[BUGS CHIRPING] [MUSIC PLAYING] SEAN CARROLL (VOICEOVER):
The diversity of animals on our
planet is breathtaking. Millions of species adapted
to all kinds of habitats. Ever since Darwin, understanding
how so many species evolved has been a major quest of
biology and biologists, like Jonathan Losos. In the Caribbean, he's studying
a remarkable group of lizards. [MUSIC PLAYING] JONATHAN LOSOS:
She will be back. SEAN CARROLL (VOICEOVER):
He's finding clues to their origins in their
bodies, their lifestyles, and in their DNA. SEAN CARROLL: There's
one out there. SEAN CARROLL (VOICEOVER):
These lizards are providing fresh insight
into both how new species form and why our world is filled
with so many creatures. [MUSIC PLAYING] JONATHAN LOSOS: Don't think
I don't see you, lizard. SEAN CARROLL (VOICEOVER):
Here in Puerto Rico, Jonathan is stalking
lizards called anoles. JONATHAN LOSOS: OK, here we go. SEAN CARROLL (VOICEOVER): With
almost 30 years of practice, he's a pro at catching them. JONATHAN LOSOS: Come on. There we go. He's OK. They've got very strong necks. This actually doesn't
hurt them at all. He's a healthy, fine
looking specimen. SEAN CARROLL (VOICEOVER):
Puerto Rico's anoles all feed on similar
food, mostly small prey like spiders and crickets. But they divide up their
habitats in a clever way. [MUSIC PLAYING] The long tailed,
slender species Jonathan caught lives in
grasses and bushes, and it's called a
grass-bush anole. On the low parts of tree
trunks and on the ground, a longer legged,
stockier species forages, called a trunk-ground anole. And higher up the tree
lives another anole species. [MUSIC PLAYING] JONATHAN LOSOS: On twigs and
small branches like these, we find very small anoles
with really short legs. SEAN CARROLL (VOICEOVER):
This slender lizard is called a twig anole. Further up the tree is
yet another species. JONATHAN LOSOS: High
up in the canopy, there's a large green
lizard with big toe pads. He lives high off the ground. There's one right there. [MUSIC PLAYING] SEAN CARROLL (VOICEOVER):
Like apartment dwellers, each species lives in a
different vertical space. But here, each floor
offers unique evolutionary opportunities to
its inhabitants. [MUSIC PLAYING] The fact that lizards differ
in leg length and toe pad size, depending on where
they live, suggests that these differences in traits
are adaptations to the lizards' habitats. JONATHAN LOSOS: Here's
a good tree over here. SEAN CARROLL
(VOICEOVER): To test whether that is,
in fact, the case, I came here to help Jonathan
conduct some experiments. JONATHAN LOSOS: Yeah, these
lizards are very cooperative. SEAN CARROLL (VOICEOVER): We
begin by comparing the running ability of two lizards-- SEAN CARROLL: Short legs. Yeah. SEAN CARROLL (VOICEOVER):
--one with long legs, the other with short ones. JONATHAN LOSOS:
Let's do some tests. Let's start with
this little lizard here and see how fast it can
run up this broad surface. SEAN CARROLL: All right. I'll catch him if he
makes it to the end. JONATHAN LOSOS: All right. Here we go. There he comes. SEAN CARROLL: Wow. She's a sprinter. JONATHAN LOSOS: Exactly. She lives at the bottom of
trees right in the open. She catches prey on
the ground, so she has to run down
quickly to get them. SEAN CARROLL (VOICEOVER): The
shorter legged twig lizard is not nearly as fast. It seems like a disadvantage. Why aren't their legs longer? Jonathan puts a twig lizard on
a thin branch to demonstrate. JONATHAN LOSOS: All right. Let's see how he does. There we go. SEAN CARROLL: Looks
pretty comfortable there. JONATHAN LOSOS: Yeah. SEAN CARROLL: Just
sort of scurrying along like a balance beam. JONATHAN LOSOS: This
is what they love. SEAN CARROLL (VOICEOVER):
Instead of speed, the twig lizard's legs
provide a firm grasp. JONATHAN LOSOS: All right. Now, let's try the other one. SEAN CARROLL: So
this is the sprinter. JONATHAN LOSOS:
This is a sprinter. Let's see how she fares
on this little stick. Look how ungainly she is. Her legs are too long for this. So you can see on
these narrow surfaces, long legs are a disadvantage. SEAN CARROLL (VOICEOVER): On
twigs, long legs only increase the chance of falling. So ground lizards
have evolved long legs and twig lizards short ones
that enabled their lifestyles. [MUSIC PLAYING] Next, we compare how well
two species can climb the slick surfaces of leaves. Anoles have different sized
toe pads on their feet. We'll see if these
help them navigate different environments. [MUSIC PLAYING] JONATHAN LOSOS: So it's time
for lizard Olympics part two. SEAN CARROLL: All right. I'm game for that. JONATHAN LOSOS: Here's
the ground lizard. Let's see if he can
hang on and move up it. Oh. SEAN CARROLL: No. JONATHAN LOSOS:
Couldn't even hang on. SEAN CARROLL: Cannot hang on. JONATHAN LOSOS:
Let's try it again. SEAN CARROLL: Here he goes. He's getting up there. JONATHAN LOSOS: He's able to
move up, but not very easily. All right. Let's do another species. SEAN CARROLL: All right. Oh, my goodness. JONATHAN LOSOS: Take
a look at this guy. SEAN CARROLL: That's an anole? JONATHAN LOSOS: This is
the big canopy lizard. Let's see how he does. SEAN CARROLL: Well,
that's not a fair contest. He's huge. There's no way for him
to hold up his weight. JONATHAN LOSOS: What do
you think now, smart guy? SEAN CARROLL: OK,
you proved me wrong. Pretty impressive. JONATHAN LOSOS: He's using
the little microscopic hairs on his toe pads to
bond with the surface, and that's what holds him up. SEAN CARROLL: And his toe pads
are bigger than other lizards? JONATHAN LOSOS: Yes, they are. He's a bigger lizard,
but even for his size, he has particularly
large toe pads. SEAN CARROLL: So this
is an adaptation. JONATHAN LOSOS: This is an
adaptation because he cannot afford to fall
out of the canopy. [MUSIC PLAYING] SEAN CARROLL (VOICEOVER): But
how do these adaptations arise? Jonathan and his
colleagues wanted to see if they could observe
the lizards' traits evolve by conducting another
kind of experiment. Their inspiration was the
rapidly changing environment of some of the smallest
Caribbean islands. [MUSIC PLAYING] [WIND WHISTLING] Hurricanes occasionally
swamp these tiny islands, scrubbing them free of lizards. The team realized they could
use the depleted islands as laboratories. They began their experiment
by capturing tree dwelling anoles on a larger island. SEAN CARROLL: Oh,
there's one out there. JONATHAN LOSOS: Yeah. [MUSIC PLAYING] SEAN CARROLL
(VOICEOVER): Then, they visited seven islands
that a hurricane had cleared of lizards. On each, they placed a
female and male anole. These islands have no
trees, only small bushes. How would the long legged
lizards fare on thin branches? The next year, the
scientists returned. JONATHAN LOSOS:
She will be back. SEAN CARROLL
(VOICEOVER): They found that the mating pairs they had
introduced not only survived, but reproduced. And the new population
had grown and taken to living on thin branches. JONATHAN LOSOS: And
now, she's in my noose. SEAN CARROLL (VOICEOVER):
The scientists collected the lizards. JONATHAN LOSOS: Every
time we found a lizard, we measured how high
it was off the ground-- SEAN CARROLL: 40 centimeters. JONATHAN LOSOS: --the
diameter of the surface, and whether it was perched head
up, head down, or horizontal. SEAN CARROLL (VOICEOVER):
They brought them back to their field lab, took
x-rays to precisely measure the length of their legs,
and scanned their toe pads. Then, they returned each
lizard to the exact spot where they had found it. JONATHAN LOSOS: OK. All right. SEAN CARROLL
(VOICEOVER): Now, they had baseline data on
the new populations. A year later, they came back-- JONATHAN LOSOS: All right. I think he gave us a slip. Excellent. SEAN CARROLL (VOICEOVER):
--and discovered that the average lizard
leg had shortened in just two generations. JONATHAN LOSOS: We
thought maybe this is just a fluke, a statistical accident. In fact, over four
years, the populations all got shorter and
shorter and shorter legs. Evolution can occur
very rapidly when natural selection is strong. SEAN CARROLL
(VOICEOVER): Adaptations like these explain how
different body types evolve, but they do not explain how
new anole species arise. It's changes in
other traits that play a key role in speciation. Two groups of animals are
defined as different species when individuals from one
group don't mate and reproduce with those from the other. So for a population to
become a new species, something has to
prevent its members from breeding with members of
closely related populations. This is called
reproductive isolation. One way a species
can split into two is for populations to
separate geographically. Over many generations, they
can undergo enough changes in their respective
habitats that if and when they come back together
again, they don't mate. So what kind of changes
keep anoles from mating? Anoles have a flap of
skin under their throats called a dewlap, which males
display to attract females. [MUSIC PLAYING] And remarkably, every
species in the same area has a different dewlap. [MUSIC PLAYING] So a change in a dewlap
is a critical step in the formation of
new anole species. SEAN CARROLL: Jonathan, why
would these dewlap colors change? JONATHAN LOSOS: Consider
this grass lizard that lives here in the forest
where it's relatively dark. And if you look
at its dewlap, you can see it's pretty
light colored. Now, suppose that a
population of these lizards ended up in an area that was
much more open and sunnier. In that case, a light colored
dewlap isn't very effective. So over time, the
population would evolve by natural selection
to have darker dewlap, and we might end
up with this one. He's got a much darker
dewlap, much more visible in a light, open habitat. SEAN CARROLL (VOICEOVER):
If for some reason these two populations
come together, the females would no
longer recognize the males as members of the same species. They wouldn't mate. They would be
reproductively isolated. SEAN CARROLL: There's a simple
connection between changes within populations,
or microevolution, and the formation of new
species, or macroevolution. When changes within populations
include traits involved in mating, like dewlap
color, then the stage is set for the formation
of new species. SEAN CARROLL (VOICEOVER):
Once new species are formed, competition drives the evolution
of different body types. Species living in the same
area compete for resources. But if members of one species
move into another habitat, they can use resources not
available to the other species. Over many generations,
natural selection favors traits that
enable species to occupy different habitats. This process has led to the body
types we see in Puerto Rico. And not just there. On each of the Caribbean's
four largest islands-- Puerto Rico, Jamaica,
Cuba, and Hispaniola-- we find the same distribution
of similar looking lizards. SEAN CARROLL: Now, you'd
think that all the lizards on the different islands would
look different, but they don't. Each island has the
same basic body types. SEAN CARROLL
(VOICEOVER): Each island has slender grass-bush
anoles with long tails, long legged trunk-ground
anoles, short legged twig anoles, and canopy anoles
with large toe pads. How did each island end up
with the same body types? Did each body type
evolve once, and then, spread to the other islands? Or did each type evolve
independently on each island? SPEAKER: So I'm going
to be sequencing some additional markers-- SEAN CARROLL
(VOICEOVER): To find out, Jonathan and his colleagues
sequence the DNA of anoles from each island. They examined the same stretch
of DNA from many species to uncover their
evolutionary relationships. JONATHAN LOSOS: Species that
are more closely related, we wouldn't expect to have
many differences in their DNA. For example, these
two species here. If you go across here,
there's only one base pair where they're different. That's because they're
very closely related. On the other hand, this species
here has many differences, here, here, here, and here. That's because this species
diverged from the other ones a long time ago. SEAN CARROLL (VOICEOVER):
After determining which two species were
most closely related, they joined them together
with a node representing a common ancestor. Then, they joined these to
the next most closely related until all the lizards were
united in a phylogenetic tree. [MUSIC PLAYING] The DNA revealed a pattern
consistent with this. The lizards on each island
tend to be more closely related to each other than to
similar looking lizards on different islands. That means that generally
the same types of lizards evolved independently
on each island. SEAN CARROLL: On all of the
large Caribbean islands, the same traits have
evolved again and again-- body color, limb
length, toe pad size. [MUSIC PLAYING] SEAN CARROLL (VOICEOVER):
Moreover, this repeated filling of habitats
on each island by anoles illustrates why our planet
has so many species. SEAN CARROLL: The
simple reason why there are so many
species in the world is that there are
so many habitats. SEAN CARROLL (VOICEOVER):
And each habitat provides numerous
ways to survive. In the Serengeti, zebras eat
the tallest, coarsest grass. Wildebeest, the
medium height grass. And Thomson's
gazelles, the shortest. In the Galapagos,
some finches primarily eat seeds on the ground, and
others, insects in the trees. SEAN CARROLL: Look around
you in your backyard or around the world. There are so many
different environments, each full of creatures making
a living in a different way. [MUSIC PLAYING]
