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Introduction to Biogeography
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0:07
Hi. My name is Uma Ramakrishnan and I'm an associate professor at the National Centre for Biological Sciences in Bangalore, India. This is a research institute where we study biology, and I've come all the way from there to tell you a little bit about biogeography and the work that we do on understanding biodiversity in the Indian subcontinent. So, anywhere you live, if you look outside your window, you'll see many different species. This is biodiversity.
0:36
And as humans we're fascinated by biodiversity. And biogeography actually is a field which studies the distribution of species and biodiversity across space. And, you know... you know immediately, even as a child, that certain places have more species than others, the species in a particular place seem to be similar, and so on. You observe these things almost naturally. And this is basically how we measure biodiversity and how we study it.
1:08
We try and quantify biodiversity and look at why places have more biodiversity than do others. So, how do we quantify biodiversity? How do we actually use mathematical ways to study patterns of biodiversity, statistical ways, across the world? So, the first thing we do is we actually count the number of species, right? This is something which has often been controversial in biology. How do we know what is a species?
1:38
A taxonomist... as humans, we have this desire to put things in boxes and say, this is one species, this is another... and in some cases this might be relatively easy. For example, with apes, you know, the gibbon... gibbons look very different from gorillas and chimpanzees and modern humans. And when we look at their DNA there are several of these differences which are reflected, allowing us to build a phylogenetic tree. We heard about this also, for example, in Scott Edwards' talk.
2:09
So, this allows us to understand that there are multiple species of apes which look different and are genetically different, distinct, as well. This may not always be the case. For example, this really beautiful examples of a ring species, salamanders in California. It shows you, here, that across their distribution these different salamanders look slightly different and yet similar, right? So, here, speciation or these different species are along a continuum, right?
2:44
They look slightly different... one looks slightly different from the other and so on. So, it's not always easy to classify species, but, overall, we use various tools: morphology, how species and individuals... how individuals in a population look; genetics, how different or similar their DNA is; and it could be several other forms of behavior -- do they have similar songs, if they're birds, and so on -- and we use all of these bits of information to
3:11
make an informed guess about whether we think sets of individuals are distinct species or not. For the sake of correctness, I should tell you that the biological species concept proposed by Ernst... Ernst Mayr is what we accept, and this suggests that a species is a group of individuals that can and do interbreed with each other in nature. So, not if you put them together in a zoo, but in nature you do find them interbreeding.
3:38
And, as we said, species could look similar or different, but at some point we take a call and we say, okay, this is a species and these two populations are not distinct enough to be a species. So, once we have catalogued species -- that's the first thing we have to do, these are our primary data -- we measure biodiversity in units of species. So, how do species actually evolve? I mean, we said, okay, they look different or, you know, they're genetically different.
4:09
Well, this has also been a field of extensive study in evolutionary biology and you made have seen a talk as part of this series by Hopi Hoekstra, who's actually trying to understand how these differences between how species look/evolve from a genetic perspective, right? Adaptation from a DNA level. But this picture, here, shows you a really simple kind of theoretical idea of speciation. So, the most basic is allopatry, over here, where you can see that there's two pop...
4:39
there's a population and suddenly, potentially, there's a barrier between two parts of this population, maybe a road came up or there was a... a... a, you know, a rift or something like that, a river. And now, individuals on either side of this barrier develop different adaptations or they become different. And so, when they meet again, say, when this barrier disappears, they don't interbreed with each other.
5:07
So, this has now resulted in the creation of two species from what was a single population, right? So, this is a case of allopatric speciation, and often what we see most commonly in nature. On the other hand, speciation could theoretically also be sympatric, where you have a new, for some reason, change... ecological change, maybe, or adaptation... of some individuals in one of this... in this population, and this results in differentiation and, over time, these differences became...
5:42
become so large that these two populations do not interbreed. An example for this is the [unknown] flies, or the apple flies, and the Hawthorn flies, which lay their eggs on different fruits. Apple and Hawthorn [flies] are just coexisting, so they're sympatric, these flies, and yet, you know, they do become... could become different species. There are kinds of shades and grades of these two examples, where you could have speciation
6:10
in peripatry or parapatry, a creation of a new niche or a certain kind of budding off of a set of individuals and so on, a range expansion. But, basically, the mechanisms are somewhere between sympatry and allopatry. While these are nice examples from a theoretical, hot perspective, what do we know about speciation? So, early on, Dodd did experiments with Drosophila where, you know, different populations of
6:41
flies were given two different kinds of food and, over time, these two flies... fly populations became differentiated and then they developed mating preferences, so flies from population 1 didn't want to mate with flies from population 2 and vice versa. So, this is very important because it shows that not only did they become different, they developed reproductive preferences and, potentially, reproductive isolation -- what's most important
7:13
for the lack of gene flow of mating, in the end, between two species. So, now I'll go... we go back to biogeography, which is basically the distribution of species on our planet, right? So, how do we actually study this? In a sense, it's one of those... the most basic things. Even before people thought about DNA or evolution, a lot of naturalists walked all over the world and did surveys and catalogued interesting things about biodiversity.
7:48
Alexander von Humboldt was one of these people and he basically did a lot of explorations and studies in South America, and this beautiful illustration he has of this mountain, Chimborazo, in South America, which is a volcanic mountain, which goes really high -- 6200 or so meters -- he shows that, actually, as you go up the mountain, the ecological environment changes dramatically. So, you can see, for example, at the top of this mountain, you have snow, right?
8:20
And that's not there at the bottom of the mountain; it doesn't seem to be covered with snow at all. So, what Humboldt did was he also catalogued the plants at the... along this mountain. He was a botanist and he said, well, it seems to be that there are very different species, very different sets of species, which occur across this mountain, and this famous and really beautiful illustration he made kind of shows this, shows the community of plants associated with these environmental gradients.
8:51
And so, Humboldt thought early on that ecology, or differences in habitat, might be important to determine where species are distributed. The father of evolutionary biology, Darwin, also had contributions to thinking about biogeography. So, Darwin, as you know, went on this really long voyage across the world on the Beagle. And he... while he was going on this voyage, he thought about all the species present along this route.
9:22
And he actually happened to go to South America and then later to the Galapagos Islands, and when he did that he noticed something really interesting. He saw many birds on the Galapagos Islands. They look similar to the ones he'd seen on the mainland and yet different, right? So, Darwin realized that, potentially, the species that occur in a location may be there because of history, because they're colonizing from somewhere close by, and so the finches
9:52
are similar to those on the mainland and yet they're different. Most interestingly, also, Alfred Russel Wallace, around the same time as Darwin... he was a codiscoverer of the theory of natural selection with Darwin, they chanced upon the idea together, in the same ti... not together, but the same... at similar times... was also a very avid explorer and he also studied distributions of species across the world.
10:22
And what he did was he actually realized that certain sets of species seemed to be most similar to each other and these sets of species seemed to occur in similar locations, okay? So, he tried to group the species in the world into bins, in a sense, not just one species but sets of species. And he, for example, suggested that all of the Orient, which is Southeast Asia and India, was one biogeographic zone or grouping.
10:55
And it's interesting to note that, very recently, in 2013, Holt and other authors actually kind of reevaluated Wallace's ideas of these biogeographic zones based on modern DNA phylogeny. So, Alfred Russel Wallace and Darwin are pre-DNA -- they didn't know anything about, you know, the kind of hereditary material that we know so much about today, which we use to study evolution. And they also found very similar results to what Wallace had found so many years ago.
11:29
So, now we can look, then... we are looking with modern tools of GIS and DNA and so on, and we can now look at biodiversity and ask questions in biogeography. And this actually shows you a distribution of vertebrate species across the world, and you can see immediately, anyone can see, that some areas have more diversity than others, and this tends to be in the tropics. So, of course, here, the colder colors or blues are areas with lower numbers of species
12:03
and the warmer reds are regions with higher numbers of species. What I'm also going to try and convince you is that if you actually were to plot onto this map mountain rangers, you would see that mountain ranges tend to correlate with areas where there are high species, and we'll come back to this in a little bit. There's other ways to look at species, right? You can just count all of them, but you can also ask whether some places have more restricted-range species, or higher endemism.
12:37
Do some places have more special species, which are not found anywhere else? This is also an important thing for us to understand when thinking about biog... biogeography and biodiversity. So, we can quantify this by quantifying endemism, or how restricted a species' range is to a geographic location. And when we do this, again, we find, you know, really interesting patterns. We find, for example, superimposing our mountains, again, that endemism, or special species,
13:08
tend to be found also in areas where there are mountains and, very interestingly, also on islands, okay? So, islands, mountains, and the tropics seem to be important for presence of species. So, how do we actually study why there's more biodiversity in a particular location? So, okay... so, why may there be more species in a particular place? Well, clearly there's more speciation there. Maybe there's less extinction.
13:40
Speciation increases the number of species while extinction might decrease the number of species. So, clearly, the balance or diversification... the balance between speciation and extinction must be high. There must be a net positive rate or diversification. And we can actually quantify speciation rate and extinction by looking at phylogenies. And so, in this paper, Rolland et al tried to do this, and what we did was they contrasted
14:09
the net accumulation of biodiversity, or diversification, amongst the tropics and amongst the temperate regions. And what you can see here, in this really beautiful graph is, if you look at speciation, it's higher in the tropics, extinction seems to be much higher in the temperate regions, and so, overall, on an average, it appears like diversification -- that's net diversification over there -- is much higher in the tropics.
14:40
So, this suggests that the tropics are cradles of diversity, which means that new species are being created here, right? They're also museums -- there's less extinction, so they retain these species over longer periods of time. So, because they're both cradles and museums, tropic regions, overall, tend to have higher biodiversity. Islands, as we pointed out earlier, are also a really interesting case. This is a very interesting figure from a review, a recent review, by Losos and Ricklefs, where
15:16
they actually suggest a model for how something like this may happen. They basically verbalized what Darwin thought those many years ago, that you may have a finch which moves from the mainland onto one island. These islands are different and so it manages to disperse to all these islands over time. And then, slowly, these finches or birds maybe differentiate over time, they become adapted to the conditions on that particular island.
15:47
And then, maybe, if there's further speciation in allopatry, you have recolonization of these different birds across these different islands, now, and further speciation. So, islands are really nice because they're actually models to study speciation. So, because they're small and they're in the ocean, environmental conditions can vary very dramatically on islands, and any of you who've been to an island know it's suddenly raining
16:13
or suddenly there's a storm, so it's... they're very easy to see environmental changes on. At the same time, because there are many islands, often, in a sea, there are natural barriers which exist between islands, so we can clearly see a role for history -- there's a mainland close by -- ecology -- differences between islands or within an island -- and things like dispersal, right, where differences can actually come based on islands across space.
16:43
So, let's look at a couple of examples of speciation and its study on islands. So, you know, one of the most biodiverse islands we have in the world is Madagascar. It's a really interesting place because it has a very unique set of species found nowhere else. And I show you, here, a map of... of... a picture of birds -- these are called vangas -- and these have diversified on Madagascar. So, you can see, below, there, there's a map which shows the different kind of ecological
17:19
conditions or habitats within Madagascar. And, here, you can see a phylogeny, a DNA-based tree, of all these birds, which allows us to infer who evolved from whom, and who colonized which environment first, and so on. And so here we can see an example of ecological speciation, right? Diversification of these birds into different niches, different habitats on this island of Madagascar. And, actually, if you look at Madagascar, there's also incredibly rich vertebrate diversity,
17:56
in terms of reptiles and amphibians, and these maps on this side actually show you those types of diversity. They show you where there are more or less species, and you can see, for example, that diversity tends to be high in some environments -- higher that, say, in other. So, wetter environments, which you can see below, actually tend to have higher diversity than drier environments. So, again, examples of ecologically driven speciation within an island, and a place with high biodiversity.
18:28
On the other hand, you can also have geographic examples of speciation. So, here we see these insects on the Hawaiian island chain. So, the nice thing about the Hawaiian island chain is the different islands have different ages right? So, the oldest island is Kauai and the youngest island is Hawaii, and you can look and see this, also, in the phylogeny. So, for example, the authors have very nicely colored these islands in different colors
19:02
and the insects found on those islands are in the phylogeny in the same color. So, if you look down, for example, you can see... you can this order, you can see green, orange, purple, blue, and red, right? So, in terms of time, the oldest island is green and then there's orange and then there's purple and then there's blue and then there's red. And you can see that reflected in the phylogenetic tree as well.
19:29
So, speciation has progressed with time and you have these different islands being colonized and these different species diverging across the Hawaiian island chain. So, lineages, or sets of individuals which are closer together from a DNA perspective, are related on an island, younger lineages are on younger islands, and genetic data kind of helps us explore the history of speciation on these islands. So now...
20:00
I've been kind of alternating between mountains and islands, slipping in the fact that mountain ranges have high biodiversity. Why was I doing this? Well, it turns out as Humboldt showed us, and this is an example from the U.S., the Southwest, mountains also have "islands", they have habitat islands. As you go up a mountain, you can see that the habitat or the ecology actually changes -- higher parts of the mountain are different than the lower parts of the mountain.
20:33
And so you might imagine that, ecologically, species would become adapted to live in these specialized environments. And then, if you looked at the mountain range, that's like a set of islands, kind of like Hawaii in a sense, right? So, it might be interesting to try and explore patterns of speciation, or the accumulation of biodiversity, in mountain chains. And this is what I'm going to talk to you about in terms of our research, but the best
21:01
part of it all is the history of this speciation is actually written in the DNA of these species, of the populations which actually live on these islands. A way to look at their past is through sequencing their DNA. Umm... this is also important; it's not just fun. But, uhh... you know, our existence as humans is critically dependent on biodiversity. There are many studies which show us, now, that human well-being, not just from an aesthetic
21:32
perspective, but in terms of our ecosystem services and many, many things, is critically dependent on biodiversity, and this allows us to really prioritize studying areas of high biodiversity. It's important to us; it's not just interesting. And these can be classified globally as biodiversity hotspots, not just regions where there is higher... high biodiversity, but also regions where there are threats to this biodiversity.
22:00
And two of the regions I'm going to talk to you about today happen to be biodiversity hotspots -- the Western Ghats, which is in India and Southern India, and the Himalayas, which is the in the northern part of Asia. So uhh... I guess what I'm trying to say is that, you know, we're really interested in understanding biodiversity, but it's also critically important from a conservation perspective. This understanding, we hope, will help us to think about how we can save these species
22:27
in the future. So, I'll just summarize and uhh... kind of give a leap into our research questions, which I'm going to talk about in the next part of this talk. Biodiversity is higher in the tropics, in mountain ranges, and in islands, and the barriers which cause allopatric speciation, or the creation of biodiversity, could be physical or ecological. But what still remains for us to explore is whether what's driving speciation is really
22:57
different or the same in various locations across the world. And you saw, for example, that study... you know... from Madagascar on vangas, right? It's a particular type of bird. But you could also think about this from the perspective of all the species which live in a location. Do they all respond similarly to these different barriers or changes in ecology? How generalizable are these patterns? How important are physical differences?
23:27
Are they more important to some species versus others, or not? And this is what I'm going to talk about in the next talk. Thank you.
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