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7. Evolution, Ecology and Behavior: The Importance of Development in Evolution
Poziom:
Temat: Nauka i technologia
Prof: Today we're going
to talk about,
or we're going to introduce the
role of development in
evolution,
and I would like to start by
asking you to make two jumps in
your head.
In the first case I want you to
think of yourself as inside a
single-celled bacterium.
If you go to a good book on the
gene networks and the
biochemical networks that can be
found inside a single-celled
bacterium,
you will be stunned by the
complexity of it and you will
deeply hope that you never have
to reproduce it on any
examination,
because there is so much of it.
Okay?
That's inside one cell.
Development doesn't arise until
we get to multi-cellular
organisms.
And if you were able to go into
the body of a multi-cellular
organism,
such as yourself,
and look at all the signaling
pathways,
all the way that information is
transferred and integrated in a
multi-cellular organism,
it's just as complex as the
whole picture down inside that
one single-celled bacterium.
So there are two huge kinds of
orders of magnitude,
levels of hierarchical
complexity of information
integration that happen in a
multi-cellular organism like
yourself.
We call the way that the
information in the genes maps
into the structure of the
genotype the genotype-phenotype
map.
It takes us through all of that
complexity to produce something
that we can then try to
understand, which is a whole
organism.
Another name for the
genotype-phenotype map is
development.
Okay?
And what I'm going to try to
show you today is that out of
this almost unimaginable
complexity,
two hierarchical layers of
orders of magnitude of
information,
biologists have been able to
extract some interesting simple
rules and show that there are
some large-scale patterns in
evolution.
However, that task is far from
done,
and the understanding of the
genotype-phenotype map,
or the understanding in general
of developmental biology,
remains probably the most
pressing issue in basic research
in biology for the twenty-first
century.
It's something that occupies
some of the best faculty in this
department and some of the best
scientists across the globe.
So it's a basic issue.
And the thing that I hope you
take home from this lecture
today is that it's important,
both for microevolution,
and for macroevolution.
Today I will be talking about
patterns that are pretty large
scale, they are more
macroevolutionary;
and next time I will be talking
about patterns that arise within
populations that reflect that
macroevolutionary history,
but that have immediate
consequences for microevolution.
So development isn't simple.
It's going on both at large
scale, over long periods of
time,
producing patterns,
and it's going on at a very,
very short scale in every
generation,
as each individual grows up and
turns into an adult.
So what's involved in
development?
It isn't really just the
production of the adult form
from an egg.
It is the living of the entire
lifecycle,
from the formation of the
gamete, through the adult,
through all the changes the
adult goes through until it dies
and produces the next
generation.
So development refers to the
entire lifecycle,
and evolution shapes the entire
lifecycle.
So something that we get from a
very important discovery in
nineteenth century biology is
that all of life is made of
cells.
It could've been organized
differently,
and in fact there are
interesting science fiction
novels,
like Solaris,
by the great Polish science
fiction novelist Stanislaw Lem,
that conceptualizes what life
would be like if it were not
cellular.
For example,
what if the whole ocean were
one living thing?
But we know that that's not the
way life is on our planet.
On our planet life is all built
out of cells,
and that means that the problem
of development is a problem of
communicating between cells.
And cells are all set up as
information signalers and
receivers.
They have cell adhesion
molecules on their surfaces.
They produce information
molecules, hormones,
and other signaling molecules
for export.
This information is used to
change the fate of a cell.
Now every cell in your body has
all the information in it that's
needed to build you,
and that is true of virtually
every organism.
The only exception in us is our
red blood cells because they
don't have any nuclei,
so they don't have any DNA in
them.
Okay?
But in every other organism
that we know of,
with minor exceptions like our
red blood cells,
all the information is in all
the cells,
and that means that development
is a matter of editing,
it's a matter of determining
which information gets turned on
in the right place at the right
time.
So the evolution of development
is about shaping those patterns
in space and time,
within the framework of an
organism,
to produce something that works.
Development does a bunch of
things in evolution.
One of the important ones is
that it has a strong role in the
course of the production of
individual organisms in shaping
the kind of variation that is
presented to selection.
Okay?
So the developmental mechanisms
that are shared by particular
organisms determine that only
certain kinds of phenotypes are
going to be produced--
and there's a lot of variation
within those phenotypes--
but they are a tiny portion of
phenotype space.
And this is why we actually
see, morphologically,
the Tree of Life.
This is why dogs look like
wolves.
This is why humans look like
chimpanzees.
This is why birds look like
birds and we can call them
birds.
It's because they share
developmental pathways that have
been inherited from ancestors
and that have constrained the
range of phenotypes that can be
presented to selection.
Now there's another important
thing about development.
You might think that you could
conceive of the body as produced
by an engineer,
but it's not really constructed
that way in evolution.
What goes on is that genes can
only build organisms out of the
materials that are available at
a certain time,
and then there is an
evolutionary memory,
of which materials are
selected, and of the control
systems that are used to shape
the phenotype with them.
Now I'll give you a couple of
examples.
What is the cell membrane?
The cell membrane is a lipid
bilayer, and it's been--it's
actually a marvelous organ now;
it has all sorts of special
channels in it,
things that are filters to let
particular stuff in and keep
other stuff out.
It's been heavily modified by
evolution.
But there is no way that you
can take simply the DNA sequence
in the genome and get a reaction
system that's going to construct
cell membranes.
All known cell membranes are
actually constructed
biologically by using
pre-existing cell membranes as
templates.
In other words,
the cell membrane itself is an
information transfer molecule.
So that's one.
There's another,
and that's bones.
Your bones are made out of a
material called hydroxyapatite,
which is a calcium phosphate
material.
And hydroxyapatite has the
following extremely convenient
feature.
If you take hydroxyapatite and
you put it under stress,
it will strengthen itself in
the direction of the stress.
That means that the genes don't
have to have sensor systems to
detect stress,
and they don't have to worry
about how they're going to make
a bone strong in the direction
of stress.
All they have to do is say,
"Hey,
I'm going to use hydroxyapatite
to make bones,
and then when that baby first
starts to toddle around and walk
on its legs,
its hips start getting
strengthened in the direction of
stress."
Now, in fact,
there are modifier genes that
then take this and use it to
their own advantage,
by building in protein
molecules that strengthen the
bone in the direction of the
stress.
Okay?
But the initial signal,
which direction is the stress
coming from, is a freebie.
It's given by the biochemical
properties of a hydroxyapatite.
So that's one, a second one.
Here's a third one,
gastrulation.
When vertebrate embryos,
and many other embryos,
grow, they grow up as a ball of
cells which then becomes a
hollow sphere of cells,
and that hollow sphere of
cells, which by the time it
gastrulates has thousands of
cells in it.
You can think of it as a little
pulsing basketball.
Okay?
It's a little sphere that's
pulsing,
and if you simply let this
thing grow to a certain size,
the tension in the actin
tubules in the cells will cause
it to spontaneously invaginate.
So it'll get a dimple in it,
like you pushed your thumbs
into it.
This happens spontaneously.
It is not as though the genes
have to say, "I am going to
construct a mechanism that's
going to make my gastrula form.
When my blastula turns into a
gastrula,
when my hollow ball of cells
turns into a thing that's got a
dimple in it,
and then forms three cell
layers, out of which I can make
muscles and bones and skin and
gut and all of that kind of
stuff,
when that happens that's a
freebie."
Okay?
That is just in the tension of
the actin filaments in an
expanding ball of cells.
So these are some of the
properties of the biological
materials that organisms are
constructed out of,
and it means that on the one
hand the genes don't have
complete control over the
phenotype,
but on the other hand they are
given certain things by the
materials that don't have to be
specified in the DNA sequence.
So where does development fit?
I'm giving you real large-scale
messages now.
Okay?
The biological disciplines that
we call Ecology and Behavior
actually deal with the processes
that reduce the cohort of
newborn organisms to the ones
that survive to reproduce.
Okay?
Ecology and Behavior actually
study the mechanics of natural
selection, while they study a
lot of other stuff as well.
But that's the level at which
that happens.
Genetics takes the genotypes of
the parents and it transforms
the genotypes of the parents
through Hardy-Weinberg
equations,
through the selection that's
operating on them,
through all of that stuff that
we just looked at quickly.
It takes the genotypes of the
parents and transforms them into
the genotypes of the offspring.
Okay?
So genetics is all about
information transfer.
Lots of people that like
computers do pretty well at
genetics.
What development does is it
takes that information in the
genotype and it maps it into the
material of the phenotype.
You can think of development as
being a big transduction
mechanism that takes
material--takes information and
turns it into material.
Okay?
And, in the process,
it places limits on what the
phenotypes can look like,
so that not every conceivable
phenotype is going to arise out
of the DNA sequence in a genome;
only a certain range.
Flies are going to look like
flies, sheep are going to look
like sheep, and daffodils are
going to look like daffodils.
If we look at what development
has been able to produce,
well.
in a large-scale it has
produced some very basic things,
and we can see that by a
comparison of the body plans of
the major groups of organisms.
I'd just like to take a moment
here and see if I can elicit
from you some sense of what it
is that we're looking at.
Okay?
I assume that you're all pretty
comfortable with chordates,
because that's what you are.
Okay?
Can anybody tell me what a
bryozoan is?
A bryozoan is a moss animalcule;
it is a moss animal.
Bryozoans produce beautiful
exoskeletons,
and you can find them on
tropical reefs.
Anybody comfortable with what a
priapulate is?
A priapulid is a deep-sea worm
that forages with a tentacle and
when you pull it out of its
hole, it looks like a penis;
which is why it's called a
priapulid.
Okay? What about a tardigrade?
Tardigrades are little water
bears.
Okay?
They look a little bit like
insects or crustaceans,
and they're very tiny,
and they're extremely cute.
Okay?
So tardigrades are water bears.
Arthropods, who's in the
arthropods?
Insects are arthropods.
What are the other big groups
of arthropods?
Student:
>.
Prof: Crustaceans;
yes spiders,
spiders and their relatives.
Arthropods are anything with
jointed legs--that's just Greek
for jointed leg,
arthro-pod.
Pogonophorans?
A tiny phylum of worms that
live in the sand and are living
fossils and haven't really
changed their morphology for
about 400 million years.
So there's a lot of big-scale
stuff here.
This is basically the animal
kingdom.
Okay?
And it's formed into these
groups.
And just to give you a little
bit of timeline,
it's about 600 million years,
700--Bilateria,
yeah, six or seven-hundred
million years ago.
A lot of stuff happens really
quickly, because from this point
to this point here is only about
a hundred million years.
At this point we're still 500
million years ago.
Okay?
So this is big-scale stuff.
The cnidarians and the
ctenophores formed trace fossils
in the pre-Cambrian ooze.
So they may go back a billion
years;
not sure about how far back.
So if you look at what happened
when multi-cellular organisms
formed and one branch of them
went off to become animals,
this is what development was
able to produce.
It could produce body
axes--front and back,
left/right;
produce a skeleton,
organ system,
symmetry and cell layers.
And figuring out the shared
general mechanisms by which
development can produce those
things,
and then how evolution can
tweak them to make them
different in these different
groups,
is evolutionary developmental
biology,
or evo-devo.
There are some big and striking
differences among these groups.
For example,
this group up here has an
exoskeleton, and this group down
here has an endoskeleton;
and that places very
fundamental constraints on
growth, size,
all kinds of things.
What can it do in plants?
Well here is an extremely
sketchy view of the plant world.
And, by the way,
I would've used a much more
complex view of the plant world
than this one,
but this is the one which is
publicly okay without copyright
protection.
Okay?
So for you plant biologists
that are worried about the view
of the plant world,
this is pretty simple.
Basically what this does is it
takes you from ferns and their
relatives through cycads,
ginkgoes, pine trees and fir
trees;
the gnetophyta,
which have some really cool
plants that live in Namibia;
Wellitschia and other things
like that are gnetophytes.
And then the magnoliophyta are
all flowering plants,
going out here.
So this is a huge group down
here.
And if you look at that,
what has development and the
evolution of development been
able to produce?
Well these guys are all
variations on the following
themes: a meristem-root axis;
where xylem and phloem appear;
where wood appears;
the kinds of branching patterns
they have;
whether they have naked or
covered seeds;
whether they have leaves;
and whether they have flowers.
So you can see that the image
that I'm trying to create here,
in both plants and in animals,
is that there's certain shared
general features,
and that what evolution has
done is that it has made many,
many different combinations of
those features,
to create the diversity that we
see,
and that this is done through
the evolution of development.
Mechanistically a lot of this
is going on at the level of gene
regulation;
and this is a recap of the
structure of the eukaryotic
gene.
And the parts that I want you
to focus on right now are the
promoters and the enhancers.
So these are parts of the DNA
molecule that receive a signal,
which says turn this gene on or
turn it off.
And I want to show you just
exactly what kind of a network
of information that results in.
You can think of there being
regulators--
and we will talk a little bit
about what kinds of regulators
are used early in development--
and that these then feed into a
signaling cascade where a signal
goes out,
there's a receiver,
there's a transducer that turns
that signal into a transcription
factor.
A transcription factor then is
going to go out and it's going
to bind to the enhancer region
of a gene, or a promoter region
of a gene.
Okay, so it's bringing in a
signal.
The signal might be either turn
it on or turn it off,
but it's bringing in a signal;
that's what a transcription
factor will do.
You can think of there being
other pathways and other signals
going through them,
and they can turn on
transcription factors,
and some of those will come
down and sit down on the same
gene.
It isn't all only one
transcription factor that can
sit down on one gene.
In an average gene in
Drosophila there are about ten
to twenty binding sites in its
control region.
That means that ten to twenty
different transcription factors
can sit down on a single gene,
and one transcription factor
can bind to the control regions
of anywhere from one gene to
several hundred genes.
Okay?
There are about 13,000 genes in
a drosophila genome.
So that gives you some idea of
the array of possibilities of
turning all those things on or
off, as you go through.
That is a huge array of
possibilities,
and I think you'll see,
when I show you later,
how you take an onychophoran
worm and turn it into a
Drosophila,
that you could imagine
evolution might have had to use
a lot of them,
in order to turn something like
a worm into something like a
fruit fly.
There is another way to think
about this.
Think about the control region
of a gene as the keys on a
piano,
and think about the
transcription factors as the
fingers on the hand of the
person who's playing the piano,
and think about evolution as
the composer who wrote the
score.
You know perfectly well that
you can play all kinds of tunes
on the keys of a piano,
and you know perfectly well
that there are traditions in
music of different kinds of
related music.
So anyone who has listened to
Bach is probably going to
recognize Telemann,
and people who have listened to
Schoenberg are going to
recognize more modernist
composers.
So you can think of those as
being clades,
and you can think of the
cultural tradition as being
inherited,
and there's constraining
variation within the range of
scores that are composed on the
piano.
Of course, what we're dealing
with in a genome is like the
biggest symphony orchestra you
ever saw in your life.
Okay?
So it's much,
much bigger than that.
So a few points about the
control of development.
At the beginning of
development, when the first cell
is getting set up to divide,
and in the formation of the
very early multi-cellular
stages,
there are concentration
gradients that are produced.
So before the first cell
divides, there will be like a
front end and a back end of the
cell,
and chemistry will get set up
to produce molecules that then
form a concentration gradient
across the cell,
and the concentration of those
molecules is positional
information on what's the front
and what's the back.
And as the cell divides,
it retains that information on
where it is in the front or the
back.
That's how the Drosophila
embryo is set up.
And we will also see that this
kind of concentration gradient
is used in the construction of
the vertebrate limb.
By the way, the signaling
center on the vertebrate limb,
when it's just a little paddle
of cells, is basically in the
armpit.
Okay?
So if you want to think of
smelly molecules being produced,
think armpits.
What then happens is that
transcription factors are used
to define specific areas where
only a precise subset of genes
is expressed.
So remember,
all the info in the whole
genome is in every cell.
You only want a certain subset
for this part of the organism
that you're making.
So the gradient,
the chemical gradient then sets
up gene expression,
and the transcription factors
appropriate to that position get
turned on.
Genes are regulated by
combinations of activators and
repressors,
and this combinatorial control
is what gives you the huge
diversity of cell specific gene
expressions.
When I say combinatorial
control, think of composers
writing notes and people playing
pianos;
that's combinatorial control
too.
All the music that's ever been
written, that can be played on a
piano, is simply a variation on
all the combinations of those
keys in space and time.
So, the control can get complex.
It can be a cascade of
information, and genes that
produce transcription factors
can be regulated by genes that
produce transcription factors.
And this sets up situations
where genes can switch their
roles.
It is not correct to think that
there are some genes that are
early development genes,
and then there are other genes
that are adult genes.
Genes are used flexibly,
in many different contexts,
depending upon the information
that's coming in to regulate
them.
Certainly there are some genes
that are quite important in the
embryo, but it turns out they
also play a role in the adult.
We'll see a case of this a
little bit later.
Not surprisingly it's the genes
that determine the general
pattern that switch on first,
and the ones that are
controlling detail switch on
later.
So when I say 'general pattern'
I mean--
for example,
in the vertebrate embryo,
the front and the back,
the top and the bottom,
the left and the right:
that gets laid down first.
Then the embryo gets chopped up
into a sequence of segments.
Some of them turn into the
head, some of them turn into leg
segments;
some of them have extremities,
some of them don't--that kind
of thing.
Okay?
So this sequence of how the
early general pattern gets set
up,
and then how the details are
developed,
that's all developmental
genetics, and that's produced in
evolution a whole lot of stuff.
Now, a little bit of vocabulary.
You're going to hear,
in this area,
about boxes.
Okay?
You're going to hear about
homeoboxes, MADS boxes,
stuff like that.
I want to tell you what boxes
are.
They are very highly conserved
sequence motifs,
and they are found in the DNA
that codes for a particular
family of transcription factors.
The reason that this
sequence--by the way,
it's about I think--I think
it's about--
I'm not sure whether its
seventy to eighty codons,
or seventy to eighty
nucleotides long.
But these boxes are not too
long.
They're conserved because they
have a very important function,
and that is to bind to the DNA.
So they have a helix twist,
helix structure,
and that means that if the DNA
molecule is here,
this part of that protein,
this part of that transcription
factor,
is going to fit right into it.
And it's because they are
transcription factors--the boxes
are found in transcription
factors--that this is a very
conserved interaction.
Because DNA hasn't changed its
structure in three billion
years.
So if they're going to bind to
it, they have to have that
structure, and so selection has
made sure that that sequence is
preserved.
They're called boxes simply
because if you lay these DNA
sequences out,
if you sequence a lot of DNA,
and you are looking for one of
these things,
what you find is that wherever
there's a transcription factor,
there is a stereotypic sequence.
And the people who were
analyzing this,
first on computer printouts,
or now with imaging on computer
screens, drew boxes around them,
to locate them.
That's why the word 'box.'
Okay?
So when you see one of these in
a DNA sequence,
you know you probably have a
gene for a transcription factor.
Here's the homeobox family.
Okay?
There are thirteen homeobox
genes that have been identified.
And this is from a number of
years ago;
this has probably been filled
out now.
And they have two--well there's
more than two striking things
about them.
But the first thing that's
striking about them is that
they're deeply conserved.
That means that they have
retained so much of their
sequence identity that you can
recognize homeobox gene 1 in a
human,
and you can see that the same
gene is there in flatworms and
in earthworms,
in priapulids and so forth.
All the way through the animal
kingdom this gene has been
conserved, and you can pick up
something like it in cnidarian.
And you wonder,
well how did there get to be
thirteen in this family?
Well here you can see a gene
duplication event right here.
Homeobox gene 1 in the
jellyfish and corals was
duplicated right here--
this was the expansion of the
central HOX genes--
and it happened here as well,
so that now there were two
copies.
And that meant that this
developmental control switch,
which was an extremely clever
piece of machinery to have
around, now existed in two
copies.
You could use the first one to
do whatever it used to be doing,
and you can now evolve a new
function for the second one.
This went on up until the time
that the vertebrates started to
evolve.
And in our closest relatives
there's one copy of the HOX
gene, and it was duplicated
twice.
It was duplicated at the level
of the Agnatha;
so the ancestors of the sharks.
And that means that all of the
vertebrates, the higher
vertebrates, have four sets of
developmental control genes.
Interestingly,
the first set is still used to
lay down the major body axis,
and the fourth set is used to
make a limb;
that's the new function.
Okay?
Now they have deeply conserved
sequences, but they are also
collinear.
By collinear, I mean this.
Look at the sequence on the
genome, and look at what part of
the body is controlled.
The parts that are on one end
control the head area,
the parts on the other end
control the tail area,
and the parts in the middle
control the stuff in the middle
of the body.
There isn't any logical reason
why it had to be this way.
It's probably simply that when
vertebrates first started--
well when animals,
prior to vertebrates,
first started to get formed as
multi-cellular things,
this happened to be one
convenient way to control
development.
But logically speaking,
giving the signaling apparatus
that's available in the genes,
there's no reason logically
that the genes have to be
collinear;
but it's a fact that they are,
and it's a marvelous fact that
they are.
Okay?
This is just to show you that
you can take homeobox genes and
look at their DNA sequences and
say,
"Oh, they have similar DNA
sequences."
And then look at what parts of
the bodies they control and see
that a homeobox gene in a fly,
that is homologous to a
homeobox gene in a mouse,
is controlling a similar part
of the body.
So the things that are
controlling the tail end,
the green genes here,
are expressed in this part of
the fly and in this part of the
mouse,
and the things that are
controlling the head end,
that are expressed here in the
fly,
are actually expressed here in
the mouse.
That's kind of interesting,
because it suggests that the
mouse has added on some stuff
that is in front of the hind
brain.
So here's the vertebrate limb.
Okay?
And this is controlled by the
fourth copy of the HOX genes,
the D copy.
And it shows you that if you
have, for example,
D9 being the only one that's
turned on, you get that.
If you have D9 and D10,
you get that.
You have D9,
D10, D11, you get that.
You have D9 through D12,
you get that.
And you get all five of these
on and you get that.
Okay?
So basically what this means is
the following.
If you just have D9 on,
make a shoulder;
D9 and D10, make a humerus;
D9 through 11,
make a radius and ulna;
D9 through D12, make a wrist;
and all five, make fingers.
How simple, how logical.
Remember when I said at the
beginning we have these orders
of magnitude of complexity
within cells,
and these orders of magnitude
of complexity between cells.
Out of all of that complexity,
this simple pattern emerges.
Oh, and I forgot,
notice that the genes are
collinear in the limb.
Okay?
The ones that are on one end of
the gene are controlling the
shoulder, and the ones that are
on the other end of the gene are
controlling the fingers.
So it's still collinear.
It's the like the body axis,
it's just been translated into
a limb.
Well what about flowers?
The MADS genes also have a
sequence in them which shows
that they're a transcription
factor;
there's a MADS box.
M-A-D-S is an acronym for the
original names that these genes
had.
Okay?
Some of them started with an m,
some with an a,
some with a d,
some with an s.
Then after all that had
happened, it was noticed that
they were related.
So people started calling them
MADS genes.
They're scattered throughout
the genome.
They are not collinear.
Okay?
In Arabidopsis,
they're on all five
chromosomes.
There's nothing resembling the
HOX genes in the way they're
genetically organized.
They fall into three groups:
the A group,
the B group and the C group.
So the A is not a single gene,
it's a group of related MADS
genes.
B is another group of related
MADS genes.
And within each of these groups
the genes are sharing
phylogenetic relationship.
That means that the members of
the A group are probably all
duplicates of an ancestral gene;
the members of the B group are
probably duplicates of an
ancestral gene for the B group;
and so forth.
Now the neat thing about the
MADS genes is the way they
control flowers.
And we're going to see that
evolutionary developmental
biology has a lot to do with the
production of beauty.
>
Two of the best understood
examples in evolutionary
developmental biology are
flowers and butterfly wings.
Okay?
So this is an area where
researchers who go out to give
talks get to use a lot of neat
slides.
The ABC model of flower
development goes like this.
If only a gene from Group A is
turned on, make a sepal;
if only A and B are turned on,
make a petal;
if only B and C are turned on,
make an anther;
if only C is turned on make a
pistol and an ovary.
So the regulation of B and C is
controlling male and female
organ development.
This is combinatorial.
Okay?
It's the same general logical
principle.
However, it's with a completely
different set of genes,
and plants evolved
multi-cellularity independently
of animals,
which means that plants
invented development in
evolution independently of
animals.
They both hit upon
combinatorial control as a
simple, logical way to control
development.
That probably means it's a very
good idea.
Okay?
It's a very simple and
economical way of expressing
information.
Of course every gene has a
history, and these MADS genes
were doing something before they
made flowers.
In fact, if you go back and you
look at the homologous genes in
the plants that do not have
flowers,
you discover that in ferns they
are controlling leaf
development,
in conifers they are
controlling cone development;
things like that.
It's not as though these genes
were invented in order to make
flowers.
They were pre-existing,
and they were co-opted by
evolution at the point where
flowers started to evolve;
and gene duplications probably
helped in that process.
So if we go back to the Burgess
Shale, back to the Cambrian,
we find lots of onychophorans
running around.
And if you go to an Australian
rainforest today,
you will find onychophorans
still running around,
and they look the same.
So 500 million years of
evolution hasn't changed
onychophorans.
onychophorans are these neat,
velvet worms.
They are, by the way,
viviparous.
If you pick them up they squirt
glue on you.
They have their own neat little
biology.
They are the ancestors of the
arthropods.
So what evolution basically did
was it took an onychophoran and,
among other things,
it turned them into fruit
flies,
and butterflies,
and horseshoe crabs and king
crabs and lobsters and shrimp.
How do you do it?
Well it was done basically by
changing the range of segments
in which particular things were
expressed.
If we go back here,
you can see that the
onychophoran has a lot of
segments.
It's got one leg on each
segment.
Okay?
The fruit fly has many fewer
segments and only six legs;
the onychophoran has fifty legs
or so.
So basically what was going on
is that the HOX genes were used
to say,
"Oh, okay,
now we're going to say that
these segments become a head,
these segments become a thorax,
these segments become an
abdomen.
We're going to make antennae on
the head,
and we're going to make wings
on the thorax,
and we're going to make legs on
the thorax,
and the abdomen isn't going to
have any wings or legs."
Okay?
The way this initially
happened--and you can find
fossils that replicate some of
these stages--
you first make a generalized
segment,
it's got both legs and wings on
it.
Okay?
You make it many times.
So you've got both legs and
wings on lots of segments.
Then you restrict the range of
expression so that only certain
segments have wings,
only certain segments have
legs.
You do that by altering the
domain of expression of control
genes,
and you use that kind of
combinatorial specificity to
say,
"Okay, this is an antenna
and not a leg.
Okay, it's an appendage growing
out of the body wall,
but I'm going to make it into
an antenna that's on this
segment and I'll make it into a
leg if it's on that
segment."
So, of course,
this goes on in seconds;
evolution took hundreds of
millions of years.
So it's not the same process.
Some of these HOX genes have
retained incredibly conserved
functions.
In a famous experiment that was
done in the 1990s,
Walter Gehring's group in
Switzerland,
took Pax6, which is a gene
that's shared by all bilateral
organisms,
and they genetically engineered
fruit flies with an extra copy
of Pax6--
it is a gene that induces the
development of eyes--
and by turning this gene on,
they were able to make that
fruit fly grow eyes in
unexpected places.
Okay?
So it could grow one on
its--this is the regular eye;
this is an eye growing on an
antenna;
this is an eye growing on a
haltier;
and so forth.
The interesting thing was that
they could do this with Pax6
from a human or a mouse;
in other words,
the DNA sequence in the gene
was so similar that it could be
used to control the
developmental pathway in an
organism in which that gene had
not been sitting for 600 million
years.
That's pretty remarkable.
Development is not easy to
evolve, and I think this gets
across one of the reasons that
it's not easy to evolve changes
in development.
Every organism has to function
and reproduce in order for a
gene to get transmitted,
and you can't tweak its
development around too much or
you'll make it fall apart.
It's like you're driving down
the road and you want to turn
your Volkswagen into a Mercedes
Benz,
and so you get out your tools,
and you're going sixty miles an
hour,
and you want to modify it,
but you can't crash.
Okay?
So that causes constraints;
there's only certain things
that you can do while you're
moving down the road.
These developmental constraints
are not permanent.
The genetic control of
development does change more
slowly than many other things,
but I would submit to you that
if we wiped out everything on
the planet--
let's first duplicate earth ten
million times,
and then let's go through and
wipe out everything on all of
those ten million planets,
except for one species,
and we leave it some food.
Okay?
It's the only thing that's
there.
But on some planets all you've
got is fruit flies.
On other planets all you've got
is redwood trees.
On other planets all you've got
is butterflies,
and on other planets you've got
albatrosses;
but they have a food supply,
they can live.
Give them long enough,
go away in your spaceship,
come back, five billion,
ten billion years later.
I would submit to you that
every one of those planets is
going to have highly diverse
life on it,
and that many of the things
that we see on this planet you
will see on each one of those
other planets.
They will contain a signature,
probably a very interesting
signature, of this huge
disturbance that has been
created on them.
But I think that it's possible
for redwood trees to evolve into
squid.
I just think it takes them a
very long time.
>
The things that change slowly
constrain things that change
rapidly, and genes don't cause
development by themselves.
They're steering the dynamics
of gene products that interact
with environmental inputs.
So the genes actually are a
fair distance away,
biochemically,
physiologically,
from the things they're
controlling.
They are working through
complicated interaction systems.
So, a few take-home points.
Development maps the
information in genotypes into
the material of phenotypes.
It's like the process that
occurs when a blueprint that an
architect has drawn is turned
into a building by the
construction company.
Developmental control genes use
combinatorial logic.
There's a lot of other stuff
that's really important in
evolutionary developmental
biology, besides combinatorial
logic.
It happens to be a pet topic of
mine.
You will find that if you talk
to people who do this for their
profession,
that they regard combinatorial
logic just as so natural,
so much a part of the
landscape, that they hardly feel
the need to mention it anymore.
But it is striking,
when you compare plants and
animals, that they both hit upon
this method of controlling gene
regulation.
The ancestors of currently
existing organisms often had an
awful lot of the genes that are
now involved in controlling
development.
Remember that phylogenetic tree
I showed you for the HOX genes.
Many of them are present back
in jellyfish,
and certainly many more of them
are present in worms and
crustaceans and things like
that.
So a lot of what has gone on in
evolution is changing the
specificity of expression in
time and space,
and the specificity of
receptors in time and space.
Rather than necessarily
evolving new genes that make new
kinds of proteins,
a lot of evolution has been
concerned with making
combinations of existing genes.
Interestingly,
there is a very good
evolutionary developmental
biologist at Wisconsin.
His name is Sean Carroll.
He is a charismatic guy.
And Sean has gone so far as to
say that most of evolution,
in the last 500 million years,
has consisted of the evolution
of gene regulation,
rather than the evolution of
new structural proteins.
And, of course,
by taking an extreme stance,
he has managed to generate a
controversy in which people are
saying,
"Oh no, it's not just that.
There are all of these other
new structural proteins that are
going on."
And this is always very good
for one's scientific career,
because both sides are
increasing their publication
rates.
Now, in fact,
both things have gone on.
Okay?
But by having a controversy,
people get motivated to pin
down the details.
So I am making a little bit of
fun of controversies,
but I also recognize that they
are very strong motivating
forces.
So next time,
we're going to talk about the
expression of genetic variation
and reaction norms.
And I want you to remember
today's lecture,
because today I have talked
mostly about the big macro
picture;
the impact and the patterns of
developmental mechanisms in the
Tree of Life,
in all plants and animals.
And next time we're going to
see what difference they make
within single populations and in
the course of the lifetime of
single individuals.
This is one way that
microevolution connects to
macroevolution;
it connects through development.
Lunch is possible if you would
like it.
