In this first session, we introduce the topic of evolution with three videos. Dr. Hale introduces Darwinian evolution and the concepts of heritable traits, genetic variation, and natural selection. She explains speciation and describes how phylogenetic trees can be constructed from fossil and DNA evidence and used to compare the relatedness of species. Dr. Hoekstra elaborates on how changes in genes can produce phenotypic variation which can be acted upon by natural selection. And finally, Dr. Gordon discusses how interaction networks evolve in different ecological settings.
All Course Materials for this Session (Educators only)
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,
00:00:14.22 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,
00:13:09.01 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,
00:15:18.20 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
00:18:33.09 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.
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.
- Melina Hale iBioSeminar: The Evolution of Neural Circuits and Behaviors
- Hopi Hoekstra iBioSeminar: The Genetic Basis of Evolutionary Change in Morphology and Behavior
- Deborah M. Gordon iBioSeminar: Local Interactions Determine Collective Behavior
After a short stint studying political science in college, Hopi Hoekstra switched her focus to biology. She received her B.A. in Integrative Biology from UC Berkeley, and her Ph.D. in Zoology from the University of Washington. She completed postdoctoral research at the University of Arizona, and, in 2003, she joined the faculty at UC San… Continue Reading
Melina Hale is a professor of Organismal Biology and Anatomy and Neurobiology and Computational Neuroscience at the University of Chicago. Using predominantly zebra fish, Hale’s lab studies neural circuits that control limb and axis movement and how that movement changes over time. Movement changes can be seen both in the short time frame of development… Continue Reading
Deborah M. Gordon received a B.A. in French from Oberlin College, a M.S. in Biology from Stanford University and a Ph.D. in Zoology from Duke University. Following receipt of her Ph.D., Gordon was a Junior Fellow of the Harvard Society of Fellows and a Research Fellow at the University of Oxford and Imperial College before… Continue Reading