Session 2: Theory Behind Evolution II
Transcript of Part 1: Biogeography: Studying the Distribution of Species Across Space
00:00:07.06 Hi. 00:00:08.06 My name is Uma Ramakrishnan and I'm an associate professor at the National Centre for Biological 00:00:12.22 Sciences in Bangalore, India. 00:00:15.18 This is a research institute where we study biology, and I've come all the way from there 00:00:20.17 to tell you a little bit about biogeography and the work that we do on understanding biodiversity 00:00:27.12 in the Indian subcontinent. 00:00:29.13 So, anywhere you live, if you look outside your window, you'll see many different species. 00:00:35.06 This is biodiversity. 00:00:36.06 And as humans we're fascinated by biodiversity. 00:00:40.16 And biogeography actually is a field which studies the distribution of species and biodiversity 00:00:47.03 across space. 00:00:48.10 And, you know... you know immediately, even as a child, that certain places have more 00:00:56.10 species than others, the species in a particular place seem to be similar, and so on. 00:01:01.05 You observe these things almost naturally. 00:01:04.05 And this is basically how we measure biodiversity and how we study it. 00:01:08.09 We try and quantify biodiversity and look at why places have more biodiversity than 00:01:14.08 do others. 00:01:15.15 So, how do we quantify biodiversity? 00:01:18.11 How do we actually use mathematical ways to study patterns of biodiversity, statistical 00:01:24.19 ways, across the world? 00:01:27.09 So, the first thing we do is we actually count the number of species, right? 00:01:33.09 This is something which has often been controversial in biology. 00:01:36.11 How do we know what is a species? 00:01:38.05 A taxonomist... as humans, we have this desire to put things in boxes and say, this is one 00:01:44.19 species, this is another... and in some cases this might be relatively easy. 00:01:49.03 For example, with apes, you know, the gibbon... gibbons look very different from gorillas 00:01:55.20 and chimpanzees and modern humans. 00:01:57.15 And when we look at their DNA there are several of these differences which are reflected, 00:02:03.01 allowing us to build a phylogenetic tree. 00:02:05.08 We heard about this also, for example, in Scott Edwards' talk. 00:02:09.21 So, this allows us to understand that there are multiple species of apes which look different 00:02:16.24 and are genetically different, distinct, as well. 00:02:20.10 This may not always be the case. 00:02:22.14 For example, this really beautiful examples of a ring species, salamanders in California. 00:02:30.03 It shows you, here, that across their distribution these different salamanders look slightly 00:02:36.08 different and yet similar, right? 00:02:38.04 So, here, speciation or these different species are along a continuum, right? 00:02:44.05 They look slightly different... one looks slightly different from the other and so on. 00:02:48.19 So, it's not always easy to classify species, but, overall, we use various tools: morphology, 00:02:55.19 how species and individuals... how individuals in a population look; genetics, how different 00:03:00.20 or similar their DNA is; and it could be several other forms of behavior -- do they have similar 00:03:06.12 songs, if they're birds, and so on -- and we use all of these bits of information to 00:03:11.19 make an informed guess about whether we think sets of individuals are distinct species or 00:03:17.10 not. 00:03:18.10 For the sake of correctness, I should tell you that the biological species concept proposed 00:03:23.06 by Ernst... 00:03:24.06 Ernst Mayr is what we accept, and this suggests that a species is a group of individuals that 00:03:29.14 can and do interbreed with each other in nature. 00:03:32.11 So, not if you put them together in a zoo, but in nature you do find them interbreeding. 00:03:38.10 And, as we said, species could look similar or different, but at some point we take a 00:03:44.14 call and we say, okay, this is a species and these two populations are not distinct enough 00:03:49.08 to be a species. 00:03:50.17 So, once we have catalogued species -- that's the first thing we have to do, these are our 00:03:55.07 primary data -- we measure biodiversity in units of species. 00:04:00.23 So, how do species actually evolve? 00:04:04.03 I mean, we said, okay, they look different or, you know, they're genetically different. 00:04:09.06 Well, this has also been a field of extensive study in evolutionary biology and you made 00:04:13.16 have seen a talk as part of this series by Hopi Hoekstra, who's actually trying to understand 00:04:18.01 how these differences between how species look/evolve from a genetic perspective, right? 00:04:24.03 Adaptation from a DNA level. 00:04:26.19 But this picture, here, shows you a really simple kind of theoretical idea of speciation. 00:04:33.09 So, the most basic is allopatry, over here, where you can see that there's two pop... 00:04:39.03 there's a population and suddenly, potentially, there's a barrier between two parts of this 00:04:45.03 population, maybe a road came up or there was a... a... a, you know, a rift or something 00:04:50.01 like that, a river. 00:04:52.00 And now, individuals on either side of this barrier develop different adaptations or they 00:04:59.16 become different. 00:05:01.13 And so, when they meet again, say, when this barrier disappears, they don't interbreed 00:05:05.23 with each other. 00:05:06.23 So, this has now resulted in the creation of two species from what was a single population, 00:05:12.24 right? 00:05:13.24 So, this is a case of allopatric speciation, and often what we see most commonly in nature. 00:05:21.13 On the other hand, speciation could theoretically also be sympatric, where you have a new, for 00:05:27.14 some reason, change... 00:05:30.12 ecological change, maybe, or adaptation... of some individuals in one of this... in this 00:05:35.16 population, and this results in differentiation and, over time, these differences became... 00:05:42.05 become so large that these two populations do not interbreed. 00:05:47.06 An example for this is the [unknown] flies, or the apple flies, and the Hawthorn flies, 00:05:53.00 which lay their eggs on different fruits. 00:05:55.23 Apple and Hawthorn [flies] are just coexisting, so they're sympatric, these flies, and yet, 00:06:02.11 you know, they do become... could become different species. 00:06:05.01 There are kinds of shades and grades of these two examples, where you could have speciation 00:06:10.13 in peripatry or parapatry, a creation of a new niche or a certain kind of budding off 00:06:16.24 of a set of individuals and so on, a range expansion. 00:06:21.06 But, basically, the mechanisms are somewhere between sympatry and allopatry. 00:06:26.21 While these are nice examples from a theoretical, hot perspective, what do we know about speciation? 00:06:34.06 So, early on, Dodd did experiments with Drosophila where, you know, different populations of 00:06:41.00 flies were given two different kinds of food and, over time, these two flies... fly populations 00:06:49.23 became differentiated and then they developed mating preferences, so flies from population 00:06:56.16 1 didn't want to mate with flies from population 2 and vice versa. 00:07:00.23 So, this is very important because it shows that not only did they become different, they 00:07:06.17 developed reproductive preferences and, potentially, reproductive isolation -- what's most important 00:07:13.04 for the lack of gene flow of mating, in the end, between two species. 00:07:18.00 So, now I'll go... we go back to biogeography, which is basically the distribution of species 00:07:26.14 on our planet, right? 00:07:28.08 So, how do we actually study this? 00:07:30.17 In a sense, it's one of those... the most basic things. 00:07:34.04 Even before people thought about DNA or evolution, a lot of naturalists walked all over the world 00:07:41.19 and did surveys and catalogued interesting things about biodiversity. 00:07:47.17 Alexander von Humboldt was one of these people and he basically did a lot of explorations 00:07:54.10 and studies in South America, and this beautiful illustration he has of this mountain, Chimborazo, 00:08:02.13 in South America, which is a volcanic mountain, which goes really high -- 6200 or so meters 00:08:09.03 -- he shows that, actually, as you go up the mountain, the ecological environment changes 00:08:14.15 dramatically. 00:08:15.15 So, you can see, for example, at the top of this mountain, you have snow, right? 00:08:19.20 And that's not there at the bottom of the mountain; it doesn't seem to be covered with 00:08:24.06 snow at all. 00:08:25.11 So, what Humboldt did was he also catalogued the plants at the... along this mountain. 00:08:32.13 He was a botanist and he said, well, it seems to be that there are very different species, 00:08:37.09 very different sets of species, which occur across this mountain, and this famous and 00:08:41.20 really beautiful illustration he made kind of shows this, shows the community of plants 00:08:46.24 associated with these environmental gradients. 00:08:50.17 And so, Humboldt thought early on that ecology, or differences in habitat, might be important 00:08:57.06 to determine where species are distributed. 00:09:02.03 The father of evolutionary biology, Darwin, also had contributions to thinking about biogeography. 00:09:08.01 So, Darwin, as you know, went on this really long voyage across the world on the Beagle. 00:09:15.21 And he... while he was going on this voyage, he thought about all the species present along 00:09:21.11 this route. 00:09:22.11 And he actually happened to go to South America and then later to the Galapagos Islands, and 00:09:28.21 when he did that he noticed something really interesting. 00:09:31.17 He saw many birds on the Galapagos Islands. 00:09:34.08 They look similar to the ones he'd seen on the mainland and yet different, right? 00:09:39.06 So, Darwin realized that, potentially, the species that occur in a location may be there 00:09:45.16 because of history, because they're colonizing from somewhere close by, and so the finches 00:09:51.19 are similar to those on the mainland and yet they're different. 00:09:59.04 Most interestingly, also, Alfred Russel Wallace, around the same time as Darwin... he was a 00:10:05.03 codiscoverer of the theory of natural selection with Darwin, they chanced upon the idea together, 00:10:10.01 in the same ti... not together, but the same... at similar times... was also a very avid explorer 00:10:16.13 and he also studied distributions of species across the world. 00:10:22.01 And what he did was he actually realized that certain sets of species seemed to be most 00:10:28.20 similar to each other and these sets of species seemed to occur in similar locations, okay? 00:10:34.24 So, he tried to group the species in the world into bins, in a sense, not just one species 00:10:41.21 but sets of species. 00:10:43.06 And he, for example, suggested that all of the Orient, which is Southeast Asia and India, 00:10:50.11 was one biogeographic zone or grouping. 00:10:55.06 And it's interesting to note that, very recently, in 2013, Holt and other authors actually kind 00:11:01.15 of reevaluated Wallace's ideas of these biogeographic zones based on modern DNA phylogeny. 00:11:09.10 So, Alfred Russel Wallace and Darwin are pre-DNA -- they didn't know anything about, you know, 00:11:15.10 the kind of hereditary material that we know so much about today, which we use to study 00:11:19.20 evolution. 00:11:21.03 And they also found very similar results to what Wallace had found so many years ago. 00:11:28.24 So, now we can look, then... we are looking with modern tools of GIS and DNA and so on, 00:11:37.12 and we can now look at biodiversity and ask questions in biogeography. 00:11:42.12 And this actually shows you a distribution of vertebrate species across the world, and 00:11:48.19 you can see immediately, anyone can see, that some areas have more diversity than others, 00:11:55.12 and this tends to be in the tropics. 00:11:57.12 So, of course, here, the colder colors or blues are areas with lower numbers of species 00:12:03.02 and the warmer reds are regions with higher numbers of species. 00:12:08.06 What I'm also going to try and convince you is that if you actually were to plot onto 00:12:13.12 this map mountain rangers, you would see that mountain ranges tend to correlate with areas 00:12:20.04 where there are high species, and we'll come back to this in a little bit. 00:12:25.01 There's other ways to look at species, right? 00:12:28.00 You can just count all of them, but you can also ask whether some places have more restricted-range 00:12:34.22 species, or higher endemism. 00:12:36.24 Do some places have more special species, which are not found anywhere else? 00:12:41.09 This is also an important thing for us to understand when thinking about biog... biogeography 00:12:46.12 and biodiversity. 00:12:47.12 So, we can quantify this by quantifying endemism, or how restricted a species' range is to a 00:12:54.12 geographic location. 00:12:57.05 And when we do this, again, we find, you know, really interesting patterns. 00:13:01.00 We find, for example, superimposing our mountains, again, that endemism, or special species, 00:13:07.23 tend to be found also in areas where there are mountains and, very interestingly, also 00:13:14.18 on islands, okay? 00:13:16.14 So, islands, mountains, and the tropics seem to be important for presence of species. 00:13:25.07 So, how do we actually study why there's more biodiversity in a particular location? 00:13:30.14 So, okay... so, why may there be more species in a particular place? 00:13:34.12 Well, clearly there's more speciation there. 00:13:38.03 Maybe there's less extinction. 00:13:39.20 Speciation increases the number of species while extinction might decrease the number 00:13:44.00 of species. 00:13:45.00 So, clearly, the balance or diversification... the balance between speciation and extinction 00:13:51.12 must be high. 00:13:52.12 There must be a net positive rate or diversification. 00:13:56.08 And we can actually quantify speciation rate and extinction by looking at phylogenies. 00:14:02.09 And so, in this paper, Rolland et al tried to do this, and what we did was they contrasted 00:14:08.16 the net accumulation of biodiversity, or diversification, amongst the tropics and amongst the temperate 00:14:16.01 regions. 00:14:17.10 And what you can see here, in this really beautiful graph is, if you look at speciation, 00:14:23.04 it's higher in the tropics, extinction seems to be much higher in the temperate regions, 00:14:30.06 and so, overall, on an average, it appears like diversification -- that's net diversification 00:14:36.07 over there -- is much higher in the tropics. 00:14:39.05 So, this suggests that the tropics are cradles of diversity, which means that new species 00:14:48.12 are being created here, right? 00:14:50.12 They're also museums -- there's less extinction, so they retain these species over longer periods 00:14:56.18 of time. 00:14:57.18 So, because they're both cradles and museums, tropic regions, overall, tend to have higher 00:15:03.05 biodiversity. 00:15:04.05 Islands, as we pointed out earlier, are also a really interesting case. 00:15:10.00 This is a very interesting figure from a review, a recent review, by Losos and Ricklefs, where 00:15:15.24 they actually suggest a model for how something like this may happen. 00:15:20.07 They basically verbalized what Darwin thought those many years ago, that you may have a 00:15:25.10 finch which moves from the mainland onto one island. 00:15:29.23 These islands are different and so it manages to disperse to all these islands over time. 00:15:35.10 And then, slowly, these finches or birds maybe differentiate over time, they become adapted 00:15:42.23 to the conditions on that particular island. 00:15:46.04 And then, maybe, if there's further speciation in allopatry, you have recolonization of these 00:15:52.23 different birds across these different islands, now, and further speciation. 00:15:56.23 So, islands are really nice because they're actually models to study speciation. 00:16:02.12 So, because they're small and they're in the ocean, environmental conditions can vary very 00:16:08.01 dramatically on islands, and any of you who've been to an island know it's suddenly raining 00:16:12.15 or suddenly there's a storm, so it's... they're very easy to see environmental changes on. 00:16:18.17 At the same time, because there are many islands, often, in a sea, there are natural barriers 00:16:24.15 which exist between islands, so we can clearly see a role for history -- there's a mainland 00:16:30.13 close by -- ecology -- differences between islands or within an island -- and things 00:16:36.22 like dispersal, right, where differences can actually come based on islands across space. 00:16:42.15 So, let's look at a couple of examples of speciation and its study on islands. 00:16:48.23 So, you know, one of the most biodiverse islands we have in the world is Madagascar. 00:16:56.15 It's a really interesting place because it has a very unique set of species found nowhere 00:17:01.21 else. 00:17:02.21 And I show you, here, a map of... of... a picture of birds -- these are called vangas 00:17:11.03 -- and these have diversified on Madagascar. 00:17:12.23 So, you can see, below, there, there's a map which shows the different kind of ecological 00:17:18.22 conditions or habitats within Madagascar. 00:17:21.12 And, here, you can see a phylogeny, a DNA-based tree, of all these birds, which allows us 00:17:28.02 to infer who evolved from whom, and who colonized which environment first, and so on. 00:17:34.12 And so here we can see an example of ecological speciation, right? 00:17:39.21 Diversification of these birds into different niches, different habitats on this island 00:17:45.23 of Madagascar. 00:17:46.23 And, actually, if you look at Madagascar, there's also incredibly rich vertebrate diversity, 00:17:55.01 in terms of reptiles and amphibians, and these maps on this side actually show you those 00:18:01.06 types of diversity. 00:18:02.09 They show you where there are more or less species, and you can see, for example, that 00:18:07.09 diversity tends to be high in some environments -- higher that, say, in other. 00:18:11.23 So, wetter environments, which you can see below, actually tend to have higher diversity 00:18:16.13 than drier environments. 00:18:17.17 So, again, examples of ecologically driven speciation within an island, and a place with 00:18:24.13 high biodiversity. 00:18:27.11 On the other hand, you can also have geographic examples of speciation. 00:18:31.05 So, here we see these insects on the Hawaiian island chain. 00:18:37.07 So, the nice thing about the Hawaiian island chain is the different islands have different 00:18:42.12 ages, right? 00:18:43.21 So, the oldest island is Kauai and the youngest island is Hawaii, and you can look and see 00:18:52.07 this, also, in the phylogeny. 00:18:54.00 So, for example, the authors have very nicely colored these islands in different colors 00:19:01.04 and the insects found on those islands are in the phylogeny in the same color. 00:19:05.16 So, if you look down, for example, you can see... you can this order, you can see green, 00:19:11.03 orange, purple, blue, and red, right? 00:19:15.17 So, in terms of time, the oldest island is green and then there's orange and then there's 00:19:21.22 purple and then there's blue and then there's red. 00:19:24.09 And you can see that reflected in the phylogenetic tree as well. 00:19:28.11 So, speciation has progressed with time and you have these different islands being colonized 00:19:34.17 and these different species diverging across the Hawaiian island chain. 00:19:39.12 So, lineages, or sets of individuals which are closer together from a DNA perspective, 00:19:45.22 are related on an island, younger lineages are on younger islands, and genetic data kind 00:19:51.16 of helps us explore the history of speciation on these islands. 00:19:56.24 So, now... 00:19:59.08 I've been kind of alternating between mountains and islands, slipping in the fact that mountain 00:20:04.21 ranges have high biodiversity. 00:20:06.21 Why was I doing this? 00:20:08.05 Well, it turns out as Humboldt showed us, and this is an example from the U.S., the 00:20:15.22 Southwest, mountains also have "islands", they have habitat islands. 00:20:21.06 As you go up a mountain, you can see that the habitat or the ecology actually changes 00:20:27.00 -- higher parts of the mountain are different than the lower parts of the mountain. 00:20:32.09 And so you might imagine that, ecologically, species would become adapted to live in these 00:20:38.10 specialized environments. 00:20:40.13 And then, if you looked at the mountain range, that's like a set of islands, kind of like 00:20:45.17 Hawaii in a sense, right? 00:20:48.00 So, it might be interesting to try and explore patterns of speciation, or the accumulation 00:20:53.15 of biodiversity, in mountain chains. 00:20:57.07 And this is what I'm going to talk to you about in terms of our research, but the best 00:21:00.20 part of it all is the history of this speciation is actually written in the DNA of these species, 00:21:07.19 of the populations which actually live on these islands. 00:21:11.11 A way to look at their past is through sequencing their DNA. 00:21:15.23 Umm... this is also important; it's not just fun. 00:21:19.12 But, uhh... you know, our existence as humans is critically dependent on biodiversity. 00:21:25.09 There are many studies which show us, now, that human well-being, not just from an aesthetic 00:21:31.03 perspective, but in terms of our ecosystem services and many, many things, is critically 00:21:35.21 dependent on biodiversity, and this allows us to really prioritize studying areas of 00:21:42.18 high biodiversity. 00:21:44.05 It's important to us; it's not just interesting. 00:21:47.16 And these can be classified globally as biodiversity hotspots, not just regions where there is 00:21:53.15 higher... high biodiversity, but also regions where there are threats to this biodiversity. 00:21:59.10 And two of the regions I'm going to talk to you about today happen to be biodiversity 00:22:03.07 hotspots -- the Western Ghats, which is in India and Southern India, and the Himalayas, 00:22:09.03 which is the in the northern part of Asia. 00:22:10.23 So, uhh... 00:22:12.00 I guess what I'm trying to say is that, you know, we're really interested in understanding 00:22:16.06 biodiversity, but it's also critically important from a conservation perspective. 00:22:20.11 This understanding, we hope, will help us to think about how we can save these species 00:22:26.02 in the future. 00:22:27.02 So, I'll just summarize and uhh... kind of give a leap into our research questions, which 00:22:32.17 I'm going to talk about in the next part of this talk. 00:22:36.09 Biodiversity is higher in the tropics, in mountain ranges, and in islands, and the barriers 00:22:42.12 which cause allopatric speciation, or the creation of biodiversity, could be physical 00:22:47.07 or ecological. 00:22:49.19 But what still remains for us to explore is whether what's driving speciation is really 00:22:56.02 different or the same in various locations across the world. 00:23:00.01 And you saw, for example, that study... you know... from Madagascar on vangas, right? 00:23:07.19 It's a particular type of bird. 00:23:10.02 But you could also think about this from the perspective of all the species which live 00:23:14.06 in a location. 00:23:15.15 Do they all respond similarly to these different barriers or changes in ecology? 00:23:22.07 How generalizable are these patterns? 00:23:24.22 How important are physical differences? 00:23:26.07 Are they more important to some species versus others, or not? 00:23:30.21 And this is what I'm going to talk about in the next talk. 00:23:34.02 Thank you.