Session 1: Theory Behind Evolution I
Transcript of Part 3: Local Interactions Determine Collective Behavior
00:00:08.01 Hello. 00:00:09.10 I'm Deborah Gordon. 00:00:10.25 I'm a professor at Stanford, 00:00:12.02 and I'd like to talk to you today 00:00:13.23 about the evolution of collective behavior. 00:00:16.15 We see collective behavior all around us. 00:00:19.15 Here's an example of collective behavior: 00:00:22.00 it's a group of starlings turning. 00:00:24.18 They have an amazingly fluid way 00:00:27.02 of moving a flock collectively. 00:00:30.02 But, of course, there's lots of collective behavior 00:00:32.25 going on around us that we don't see. 00:00:35.08 Gene transcription networks 00:00:37.15 are a form of collective behavior. 00:00:40.05 Cells work collectively, for example, 00:00:42.15 in an embryo, 00:00:45.04 the development of an embryo and differentiation 00:00:47.25 is the result of collective behavior among cells. 00:00:51.01 Cancer cells work collectively 00:00:53.09 to establish tumors. 00:00:56.00 In a brain, 00:00:57.27 neurons work collectively to produce perception, 00:01:01.05 and memory, 00:01:02.18 and all of the functions of brains. 00:01:05.12 What all these systems have in common 00:01:07.08 is that there's no central control. 00:01:09.11 There's nobody in charge, 00:01:11.00 nobody telling anybody what to do. 00:01:13.01 I study collective behavior in ants. 00:01:15.21 An ant colony consists of sterile female workers, 00:01:19.27 those are the ants you see walking around, 00:01:22.01 and although there are reproductive females called queens, 00:01:26.22 they don't give any instructions 00:01:28.11 or tell anybody what to do. 00:01:30.17 So, instead, 00:01:32.07 ant colonies work through local interactions. 00:01:35.06 Although this is the way 00:01:37.16 that most people think about ant colonies 00:01:39.26 -- this is a silly picture that's been staged -- 00:01:42.03 in fact this never happens. 00:01:44.01 There's no foremen, 00:01:45.28 there's no bureaucrats, 00:01:47.14 there are no managers... 00:01:49.19 somehow the behavior of the colony, 00:01:51.21 the way that it can respond to its environment, 00:01:54.23 arises through interactions among ants. 00:01:57.26 Systems without central control 00:01:59.27 always use networks of local interactions. 00:02:03.22 In ants, those are networks 00:02:06.07 of antennal contact 00:02:08.16 and chemical interactions. 00:02:10.24 In cells also, 00:02:12.27 those are networks of chemical interactions between cells, 00:02:15.13 and between cells and their environments. 00:02:18.09 And so, all of these interactions together 00:02:20.25 create a network. 00:02:25.14 The history of biology, 00:02:27.22 especially in the last hundred years, 00:02:30.11 has been to try to understand 00:02:33.00 the function and dynamics of networks, 00:02:36.09 and it began with trying to associate 00:02:39.13 function with type. 00:02:42.11 So, illustrated here, for example, 00:02:45.29 is the idea of one gene - one protein. 00:02:50.18 Then, in studies of neuroscience, 00:02:54.11 early on, we hoped to find particular parts of the brain, 00:02:59.25 each of which would do a certain function, 00:03:02.11 and the study of social insects 00:03:04.02 proceeded in the same way, 00:03:06.25 by looking at the minority of species 00:03:09.25 in which workers come in different sizes, 00:03:12.29 and trying to assign a function 00:03:15.05 to each type of worker. 00:03:18.12 But over time, 00:03:20.05 we've understood that, instead, 00:03:22.02 function and dynamics are produced by interactions. 00:03:24.29 In genes, there are very complex regulatory processes 00:03:28.25 that determine the relationship 00:03:30.24 between genotype and phenotype. 00:03:32.29 The function of brains arises 00:03:35.13 from interactions among many different groups of neurons 00:03:39.06 in the brain 00:03:40.22 that form circuits that interact with each other, 00:03:42.26 and in the same way, 00:03:44.20 in ant colonies, 00:03:46.13 we can see how local interactions 00:03:48.12 produce the behavior of the system. 00:03:52.06 So, ants operate mostly by smell. 00:03:55.01 Most ants can't see. 00:03:57.19 And, they smell with their antennae, 00:04:00.10 so one very important interaction among ants 00:04:03.07 is when one ant touches another with its antennae, 00:04:06.02 and when one ant touches another with its antennae, 00:04:08.23 it can tell by the odor 00:04:10.26 whether the other ant belongs to the same colony 00:04:13.17 and what task it's been doing. 00:04:16.28 So here, we see a laboratory arena. 00:04:20.10 The ants are moving around and interacting. 00:04:22.22 In this arena, 00:04:24.22 there are two tubes connecting to other arenas. 00:04:28.25 When one ant meets another, 00:04:30.28 it doesn't matter which ant it's meeting, 00:04:33.02 they're not exchanging any complicated signals or messages, 00:04:37.11 all that matters to the ant 00:04:38.29 is the rate at which it meets other ants. 00:04:44.19 Taken together, 00:04:46.11 all these interactions produce a network. 00:04:48.17 This illustrates the network and the paths 00:04:51.20 of all the ants that you saw in the film 00:04:54.04 in the previous slide. 00:04:56.01 And it's this constantly shifting 00:04:58.08 network of interactions 00:05:00.01 that produces the behavior of the system. 00:05:02.25 A brain works the same way, 00:05:04.15 but the great thing about ants 00:05:06.04 is that we can see all of the interactions 00:05:08.03 as they're happening, 00:05:09.26 and so we can see how this network of interactions 00:05:12.19 is related to the function of the system. 00:05:17.16 4 I study ants in a desert in Arizona 00:05:21.13 and I'm going to be telling you about 00:05:23.16 some of the work that I've done with harvester ants in the south... 00:05:27.24 I'm gonna be telling you about some of the work 00:05:29.21 that I've done with harvester ants 00:05:31.25 at a study site in southeast Arizona. 00:05:34.20 This is what the nest of a mature colony looks like. 00:05:39.21 You can see the nest entrance, 00:05:41.18 and then there's a trail leading away from the nest entrance, 00:05:44.15 sometimes cleared and sometimes not, 00:05:47.01 that goes about 20 meters, 00:05:49.03 and these ants are called harvester ants 00:05:51.01 because they eat seeds. 00:05:52.16 So, they travel along this trail, 00:05:54.00 collect seeds, and bring them back to the nest. 00:05:57.06 And, I divide all the behavior that I see outside the nest 00:06:00.00 into these four categories: 00:06:02.14 foraging, that's going out and collecting seeds 00:06:05.02 and bringing it back, 00:06:06.16 then the patrollers, shown here with a magnifying glass, 00:06:09.22 are an interesting group of workers 00:06:11.20 that go out early in the morning, 00:06:13.19 they move around the foraging area, 00:06:15.17 they meet the neighbors... 00:06:18.11 they meet the ants of the other neighboring colonies, 00:06:21.21 and it's their safe return 00:06:24.02 that signals the foragers that it's time to go out. 00:06:26.26 The nest maintenance workers work inside the nest. 00:06:29.20 They line the walls of the chambers 00:06:31.16 with moist soil that dries to a kind of adobe finish, 00:06:34.28 and then they carry out the dry soil. 00:06:37.07 So, you see nest maintenance workers 00:06:39.11 coming out, putting down soil, and going back in. 00:06:42.15 And finally, the midden workers 00:06:44.03 work on the refuse pile, or midden, 00:06:46.02 where they put a colony-specific odor 00:06:48.03 that helps guide foragers back into the nest. 00:06:52.24 It's only about 25% of the colony 00:06:55.02 that works outside the nest, 00:06:57.07 so these four task groups that I told you about 00:06:59.07 are only 25% of the colony. 00:07:01.09 Deep inside the nest, 00:07:03.01 which goes down a meter, sometimes two, 00:07:06.10 there are ants that are storing the seeds 00:07:11.00 and processing the seeds. 00:07:13.02 The queen is down somewhere, 00:07:15.05 she just lays the eggs. 00:07:17.07 Then, there are ants 00:07:19.07 that are feeing the larvae and brood. 00:07:20.27 It's actually the larvae that consume most of the food. 00:07:23.21 And, despite what it says in the... 00:07:26.25 and, despite what it says in the bible 00:07:28.15 about how hard-working ants are, 00:07:30.08 there are a lot of ants 00:07:32.12 that are just hanging around doing nothing, 00:07:34.02 and it's a very interesting question about the function of the network, 00:07:36.18 why the colonies... 00:07:39.28 it's a very interesting question 00:07:41.17 about how that group of reserve, 00:07:44.19 or inactive colonies 00:07:46.20 might function to contribute to regulating 00:07:48.20 the network of interactions. 00:07:52.00 In this species, as in most ant species, 00:07:55.05 all of the ants are the same size, 00:07:57.18 so you can't identify the task of an ant by its size, 00:08:00.29 but you can identify the task of an ant 00:08:02.26 by what it's doing. 00:08:04.25 And it turns out that ants change tasks. 00:08:07.14 So, this shows the results of experiments 00:08:10.07 in which I created a need 00:08:12.13 for more ants to do a certain task. 00:08:14.18 So, the arrows point to the... 00:08:18.11 the arrows show the outcome 00:08:20.08 of experiments where I created a need for more ants 00:08:23.01 doing that task. 00:08:24.03 So, for example, 00:08:26.04 if more ants are needed to forage, 00:08:28.01 then the patrollers will change to forage, 00:08:29.23 the midden workers will change to forage, 00:08:31.25 and the nest maintenance workers will change to forage. 00:08:34.22 In response to a lot of really exciting new food 00:08:38.09 that I put out there, 00:08:40.03 everybody will switch to forage. 00:08:42.15 If there's a need for more patrollers, 00:08:44.05 so for example, if I create a disturbance, 00:08:46.10 then the nest maintenance workers 00:08:48.13 will switch to do patrolling, 00:08:50.08 but if more ants are needed to do nest maintenance, 00:08:53.07 so for example, if I create a mess that the nest maintenance workers 00:08:55.19 have to clean up, 00:08:57.23 then none of the others will switch back 00:08:59.15 to do nest maintenance work, 00:09:01.07 and they have to go new nest maintenance workers 00:09:03.09 from the younger ants inside the nest. 00:09:05.13 So, there's a one-way flow of ants 00:09:08.06 from the younger ants inside the nest, 00:09:09.29 through nest maintenance, 00:09:11.20 some of them become patrollers, 00:09:13.10 and everybody eventually ends up as a forager, 00:09:15.27 and once an ant is a forager, it doesn't come back. 00:09:20.13 All of this is regulated 00:09:22.12 by the interactions of the ants 00:09:24.10 as they come in and out of the nest, 00:09:26.22 and so this process 00:09:28.22 that I call task allocation 00:09:30.24 is the process at the level of the colony 00:09:33.01 that gets the right numbers of ants 00:09:35.12 to each task in a given situation, 00:09:38.19 and so now we understand 00:09:40.07 that it's this regulatory process of task allocation 00:09:42.28 that determines how the system functions, 00:09:45.29 not the inherent characteristics of any particular ant, 00:09:50.17 but the regulation of the whole system 00:09:53.05 that shifts ants around 00:09:55.11 into different tasks 00:09:57.02 as they're needed in changing conditions. 00:10:01.23 So, this raises the question, 00:10:03.14 how do these interaction networks evolve? 00:10:06.00 How does evolution shape the regulation 00:10:08.11 of a system with no central control? 00:10:12.12 I'd like to begin with a quote from Dobzhansky, 00:10:15.03 that "nothing in biology makes sense 00:10:16.23 except in the light of evolution," 00:10:18.22 and to modify that in my own way 00:10:20.09 by saying that nothing in evolution makes sense 00:10:23.03 except in the light of ecology, 00:10:24.26 and what I'd like to do now 00:10:26.13 is to explain that in more detail. 00:10:29.27 So, ecology comes from the words for village. 00:10:33.13 Ecology is about how different parts of a system interact, 00:10:38.03 and we think of ecology 00:10:40.04 in terms of interactions at different levels. 00:10:42.18 First, interactions among individuals 00:10:44.24 like the trees shown here, 00:10:47.00 and then interactions within a population 00:10:50.01 and between populations, 00:10:51.21 where a population is the set of all individuals 00:10:54.28 that may reproduce with each other. 00:10:58.15 So, populations are defined in terms of reproduction. 00:11:02.00 And then a community 00:11:04.07 is all of the populations that are living together 00:11:06.11 and interacting in a certain place, 00:11:09.00 so here, there are some birds in the forest. 00:11:11.25 Of course there are many, many kinds of organisms 00:11:15.12 in the community that that forest is part of. 00:11:18.14 And we could go even one level up 00:11:20.13 and think about the interactions 00:11:22.19 between all of those organisms 00:11:24.14 and all of the other factors that affect them, 00:11:27.01 like the air, and the water, and the wind, 00:11:29.29 and all of the chemicals that are circulating 00:11:32.05 through the system. 00:11:34.04 Ecology is the science 00:11:36.06 that helps us to understand 00:11:38.12 how all of the interactions lead to changes in a system. 00:11:42.22 And so when we want to ask, 00:11:44.07 how do interaction networks evolve, 00:11:45.28 we really have to ask, 00:11:48.12 how does evolution 00:11:50.25 react to the interactions within the system? 00:11:56.13 So, we have to think about ecology to understand evolution. 00:12:01.19 Another way to think about the relationship 00:12:03.21 between ecology and evolution 00:12:05.23 is to think about what natural selection really is. 00:12:09.01 Natural selection requires three conditions. 00:12:14.14 First, there has to be variation in a trait, 00:12:17.23 and we can think of a trait as anything. 00:12:19.09 It could be eye color, 00:12:20.23 or the height of a tree, 00:12:23.04 or the time of year that the tree flowers, 00:12:25.03 or how the thick the polar bear's fur is. 00:12:27.10 A trait could be anything, 00:12:29.09 including how individuals within a system 00:12:32.13 interact to regulate that system. 00:12:34.25 So, first of all, there has to be variation. 00:12:37.17 Then, that trait has to be heritable, 00:12:39.28 because natural selection 00:12:41.18 acts over many generations, 00:12:43.19 and if the trait isn't heritable, 00:12:45.19 if the offspring don't resemble their parents, 00:12:49.00 then over many generations 00:12:50.28 things might reshuffle 00:12:52.25 but there will be no trends. 00:12:54.25 And finally, 00:12:56.09 and here's where the ecology comes in, 00:12:58.04 there has to be... 00:12:59.22 and finally, here's where the ecology comes in, 00:13:01.20 there have to be differences 00:13:03.14 in reproductive success 00:13:05.12 related to the trait. 00:13:06.27 The trait has to make a difference ecologically 00:13:08.27 to how the organisms survive and reproduce. 00:13:14.26 So, here's a diagram to illustrate that. 00:13:17.18 So here, 00:13:20.01 let's imagine that we have lots of individuals in a population, 00:13:22.27 and each row in this diagram 00:13:24.23 is gonna be a different generation, 00:13:27.11 and the different colors represent the trait, 00:13:29.29 so the trait here is color, 00:13:31.19 and some of them are blue, 00:13:33.02 some are red, 00:13:34.12 some are orange. 00:13:35.23 That's the variation in the trait, 00:13:37.06 so there's variation to start out with. 00:13:39.14 And then, that trait is heritable, 00:13:43.00 so blue circles make more blue circles, 00:13:45.13 red circles make more red circles, 00:13:47.25 orange circles make more orange circles... 00:13:50.10 the offspring resemble their parents, 00:13:52.20 so it's heritable. 00:13:54.07 And now, here's where the ecology comes in. 00:13:56.08 In order for their to be any change over generations, 00:14:01.00 some of these individuals have to reproduce more than others, 00:14:05.13 and in this story let's say that it's really great, 00:14:08.05 ecologically, to be orange. 00:14:10.04 And so, over generations, 00:14:11.23 there will be more orange individuals, 00:14:13.19 because their ecology is such 00:14:16.22 that they can reproduce more. 00:14:18.28 And here we get natural selection, 00:14:20.24 a change over many generations, 00:14:22.17 in the frequency of individuals that are orange. 00:14:25.01 That's how natural selection works, 00:14:27.12 and it always requires an ecological process 00:14:30.21 that affects the reproductive success 00:14:33.10 of the trait 00:14:35.07 in such a way that some individuals 00:14:37.15 reproduce more than others. 00:14:41.00 So, when we ask, 00:14:42.20 how do interaction networks evolve, 00:14:44.14 really we're asking an ecological question 00:14:46.19 about why networks function differently 00:14:50.04 in a given environment 00:14:52.13 so that some forms of an interaction network 00:14:55.10 allow more reproduction than others. 00:14:59.25 So, we could ask this question about cancer cells. 00:15:02.27 We could ask, 00:15:05.11 why is it that certain ecological conditions 00:15:07.14 within a body 00:15:10.02 allow the evolution of particular types of cancer, 00:15:13.24 and allow the cancer cells 00:15:16.29 to change over generations of cells? 00:15:20.07 That's a difficult question, although it's one that's really important, 00:15:23.23 and we can also ask the same kind of question 00:15:26.09 about ant colonies. 00:15:28.00 And there, because we can see the interactions 00:15:30.09 among ants 00:15:32.07 and we can look at them in their environments, 00:15:34.02 we have the opportunity to learn 00:15:35.26 how interaction networks 00:15:38.01 are evolving in certain environments. 00:15:40.04 So, there are more than 12,000 species of ants. 00:15:43.02 They live in every conceivable habitat on Earth, 00:15:45.26 and they all have to solve ecological problems 00:15:48.01 because they have to explore their environments, 00:15:50.12 they have to get resources, 00:15:52.03 and then they have to reproduce. 00:15:54.07 And when we look at different species, 00:15:56.02 we can see how interaction networks 00:15:58.11 are evolving to work differently in different environments. 00:16:01.09 So, one species that we study in my lab 00:16:03.13 is the Argentine ant. 00:16:05.03 It's an invasive species. 00:16:06.29 They came from Argentina, 00:16:08.16 they have spread around the world, 00:16:10.04 and everywhere that there's a Mediterranean climate, 00:16:12.22 there now are Argentine ants, 00:16:14.21 on the coast of California, 00:16:16.22 the coast of South Africa, Australia, 00:16:19.08 the whole Mediterranean coastline, 00:16:21.03 Japan, Hawaii... 00:16:24.22 And one of the interesting things 00:16:26.05 about how their interaction networks operate 00:16:28.01 is how different they are from other ant species. 00:16:32.24 Many ant species use interaction networks 00:16:35.23 to create what is called central place foraging. 00:16:39.12 So, you can think of it like a broom. 00:16:41.11 The ants all live in one nest, 00:16:43.09 they go out to forage, and they bring their resources 00:16:45.20 all back to the central nest. 00:16:48.07 But, Argentine ants, like other species, 00:16:50.16 are really good at a different kind of strategy. 00:16:53.11 They make circuits, 00:16:55.09 it's more like a vacuum cleaner. 00:16:57.11 They move from nest to nest, they have many different nests, 00:17:00.00 they move from nest to nest 00:17:01.28 and they sweep up resources as they go. 00:17:04.20 So, they create a different kind of interaction network 00:17:06.29 that functions differently 00:17:09.11 from the species that use central place foraging. 00:17:12.15 And one of the things that we've been studying 00:17:14.17 is how new paths form, 00:17:17.03 and we find that Argentine ants 00:17:18.28 actually recruit from the trail and not from the nest. 00:17:21.18 That is, they create a large network like the vacuum cleaner 00:17:23.17 going around. 00:17:25.05 If you're operating a vacuum cleaner, 00:17:27.03 instead of going all the way back 00:17:29.19 to a corner of a room all the time, 00:17:31.13 you keep moving the vacuum cleaner from wherever you are, 00:17:33.13 and that's the way that Argentine ants work also. 00:17:37.26 In the tropics, we see different kinds of interaction networks. 00:17:41.12 One of the species that we're studying 00:17:43.05 is the turtle ant, 00:17:45.10 and here you can see ants marked... 00:17:49.18 those green ants 00:17:51.20 and the pink ants 00:17:53.21 are marked with nail polish by us, 00:17:55.14 they're not really that color, 00:17:57.02 and we did that in order to see how ants 00:17:59.09 are allocated on different trails. 00:18:01.06 And what we find is that ants create circuits in the trees, 00:18:05.24 ongoing circuits, again, like a vacuum cleaner, 00:18:08.10 from a nest to another nest, 00:18:10.13 fort to a food source, 00:18:12.07 so the ants create a circuit in the trees 00:18:14.06 from nest to food sources and back again, 00:18:16.28 around and around, 00:18:18.22 and those circuits are shaped by negative interactions 00:18:21.13 with the ants of another colony. 00:18:24.17 So for example, here you can see 00:18:27.03 another Cephalotes species 00:18:29.12 plugging up its nest entrance with its head 00:18:32.07 in response to negative interactions 00:18:35.11 with a different species, 00:18:37.17 those smaller ones running around with their abdomens in the air, 00:18:44.06 and so this species has a form of security 00:18:47.14 that's based on interactions with ants of other species, 00:18:51.15 and they regulate the flow of ants 00:18:53.15 in and out of the nest based on those interactions. 00:18:58.17 So, ants are using interactions differently 00:19:01.23 in a huge range of environments, 00:19:03.18 and we can study the evolution of collective behavior 00:19:06.19 by understanding, ecologically, 00:19:09.28 how the function of a network in one environment 00:19:13.07 affects the reproductive success of the colonies 00:19:16.10 using that network. 00:19:20.23 Interaction networks 00:19:22.20 evolve in response to environmental challenges, 00:19:24.28 and so the way to study the evolution of collective behavior 00:19:27.27 is to try to understand 00:19:30.16 how interaction networks 00:19:32.17 are operating in particular environments, 00:19:35.02 and what that means for the survival 00:19:37.00 and reproductive success 00:19:38.29 of the system using those networks. 00:19:41.20 Thank you.