[MUSIC PLAYING] [MUSIC PLAYING] SEAN CARROLL: Across
the American southwest, golden deserts dotted by cacti
and brush stretch for miles. Yet here in New
Mexico's Valley of Fire, the landscape
changes dramatically. Patches of black rock
interrupt the sand-- remnants of volcanic
eruptions that occurred about 1,000 years ago. The eruption spewed a river of
lava more than 40 miles long across the desert. As the molten rock
cooled, it darkened, leaving any creature
dependent upon camouflage in serious trouble. [BIRDS CAWING] [SNAKE HISSING] In the complex battle of
life, one of the constant struggles between seeing
and not being seen-- the evolutionary game
of hide and seek. And we've come
here to the Valley of Fire in New
Mexico, a battlefield, to find one of the
tiniest soldiers and what it can teach us
about how evolution works. On the desert sands, the
rock pocket mouse blends in perfectly. Its light colored fur
concealing it from predators. But on dark lava, the same
fur makes the mouse stand out, attracting the many creatures
that see it as food. [MUSIC PLAYING] [BIRDS CAWING] MICHAEL NACHMAN: These mice are
the Snickers bar of the desert. They're eaten by foxes and
coyotes and rattlesnakes, and certainly by owls, and
maybe even occasionally hawks. And most of those predators
are visual predators. [BIRDS CAWING] You know, we haven't been-- SEAN CARROLL: So what happened
to the pocket mice that found themselves on this new terrain? MICHAEL NACHMAN: --and when
we did this big cross-- SEAN CARROLL: When I accompany
biologist Michael Nachman onto the lava, it doesn't
take long to find out. Huh? MICHAEL NACHMAN: Oh,
this one's closed. SEAN CARROLL: Does it have-- Nachman has been collecting
mice unharmed in traps. And it's a dark one. MICHAEL NACHMAN: It is. SEAN CARROLL: Now are most of
the ones you find up here dark? MICHAEL NACHMAN:
Almost all of them. SEAN CARROLL: Not only
have the mice here evolved to be as
dark as the rock-- MICHAEL NACHMAN: Shake
them into the bag. SEAN CARROLL: --the color change
has occurred precisely where it will conceal them from hunters. [MOUSE SCREECHING] And a bit of a white
underbelly too. MICHAEL NACHMAN: That's right. All of the dark ones, here
and on other lava flows, have a white underbelly. And presumably, there's no
selection for dark underbelly because predators are
coming from above. SEAN CARROLL: Left
to themselves, the mice show no preference
for light or dark rocks. It's the predators that
have made the difference. MICHAEL NACHMAN:
The change in color over evolutionary
time in the population is driven by predators
weeding out the mice that don't match their background. [BIRDS CAWING] SEAN CARROLL: But
how did the dark mice arise in the first place? MICHAEL NACHMAN:
When a black mouse appears in a light
population of mice, that is usually going to
be due to a new mutation. And those are random
and rare events. [MUSIC PLAYING] SEAN CARROLL: To fully
understand the pocket mouse transformation, Nachman moves
from the lava to the lab. He and his team extract DNA
from light and dark mice taken from one desert region. The aim? To find one or more
genetic mutations that cause dark coloration. A mutation is a change
in the chemical letters that make up our genes. It's a copying error that may
occur when our cells divide. Mutation seems to mean that
something bad has happened. Well, mutations are
neither good or bad. Whether they are favored or
whether they are rejected, or whether they're just neutral,
depends upon the conditions an organism finds itself. So for the pocket
mouse, a mutation that caused a mouse to turn
black, that is good if you're living on black rock. It's bad if you're living
out in the Sandy Desert. [MUSIC PLAYING] MICHAEL NACHMAN: The light
mice are all on the bottom. Here, here-- SEAN CARROLL: Fur color is a
trait controlled by many genes. To figure out how
dark mice evolved, Nachman focuses
on how these genes differ in dark and light mice. One by one, the genes
prove identical. But, at last,
something does turn up. The difference between
dark and light mice boils down to a difference
of four chemical letters in a gene called Mc1r. Because the gene
controls the amount of dark pigment in a
mouse's hair follicles, a mouse with these
mutations grows dark fur, which gives it an
advantage on a dark background. But, still, that's one mouse. How would its dark fur
spread to a whole population? MICHAEL NACHMAN: This lava
flow is about 1,000 years old. And so you may wonder is,
has there been enough time? It's only been 1,000 years. It's a very short period
of time for a new mutation to come along and spread so that
all of the mice on this lava flow are black. Because, really, they all are. SEAN CARROLL: Indeed, such a
rapid spread of the mutation may seem unlikely-- until you do the math. And the reason is that while
only one new mouse born in 100,000 may be black,
hundreds of thousands of mice are born in any given year. And then those
mice that are black have enough advantage that
their babies do better, and they have more offspring,
and their offspring have more offspring. And just about a 5% advantage,
compounded year in and year out, can very quickly turn
the whole population black, as we see today. [MUSIC PLAYING] If dark color gives mice a
1% competitive advantage, and you start with 1% of
the population being dark, in about 1,000 years, 95%
of the mice will be dark. If instead the dark color
gives them a 10% advantage, then it only takes 100 years. Thanks to Nachman's
mice, science has an example of evolution
crystal clear in every detail. MICHAEL NACHMAN: What's
exciting about this is that we have a system that's
very simple ecologically. You have dark rocks and
you have light rocks, and you have dark
mice and light mice. It couldn't be simpler. SEAN CARROLL: We know
who the predators are with the selective forces. We know precisely
the genetic basis of what makes the mice have
an advantage or a disadvantage depending upon where they live. All the pieces are
finally together. It's a perfect illustration
of Darwin's process of natural selection. In fact, it's more than that. For Nachman's mice also counter
a common misconception-- that evolution is
a random process. Well, there is one
random component, and that's the
process of mutation. Mutations occur at random
throughout our DNA. Every new organism is born
with a new set of mutations. But while mutation is random,
natural selection is not. Natural selection sorts
out the winners and losers. That's really what the process
of evolution is driven by. [BIRDS CAWING] [MUSIC PLAYING] But if natural
selection is not random, would it produce the same result
under the same conditions? It does, and here's proof. Rock pocket mice
collected by Nachman from other lava flows in
other parts of the southwest. MICHAEL NACHMAN: These are
two different black mice, and they each evolved
on different lava flows. And the lava flows over
hundreds of miles apart, but the changes,
the genetic changes that made these mice black,
were different in each case. And what's amazing to me is
how similar the black mice are. We didn't know when
we started this whether we would find that
they were the same genes or different genes. And we were really surprised
to find that they were completely different genes. And yet, if you
look at the mice, they look almost identical. [MUSIC PLAYING] SEAN CARROLL: Clearly, there
are different genetic ways to make a mouse dark. But once the beneficial
mutations appear, natural selection, the
non-random part of evolution, can, under very
similar conditions, favor very similar adaptations. MICHAEL NACHMAN: In effect,
each of these lava flows is like rewinding
the tape of life and allowing evolution
to occur again and again. And, in each case, we find
the dark mice have evolved. SEAN CARROLL: The rock
pocket mice show us that evolution can and
does repeat itself, and why evolutionary
change is never ending. As environments
transform, so must the species that inhabit
them, adapting and re-adapting in the great and
complex battle of life. [MUSIC PLAYING]