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.