Już 62 334 użytkowników uczy się języków obcych z Edustation.
Możesz zarejestrować się już dziś i odebrać bonus w postaci 10 monet.
Jeżeli chcesz się dowiedzieć więcej o naszym portalu - kliknij tutaj
Jeszcze nie teraz ZAREJESTRUJ SIĘlub
Zaloguj »
ZAREJESTRUJ SIĘ
Nie masz konta?
1. Evolution, Ecology and Behavior: The Nature of Evolution- Selection, Inheritance, and History
Poziom:
Temat: Edukacja
Prof: Biological
evolution has two big ideas.
One of them has to do with how
the process occurs,
and that's called
microevolution.
It's evolution going on right
now.
Evolution is going on in your
body right now.
You've got about 10^(13th)
bacteria in each gram of your
feces, and they have enough
mutations in them to cover the
entire bacterial genome.
Every time you flush the
toilet, you flush an entire new
set of information on bacterial
genomes down the toilets.
It's going on all the time.
Now, the other major theme is
macroevolution.
This process of microevolution
has created a history,
and the history also constrains
the process.
The process has been going on
for 3.8 billion years.
It has created a history that
had unique events in it,
and things happened in that
history that now constrain
further microevolution going on
today.
That's one of the tricky things
about evolution.
It has many different scales.
My wife always gets frustrated
with me.
She says, "Well when did
that happen?"
I say, "Oh not too long
ago, only about 20 million
years."
And, you know,
that's what happens when you
become an evolutionary
biologist, you zoom in and out
of deep time a lot.
And this process of
microevolution is going to be
the first thing we examine.
It's the nuts and bolts.
It's what's really created the
patterns.
But the patterns of
macroevolution are also very
important because they record
the history of life on the
planet and they constrain the
current process.
So the evolution part of the
course is set up basically with
two introductory lectures.
Then I'm going to spend six
lectures talking about
microevolutionary principles.
So these are things that you
can always return to if you are
puzzled about a problem.
Then there'll be five lectures
on how organisms are designed
for reproductive success.
This includes cool stuff like
sexual selection,
mate choice,
that kind of stuff.
I usually manage to give the
sexual selection lecture just
about on Valentine's Day.
Then we'll do macroevolutionary
principles.
This has to do both with
speciation,
how new species form,
and with how biologists now
analyze the tree of life to try
to understand and infer the
history of life on the planet.
Then we'll take a look at that
history,
looking at key events--and this
includes both fossils and the
diversity of organisms--
and some abstract organizing
principles about life.
So all of those are part of how
we can analyze the history of
life on the planet.
And then, just before Spring
Break, we will integrate micro
and macroevolution.
We'll do it in two different
ways.
We'll do it with co-evolution,
where micro and macro come
together,
and we'll also do it with
evolutionary medicine,
where both kinds of thinking
are necessary really to
understand disease and the
design of the human body.
So where did this idea of
evolution come from?
Well, there are always ideas.
You can go back to Aristotle
and find elements of
evolutionary thought in
Aristotle.
But really it's a nineteenth
century idea,
and in order to see how it
developed let's go back to about
1790 or 1800;
so at the end of the Century of
the Enlightenment.
At that point,
if you were to ask a
well-educated person living in a
Western culture how old the
world is,
they would say,
"Oh thousands of
years."
And if you were to ask them,
"Well, where did all these
species on the planet come
from?"
they would say they were all
created just the way they look
now and they've never changed.
And if you asked them,
"Have there ever been any
species that went extinct?"
they would say,
"No, everything that was
created is still alive and can
be found somewhere on the
planet."
So when Alexander von Humboldt,
who was certainly a creature of
The Enlightenment,
sets out to explore South
America, he thinks that he might
encounter some of those strange
fossils,
that the French have been
turning up in the Paris Basin,
on top of Tepuis in Venezuela.
So he really thought that there
was a lost world.
Of course, Arthur Conan Doyle
later wrote a novel about that.
But these guys actually
thought, "Hey,
I go to Venezuela or I go to
the Congo, I might meet a
brontosaurus."
That was what they thought at
that time.
They thought that adaptations
were produced by divine
intervention.
They did not think that there
was a natural process that could
produce anything that was so
exquisitely designed as your
eye.
We now know that your eye is in
fact very badly designed,
but it looked pretty good to
them.
Anybody here know why the eye
is badly designed?
What's wrong with your eye?
Student: The blind spot.
Prof: It's got a blind
spot and--?
Student:
>
Prof: It's got--the
nerves and the blood vessels are
in front of the retina.
The light has to go through the
nerves and the blood vessels,
to get to the retina.
The octopus has a much better
eye.
Okay, now by the time that
Darwin published his book in
1859, people thought that the
world is very,
very old;
how old they weren't sure.
We now know about four-a-half
billion,
but at that point,
based on the rate of erosion of
mountains and on the saltiness
of the ocean,
assuming that the ocean had
been accumulating salt
continuously,
and that it hadn't been getting
buried anywhere,
which it does,
people thought hundreds of
millions of years.
They weren't yet in the
billions range,
but they thought hundreds of
millions.
They knew that fossils probably
represent extinct species.
That was Cuvier's contribution.
He did it for mammal fossils in
the Paris Basin.
Geoffrey Saint-Hilaire had had
a big debate with Cuvier about
homology, and that was in 1830.
By the way, it was one that
many people throughout Europe
followed very closely--
this was a very,
very key intellectual topic at
the time--
and it was about homology.
Basically it was about the idea
that Geoffrey Saint-Hillaire had
had that if my hand has five
fingers then--
and a bat's wing has five
fingers and the fin of a
porpoise has five fingers--
that that indicates that we all
got those five fingers from a
common ancestor,
and therefore we are related
because we had a common
ancestor.
So you could see that in 1830.
That's before Darwin publishes
his book.
Okay?
Then of course we have the idea
that adaptations are produced by
natural selection;
and we owe that to Darwin.
And I will run through the
process he went through between
1838 and 1859 very briefly.
This is one of the most
important ideas about the nature
of life,
and therefore about the human
condition,
that's ever been published,
andI strongly recommend that,
if you have a chance,
read The Origin of
Species.
Darwin actually was quite a
good writer.
It's Victorian prose,
so it's a little bit like
reading Dickens.
But it's good stuff,
he has a nice rolling style.
How did he come to it?
Well Darwin was a med school
dropout.
Went to Edinburgh,
didn't like med school;
loved beetles and became
passionate enough as a
naturalist to become known,
as a 22-year-old young man,
as a guy who might be a good
fellow to have on an expedition.
And the British Admiralty was
sending Fitzroy around the world
to do nautical charts and Darwin
got on the ship.
So at an age not very much
greater, or perhaps even a bit
younger than some of you,
Darwin sets off.
He's 22 years old.
He wants to know how species
form.
He has set himself that goal.
So he's ambitious.
He's set a clear goal.
The goal is to solve one of the
most pressing problems that
biology has at that time:
where do species come from?
Now the stimulus that he has is
in part from Charles Lyell,
the geologist,
who had discovered deep time,
and that convinced Darwin that
there would've been enough time.
He stops in Argentina.
In the banks of a river in
Argentina he can see giant
fossil armadillos,
and then right on top of that
same bank he can see the current
armadillos walking around,
up on top of the bank.
There they are;
the live ones are right above
the fossil ones.
They look the same but--I mean,
they look similar--but they're
not the same.
So there's some connection
there.
He gets on a horse in Chile and
he rides up into the Andes and
he sees marine fossils lifted
thousands of feet above sea
level;
clearly some dynamic process is
going on that had lifted those
marine fossils up.
He doesn't know about
continental drift
yet--right?--but there the
fossils are.
In the harbor at Valparaiso he
sees the effects of an
earthquake that had happened
just before they arrived.
It was a big one.
It was probably as large as the
earthquake that recently caused
the big tsunami in Indonesia--
so it was probably an 8.5,8.6
earthquake--
and it had caused an uplift in
the harbor of maybe 50 feet.
So he began to see the world as
dynamic.
Things hadn't always been the
way they are.
Then he goes to the Galapagos,
and please navigate the
Galapagos website and have a
look at some of these
differences.
The thing that Darwin noticed
is that the mockingbirds are
different on the different
islands.
If you go to the Galapagos what
you'll notice is that if you
land on Espanola,
the mockingbirds really want
your water supply,
and they will hop onto your
head or your knee to try to get
at your water supply.
But, in fact,
the mockingbirds all look a
little bit different on the
different islands,
and that's what Darwin noticed.
He could also see that that the
marine iguanas look a bit
different, and the land iguanas
look different.
Interestingly,
he didn't notice the
differences in the finches,
until he got back to England
and gave his collection to the
British Museum,
and the ornithologists at the
British Museum came in and said,
"Hey Darwin,
do you realize that the finches
on these islands are
different?"
And that was when he began to
really see how many differences
could accumulate,
how rapidly,
when you take a migrant from
Central America and put it on an
isolated archipelago.
So he goes back to London.
He's been onboard ship for
about four years.
He has a problem with
seasickness.
He never again sets foot on a
ship.
He doesn't want to go near the
water after being four years on
this ship.
He had a few issues with the
captain too, Fitzroy,
but mainly it was that he had a
very bad upset stomach onboard
the Beagle.
He reads the Reverend Malthus
on population growth.
Malthus's book had come out in
1798.
Malthus said basically that
populations grow exponentially
but agriculture grows linearly.
Therefore populations will
always outstrip their resource
base.
This convinced Darwin that all
organisms are in a competitive
struggle for resources,
and that that must inevitably
be the case.
He saw very clearly how
powerful reproduction is at
generating exponential
population growth.
We will come back to that in
the ecology portion of the
course.
And we now know that organisms
are in competition really
essentially not just over food
resources,
they are in competition over
anything that will get their
genes into the next generation.
So that can be competition for
mates.
It can be competition for
nesting sites,
competition for food;
lots of different things.
But at any rate this primed
Darwin's thinking.
So he writes down the idea of
natural selection.
It comes to him in 1838;
it's in his notebooks in 1838.
Basically, I'll run through
natural selection in a minute.
It's a deceptively simple idea
because the mechanism looks so
simple, but the consequences are
so wide ranging.
Darwin recognized what the
consequences were.
And he didn't publish
immediately.
He did other things.
He went off and he worked five
or six years on barnacles.
He wrote down lots of ideas
about things unrelated to
natural selection,
and he wasn't really jogged out
of this until a letter arrived
in 1858 from Alfred Russel
Wallace,
a young British naturalist who
had,
in a fit of malarial fever,
had the same idea,
in Indonesia.
And Wallace knew that Darwin
had been thinking about these
things, and he sent Darwin a
letter.
And at that point Darwin,
British gentleman as he was,
had to decide whether he would
do the sort of gracious,
honorable thing and let Wallace
have the idea,
or do the honest thing,
which, his colleagues knew,
was that he had already had the
idea.
And what they decide upon is
that they will do a joint
publication.
So if you go to the Biological
Journal of the Linnaean Society
for 1858,
which is in the Yale Library,
you can look up the back to
back papers by Alfred Russel
Wallace and Charles Darwin in
which the idea of Natural
Selection is laid out.
And then Darwin rushes his book
into print.
So he has been working on a
book that was probably going to
be about 1200 pages long,
and instead he publishes an
abstract of it,
which he calls "The Origin
of Species",
which is about 350 pages long.
And it sells out on the first
day, sold all 6000 copies on the
first day, and has remained in
print ever since.
That's The Beagle.
Darwin slept in a hammock in
the captain's cabin,
at the back of the ship,
which rocked horribly.
And that's essentially all I
want to do about the development
of the idea of Evolution.
Basically what I did was I
wanted to give you the feeling
that there was somebody like you
who went out and knew what a
deep problem was,
and happened to have the luck
to get into a special situation
where they were stimulated,
and came up with an idea that
changed the world.
No reason it can't happen again.
So now I'm going to give you a
brief overview of microevolution
and macroevolution.
Here's Natural Selection;
here's Darwin's idea.
If, in a population,
there is variation in
reproductive success--what does
that mean?
Would everybody in the room
raise their hand if they're an
only child?
Look around.
There are about five or six.
How many of you come from
families with two children?
Lots.
How many with three?
Quite a few.
How many with four?
Quite a few,
but not as many as there with
only children.
Anybody with five?
Yes, a couple.
Anybody with six?
No.
If we were, by the way,
in the nineteenth century,
at this point there would still
be lots of hands going up.
What you've just seen is the
amount of variation in
reproductive success represented
by the families in this room.
Variation in reproductive
success basically means that
different families have
different numbers of offspring,
or different individuals have
different numbers of offspring.
Then there has to be some
variation in a trait.
How many of you are under 5'5?
Raise your hands.
How many between 5'5 and 6 feet?
How many over 6 feet?
Lots of variation in height in
this room.
So we got lots of variation in
reproductive success;
lots of variation in height.
There has to be a non-zero
correlation between reproductive
success and the trait.
On this particular trait
there's been some research.
Turns out that taller men have
more children.
I don't know whether that's
just an NBA effect or what that
is but it turns out to be true
in many societies.
So there is a non-zero
correlation between the
reproductive success and the
trait.
Then there has to be
heritability for the trait.
The heritability of height in
humans is about 80%.
So all of the conditions for
natural selection on height are
present in this room.
All you have to do is go out
and have kids and it will
happen.
So if you're ever in doubt
about whether evolution is
operating in a population,
go back to these basic
conditions.
You can always decide whether
it's likely to be operating or
not.
We can turn natural selection
off by violating any of these
four points.
If there's no variation in
reproductive success--
for example,
if there is lifetime monogamy
and a one-child policy,
there will be zero-variation in
reproductive success if
everybody just has one child;
of course some people will
still have zero,
but that's about as close as
you can get.
If there's no variation in the
trait--if the trait is like five
fingers;
there are very few people with
six fingers;
there are some, but very few.
If there's a non-zero
correlation between reproductive
success and the trait;
if there is a zero correlation
between reproductive success and
the trait.
We'll go into all the
conditions for that.
That results in neutral
evolution.
Okay?
Then things just drift.
Well have a whole lecture on
that.
Or if the trait is not
heritable, if there's no genetic
component to it,
then it won't evolve.
So Natural Selection-I wonder
why it's doing that?
Sorry- Natural Selection does
not necessarily happen.
It only happens under certain
conditions.
Essentially in this picture,
this is what I've just told you
about Natural Selection.
If there's variation in the
trait, represented on the
X-axis,
and there's variation in
reproductive success,
based on the Y-axis,
and there is a correlation
between the two,
represented by the fact that I
can just about draw a straight
line between these points,
Natural Selection will occur
and it will push the trait to
the right.
If all of these conditions,
except the correlation,
occur--so you have variation in
the trait,
variation in reproductive
success but no correlation--
then you get random drift.
And these two situations result
in radically different things.
This situation produces
adaptation, it produces all of
the fantastic biology that
you're familiar with.
It's produced meiosis;
it's produced your eye;
it's produced your brain.
It's extremely powerful.
This situation on the right,
the random drift situation,
is what connects microevolution
to phylogenetics,
and it's what allows us to use
variation in DNA sequences to
infer history.
And I'll get to that.
That statement right now is
opaque.
Don't expect that one to be
transparent at this point.
But two or three lectures from
now I will go into that in
detail and you will see that we
need to have a process of drift
in order to generate a kind of
large-scale regularity that
gives us timing and relationship
in macroevolution.
So both are driven by variation
in reproductive success.
The difference is in whether
there's a correlation between
the variation of the gene or the
trait and the variation in
reproductive success.
If we have strong selection,
we can get pretty amazing
things.
I could illustrate adaptation a
lot of different ways.
I could do it say with the leaf
cutting ants that were the first
farmers;
they domesticated a fungus 50
million years ago and have been
cultivating it ever since.
That would be one way I could
do it.
I could do it with the
exquisite morphology of the deep
sea glass sponges and how
efficient they are at filtering
stuff out of the water.
I could do it with the design
of a shark's body.
Lots of stuff.
I'll do it with bats,
in part because when I was a
Yale undergrad I worked on bats
in this building.
We had a guy that did research
on bats at that time.
Now a lot of bats are
insectivores,
and they will hunt moths at
night, in complete darkness.
They do it with sonar.
The bat only weighs about say
50 to 100 grams,
and it is making a sound that
is as loud as a Metallica
concert when you're standing
right next to the lead guitar's
speaker system.
Okay?
Or it's as loud,
if you like,
as a Boeing 747 taking off from
a runway.
It's this tiny little creature.
It's making an incredibly loud
sound.
It's 130 decibels.
It does that because the
intensity of sound,
the amplitude of sound,
decreases with the square of
distance,
and it needs to detect an echo
coming back from the moth.
The echo coming back from the
moth--
which by the way it can pick up
at a distance of about 20 feet--
is about a million times less
loud,
and it's only coming in about
one to two milliseconds later.
So imagine, there you are,
you've gone "woo"--
except a lot louder than
that--and milliseconds later you
hear "click",
and you haven't deafened
yourself.
That's exquisite.
It has all kinds of physiology
in its ear to hear the returning
echo,
and it can actually discern
whether or not it's looking at a
kind of a fuzzy moth or a smooth
beetle.
The moth has all kinds of
adaptations to try to get away
from the bat.
It hears the bat.
The bat's cruising around,
the moth hears the bat.
The moth goes into a desperate
spiral, diving towards the
ground--okay--the bat starts to
swoop in.
There is a mite that lives in
the ear of moths.
I think you begin to understand
the problem that this mite has.
If the moth gets caught,
the mite will be eaten.
The mite's solution?
It only lives in one ear.
If you collect moths and you
look for mites in their ears,
you will find that they are
always only on one side.
So the moth always has a clear
ear so it can hear the bat.
There's stuff like this all
through biology.
There's another kind of a bat,
called a Noctilio,
hunts fish.
A Noctilio basically detects
ripples in the water surface,
and then it swoops down and it
gaffs the fish with its hind
legs.
It can detect a wire 1/10^(th)
of a millimeter in diameter,
sticking 1/10^(th) of a
millimeter above the water
surface.
When I was taking care of bats,
I'd never seen a Noctilio.
I thought, "God,
this must be the greatest bat
in the world."
About four years ago,
on the Amazon,
my wife and I went out in a
canoe, at sunset,
on a lake, just off the Amazon
River.
It was starting to get dark.
All day long the kingfishers
had been fishing on that lake,
and during the day the lake had
gotten covered with a lot of
food that the fish wanted,
but they were afraid of the
kingfishers.
As it got darker the
kingfishers couldn't hunt
anymore and the whole surface of
the lake dimpled with the fish
coming up to eat the food.
So their timing was exquisite.
They knew exactly how dark it
had to get before they were
safe.
The fish came up and started to
eat the food.
At that point--it was just
shortly after sunset--the bat
falcons were still stationed
around the lake.
You could see,
up on the trees,
falcons sitting up on the limbs
and making flights off of the
limbs.
About 15 minutes after the fish
started to eat,
it got dark enough so that the
bat falcons couldn't hunt
anymore,
and at that point Noctilio came
out,
and the water was covered with
hundreds of bats that were
catching the fish.
They were catching the fish
within a meter of us.
Now there are a couple of
things about that story that I
think, uh, I'd like to
underline.
One is that that entire
community is exquisitely
adapted.
Every element in it knows when
everything is going on and what
the risks are,
and what the costs and the
benefits are.
The other thing is that I had
benefited from a liberal
education,
and when that bat came out,
and was flying around a meter
away from my canoe in the
Amazon,
my life was so much richer
because I had been waiting to
see it for 40 years.
I had heard about it in a
course at Yale.
I knew where it fit in.
I knew what kinds of
adaptations it had,
and boy was I happy to see it.
So adaptation can be impressive.
Drift is something that
actually appeals to the geeks
among us.
I have a geeky side too, okay?
Drift isn't such a
morphologically or artistically
beautiful thing.
It's a mathematically beautiful
thing.
Drift happens whenever there is
no correlation between
reproductive success and
variation in a trait,
and it produces patterns like
this.
So here we start off with 20
populations,
and we start them all with a
gene frequency of 0.5,
and we let meiosis--which is
like flipping a fair coin--
and we let variation and
reproductive success take their
course,
and we just run these
populations for 20 generations,
and you can see that there's
just about an equally likely
distribution of end-states out
here.
So we all start off at 0.5,
and it gets noisy as we go
along.
So this is an image of the
process of drift,
and if any of these populations
happens to get up to 1,
or down to 0,
in terms of gene frequency,
the process will stop,
because those are absorbing
states.
If the frequency becomes 1,
then everybody's got it and
there can't be any change,
and if the frequency becomes 0,
then nobody's got it and there
can't be any change.
So that's what's meant by
absorbing state.
Now to a first approximation,
whole organism traits are the
products of Natural Selection.
Maybe not in the immediate
past, but usually at some point
in the history of life,
a whole organism trait will
have been under Natural
Selection.
So it will have been shaped and
designed by this process.
And to a first approximation,
a lot of DNA sequences have
been shaped by drift.
So we see design in the whole
organism and we see noise in the
genome--to a rough cut;
lots of exceptions.
There are DNA sequences that
have clear selective value;
in fact, there's a whole
literature on this now.
If any of you want to write an
essay on signatures of selection
in the genome,
you can find lots of stuff on
that now,
on how to recognize that a
chunk of genome has recently
been under selection.
There are whole organism traits
that have no apparent selective
value;
for example, the chin.
The chin actually is the result
of evolution,
operating on development,
to take a face,
which is like that of a gorilla
or a chimpanzee,
which bulged out like this and
essentially flattened it out;
so that we are vertically much
flatter than a chimp or a
gorilla,
and as a result of this being
pushed back,
something that was there,
but kind of covered up,
stuck out.
So that's where the chin came
from.
That doesn't mean chins were
selective.
Now it may be that after they
originated, that there could've
been a little bit of sexual
selection operating on them.
But certainly the developmental
process that originally produced
them didn't have to be adaptive.
It could just be a byproduct of
something that was going on,
basically from the mouth up.
So the themes of microevolution
are selection and drift.
Natural selection is driven by
variation in reproductive
success.
The strength of selection is
measured by the correlation of
variation in a trait with
reproductive success.
When there's no correlation,
there's no systematic change,
and then things just drift,
okay?
Now macroevolution;
the big scale process,
the big picture.
Well here are sort of the basic
statements about macroevolution.
If anybody asks you,
"What does this fancy word
macroevolution
mean?", tell them basically
this is it.
There's one tree of life.
Everything on the planet had a
common origin.
Everything is related to
everything else,
with the possible exception of
the viruses, which are too small
for us to decide;
their genomes are too small.
The branch points in the tree,
speciation events--that's when
new species were formed.
This history is marked by
striking major events.
There have been mass
extinctions.
There have been meteorite
impacts.
There have been major changes
in the organization of the
information structure of life.
And the biological disciplines
that you may encounter map onto
this timeline.
So actually different parts of
biology study different parts of
this process.
The tree looks like this.
This is the large-scale tree.
So at this scale,
what you see here are the three
kingdoms of life,
which are the bacteria,
the archaea,
and the eukaryotes,
up here;
the root's at about 3.7 billion
years, not million years.
And at one point a purple
bacterium got into the
eukaryotes and became a
mitochondrion,
and at another point a
cyanobacterium got into various
plant lineages,
three times,
and became a chloroplast.
So that's the large scale.
And you're probably searching
around on that to find out where
you,
the most important thing in the
universe are,
and you're way up here,
on a little twig.
Okay?
Now if we blow that up and just
look at the multi-cellular
organisms,
multi-cellularity looks like it
originated around 800 million to
a billion years ago.
And these are the fungi,
these are the things we call
the plants,
multi-cellular plants,
and then off in this direction
we have got a fairly complicated
series of branches that end up
with us being up here.
Okay?
The things that are--this was
done by Tom Pollard,
at MCDB, about five years ago,
and at that point the things in
yellow had genomes that had been
completely sequenced.
Now there are hundreds of
completely sequenced genomes.
So for the first two billion
years of life most of the action
is down in the basal radiation.
So going on with bacteria,
archaea and eukaryote ancestor;
single-celled things.
At that scale--we're just way
up at a small twig on the
tip--and symbiotic events
brought mitochondria and
chloroplasts into eukaryotic
cells.
Already this is telling you
something interesting about
yourself.
You are a community of genomes.
You are not a unitary genome.
You've got that mitochondria in
you.
The main themes are basically
that the speciation events that
have occurred,
particularly over the last
billion years or so,
have created a tree of life
that describes the relationships
of everything on the planet.
Systematic biology,
phylogenetics,
tries to infer the history of
life by studying those
relationships.
And there's a real deep issue
here of how do we infer the
tree?
The tree--organisms don't come
with a barcode on their
foreheads telling us who they
are related to.
We have to try to figure out
who they're related to,
and when we understand the
relationships,
then we know the history,
because the relationships
define the history.
So we work with hypotheses
about history,
and we test these hypotheses
against each other and try to
come up with the one that's most
consistent with the data that
we've got.
And they give us a historical
framework within which we can
then interpret what's happened.
There are major events that
have happened.
Briefly these are they.
Life originates about 3.6 to
3.9 billion years ago.
And, by the way,
it seems to have originated
fairly quickly.
Within probably about 100
million years--
see I'm being an evolutionary
biologist again--
within just a hundred million
years,
uh, after water could exist on
the surface of the planet in
liquid form--
so following the meteorite
bombardment,
when the surface of the planet
cools down enough for water to
be liquid--
life probably originates pretty
quickly.
And arguably,
within the first hundred
generations, the first parasites
were around.
So those things happened pretty
quickly.
Then eukaryotes and meiosis,
which is how a biologist refers
to organized sex,
happened about 1.5 to 2.5
billion years ago;
multi-cellularity,
which gives us developmental
biology, about a billion years
ago.
All the major body plans for
animals appear to have,
with the exception perhaps of
the, uh,
jellyfish and a few of their
relatives,
they all seem to have
originated about 550 million
years ago.
There was a near loss of life
on the planet in the Permian
mass extinction.
We will study that later in the
course.
You're welcome to write an
essay on mass extinctions if you
want to;
you know, big death is kind of
exciting.
It seems to have occurred
basically by a process of
poisoning of the oceans.
The flowers radiate about
between 65 and 135 million years
ago.
Language is important because
once language occurs,
then we have an independent
kind of information transmission
from generation to generation;
we get cultural transmission.
That's probably about
60-100,000 years old;
at least with syntax and
complicated information storage.
Writing is only about 6000
years old.
And of course the important
stuff is quite recent.
So this is a view of life that
goes from bacteria to dinosaurs
to rock and roll;
and that all can be studied
with evolutionary principles.
How do the biological
disciplines map onto this?
Well microbiology and
biochemistry try to study things
that are common to all life.
That means that the same
chemical reactions that go on in
bacteria go on in the human
liver,
and that's about one-and-a-half
to four billion years old.
Okay?
Genetics and cell biology study
stuff that follows the
evolutionary invention of
meiosis;
to a large degree.
There is bacterial genetics,
but eukaryotic genetics is
something which is studying
things that are about 1.5
billion years old.
Developmental biology and
general physiology,
those are multi-cellular
disciplines;
they depend upon the existence
of a multi-cellular organism.
That thing didn't come along
until about a billion years ago.
Neurobiology,
you need a complex--you need
cephalization--you need to have
a complex nervous system.
That studies phenomena that are
probably about 500 to 600
million years old.
Same for behavior.
There are several
anthropologists in the class.
You guys are studying things
that probably originated along
our branch of the tree,
within the last 15 to 20
million years.
So there is a temporal assembly
of biology, as a discipline,
as well as there is of life,
on the planet.
So the key concepts from this
lecture are that there are two
kinds of explanation in biology.
One is the proximate or
mechanical question,
which is answered by studying
how molecules and larger
structures work.
Those are basically physical
and chemical explanations.
And then there are the
evolutionary questions,
which is why does the thing
exist;
why did it get designed this
way?
And that could be answered
either through selection or
through history;
or the best way to do it is to
use both and combine those
explanations.
The thing that distinguishes
biology from physics and
chemistry is Natural Selection.
This is not a principle that
you can find in a physics
textbook or in a chemistry
textbook.
This is something that is a
general principle that actually
applies to lots of things
besides biology,
but it's not contained within
physics and chemistry.
And there is a pattern in
biology that unites biology with
geology and astronomy,
and that's history.
So there is an important
element of historical thought in
evolutionary biology,
as well as the more abstract
action of natural selection on
designing organisms for
reproductive success and shaping
changes and gene frequencies.
Now I want to end the lecture
by telling you something
astonishing.
I won't always be able to tell
you something astonishing in
every lecture.
But one of the great privileges
of teaching Introductory
Biology,
or being in an Intro Bio class,
is that there are certain big
things that never get discussed
again.
Okay?
This is one of them.
We are continuous with non-life.
Here's how I'm going to
convince you of that.
Think of your mother.
Now think of her mother.
Now think of your mother's
mother's mother.
Now I want you to go through a
process like you've done in math
where you do an inductive proof;
you just go back.
Just let that process go.
Okay?
Back you go in time.
Speed it up now.
Okay?
We're back at ten million.
Now we're at a hundred million.
Now we're at a billion years.
Now we're at 3.9 billion years.
Every step of the way there has
been a parent.
3.9 billion years ago something
extremely interesting happens.
You pass through the origin of
life, and there's no parent
anymore.
At that point you are connected
to abiotic matter.
Now this means that not only
does the tree of life connect
you to all the living things on
the planet,
but the origin of life connects
you to the entire universe.
That's a deep thought.
Every element in your body,
which is heavier than iron,
and you need a number of them,
was synthesized in a nova,
uh, supernova.
The planet that you're sitting
on is a secondary recycling of
supernova material,
and your bodies are constructed
of that stuff and they use it in
some of their most important
processes.
So the vision that evolutionary
biology gives you is not only
the practical one of how to
think about and analyze how and
why questions in biology,
it's also a more general
statement about the human
condition,
and I hope it's one that you'll
have time to reflect on.
Next time we'll do basic
genetics.
