Session 1: Theory Behind Evolution I

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.

00:00:00.18 Hi, my name is Hopi Hoekstra and I'm a professor
00:00:03.10 at Harvard University. Today what I'm excited to do is to
00:00:06.23 tell you about the field of evolutionary genetics, and in particular,
00:00:09.26 the genetic basis of evolutionary change. I'm going to tell you two
00:00:13.29 stories, one about morphology and one about behavior.
00:00:16.23 So, here's the outline of the 3 segments of my presentation.
00:00:21.12 So what I'm going to do now is give you an introduction
00:00:24.10 and introduce you to some of the longstanding questions
00:00:27.17 in the genetics of adaptation, and give you a sense of how
00:00:30.14 we're addressing these questions. And then the second segment,
00:00:33.21 in particular, I'll tell you a story about how we're tracking down
00:00:38.09 the genes and developmental mechanisms involved in camouflaging
00:00:42.11 and color differences between two species of wild mice.
00:00:45.08 And then in the third segment, I'll tell you about how we're using
00:00:47.25 very similar approaches to track down genes involved in burrowing behavior
00:00:51.25 differences in these same mice.
00:00:55.26 So I want to start today by talking about Darwin. Because in
00:01:00.06 2009, we had a number of celebrations celebrating everything that
00:01:05.07 Darwin knew on his 200th birthday and on the 150th anniversary
00:01:09.12 of his magnum opus, On The Origin of Species. Now Darwin
00:01:13.19 certainly knew a lot about evolutionary change, but there is one thing
00:01:19.00 that he didn't get quite right. And that is the mechanism
00:01:22.09 of evolutionary change, or the genetic nuts and bolts about how
00:01:26.02 organisms adapt to their environment. Now Darwin knew the traits were
00:01:29.04 inherited, he knew that offspring resembled their parents, but he didn't
00:01:32.14 know how. And this maybe isn't surprising because during this
00:01:36.02 time, of course we didn't know about DNA or genes, much less
00:01:39.08 the whole genome. And that's really what I want to focus on
00:01:43.21 today, is this mechanism of how changes in genes actually produce
00:01:47.26 variation in phenotypes on which natural selection can act.
00:01:52.08 So I'm going to start by telling you a brief anecdote that links
00:01:55.22 Darwin to a second great discovery, and that is the discovery of
00:01:59.06 DNA. So what I'm showing you in this next slide is Darwin's
00:02:04.24 last publication. Now I don't expect you to read it, but i just want
00:02:08.05 you to appreciate the fact that you're looking at his last publication.
00:02:12.08 It was published in 1882, just two weeks before he died
00:02:15.03 in a prestigious journal called Nature. The title of this article
00:02:18.23 is called, "On the Dispersal of Freshwater Bivalves."
00:02:21.28 And what this really is, is a report of the finding of a freshwater
00:02:26.10 beetle clamped to its leg was a freshwater clam, or a cockle.
00:02:30.15 So why you may be wondering was this published in such a
00:02:34.14 prestigious journal even 100 years ago? Well, this actually
00:02:40.06 resolved this great debate about why freshwater cockles were so similar
00:02:44.10 among disjunct lakes in the British midlands. One hypothesis
00:02:49.09 was that these cockles could migrate from lake to lake,
00:02:52.17 thereby homogenizing the populations and thereby, making them
00:02:56.23 very similar in size and shape. But the big question always was,
00:03:00.15 well how do they get from lake to lake if they can't cross
00:03:03.07 terrestrial habitats? Well here was a mechanism. They could
00:03:06.15 hitchhike by attaching to things that could fly or traverse
00:03:11.29 this terrestrial habitat -- in this case by clamping to the leg of a
00:03:15.22 beetle. But that actually isn't the point of telling you this story.
00:03:19.16 The point of telling you this is to mention that Darwin was sent this
00:03:24.08 beetle with a cockle clamped to its leg by a shoemaker who
00:03:28.22 was working in the British midlands, who was an amateur naturalist.
00:03:31.21 And his name was Walter Drawbridge Crick. Now this name should ring a
00:03:36.12 bell, because his grandson was the one with his colleague, Jim Watson,
00:03:41.04 that made the second great discovery. That is the discovery of the 3-dimensional
00:03:45.09 structure of DNA. And it's in this DNA text that we find even more
00:03:52.14 evidence for Darwin's great theory, that is our 3 billion year
00:03:56.20 existence, the shared evolutionary history of all living organisms,
00:04:00.18 and the subject of what I want to talk about today. And that is the
00:04:03.20 mechanistic basis for evolutionary change.
00:04:07.17 So like Darwin, one of the big questions in evolutionary biology
00:04:11.12 today is what gives rise to this amazing diversity? How is variation
00:04:16.02 generated and maintained in natural populations?
00:04:19.10 But thanks to Watson and Crick, we can look for that answer
00:04:22.11 in the genetic code. So the big question that we're focusing
00:04:26.02 on is what is the genetic basis of fitness-related traits?
00:04:30.14 By fitness-related traits, I mean traits that improve the probability
00:04:35.10 of survival or reproduction of organisms in natural populations.
00:04:39.07 So finding the genetic changes or the precise DNA changes
00:04:44.04 that contribute to variations either between populations or between
00:04:47.15 species, is a fun endeavor. And we certainly can learn things about the mechanistic
00:04:53.14 aspects of evolutionary change. Like how do changes in genes
00:04:57.15 actually produce changes in phenotype? But I'd like to argue that
00:05:00.21 we can actually learn even more about the evolutionary process.
00:05:04.11 So what can finding genes tell us about how evolution works?
00:05:09.03 Well there are a number of longstanding questions that I think we're
00:05:12.13 just now starting to be able to answer, because we're armed
00:05:16.06 with molecular biology and the ability to link genotype and phenotype.
00:05:20.17 So I'm just going to list a few of these big questions.
00:05:24.03 So for example, how does evolution proceed? Is it through
00:05:28.27 many small changes? Many small mutations, each that have a small
00:05:33.13 effect on the trait, or can evolution take big leaps?
00:05:37.08 That is, can mutations have large effects that are beneficial?
00:05:40.08 We also want to know about the dominance of these mutations.
00:05:45.07 So, do adaptive alleles or mutations that appear, do they tend to be
00:05:49.21 dominant or recessive? So J.B.S. Haldane, one of the founders
00:05:53.28 of population genetics, argued that adaptive mutations tend to be
00:05:57.19 dominant. Because when they first appear, they're visible to selection
00:06:01.03 and then can quickly spread through the population.
00:06:03.01 Compared to a recessive mutation, which would have to build up enough
00:06:07.14 number in a population to be contained in the same individual,
00:06:12.12 and that recessive trait then expresses. We also want to know, how
00:06:16.23 many -- how do these mutations interact? So if multiple mutations
00:06:19.26 are responsible for changing the phenotype, do they interact
00:06:24.03 in a complex way? Or does each mutation additively affect
00:06:28.07 that trait? We also want to know where these mutations, these beneficial
00:06:35.08 mutations are. Do they occur in the protein itself? For example,
00:06:39.17 amino acid changes that affect that structure and function of that
00:06:42.15 protein. Or do they occur in what we call non-coding DNA,
00:06:46.22 which affects the regulation, let's say the timing or place of expression
00:06:51.21 of that protein. And then we want to know where these mutations come
00:06:55.25 from. So for example, if there's a change in the environment, do we have to
00:07:00.13 wait around for new mutations to appear in that population?
00:07:03.08 Or are there these mutations maybe at a low frequency in the
00:07:07.18 population already that are pre-existing that can be selected
00:07:10.27 on almost immediately? And then finally, if we find mutations in one
00:07:16.03 population that are responsible for an adaptive trait, and we
00:07:19.01 have a similar trait involved in another population, is it the same
00:07:22.08 mutations and same genes that are responsible for those
00:07:25.16 convergent traits? Now importantly, all of these questions that I've listed
00:07:29.17 don't have simple yes or no answers. And in fact, we're more
00:07:33.08 interested in the frequency, whether for example, more often
00:07:37.13 beneficial mutations occur in regulatory regions versus structural
00:07:41.17 regions. But even more importantly than that, we want to know
00:07:44.26 why. Why in some cases do we see protein changes and in other
00:07:49.09 cases we see regulatory changes. Now these I would argue are
00:07:55.01 still largely open questions, but questions we can start to answer
00:07:58.25 by making the connection between genotype and phenotype.
00:08:01.20 So the context in which we're studying the genetic basis of
00:08:05.15 adaptation looks like this. That is, we're trying to make the connection between
00:08:09.16 environment and phenotype. In other words, trying to implicate
00:08:12.21 a role for natural selection in driving that phenotypic variation.
00:08:16.00 That is, suggesting that the phenotypic differences affect fitness.
00:08:20.02 But we also want to understand the genes underlying that phenotypic
00:08:23.29 variation, and not just what those genes are, but how those genes
00:08:27.12 through let's say development, actually produce the differences
00:08:30.28 in variation. And then once we make those links, we'll have a much
00:08:34.18 more complete picture of the adaptive process. I think this is where things
00:08:38.09 can get really fun, because we can go back out in the wild and ask how traits
00:08:41.21 evolved in nature. So, to make these links between environment
00:08:47.07 and phenotype and genotype, my lab group is studying one particular
00:08:51.26 group of wild mice, commonly referred to as deer mice.
00:08:55.05 Or mice in the genus peromyscus. These are the most abundant
00:08:59.10 mammal in North America. And the reason we study them is because
00:09:04.04 first, they're found in almost every habitat type. So from the top of the Rocky
00:09:09.03 Mountains out to the shores of Maine, to the plains of Kansas, to the deserts
00:09:15.21 of Arizona. So they're very widespread in their distribution
00:09:19.12 and because they live in all sorts of different habitat types,
00:09:22.06 there's a lot of opportunity for local adaptation.
00:09:24.23 So in addition to all the variation that we find in the wild,
00:09:28.14 they also can be treated much like laboratory mice. That is
00:09:32.14 we can bring them into a controlled laboratory environments. They
00:09:35.24 breed in the lab just like laboratory mice, and we can do controlled
00:09:39.22 experiments. And finally, while we're still behind traditional
00:09:45.05 model organisms, my group, as well as others, is building a series of
00:09:49.27 genetic and genomic tools that are going to be useful in trying to
00:09:54.17 make these connections between genotype and phenotype. But I would argue
00:09:58.00 one of the main reasons for studying these mice is because
00:10:01.11 we have this amazing literature of natural history studies on their
00:10:08.02 ecology. That is, these mice have been studied for nearly a century
00:10:11.13 by natural historians who have described morphological, physiological,
00:10:15.05 and reproductive behavioral variation in natural populations.
00:10:19.21 Just to give you a sense of how these mice vary, here are just
00:10:25.01 a number of traits that I picked out of the literature that describe
00:10:29.01 traits that have been studied and traits vary either between
00:10:32.14 populations or between species of peromyscus species.
00:10:36.12 So they vary in body size, tail length, foot size, color patterning,
00:10:39.27 testis size, sperm morphology, et cetera. They vary in morphological
00:10:43.05 traits, physiological traits, and behavioral traits. So, using these
00:10:48.20 mice, we're trying to make those connections between genotype and
00:10:52.01 phenotype. And the next two segments of my presentation, I'm
00:10:54.23 going to focus on two of these traits. One morphological trait,
00:10:58.04 color patterning, and a second trait, burrowing behavior.
00:11:01.18 So, the second part of my presentation, what I'd like to do
00:11:07.05 is focus on the morphological trait. And in particular, camouflaging
00:11:10.28 and color differences between subspecies of peromyscus polionotus.
00:11:16.02 Both to understand the ultimate reasons why these color differences
00:11:20.10 evolved, as well as the mechanisms or the underlying genetics
00:11:24.21 contributing to these differences in camouflage and color.
00:11:28.08 And for the third part, we'll switch gears and focus now using
00:11:32.24 very similar approaches. But instead of studying a morphological
00:11:34.26 trait, we've substituted in a behavior where we're taking advantage of these dramatic
00:11:39.18 differences in burrowing behavior; there are species that build these
00:11:42.24 large burrows versus those that build small burrows. To try and
00:11:47.11 understand how genes can affect behavioral variations in natural
00:11:50.21 populations. So I've hoped to have gotten you excited about
00:11:54.28 biology, in the sense that we're at this amazing time where
00:11:59.11 we can use approaches like Darwin first did, that is studies of
00:12:03.29 natural history, observation and experiment in the wild,
00:12:07.14 but combine that with studies of modern day molecular
00:12:10.15 genetics. To try to understand the genetic basis of what Darwin
00:12:14.26 referred to as that perfection of structure and coadaptation which
00:12:19.01 most justly excites our admiration. So thank you very much
00:12:23.00 for your attention, and I hope you'll join me for the next two
00:12:26.24 segments, where I'll tell you about more detailed studies from
00:12:29.19 laboratory group, trying to connect genes and phenotypes for both
00:12:33.11 morphological and behavioral traits. Thank you.

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.