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Living World / Evolution
Discover
Interview
DNA Agrees With All the Other Science: Darwin
Was Right
Molecular
biologist Sean Carroll shows how
evolution happens, one snippet of DNA at a time
by
Pamela Weintraub
published online February 19, 2009
http://tinyurl.com/dna-science-darwin-was-right
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Excerpts:
Click
here to see DISCOVER's special package
on Darwin in honor of his 200th birthday and the
150th anniversary of The Origin of Species.
http://discovermagazine.com/2009/mar/04-discover-does-darwin/
When
Sean Carroll was a graduate student at Tufts
School of Medicine in Boston, he found himself
seduced by spectacular new studies of the humble
fruit fly. That work, which eventually won a Nobel
Prize for its principals, showed that modifying a
single gene during a fly's embryonic development
could transform the insect's body plan:
Instead of becoming an antenna, a body
extension could develop into a
leg.
Carroll
continued to study these genes and, some
years later, found that they were not restricted to
fruit flies; they turned out to be part of a master
tool kit that sculpts the body structures of all ani-
mals, ranging from humans to nematode worms.
The
discovery of this small set of universal body-
building genes gave Carroll and others a fresh way
to explore the inner workings of evolution.
By
observing how the genes changed during the
course of embryonic development, scientists
could track the emergence of a novel physical
trait, the first step toward the creation of a new
species. For the first time, researchers had direct
access to the machinery of evolution and could
actually watch it in the act.
A
new science, known as evolutionary develop-
mental biology, or evo devo, was born.
One
of the great triumphs of modern evolutionary
science, evo devo addresses many of the key
questions that were unanswerable when Charles
Darwin published On the Origin of Species in
1859, and Carroll has become a leader in this
nascent field.
...
It
has been 150 years since Charles Darwin pro-
posed his theory of evolution in On the Origin
of Species, yet in some ways the concept of
evolution seems more controversial than ever
today. Why do you think that is?
It
is a cultural issue, not a scientific one.
On
the science side our confidence grows yearly
because we see independent lines of evidence
converge.
What
we've learned from the fossil record is
confirmed by the DNA record and confirmed
again by embryology.
But
people have been raised to disbelieve evo-
lution and to hold other ideas more precious
than this knowledge.
At
the same time, we routinely rely on DNA to
convict and exonerate criminals. We rely on
DNA science for things like paternity. We
rely on DNA science in the clinic to weigh
our disease risks or maybe even to look at
prognoses for things like cancer.
DNA
science surrounds us, but in this one
realm we seem unwilling to accept its facts.
Juries
are willing to put people to death based
upon the variations in DNA, but they're not
willing to understand the mechanism that cre-
ates that variation and shapes what makes
humans different from other things.
It's
a blindness. I think this is a phase that
we'll eventually get through.
...
In
your new book, Remarkable Creatures, you
relate how Darwin arrived at his theory of evo-
lution. Can you connect the dots?
As
a college student Darwin collected beetles.
He was looking for more opportunities to col-
lect when there came this opportunity to be a
naturalist on the British ship the HMS Beagle.
It
was seductive. He could go to faraway places-
visit the tropics, places of incredible richness of
life relative to cold, damp, gray England.
...
Two
stops in this five-year journey were pivotal.
The
first came early in the voyage when he ar-
rived on the coast of Argentina and unearthed
fossils of many species, including some un-
known to science-for instance, fossils of giant,
extinct sloths that had been enormous com-
pared with the living sloths he saw in the South
American forests. So it planted the seed in his
mind that life had changed.
Then
Darwin got to the Galápagos Islands. He
went from island to island collecting birds-mock-
ingbirds and then finches-and realized that even
when the birds appeared to be similar, on each
island they were slightly different. After he left
the Galápagos, on his way home to England,
the lightbulb went on.
He
realized that if these birds lived on such sim-
ilar islands but were slightly different from one
another, there could be just one explanation:
They had started out as a single species, but
over time and with separation they had drifted
apart and changed.
This
insight was widely regarded as heresy, but
why?
The
prevailing idea was called special creation:
that every species was created by a supernatural
power and put in place on the earth for a speci-
fied role in a specified time by a completely
mysterious process. It wasn't open to natural
science.
Instead,
Darwin said no, species are changeable,
and the introduction of new species is a com-
pletely natural process that follows natural laws
just the way physics does.
A
fundamental aspect of human existence has
been to ask how we got here. Evolution is the
big answer to that big question.
Obviously
there are alternative answers that
have prevailed for a very long time, but evo-
lution has replaced a supernatural explanation
of human origins with a naturalistic one.
Beneath
diverse exteriors, all animals share a
set of body-building genes. If I had five min-
utes with Darwin, I would start right there. It
would blow his mind.
...
What
piqued your own interest in evolution?
As
a kid, I was fascinated by zebras and gir-
affes and leopards. I kept snakes, and I loved
their color patterns. As I got older I asked
deeper questions-mainly, how are pattern and
form generated?
One
of the most spectacular pageants on earth
involves a complex creature developing from
a single fertilized egg. Anyone who's a parent
is still amazed that it works.
When
I was a graduate student, we could watch
this happen, but we didn't understand the me-
chanics. What was going on inside that would
put limbs in the right place, put eyes in the right
place, carve the circulatory system and the back-
bone?
It
was an irresistible mystery, made even more
irresistible with the realization that what makes
a snake different from a lizard, what makes a
zebra different from a giraffe, are changes within
that developmental process.
Understanding
development was a passport to
two fundamental questions: How does a com-
plex creature form from an egg, and how have
different types of creatures evolved?
These
seem like two very disparate ideas: the
embryonic development of a single specimen
and the evolution of a whole species. How did
they get connected?
At
first paleontologists were studying evolution
on vast timescales through fossils. Then geneti-
cists came on the scene, and they were studying
small-scale differences within species based on
mutations in genes.
What
has been called the modern synthesis of
the two fields emerged in the 1940s with the
idea that the sorts of genetic differences you
could observe in populations, right out your
window, when compounded and extrapolated
over vast periods of time, could account for
the large-scale changes we see in the fossil
record.
So
the modern synthesis was a harmonization
of those two scales.
But
the modern synthesis did not explain evo-
ution in full. It was still just a theory. Where
was the empirical evidence?
Darwin's
theory of descent was a black box.
You could not see exactly what kinds of changes
were taking place to account for the differences
in forms. But the study of embryonic develop-
ment has allowed us to peer into the machinery
of making these creatures.
We
can study their DNA text and their develop-
ing embryos and ask, where do the differences
arise?
That
gave us the empirical data for the theory.
You
can't necessarily see the change happening
in the adult, but you can see that if you change
that nucleic acid base right there in that gene, at
that particular point in embryonic development,
that animal is darker.
If
you change those three bases over there, that
limb is longer.
This
is the fundamental basis of evolution:
changes in DNA.
...
You've
said evolution is like compounding inter-
est. How so?
Just
like a good money market account, evolu-
tion works through incremental change. If vari-
ants within a species provide an advantage, no
matter how slight, then that form, that capacity,
will be favored.
If
evolving spots on wings makes you more
attractive to mates or more evasive to preda-
tors, those patterns will dominate. Those vari-
eties will have more offspring. Added up over
centuries, millennia, and longer periods of time,
natural selection-the competition that takes
place in nature between variant forms-is power-
ful enough to forge all the changes that we've
seen on the face of the earth.
It
is hard for most people to wrap their brains
around such vast stretches of time.
A
century ago, Teddy Roosevelt was president
and cars were barely in use.
That
seems like an unimaginable amount of time
ago, but biologically and geologically speaking,
it was a split second. A million years is just a
fraction of the time that upright hominids have
had to evolve.
It
takes time for sea levels to rise, for rivers to
cut their course. As temperatures change, as
rain forests grow up or deserts emerge, the
creatures that live in these regions are adapting
and changing too.
You
call the combination of evolution and embry-
onic development evo devo.
What
is that, exactly?
It
is just shorthand for "evolutionary develop-
mental biology," a mini-syllabic description of
this field that's concerned with the evolution
of development.
It's
related probably to Devo, the new-wave
band of the early 1980s-those were the guys
who played with dog dishes on their heads.
Before then you could describe evolution as
change over time, but we did not have any
grip on that process until the 1980s.
...
You
saw the fruit fly as a window into evolu-
tion and development. How did you make the
connection?
It
was not an obvious call, because the expec-
tation was that fruit flies didn't have anything
to do with the development of furry creatures.
...
it became clear from our research and
others' that these body-building genes were
not restricted to fruit flies; they were shared
throughout the animal kingdom. It was a real
jolt.
All
of a sudden we could do deep experiments
at the most fundamental level to understand
how form actually evolved.
So
scientists were seeing the same master
genes at work in many different species?
Yes.
One shocking discovery was the relation-
ship between our eyes and bug eyes. You
wouldn't think they had anything in common,
right?
Bug
eyes, with 800 facets, work by different
optical principles than human eyes. For almost
a century and a half, biologists thought that
they had evolved independently, from scratch,
and that eyes had been invented many times
in the animal kingdom by completely different
means-different recipes in different groups of
animals.
We
have now discovered that these eyes are
formed by what is recognizable as the same
gene, even though those animals have been
evolving separately for 500 million years.
When
we took the mouse version of this
gene-the same gene we find in the human-
and put it in the fly and tweaked it, we in-
duced fly eye tissue.
Our
team showed that the same common
gene is critical to building limbs in humans
and fruit flies. It turns out that this gene is
critical to building virtually everything that
sticks out of the body: antennae, legs, horns,
whatever.
These
kinds of experiments shattered our
preconceptions and forced people to think
differently.
Beneath
these extremely diverse exteriors
was a deeply shared common genetic tool
kit.
If
I had five minutes with Charles Darwin,
I'd start right there. It would blow his mind.
...
Can
we apply these discoveries to the human
realm?
We
now know that the human genome and
the chimp genome differ by only about 1 per-
cent. Yet our bodies and brains are so differ-
ent. How can we be so different from other
primates if our genes are so much the same?
How
did we get the dexterity in our hands?
How
do we walk upright? How are we able
to hold this conversation? How did we get
big brains?
Once
you identify the meaningful functional
changes that have taken place between us
and chimps, you realize that pretty big dif-
ferences in anatomy and behavior can result
from a small degree of genetic divergence.
Evo
devo has given us the tools to explore
this mystery: The same genes are being regu-
lated and then used in a different way. Some-
thing is happening a little earlier or in another
place or is staying on a little bit longer. These
are the time and space dimensions of devel-
opment. It's like choreography. You've got
the same dancers, but the ballet is different
based on different cues.
In
your book Endless Forms Most Beautiful,
you refer to the Cambrian explosion, a time
when a vast number of new life-forms ap-
peared at nearly the same time. Evolutionary
skeptics often point to this kind of abrupt
shift-doesn't such rapid change contradict
your description of a single master tool kit
and slow evolution over long stretches of
time?
Prior
to about 543 million years ago, you
saw things like jellyfish and spongelike
creatures, but you didn't see bilateral crea-
tures: worms and trilobites and things like
this.
Then
in the Cambrian explosion, large and
complex animal forms erupted.
These
forms in the Cambrian represent a lot
of the major divisions of the animal kingdom
we see today.
The
Cambrian explosion looks abrupt in the
fossil record, but the surprising message
from evo devo is that all the genes for build-
ing big, complex animal bodies long predated
the appearance of those bodies.
Most
of what was needed to create this incred-
ible complexity already existed. The genes
were expressed prior to the Cambrian in those
more modest, soft-bodied creatures, but they
had fewer jobs to do.
Complexity
evolved by expanding the uses
of these genes rather than inventing lots more
of them.
It
makes you wonder what kind of potential
is just waiting to burst out today.
Dinosaurs
were the dominant vertebrates right
up until the end of the Cretaceous.
Mammals
existed, but they were smaller, carv-
ing lifestyles out of the dinosaurs' way.
Take
out the dinosaurs and in 10 or 15 million
years mammals had evolved into all sorts of
large forms and dominated terrestrial ecosys-
tems.
When
genetic potential met ecological oppor-
tunity, you got elephants and bison and giraffes.
Think
about ecology as corking the bottle; take
the cork out and things explode.
...
You've
presented an avalanche of irrefutable
evidence, yet opponents of evolution seem to
refute it all. How do you respond?
You
can hear me almost chuckling, because
it's not reasonable, it's not rational, and as the
years click by, it's ever more preposterous,
but people still stick to their guns.
Is
there anything we can do to help persuade
the skeptical public to accept the evolutionary
way of looking at life?
Seriously,
teach evolution as a core theme in
science from the early grades.
The
universe changes, the earth changes, and
life changes with the evolving earth.
Where
do you see evolutionary biology going
next?
Today
we're in a second golden age.
We're
not collecting the menagerie of critters
that Darwin did or hauling them back to a
museum. Instead, we're collecting the genetic
recipes of creatures across the planet and
trying to figure out how they came to be.
We're
looking right into the text of evolution,
and even into the text of extinct creatures like
woolly mammoths and Neanderthals, and
we're asking what made them similar to or
different from elephants or from us.
A
third golden age will come when we under-
stand life beyond earth. How many times has
life evolved, and how many origins have there
been? Has life movedfrom planet to planet? Is
the chemistry of extraterrestrial life different
from that of life on earth?
This
will be difficult work, but we have to look
ahead. Finding life elsewhere in the universe
would bring a scientific revolution as big as
any we've ever had.
Click
here to see DISCOVER's special package
on Darwin in honor of his 200th birthday and the
150th anniversary of The Origin of Species.
http://discovermagazine.com/2009/mar/04-discover-does-darwin/
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