The Phylogenetic Tree of Anole Lizards — HHMI BioInteractive Video
Summary
TLDRThis video script explores the biodiversity of anole lizards in the Caribbean, focusing on their unique adaptations to different habitats. Renowned biologist Jonathan Losos investigates how these lizards' physical traits, such as leg length and toe pad size, have evolved to suit their environments. The script also delves into the concept of speciation, highlighting how changes in traits like dewlap color can lead to reproductive isolation. Through experiments and DNA analysis, Losos and his team reveal the rapid evolution of these lizards and the repeated pattern of adaptation across various Caribbean islands, illustrating the diversity of life on Earth.
Takeaways
- 🌍 The diversity of animal species on Earth is vast, with millions of species adapted to various habitats.
- 🧬 Understanding the evolution of these species has been a significant pursuit in biology, especially since Darwin's time.
- 🐉 Jonathan Losos studies lizards in the Caribbean, focusing on their origins and how their bodies and DNA provide clues to their evolution.
- 🏝️ Puerto Rico's anoles demonstrate niche partitioning, with different species occupying different vertical spaces in their environment.
- 🦎 Adaptations such as leg length and toe pad size in anoles are linked to their specific habitats, affecting their ability to run, climb, and grasp.
- 🔬 Experiments show that ground-dwelling anoles with long legs are fast runners but poor climbers on thin branches, while twig anoles with short legs excel in the latter.
- 🌪️ After hurricanes clear small Caribbean islands of lizards, reintroduced anoles adapt rapidly, with leg lengths changing significantly over just a few generations.
- 🌿 Changes in traits like dewlap color can lead to reproductive isolation, a key factor in the formation of new species.
- 🌳 Competition and resource availability drive the evolution of different body types among species, as seen in the various anole species across Caribbean islands.
- 🌿 The repeated evolution of similar body types in anoles across different islands illustrates the diversity of species driven by the availability of habitats and ecological niches.
Q & A
What is the main focus of the study conducted by Jonathan Losos on lizards in the Caribbean?
-The main focus of the study is to understand how new species form and why the world is filled with so many creatures by examining the diversity and adaptations of anole lizards in the Caribbean.
How do the anole lizards in Puerto Rico divide their habitats?
-The anole lizards in Puerto Rico divide their habitats based on their physical characteristics and preferred microhabitats: grass-bush anoles live in grasses and bushes, trunk-ground anoles forage on low parts of tree trunks and the ground, twig anoles live on twigs and small branches, and canopy anoles inhabit higher areas of the trees.
What adaptations do the different anole species exhibit according to their habitats?
-The different anole species exhibit adaptations such as varying leg lengths and toe pad sizes. Grass-bush anoles have short legs for quick sprinting on the ground, while twig anoles have very short legs for a firm grasp on narrow branches. Canopy anoles have large toe pads with microscopic hairs for bonding with surfaces on slick leaves.
How do the experiments conducted by Jonathan Losos and Sean Carroll demonstrate the importance of leg length in anole lizards?
-The experiments show that long-legged anoles are fast runners on the ground but ungainly on thin branches, while short-legged twig anoles are slow runners but have a firm grasp on narrow surfaces, indicating that their leg lengths are adaptations to their specific habitats.
What role do toe pads play in the climbing ability of anole lizards?
-Toe pads play a crucial role in the climbing ability of anole lizards. Larger toe pads with microscopic hairs allow canopy anoles to bond with the surface and climb slick leaf surfaces effectively, which is an adaptation to their high-off-the-ground habitat.
How does the experiment on hurricane-hit islands contribute to the understanding of anole lizard evolution?
-The experiment on hurricane-hit islands demonstrates rapid evolution in action. By introducing long-legged tree-dwelling anoles to islands with only small bushes, the researchers observed that over just two generations, the average leg length of the lizards shortened, showing how natural selection can lead to rapid adaptation to new environments.
What is reproductive isolation, and how does it relate to the formation of new anole species?
-Reproductive isolation is a mechanism that prevents members of one population from breeding with members of another population, leading to the formation of new species. In anoles, changes in traits like dewlap color can lead to reproductive isolation if populations evolve different mating signals, preventing interbreeding even if they come into contact.
How do the dewlap colors of anole lizards influence their speciation?
-The dewlap colors of anole lizards are critical in mating and can influence speciation. If populations of anoles are separated geographically and their dewlap colors evolve to better suit their new habitats, females may no longer recognize males with different dewlap colors as suitable mates, leading to reproductive isolation and the formation of new species.
What does the DNA sequencing of anoles from different Caribbean islands reveal about their evolutionary relationships?
-DNA sequencing reveals that anoles on each Caribbean island tend to be more closely related to each other than to similar-looking lizards on different islands, suggesting that the same types of lizards evolved independently on each island rather than spreading from one island to another.
How does the distribution of anole lizard body types across the Caribbean islands illustrate the diversity of species on Earth?
-The distribution of anole lizard body types across the Caribbean islands shows that similar adaptations evolve repeatedly in response to similar environmental pressures. This repeated pattern of adaptation to different habitats on each island illustrates the principle that the diversity of species on Earth is a result of the diversity of habitats and the many ways organisms can adapt to survive in them.
Outlines
🐉 Studying Anoles in Puerto Rico
The video script begins with a focus on the biodiversity of animals on Earth, emphasizing the quest to understand evolution since Darwin's time. It introduces Jonathan Losos, a biologist studying lizards in the Caribbean, specifically anoles in Puerto Rico. The script describes how these lizards have adapted to their environments, with different species occupying different vertical spaces within the habitat, each with unique evolutionary opportunities. The anoles are categorized based on their physical characteristics and the specific areas they inhabit, such as the grass-bush anole, the trunk-ground anole, and the twig anole. The script also details an experiment comparing the running abilities of anoles with long and short legs, demonstrating how their physical traits are adaptations to their specific habitats.
🌿 Adaptations and Rapid Evolution
This section delves into the specific adaptations of anoles, such as the size of their toe pads, which enable them to navigate their environments effectively. It describes an experiment where different species of anoles are tested on their ability to climb slick surfaces like leaves, highlighting how their toe pad size is an adaptation to their specific ecological niches. The script then transitions into a discussion about how these adaptations arise, describing an experiment where anoles are introduced to islands cleared of lizards by hurricanes. The study observes rapid evolution in just two generations, with the anoles' legs shortening to better suit their new habitats. The segment concludes with a reflection on how these adaptations are not only about survival but also about the formation of new species.
🔬 Speciation and Reproductive Isolation
The script explores the concept of speciation, focusing on reproductive isolation as a key factor in the formation of new species. It explains that when populations of anoles are geographically separated and evolve in different environments, they can develop traits that prevent interbreeding if they ever come into contact again. The example of the dewlap, a flap of skin under the throat used by male anoles to attract females, is used to illustrate how changes in this trait can lead to reproductive isolation. The script discusses how different species in the same area have different dewlap colors, which could be a critical factor in speciation. It concludes by connecting microevolution, or changes within populations, to macroevolution, or the formation of new species, particularly when these changes involve mating traits like dewlap color.
🌳 Convergent Evolution and Biodiversity
The final section of the script discusses the phenomenon of convergent evolution, where similar traits evolve independently in different populations due to similar environmental pressures. It describes how the same body types of anoles have evolved on each of the Caribbean's four largest islands, suggesting that these traits are not just advantageous but also recurrent in the face of similar ecological challenges. The script uses DNA sequencing to trace the evolutionary relationships among anoles from different islands, revealing that similar-looking species on different islands are not closely related, indicating independent evolution. The segment concludes with a broader reflection on biodiversity, suggesting that the diversity of species on Earth is a result of the multitude of habitats and the various ways in which organisms can adapt to survive within them.
Mindmap
Keywords
💡Adaptation
💡Evolution
💡Species
💡Habitat
💡Natural Selection
💡Reproductive Isolation
💡Dewlap
💡Phylogenetic Tree
💡Caribbean
💡Speciation
💡DNA
Highlights
The diversity of animals on Earth is vast, with millions of species adapted to various habitats.
Biologists like Jonathan Losos study how these species evolved, focusing on a group of lizards in the Caribbean.
Lizards in Puerto Rico are studied for their adaptations, providing insights into species formation.
Anoles in Puerto Rico exhibit habitat partitioning, with different species occupying different vertical spaces.
Leg length and toe pad size in lizards are adaptations to their specific habitats.
Experiments show that long legs are advantageous for ground-dwelling lizards but a disadvantage for those living on twigs.
Toe pad size in anoles is crucial for climbing on slick leaf surfaces.
Lizards with larger toe pads have a better grip and are less likely to fall from the canopy.
Evolutionary experiments on depleted islands show rapid adaptation of lizards to new environments.
Lizards introduced to islands without trees adapted to living on thin branches within a few generations.
Reproductive isolation is a key factor in the formation of new species, as seen with anoles' dewlap colors.
Dewlap color changes can lead to reproductive isolation and the formation of new anole species.
DNA sequencing reveals that similar anole body types evolved independently on each of the Caribbean's largest islands.
The repeated evolution of similar traits across different islands illustrates the diversity of species on Earth.
The variety of species is a result of the multitude of habitats and ways to survive in each.
The study of anoles in the Caribbean demonstrates the process of microevolution leading to macroevolution.
Transcripts
[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]
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