Congratulations! This is our last episode of our section on
Evolution and Genetics, which puts us at the halfway mark
of CrashCourse Biology.
So far we've learned about DNA, genetics, natural selection,
how cells multiply, populations, speciation, replication,
respiration, and photosynthesitation.
I'm so proud of you.
But I couldn't let this section end without discussing the
iscussion that everybody can't help but discuss these days:
Evolution.
It's a thing.
It's not a debate.
Evolution is what makes life possible.
It allows organisms to adapt to the environment as it changes.
It's responsible for the enormous diversity and complexity of life
on Earth, which not only provides organisms with sources of food and
some healthy competition.
It also gives us some truly awesome stuff to marvel at.
And even though evolution makes living things different from
one another, it also shows us how we're all the same.
All of life, every single thing that's alive on the Earth today,
can claim the same shared heritage, having descended from the very
first microorganism when life originated on this planet
3.8 billion years ago.
There are people who will say that this is all random-
It's not.
And that this clumsy process could not be possible for the majestic
beauty of our world.
To them, I say, well at least we agree that our world is beautiful
but, well you're probably not going to enjoy
the rest of this video.
To me, there are two sorts of people in the world,
those who are excited about the power and beauty
and simplicity of the process of evolution,
and those who don't understand it.
And somehow, I live in a country where only 40% of the population
believes that evolution is a thing.
The only possible reason for that that I can accept is that
they just don't understand it.
It's time to get real, people.
First, let's understand what we mean when we talk about
the theory of evolution.
Evolution is just the idea that gene distribution changes
over time, which is an indisputable fact which we observe all the time
in the natural world.
But the THEORY of evolution is a large set of ideas that integrates
and explains a huge mass of observations from different
disciplines including embryology, paleontology, botany, biochemistry,
anatomy and geophysics.
In every day language, the word "theory" means "hunch"
or even "hypothesis."
But in science, a theory is an idea that explains several
phenomena at once.
Thus, The theory of evolution is a bunch of ideas that explain
many things that we, as humans, have observed for
thousands of years.
It's the theory that meticulously and precisely explains the facts,
and the facts are indisputable.
So let's spend some time going through the facts, and how
evolution explains them all so well.
First, fossils:
The fossil record shows that organisms that lived long ago
were different from those that we see today.
Sounds obvious, but two hundred years ago it seemed
a little bit crazy.
When scientists first started studying
dinosaur fossils in the 1820s, they thought that all dinosaurs
were basically giant iguanas.
That's why the first fossil dinosaur was named Iguanodon.
It wasn't until the fossils of two-legged dinosaurs started
showing up in the 1850s that scientists had to grapple with
the idea that organisms of the past were somewhat similar to ones today
like, dinosaurs were reptiles, but many of them took on a diversity
that's barely recognizable to us.
And of all those ancient not-really-iguanas
were all extinct, either dying out completely or evolving into
organisms that survive today, like birds.
Fossils make it clear that only evolution can explain the origin
of these new kinds of organisms.
For instance, fossils taught us that whales used to walk.
Whales are cetaceans, a group of mammals that includes porpoises
and dolphins, and biologists long suspected that whales descended
from land mammals.
Partly because some modern whales still have the vestigial remnants
of a pelvis and hind-limb bones.
But it wasn't until recently, the 1990s and 2000s, that
the pieces really came together.
First, paleontologists discovered fossils of DOR-oo-dons,
cetaceans that had different skulls from modern whales but
still had the same vestigial leg bones.
Then they found even older fossil remains of another cetacean that
actually had hind legs and a pelvis.
The pelvis wasn't fused to the backbone like ours is,
so it did swim like a whale, but more importantly,
it still had ankle bones
And they were ankle bones that are unique to the order that
includes bison, pigs, hippos and deer.
So by following these clues left behind in fossilized bones,
paleontologists were able to track the origin of whales back to the
same origin as bison and pigs.
This leads us to another series of facts that evolution explains:
Not how animals were different, but how they are incredibly similar.
Last week we talked about Carl Linnaeus and how he
classified organisms by their structural similarities.
Well he didn't know anything about evolution or genetics,
but when he began grouping things in this way,
he hit upon one of evolution's most prominent clues:
homologous structures.
The fact that so many organisms share so many finely detailed
structures shows us that we're related.
Let's go back to the whale.
Like my dog, Lemon, and me, the whale has two limbs at the
front of its body, its front flippers.
And so does this bat, its wings.
Inside our limbs we all have the very same structure: one longish
bone on top, connected to two thin bones at the joint, followed by a
cluster of small bones called the carpals, and then our fingers,
or digits.
We each use our forelimbs for totally different purposes:
the bat flies, the whale swims, Lemon walks and I...
you know,
jazz hands!
Building limbs like this isn't the most efficient way to swim
or fly or walk.
Our limbs have the same structure because we descended from
the same animal, something like this more-gan-uh-cah-don here,
which, yeah, has the same forelimb structure.
In the first stage of our existence, every
vertebrate looks almost exactly the same.
Why?
Because we're all descended from the same initial vertebrates.
So our structures are the same as other mammals and
other vertebrates, sure, but it also turns out that our molecules
are the same as, like, everything.
In fact, if we were ever to find life on Mars or something,
the sure fire way of knowing whether it's really
extra-terrestrial is to check and see if it has RNA in it.
All living things on our planet use DNA and/or RNA to encode the
information that makes them what they are.
The fact that we all use the same molecule itself suggests that
we are all related, even if very distantly.
But what's more, by sequencing the DNA of any given creature,
we can see precisely how alike we are.
The more closely related species are, the more of
the same DNA sequences they have.
So the human genome is 98.6% identical to that
of the chimpanzee, our closest evolutionary relative,
and fellow primate.
But it's also 85% the same as a mouse.
And I wonder how you're going to feel about this, about half of our
genes are the same as in fruit flies, which are animals, at least.
So, just as your DNA proves that you descended from your parents,
your DNA also shows that you descended from other organisms
and ultimately, from that one prokaryotic microorganism
3.8 billion years ago that is the grandparent of us all.
Now when it comes to species that are very similar,
like say, marsupials, their distribution around the world
or their biogeography, is also explained extraordinarily well
by the theory of evolution.
Animals that are the most similar, and are the most closely related,
tend to be found in the same regions, because evolutionary
change is driven in part by geographical change.
As we talked about in our speciation episode,
when organisms become isolated by physical barriers, like oceans
or mountains, they take their own evolutionary courses.
But in the time scales we're talking about,
the geographical barriers are much older,
and are often even the result of continental drift.
So, marsupials.
You know about marsupials.
They can be found in many places, but they aren't evenly distributed
around the world.
By far the highest concentration of them is in Australia.
Even the majority of mammal fossils in Australia are marsupials.
So why is Australia rife with kangaroos, koalas and wombats
while North America just has, opossums?
Fossils show us that one of marsupials' earliest ancestors
found its way to Australia before continental drift turned it into
an island 30 million years ago.
More importantly, after Australia broke away, placental mammals like
us evolved on the main landmass and quickly outcompeted most of the
marsupials left behind, in what would become
North and South America.
So, very few marsupials remain in the Americas,
while Australia has been drifting around like some kind of
marsupial Love Boat.
Darwin's finches are another example of biogeographical evidence
As he wrote in The Origin of Species, Darwin observed that
different species of finches on separate Galapagos islands were
not only similar to each other but were also similar to a species on
the South American mainland.
He hypothesized that the island finches were all descendants of
the mainland finch and changed over time to be more fit
for their environments, a hypothesis that genetic testing
has since confirmed.
Now, you'll remember, I hope, a few weeks ago, when I told you
about Peter and Rosemary Grant, the evolutionary biologists/lovebirds
who have studied Galapagos finches since the 1970s.
One of their greatest contributions came in 2009 when studying finches
on the island of Daphne Major.
They discovered that the offspring of an immigrant finch from another
island and a Daphne Major finch had become a new species
in less than 30 years.
This is just the latest example of our fourth body of
evolutionary evidence: direct observation of evolution.
The fact is, we have seen evolution take place in our own lifetimes.
One of the fastest and most common changes we observe is the growing
resistance to drugs and other chemicals.
In 1959, a study of mosquitos in a village in India found that DDT
killed 95% of the mosquitos on the first application.
Those that survived reproduced and passed on their genetic resistance
to the insecticide.
Within a year, DDT was killing only 49% of the mosquitos,
and it continued to drop.
The genetic makeup of the mosquito population changed because of the
selective pressures caused by the use of DDT.
But it's not just tiny changes in tiny animals,
we've also observed larger animals undergoing some
pretty striking changes.
In 1971, for instance, biologists transplanted ten Italian
wall lizards from one island off the coast of Croatia to another.
Thirty years later, the immigrant lizards' descendants had undergone
some amazing, fundamental changes like, even though the original
lizards were mainly insect eaters, their digestive systems had changed
to help them exploit the island's most abundant food source: plants.
They actually developed muscles between their large and small
intestine that effectively created fermenting chambers, which allowed
them to digest vegetation.
Plus, their heads became wider and longer to allow them to better bite
and chew the grasses and leaves.
These are all great examples of microevolution, allele frequency
changes that happens rather quickly and in small populations.
Macroevolution is just that microevolution on a much
longer time scale.
The sort of thing that turns hippos into whales is a lot harder
to observe for a species that, 200 years ago, thought dinosaurs
were big iguanas, but part of the power of the human mind is being
able to see far beyond itself and the time scales that our own
individual lives are limited to.
And I for one, am pretty proud of that.
Let's all at least agree that the world is a beautiful
and wonderful place.
And life is worth studying and knowing more about,
and that's what Biology is.
If you want to go back and watch parts of this video again please
click on the annotations in the little table of contents over there.
If you have questions for us, please leave them on Facebook
or Twitter or in the YouTube comments below.
Thanks to everybody who helped put this together.
And we'll see you next time.