cc >> Tom Zinnen: Welcome to
Wednesday Nite at the Lab. I'm Tom Zinnen.
I work here at the Biotechnology Center.
I also work for UW Extension Cooperative Extension, and on
behalf of those two organizations, thanks for coming
to Wednesday Nite at the Lab. We do this every Wednesday
night, 50 times a year. Tonight I'm delighted to be able
to introduce to you David Baum who's a professor of botany here
at UW-Madison. He's also the director of the
James F. Crow Institute for the Study of Evolution.
He was born in London, England, and obtained a bachelor's degree
in botany from Oxford University.
He then moved to St. Louis, Missouri, and obtained a PhD
in population and evolutionary biology at Washington
University. His research interests are in
plant evolutionary biology and evolutionary theory.
His research team conducts both laboratory and field research
aimed at understanding the evolutionary diversification of
several plant groups and at pinpointing the underlying
molecular changes that differentiate plant species.
Please join me in welcoming David Baum to Wednesday Nite at
the Lab. [APPLAUSE]
>> David Baum: Thank you very much, Tom.
And thank you all for coming. My intent tonight is to tell you
something very old but try and give it a new spin.
So Darwin's Origin of Species is hardly a new piece of work, but
what I'm going to try and do today is to tell you a little
bit about some relatively new ways to think about Darwin's
insights and to kind of shape it, and what I'm going to do is
try and connect for you two of his important ideas: common
ancestry and the tree of life. So Darwin's most famous work was
published in 1859, The Origin of Species, when he was 50 years
old, and I thought I would start by sharing with you the very
first sentence for The Origin of Species because I think it will
shape what I'm going to talk about tonight.
So he says, "When on board HMS Beagle, as naturalist, I was
much struck with certain facts in the distribution of the
inhabitants of South America and in the geological relations of
the present to the past inhabitants of that continent."
So why did he start The Origin of Species with this sentence?
Now, let's just think about a second what he's writing about.
He's writing about his voyage on the Beagle.
So his voyage on the Beagle took him around the world.
He started off in Great Britain, he traveled around South
America, and went around the world coming ultimately back to
Great Britain, to England. And this trip was clearly
essential. Before he left he was already
interested in the problem of diversity, the problem of where
did all these species come from, why do they have the traits that
they have, and a very strong case can be made that had he not
traveled around the world, he would never had developed this
theory. And it's notable the other
naturalist who came up with similar mechanisms on the
understanding of evolution, Alfred Russel Wallace, also
traveled great long distances around the world.
So traveling around the world was key, and this sentence
explains why it's key. It's key because as he moved
around the world he saw geographical patterns, and
specifically what he saw that jumped out at him was that when
he would go to a place on the planet he would find that the
species that occupied that area appeared to have these deep
structural similarities to one another.
They had some underlying similarity that he interpreted
as relatedness. They were seen to be relatives.
He also noticed relations between the fossil and the
living species that lived in an area.
So, for example, in South America, one of the first
excavations he did, paleontological excavations
he did, involved glyptodons. glyptodons are these giant
things that we now know are close relatives of armadillos.
He noticed the great similarities between glyptodons
and armadillos, and he was struck by the fact that
armadillos are this uniquely South American group, and the
only place in the world that anybody finds glyptodons are in
South America. So he was struck by these
geographic patterns. And thanks to these
observations, he had this great idea, this idea of common
ancestry. He concluded that when he looked
at organisms, it didn't look like they had all been
separately created. It appeared that they had common
ancestry. And he first sketched out the
idea of common ancestry in a very famous sketch in 1837 that
you can kind of see in the background of this slide.
So I'm going to be talking a lot about that idea of common
ancestry because I feel that it often got lost in the mix.
When people talk about evolution currently, people talk a lot
about natural selection, they talk a lot about fossils and
DNA, but people forget the central importance that common
ancestry plays in evolutionary theory, and I'm going to try and
spend today emphasizing why we really should focus on common
ancestry very clearly. So let's just start.
What is common ancestry? Common ancestry is a really
simple idea, but let me spell it out, nonetheless.
So take any set of living species, and I just grabbed five
different vertebrate species that are alive today, and play a
mental game. Take that organism, an actual
individual organism, and go to its parents.
And now go to that parent, that organism's parent, the
grandparent of the first organism.
Now go to the great-great grandparent and the
great-great-great grandparent, and keep doing that backwards
through time. At some point, those lineages
for those separate species will converge on a single common
ancestor. So that means that there's one
or a set of organisms at a point in time, all of whom are
ancestors to both in this example, why can't I find my
mouse, to the mouse and the human.
So they all trace back to a common ancestor.
And if we keep walking backwards through time, of course, we find
further common ancestors. This common ancestor that we've
come to down here is the last common ancestor of the human,
the mouse, and the lizard. We walk further down through
time, we come to the common ancestor of the frog, the
lizard, the mouse, and the human.
And on this tree, we ultimately will converge on this ancestor
at the bottom here that is an ancestor composed of organisms
that are ancestors, lineal ancestors, of all of these
living things. There's a simple idea, but it's
also a really important idea. It's hard to overestimate the
importance of common ancestry for thinking clearly about
evolution. And the main reason the common
ancestry is so important is that if common ancestry holds, then
all the differences that we see between living species had to
accumulate over time. In other words, evolution must
have happened. You can take any two living
species that you wish to think about, if you picture them, and
think about all the ways they differ.
Now imagine that they do trace back to a common ancestor.
All those differences must have accumulated somewhere on the
time since common ancestry. The differences in this example
between the mouse and the human will either represent changes
over here on the lineage to humans or over on the lineage to
mouse or both. So if common ancestry is true,
evolution has to have happened. And the nice thing, as you all
see, is that's actually very straightforward to find evidence
for common ancestry. In fact, Darwin's claim that
evolution happened is really the claim that common ancestry
holds. So it's really an essential idea
in evolutionary theory. So, in fact, the only way you
could entirely deny evolution would be to claim that there
were separate ancestry. In other words, if you walk
backwards through time, generation by generation from a
set of living species, they would all have a separate
origin. In that case, evolution might
not have happened. But the minute you say that two
very different living species share a common ancestor, then
evolution happens, substantial amounts of evolution if they're
substantially different like a human and a mouse.
So it's a very important idea. So, to reiterate, evidence of
common ancestry is evidence of evolution.
So, in The Origin of Species, which was finally published in
1859, Darwin documented evidence for common ancestry, and I'm
going to go through some of the evidence in a few minutes.
It came from an analysis of the structure of the different
organisms embryology, by geography, paleontology,
the fossil record, and from classification.
But before I can get to the evidence, I just thought I would
pose a question, a thought process, which is why did he
take so long to publish The Origin of Species?
His sketch showing common ancestry was 1837.
So why did he wait 22 years from that insight to write down on
paper the evidence he saw for common ancestry?
And the reason he waited so long was clearly because he felt that
he needed a mechanism. It wasn't sufficient to show
that evolution happened. He wanted to be able to say how
it happened because otherwise he felt people would say, well,
okay, it's ridiculous, it can't happen, so there must be
something wrong with your evidence.
He articulated this very well in the preface
to The Origin of Species. I've got another quote here.
He said, "In considering the origin of species, it is quite
conceivable that a naturalist might come to the conclusion
that each species had not been independently created.
Nevertheless, such a conclusion would be unsatisfactory until it
could be shown how the innumerable species inhabiting
this world have been modified, so as to acquire that perfection
of structure and co-adaptation which most justly excites our
admiration." So, of course, I love Darwin's
writing. It's beautiful writing, but the
idea is very clear here. The idea is that you need a
mechanism too. And so he was spending those
22 years trying to develop a clearly defensible mechanism for
evolution. And so the consequence of that
is that if you look superficially at The Origin of
Species, what jumps out at you is natural selection, and I can
tell you from experience in classes that if I asked the
question, tell me what you think of when you think of Darwin,
they always say natural selection.
Sometimes they'll say survival of the fittest which was a
phrase coined a little after Darwin, but it's the same idea.
People rarely say, oh, common ancestry.
And I think the reason that he focused on the mechanisms of
evolution was almost because common ancestry was so obvious.
It was too apparent to him. The minute you see the world
through the lens of common ancestry it is about as obvious
a thing as you can image. And so I think that's why in
The Origin of Species, while the evidence is all in there,
it almost takes second place to natural selection and the
writing of The Origin of Species.
And, in fact, he was quite astute in making that decision
because it really was natural selection that people tripped up
over, and so people, on reading The Origin of Species,
scientists were pretty much convinced of common ancestry
almost immediately. They were convinced that
evolution did happen, but they were very skeptical about the
mechanism. Just to give you kind of a
contemporary quote, this is Paul Broca, who is well-known for his
neurological work, there's an area of the brain named after
him, but he wrote in 1866, so just seven years after the
publication of The Origin of Species, "The fixity of species
is almost impossible, it contradicts the mode of
succession and of the distribution of species in the
sequence of extant and extinct creatures.
It is therefore extremely likely that species are variable and
are subject to evolution. But the causes, the mechanisms
of this evolution are still unknown."
So I think that it's inevitable for the biologist who
saw common ancestry became so obvious that the question was
all about natural selection. The downside of that is that for
150 years evolutionary biology perhaps is overly focused on the
mechanism, and, particularly in an educational context, we
haven't placed sufficient emphasis on common ancestry in
my opinion. And I'm going to try and explain
why I think that's important as we go.
So, let's just take sort of a digression, but let's just
clarify what is the evidence for common ancestry.
And I'm only going to do a broad sweep, broad strokes here,
nothing very detailed. And I'm going to use a fair
number of historical references. I find that useful when I'm
telling history. So I think it's Darwin's great
grandfather, Erasmus Darwin, obviously he didn't develop a
fully fledged theory of evolution or he would be the
Darwin we talk about, but he did think a lot about diversity and
he wrote this epic poem, sort of medical, scientific poem.
But he also thought about the patterns he saw, and the one
thing he pointed out, which actually resonates to the modern
understanding of evolution, is that when you see deep
similarities among organisms, it suggests that they have some
degree of relatedness. Of course he's putting it in a
more theological framework, but this is the word that he used:
"The great Creator of all things has infinitely diversified the
works of his hands, but has at the same time stamped a certain
similitude on the features of nature that demonstrates to us
that the whole is one family of one parent."
So it's that one family of one parent, that's a very familiar
idea. When we see family resemblances
in families, sort of same idea that when we see these deep
similarities among species, they tell us that they have some
degree of relatedness, some common ancestry.
So, similarity, in general, is not particularly good evidence
for common ancestry over separate ancestry.
The similarity becomes particularly compelling when we
have similarities in structure despite differences in function.
So if we have two structures that are doing the same thing
and they're similar in two different species, you could
easily argue that they're structured that way because
that's the best way to build an ax.
Okay, to build a structure for that function.
But when we find something like this where we have very similar
structures in the frog forelimb and the bat forelimb and the
porpoise forelimb, similar bone arrangements with a similar
developmental trajectory, given that these three structures
are used for very different functions--
We wouldn't expected them to have similarities,
so the fact that they do have similarities,
it becomes stronger evidence that they have common ancestry.
Similarly, we've got sort of a molecular example here in the
middle. Here we have two, be them actual
representations of protein complexes in bacteria that are
used for entirely different functions.
The flagellum here is basically a bacterial motor that is used
with a little propeller around, bacteria that you can use to
swim with. Whereas the type two secretion
system which is very similar but is used for completely different
structure and function, is used to inject toxins into host
cells. So you can just visually see the
similarities there. We also have many good cases of
structures that show similarity despite a lack of function.
We call these vestigial structures.
So you can see over here a good example of that.
There are these vestigial hip bones in a whale.
So whales lack hind limbs. So they don't have functional
hips but they have these residual hips embedded in the
muscle on their flanks. It's sort of shown there on the
little picture. So similarity despite functional
difference provides the best evidence for common ancestry
among similarities. Extending that idea, very strong
evidence for common ancestry arises when we find similarities
improbable.
[replacing bad mic] Okay, better, no cracking.
So we see these kind of patterns all over the place.
Cases where we see, for example, similarities despite what
appears to be very unintelligent design.
So let's give one of my favorite examples.
We have a nerve that feeds our voice box that originates from
the brain stem, travels down into the chest cavity,
underneath aorta, the major blood vessel serving the heart,
back up to the voice box. And this odd wiring pattern is
seen not just in us but in all mammals.
So, for example, in a giraffe this nerve runs from the brain
stem down, way, way down into the chest, underneath the aorta,
back up to the larynx. That's hard to explain without
invoking a common ancestor of the mammals.
Similarly, the blind spot in our eye is bad wiring again.
Our retina is basically wired backwards, and as a result of
that, we have a blind spot in the middle of each retina, and
that's shared by all vertebrates.
Why would that be were it not for common ancestry.
And we also see this very clear at the molecular level.
So in theory there's an almost infinite number of nitrogenous
bases, yet all of life uses the same five, four for DNA and an
overlapping four for RNA. Likewise, all of life uses the
same 20 amino acids even though there's actually an infinite
number of possible amino acids. That seems hard to understand
without common ancestry. And the code that matches the
DNA to the protein coding sequence to the amino acids is
also almost identical across life, even though there's not
particular functional benefit to this arrangement.
So, similarities among organisms are just compelling evidence of
common ancestry. Fossils are often misunderstood.
The role of fossils as evidence of evolution I think is often
misunderstood. But there is an important sense
in which they provide evidence for common ancestry which is
when we find what are called transitional fossils.
So I thought I would just take a second to clarify what that
means because I do see a lot of misunderstandings about the
nature of fossils. We rarely see in the fossil
record continuous series of fossils.
Fossil forms gradually morphing one into the other through time,
and there are good reasons why we don't see that.
Nonetheless, fossils provide very compelling evidence for
common ancestry when we find transitional fossils.
So what is a transitional fossil?
So think of a living group. Most living groups you can think
of have a number of distinct features that make them,
differentiate them from anything else.
So here is a distinctive living group represented by this
triangle. And here are a whole bunch of
traits that are different between this group and
everything else. The group of the living mammals
you can list at least 10 traits that we have that no other
living group has. We have three middle ear bones.
We have hair. We have milk, etc.
So given that all these traits couldn't have sprung into
existence all at once, there must have existed forms in the
past that had some but not all of those unique traits.
And so whenever we find those transitional forms, that's
consistent with the expectations under descent from common
ancestry. But you wouldn't predict
necessarily finding those forms if there were separate ancestry.
So just to kind of give you a concrete example, birds, here's
a list of some of the many, many features that are unique to
birds. And when you look in the fossil
record, you find these traits. You find ancestors that had
hollow bones but didn't have feathers.
You find ones that had hollow bones and feathers but didn't
have any of these. You find some like this one.
This is an example of a fossil image, an image of a fossil with
a very initial wishbone. The wishbone is a fusion of the
collarbone. We have two separate clavicles
here on either side. The fusion of them into the
wishbone is what helps birds achieve the springiness to
spring their wings back up, but it evolved in terrestrial
dinosaurs halfway up there. Okay, so traditional fossils.
I'm going through these kind of different kinds of evidence for
common ancestry. And then the last one I'm going
to talk about tonight, although there are more, but the last
I'll talk about is biogeography because this is really where
Darwin started. So Darwin's, most famously, when
he got to the Galapagos Islands, he was struck by the fact that
there was a group of birds there, represented by several
species, but they all had these rather striking anatomical
similarities. And so when he saw them, he
said, "Seeing this gradation and diversity of structure in one
small, intimately related group of birds, one might really fancy
that from an original paucity of birds in this archipelago, one
species has been taken and modified for different ends."
So he could see the biogeographic structure
suggested common ancestry. Of course, we know this very
different conventional, every day, we know this without really
thinking about it. We all know, most people with
any biological knowledge know that there's something wrong
with this picture. What's wrong with this picture,
of course, is that these two species would never encounter
one another in nature. They also wouldn't drink Coke,
but that's not the point here. So, why is that?
Well, you would have thought that a polar bear is very well
adapted to a cold, wintery environment, why doesn't it live
in the Antarctic? Why is it that penguins, which
seem supremely good at living in extremely snowy, cold
environments and finding fish to feed on, only live in the
southern hemisphere? And the common ancestry, this is
relatively easy to understand because penguins, for example,
you just have to imagine that their ancestors originated in
the southern hemisphere, and they have never had an
opportunity to disperse through the warm tropics to the northern
hemisphere, the northern frozen zone.
Likewise, the bears gave rise to the polar bear on the northern
hemisphere. There are many, many such
patterns. Another one I've illustrated,
there are hummingbirds, hummingbirds being wonderfully
well adapted, nectar feeders, they're restricted to the
New World, to North and South America.
Even though they ought, by all logic, to do very well as nectar
feeders in Africa or in Asia, they're not there.
So biogeography is the last of these major lines of evidence.
So, to wrap up that point, common ancestry is extremely
apparent, and, as a result, evolution is very obvious when
looking at all these aspects of biology.
So, what about the tree of life? It was in the title so I want to
kind of get to it now. So, Darwin very clearly used a
metaphor of the tree of life to organize his thinking and his
work by biology. So he said that all the beings
of the same class are represented by a great tree.
I believe this similarly largely speaks the truth.
The great tree of life covers the Earth with ever-branching
and beautiful ramifications. So he's basically equating the
idea of common ancestry very naturally with the idea of a
tree. Like a family tree but a tree of
relationships among species. So what I want to do now is ask
the question, why does the tree metaphor work?
Why is that a useful way to think about evolution in the big
scale? So, it doesn't actually, you can
have common ancestry without a tree.
You can have a structure such as the one on the left which is
sort of common ancestry but it's sort of like a star burst.
There's not really branching within from other branches.
You could also have common ancestry with some sort of
network of relationships. So why do we use a tree
metaphor? Remember that what's distinctive
about a tree, within a tree branches diverge and then those
branches never come back and fuse again.
So once two branches separate, they go on their own way which
means, for example, that if you pick two leaves at random on a
tree, there is only one path that the squirrel could take to
get from one to the other. Whereas, in a network there are
many paths between any two points.
So why does the tree metaphor work?
Why is it appropriate? So to kind of explain this, I
want to just take a moment kind of getting you to think about
the tree and what it represents. I'm going to walk you through
how to think about what the tree actually represents in biology,
and I think then you'll see why the tree form is expected.
So, evolution is all about reproduction.
It's all about organisms giving rise, parents giving rise to
offspring and so on, and passing on their genetic material to
their offspring. So I wanted to start this little
sort of vision in a field, and just imagine a small group of
plants. Five parents giving rise to five
offspring. They're exchanging pollen and
giving rise to offspring. And now I want to zoom out a
second and just expand our picture.
Now we're going to look at, instead of just being a little
patch in the field, we're going to think maybe a bigger patch.
So now instead of having five individuals, we have 15
individuals, and now instead of looking at two generations,
we're looking at five generations.
It's already getting a little busy on the slide, and so from
here on out let's get rid of the organisms.
The organisms are relatively transient.
It's one of those sad realities that in evolutionary times the
organisms are transient, and it's the lines of descent that
really matter because it's through the lines of descent
that the genetic material is propagated.
So I've drawn down here the same diagram, but I just zipped out
the organisms. So now let's expand, zoom out
even further. Now let's visualize not just
this sort of field maybe the size of this lecture hall, let's
imagine a field maybe the size of this end of campus.
So now we've got, instead of 15 organisms, I think we have about
80 organisms here. Instead of five generations,
we've got 80 generations. And you can see that as you
start moving out, those connections start blurring into
just sort of a mess of lines of descent.
And that's what we're looking to communicate here that if you go
far enough out you don't see the individual lines, you just see
the pattern through which genes are passing.
And if you kept zooming out, you would find that this little
patch of this plant in this part of campus is part of a braided
network of populations connected, disconnected maybe
for a period of time, and then maybe connected once in a while
by seeds or pollen flow. So this structure shown here is
a good way to think of a species lineage.
It's what a species looks like viewed through time.
And then what a tree is such a species lineage that has
undergone branches events. So, in this case, starting with
one ancestral species lineage we've divided to give rise to
here four living species. So that's what the tree is.
It's kind of basic but it's worth reviewing it.
So why, however, do we get a tree?
Why is it not a network? Notice it is a network in here.
This is a network. But that is a tree.
Why do we get a tree? So I flipped it over to look
more like a tree. So time is now running from the
bottom to the top. And let's ask the question, what
happened there? Why did lineages split?
So it turns out that we now understand that the main reason
lineages split is because they get separated by external
forces, geography. So to give you an example,
imagine we have a widespread population lineage before this
split, before here. And I'm just showing it with
little trees. And imagine that there's a
canyon that's worn through this population.
So the population is now split into two populations, and
they're separated far enough that genes are not readily able
to flow from one side to the other.
If you wait long enough, you expect changes to accumulate on
the two sides of the canyon. So some of those changes might
be visible as represented here. So what's happening here is that
the two lineages are now evolving independently, and if
they're independent when a mutation arises in one and it
maybe increases in frequency and becomes fixed in one, that
mutation is not able to get into the other one and vice versa.
So over time those two lineages are guaranteed to begin to
separate. So as long as they remain
separate they will become more and more different the longer
you wait. The reason that is significant
is that if they wait long enough, if they're isolated for
long enough, they will lose the ability to ever fuse again.
And that is because the ability for organisms to reproduce
sexually is rather complicated. It requires a certain match
between the communication systems of the two individuals
that are going to reproduce. It requires that their
reproductive structures match and fit and it all works.
It requires that the zygote produced in the next generation,
the genes are compatible, it can develop properly, and that it
can be itself able to reproduce. And so I think most of us kind
of know intuitively that if you wait long enough like the lion
and the tiger, they actually cannot reproduce to make fertile
offspring. They can make offspring, they're
just not fertile. And if you wait long enough, any
two lineages are guaranteed to not reproduce.
And we sort of forget this. Here we have squirrel on a tree.
We all know without even thinking about it they cannot
reproduce with one another. But under the thesis of common
ancestry, they do share a common ancestor.
So clearly what's happened is the inability to reproduced is
inevitably acquired over time, and that's why the predominate
pattern in evolution is a tree. It's not a network because
reproductive isolation arises inevitably given long enough.
Okay, so to kind of encapsulate that, the tree for was
inevitable because lineages are going to become separated during
evolutionary history. And once they're separated, they
are almost inevitably going to acquire, eventually acquire
an inability to integrate. So to give you a way to think
about it, think about languages, the ancestral language for
French and Italian was a single language.
Those two populations, the Italians and the French,
were separated long enough. They were speaking internal to
the group. There was no pressure to
not speak to the other group, but nonetheless, you come to
the present, and a Frenchman and an Italian can't communicate.
So it's the same basic principle with evolutionary lineages.
Inevitably they lose the ability to communicate, to reproduce.
>> Except with gestures. >> Yeah.
Well, gestures will work anyway. >> Why did this vary from
species to species? We know now that polar bears
and brown bears can hybrid. And all canines--
>> So the question was why do they vary from species to
species. It actually does vary in the
sense that the rate in which two lineages lose the ability to
interbreed is going to very depending on how complicated
their mechanisms of interbreeding are and how prone
they are to change. So in some groups, like in
plants, we know that plant lineages often retain the
ability to interbreed for quite a long time.
So you can actually force a cross between almost any two
species of orchid even though their last common ancestor might
be many millions of years ago, where some other groups very
quickly lose the ability to interbreed, like fruit flies are
known to be quick. Okay, so what we've learned from
all this is that there is this tree of life and it is largely a
tree. There are a few of these naughty
reticulations but it's very tree-like, and that it connects
all of life. That pattern I just showed were
those little plants in the plot, you keep blowing out, you keep
stepping out, zooming further and further out, eventually you
get to the entire tree of life that connects every living
species around us today. Every living species around us
today is, every living organism around us today is connected by
an unbroken line of parents, grandparents, and so on, back to
common ancestors that lived probably three billion years
ago, an unimaginably long period of time.
So it's this kind of, to me, really wonderful insight that we
have in biology. It explains why we have a
science of biology because all of life is part of the same
tree. We don't have separate biologies
for each kind of organism. We have one biology because we
have one common ancestry. And I personally find it very
exciting to think of the connectedness, the unity of
life, but I can also understand why this is a cartoon that I've
been given permission to use which, I think, captures the
fact that for many people it's actually an uncomfortable idea,
the idea that we share common ancestry with all other life
forms. But, as I said, you can flip it
around very easily and say it's great, it's useful, it means
that research done on a mouse is relative to medicine in humans.
Indeed, research done on a plant can be relative to humans or
bacteria. And so I think it's something
that biologists are very comfortable with even if
not everybody in the public are comfortable with this idea.
Okay, and the exciting thing for me as a practicing scientist in
this discipline is that over, particularly, the last 20 years,
thanks to the ability to readily sequence DNA, we've made great
strides in putting the tree together, in figuring out where
each species fits on this great tree.
And it is really a triumph of modern science that we can,
almost without fail, tell you where a species fits on the tree
of life. Okay, so I hope I've showed you
so far that there is a tree and why there's a tree and that it
was important as a lynchpin for evolutionary thinking.
So what I'm going to sort of, my last little section of this talk
is really trying to get across the idea of why it's actually
important to communicate trees today, and particularly in an
educational context. Why is it important that I come
out and tell you about trees and tree thinking.
The tree metaphor are very old ideas.
These are some things you'll find if you look up the tree of
life on the Internet. Various mystical ideas.
But, of course, today the tree of life is used as a catchphrase
to refer to this idea of the connectedness of all life
through common ancestry. So why is this so important?
>> Were those Hebrew words? >> Yes.
Yes, they are. That's Hebrew.
Appropriate for today, I guess, given that it's Hanukkah.
So why is the tree metaphor so important?
The main reason that I think it's so important is I would
argue that the only way to think clearly about evolution at large
is to use the tree metaphor and to deploy what I call
tree thinking. And so one of the activities I'm
engaged with pretty actively is trying to develop ways to assist
teachers to teach tree thinking, which I'll come to shortly.
So why is tree thinking important to avoid
misconceptions? Well, it tends to be pitted
against an alternative metaphor. So the tree metaphor is a way
that evolutionary biologists think about evolution.
I would contend that most non-evolutionary biologists,
their mental image of evolution is not that dissimilar from this
other idea called the ladder of life, or scala naturae, which
goes back really to the Greeks, to the ancient Greeks, but it
proposes that there is this sort of series of living things of
different degrees of advancement.
And so people spend a lot of time, in the 18th century
especially, trying to construct these hierarchies of
advancement, this ladder of life.
And so they would play these games to try to connect things
into one unbroken line where you do clever things like you would
go from worms to insects. Shellfish, they've got kind of a
shell, they link you to reptiles.
Snakes conveniently link you to eels which link you to fish
which link you to flying fish which link you to birds and so
on. But of course the clear theme in
all of these ladders of lives was that there were certain
organisms that were advanced, superior, and of course in every
case among the living things, the top was humans.
And in many models you would continue beyond humans up to
angels and demons and so on. You could also proceed down
below what were perceived to be the lowest form, plants to
minerals and non-being. So this is the competing view,
and I think you can see if you present it like this it seems
very foreign. But nonetheless, this basic way
of thinking about evolution as a story of advancement is
absolutely embedded in our culture.
And it is, I would contend, the way that evolution is
conventionally presented. So if you look at cartoons about
evolution, and there are almost an infinite number of them, they
tend to have something like this.
They tell a narrative story of advancement, often with a little
joke at the end of regression of some kind, but they usually
depict organisms that, at least in a cartoony sense, resemble
other living organisms. This looks like a living monkey.
This might be argued to look like amphibian.
But this certainly looks like a living fish.
And you can see these all over the place.
Here's a number of other examples all showing ancestral
apes that look very much like modern apes giving rise to
moderns humans. And this is, I think, partly in
the culture. I think you probably all
recognize it. This is how we tell the story of
evolution. It's advancement with humans at
the end. But it turns out this is an
inaccurate way, there's another manifestation of it, even when
people draw trees they often add a kind of, editorialize them
with a certain property. So you'll see here that here we
have this lineage here. There are the vertebrates right
at the top there, and these are kind of drawn as shorter
branches. They're not actually ancestors
but they haven't changed much. And if you actually look at,
this is a famous drawing from Haeckel who drew a lot of these
beautiful trees. There's mentioned man at the top
up there, of course. But if you look along this stem,
you find a lot of living things that are perceived to be
ancestral to us. We're the top, everybody else is
down there. So this is a very -- way of
thinking, and the question is, why is ladder thinking so
abundant? I'm not going to quiz you on
this, but one explanation could be that people just don't really
understand evolution and that certainly there is some of that
out there. I think a very important point,
as I do think we have a predisposition to linear
narratives, if you open a typical novel, it has a
beginning, a middle, and an end. It doesn't have one beginning
and 20 ends like a tree does. As I'm going to show you in a
moment, tree thinking, I think, is hard.
So the bottom line is, I think, that for all these reasons,
trees have not permeated into the common understanding of
evolution to the extent that they should if we want people to
really understand evolution, evidence for evolution, and the
connectedness of life. So why is tree thinking so hard?
Well, the first thing that is counter-intuitive for many
people is that if there is a tree, all tips of that tree, all
living species that are around today, are equally evolved.
All of them. They've all been around the same
amount of time. Take a common ancestor of two
living things and you say, well, they had a common ancestor at
some point in the past, those two things are equally distant
in time from that common ancestor.
They've all been equally vetted by selection.
So there really is no grounds for looking at any one tip and
saying it is more evolutionary advanced than any other.
I'm not going to read that quote but it just gives an example of
a good way of thinking about it, that of course humans are going
to put ourselves at the pinnacle because we're humans, but there
really isn't a biological basis for doing that.
There isn't an evolutionary basis for doing that.
I think this is put very clearly by Asa Gray.
Asa Gray was the father of American botany at Harvard
University, and he wrote in a letter to Darwin, "We have,
that I know of, no philosophical basis for high and low."
So what he's basically saying is when we look at these
organisms that are conventionally interpreted as
being primitive, so bacteria, amoeba, lungfish, in fact, in an
evolutionary sense, they are no lower than one another
nor than Asa Gray himself. All living things are equally
the tips of the tree of life. So evolutionary advancement we
have to get rid of, and I think people find that hard which is
one reason that tree thinking hasn't permeated as readily out
of the evolutionary biology community itself.
Another implication, or a closely related implication of
tree thinking, is that there is no one pinnacle or purpose or
privileged point for evolution. Mark Twain, brilliant kind of
sardonic way of putting it. If the Eiffel Tower were now
represented the world's age, the skin of paint on the pinnacle
knob at its summit would represent man's share of that
age. And anybody would perceive that
that skin was what the tower was built for.
So that's a challenge to us in our conventional way of
thinking. And then it also challenges us
in a more biological way which is that it challenges certain
conceptions that we might have about organisms and how they are
related to one another. So relatedness in every day
culture refers to actually how recently you share a common
ancestor. Your degree in kinship with your
first cousin is greater than with your second cousin because
you share a more recent common ancestor, namely your
grandparents, with your first cousins than you do with your
second cousins, which is your great-grandparents.
And, yet, relatedness, when we turn to biology we have a
tendency to equate relatedness with similarity, but they're not
the same thing. To give you a concrete example
here, this is the phylogenetic tree, the evolutionary
relationships that are well-established now for the
reptiles. So here are the turtles.
There are the lizards and snakes.
There's the crocodiles, and there are the birds.
Now, what this tree says is that the crocodile is more closely
related to the bird than it is to the lizard.
And that's because there's piece of its evolutionary history down
here that is uniquely shared between the bird and the
crocodile that's not shared with the lizards and snakes.
And this is actually supported by a couple of traits including
laying and defending nests which is shared between crocodiles and
birds. But I think anybody would agree
that in terms of similarity, a crocodile is more similar to
a lizard, just superficially at least, even though it's more
closely related in evolutionary terms to the birds than it is to
the lizards. So there's another sense in
which trees are challenging, they challenge our
preconceptions. So here's a question.
I'll do a quick count. I'd actually like you to look at
this a second. Here's a tree and this
represents relationships. I showed you, basically, this
tree earlier. So the question I just want you
to think about for a second is given this tree, assume this
tree is correct, is a frog more closely related to a trout or a
human? Now, I'm going to take a little
look out and see what people say.
If you think it's a trout, maybe you can do a sign like this.
If you think it's a human, you can maybe tap your own chest.
Okay? And then I can get a sense of
the vote here. I'll give you a second to do
that. What do you think?
Is a frog more closely related to a trout or a human?
Okay, I'm see, actually, a mix. I'm seeing both.
I can tell you from experience having done this many times in
classes and other situations that this audience is a better
educator than average audiences. I can tell you that because it
was about 50/50. In a typical audience, 80% of
people would look at this tree and they would say a frog is
more closely related to a trout than to a human.
But from an evolutionary point of view, a frog is more closely
related to a human than it is to a trout.
So let me explain that just to give you a feel for how this
works. To ask the question of whether
the frog is more closely related to the human or the trout,
you have to find the common ancestors.
So let's first find the common ancestor of the trout and the
frog. There it is at the bottom of the
tree. Now we can find the common
ancestor of the frog and the human.
There it is. Since this common ancestor is a
descendent of this common ancestor, it's closer to the
present, the present's at the top, the time is down here, so
this common ancestor is a descendent of this common
ancestor, that shows us that according to this tree the frog
is more closely related to than human than it is to the trout.
Now, my point of showing that exercise is not to teach you how
to read trees, although I do like to do that in classes, my
point was to show you that for many people it's not obvious
how to read a tree. Trees are surprisingly tricky.
And this is why I'm a believer in the importance of educating
at various levels, college level and at high school and before,
children and everybody, to think clearly about trees because it's
not easy. It takes actual effort to do
this, but by doing it, one achieves a much clearer
understanding of evolution and avoids some of these important
misconceptions. So what I would say looking back
over the 150-year story that I've sort of packed into
50 minutes is that it took 150 years for us to actually
realize that the common ancestry part of The Origin of Species
is really important, and we shouldn't forget it underneath
this mess of discussion about natural selection.
And that because evolutionary trees are becoming so important
in functioning biology, from things like tracking diseases,
determining how different traits evolved, that we should be
making much more of an effort, and because trees are difficult
we should be making a real concerted effort to help people
understand evolutionary trees and their place in modern
biology. So this is something that I'm
engaged with and concerned about, and so me and my group
and a lot of colleagues here on campus, we spend quite a lot of
time working with teachers, doing workshops for teachers and
developing curriculum materials, like pipe cleaners are a great
way to introduce the concept of a tree and how to read it
and not read it. And my hope is that over the
next 10-20 years, we will gradually get good tree thinking
introduced not only into the college level
but into the K-12 level. The hope being that when I see
students coming into an intro biology class at the college
level, they will not have this ladder thinking view stuck in
there too strongly, and instead they're going to have a greater
ability to see that the tree of life is made up of many living
tips, all of which have a certain equality to one another,
none of which is more advanced or the pinnacle of life, and
that evolution is ongoing, that all those lineages are
continuing the evolutionary process even now.
So that's sort of my hope for the future.
So I thought I would just end, I know I've read a lot of quotes,
but I just think that I couldn't do better than ending with a
quote, the last sentence, I began with the first sentence of
The Origin of Species, and I'm going to end with the last
paragraph. So this is how Darwin ended
The Origin of Species. "It's interesting to contemplate
an entangled bank, clothed with many plants of many kinds, with
birds singing on the bushes, with various insects flitting
about, and with worms crawling through the damp Earth, and to
reflect that these elaborately constructed forms, so different
from each other and dependent on each other in so complex a
manner, have all been produced by laws acting around us.
There is grandeur in this view of life, with its several
powers, having been originally breathed into a few forms or
into one, and that whilst this planet has gone cycling on
according to the fixed law of gravity from so simple a
beginning, endless forms most beautiful and most wonderful
have been and are being evolved."
So I will leave it there. Thank you for your attention.
[APPLAUSE]