking at shared characteristics, 00:16:12.22 or similarities and differences among organisms, 00:16:15.20 are anatomical features, 00:16:17.22 things like the shape of bones 00:16:20.06 or where sutures 00:16:21.14 -- where bones connect to one another -- 00:16:23.04 or where we see holes through our skull 00:16:25.03 or other parts of our anatomy. 00:16:27.12 Bone and other structures 00:16:29.09 from the body 00:16:30.26 provide really nice characters 00:16:32.11 that we can use to try to figure 00:16:34.03 the relatedness of organisms. 00:16:36.11 In addition to using anatomical features 00:16:39.10 to try to understand the evolutionary history 00:16:41.20 of organisms and their relatedness, 00:16:43.24 DNA is now also 00:16:46.14 providing a really powerful way 00:16:48.24 of generating characters 00:16:50.17 to try to understand 00:16:52.20 how organisms have evolved. 00:16:54.08 In particular, we can compare a single gene 00:16:56.24 among different organisms, 00:16:58.14 different animals and species, 00:17:00.17 and see how it varies and how it's similar, 00:17:03.05 and look for changes in that 00:17:07.17 organization of the gene itself 00:17:09.05 that might give us signals 00:17:11.07 about how close a species is 00:17:12.27 to another species 00:17:14.20 and the relationship among them 00:17:16.28 and to different species. 00:17:18.18 Now, another set of data 00:17:20.12 that's been useful in understanding evolutionary history, 00:17:22.28 of course, is fossils. 00:17:24.22 They're really important. 00:17:26.01 Now, fossils provide information 00:17:28.16 about when and how features arose. 00:17:30.26 They won't, though, 00:17:32.20 provide the common ancestor. 00:17:34.07 It would be very unlikely 00:17:35.21 to actually dig up a fossil 00:17:37.12 that gives you the exact common ancestor of a species 00:17:39.24 but, nevertheless, 00:17:41.20 what they can provide us, 00:17:42.28 how they can ground our understanding 00:17:45.29 of when an organism 00:17:47.24 or particular elements and characteristics 00:17:49.11 of an organism arose, 00:17:50.28 is incredibly important. 00:17:53.25 So, to summarize 00:17:56.23 our introduction to evolution 00:17:58.09 and some of the major points we've talked about... 00:18:00.11 first, evolution is change 00:18:02.13 in the heritable characteristics of organisms 00:18:05.00 from generation to generation, 00:18:06.17 descent with modification 00:18:08.21 as proposed by Darwin. 00:18:11.08 Variation in characteristics 00:18:13.09 allows some subsets of populations 00:18:15.18 to be selected for or against. 00:18:18.20 And selection can cause change 00:18:20.14 in the characteristics 00:18:22.11 that persist in a population, 00:18:23.26 and this can allow for populations to diverge. 00:18:28.27 Reconstructing how the diversity of organisms 00:18:31.19 evolved 00:18:33.09 involves making trees, 00:18:34.23 or these phylogenies that I talked about, 00:18:37.09 that show different organisms 00:18:39.12 are related to one another. 00:18:41.19 And phylogenies, though, 00:18:43.08 depend on identifying characteristics 00:18:45.29 that are shared between organisms 00:18:48.06 and that can suggest their common ancestry. 00:18:50.07 And, again, we can get those characteristics 00:18:52.22 from morphology, from genes, 00:18:54.24 from all sorts of different sources. 00:18:57.19 Thank you.