- [Voiceover] We've done many videos on Khan Academy
on evolution and natural selection explaining them,
but I thought I would do a video going a little bit
more in depth in evidence for evolution
and natural selection.
And I started with this quote "Nothing in biology
makes sense except in the light of evolution."
This is by Theodus Dobzhansky, who's a famous
biologist, he's passed away now, and what he's
saying is absolutely true and this is why it's
so important to appreciate the evidence for evolution
and natural selection and to understand them,
because before the theory of evolution, biology was
just about observation and classification without
having a cohesive narrative for how all of this
came about.
And since Darwin had come up with this theory
in the mid-19th century, we've had far more tools
to back it up beyond just the observations we had
up until that point.
We have our tools around dating and the fossil
record, which gives us much more evidence.
We have our tools of microbiology and genetics,
which gives us even stronger evidence, so a lot
of times people say "Oh, it's a theory, evolution
"is it just a theory?"
Well, it's about as strong as theories get
and without it, as Theodus Dobzhansky said, biology
as we know it and all of the progress we've made
in biology frankly wouldn't make any sense
and probably would not have happened.
Now I'm going to broadly go into three types
of evidence in this video for evolution
and natural selection.
The first is structural.
And these are the types of things that folks like
Darwin would have observed, that people have been
observing in biology for a long time
but evolution and natural selection starts to make
a lot more sense of it, and here we're talking
about the macro structure, things that we can
for the most part observe with our eyes or with
a very simple microscope.
The next level is what we've learned, really over
the last 100 or so years, at the micro level,
in microbiology.
Microbiology and especially in genetics.
So this has really firmed up the theory of evolution.
And then the last dimension we'll look at
is direct observation, direct observation,
and this is really where it goes beyond a theory.
We are seeing it happen.
A lot of times people say "Oh, it's a theory,
"you know, the theory says it happened over tens
"of millions of years but no one was around to really
"observe, even if we see a lot of evidence, no one
"knows if it for sure happened."
But if you're directly observing things, well you
know it's for sure happening and as we'll see,
evolution does not only occur over time scales
of millions or tens of millions of years,
it actually can occur and we see it occurring
all the time on scales well within a human
observational capacity, within just a matter
of months or years.
So let's go through each of these.
So first let's talk about structural and this is
a very high-level overview.
I encourage you to do more research on it.
You will find loads and loads and loads of any
type of this evidence.
So the first thing I want to talk about is
homologous structures, homologous structures,
that you see throughout the biological world.
Hom-ol-o-gous, homologous structures.
And the word homologous means things that have
similar structure, similar position, similar
ancestry but not necessarily the exact same function.
And here you see examples of a, well as a human,
we would consider a forearm.
You see the human forearm and wrist and then you
see the homologous structures in dogs and birds
and whales.
And even though this part of those animals have
very different functions, a human does not walk
on its hands for the most part; a dog does walk
on its front legs; a bird isn't walking at all,
it's using them to flap its wings; and a whale,
this is making up its actual fins,
it's using them to propel or to control
their movement inside of the water.
And even though they have these very, very
different functions and at first when you look
at a human and a bird and a whale on the outside,
they might look reasonably different, when you look
at these bone structures, they are eerily similar,
especially color-coded the way it is.
So these are, this is a very strong hint that
maybe humans, dogs, birds, and whales share
a common ancestor, more recently in the past
than say other animals or organisms, I should say,
that don't have, whose structures aren't as
homologous as these are right over here.
And if you were independently trying to create
structures for what each of these different species
are doing, it's not obvious that you would have
such homologous structures actually be involved.
Now these are all species that exist today,
these are all species that exist on the planet
at the same time, but we also see structural
evidence by going into the fossil record.
In the last few hundred years, or early in the last
hundred years is where we've gotten really good
at it, we've gotten good at looking at different
layers of rock strata and being able to date them
and say "Okay, that layer was laid down x-million
"years ago, that layer was laid down a little bit
"more recent, this one was even more recent,"
and then looking at fossils within that to say
"Okay, 20 million years ago there were species
"around that looked something like that,
"and then 10 million years ago there were species
"that looked like that."
And one example is if you look at a horse-like animal.
So this is right over here, we're talking about
horses, zebras, donkeys, mules, things like that,
the modern ones, well this is their bone structure
but if you look at the fossil record from 12 to 5
million years ago, you see fossils that look like this
and they're very close so you see, it's very
believable that you could have evolution from this
to that, but then you go further back and once
again, it draws, it seems like a very gradual process.
And once again, this happening over, these are from
12 to 5 million years ago, these are from 16 to 12
million years ago, these are from over
34 million years ago.
And so you can see how this is happening at a very,
very gradual pace and the mechanism, and we go
into some depth in other videos in Khan Academy,
you have variation in species, you have
the environment selecting for it.
The environment might change or different things
happen so you have different forms of selection,
different types of combinations sprout up,
they're more suitable for the environment,
they start to reproduce better, they become
the dominant species or they take over certain
parts of a niche or an ecosystem, and so you have
this change, this heritable change of traits over time.
And so when you look at the fossil record, it makes
a lot of sense that, okay, this is strong evidence
for evolution, that the animals that we see today
weren't just put on, just created all of a sudden
and haven't changed since then, but there's
a constant change and we can see it directly
through the fossil record.
Now the next point of evidence, I will put
a bit of a caveat because the gentleman who first
created this, his name was Haeckel, he was
a controversial figure, he had some spurious
theories, and even this diagram that he created,
it seems like he fudged a little bit of the drawings
in order to make a stronger argument,
but even with modern observations, these drawings
are pretty close to being correct.
And it's very, very compelling.
It shows the embryonic development of a whole
series of species, from a fish on the left
to a reptile to birds to mammals
and another mammal, to non-human mammals
and, of course, to humans.
And you can see at the early stages,
they look eerily similar.
In fact, you see proto gill slits
in all of these animals, which later
differentiate into things that are more suitable
for what that animal actually becomes.
And Haeckel, he's the guy who claimed ontogeny
recapitulates phylogeny, which is a very fancy
way of saying that your embryonic development
is telling the story of the evolutionary path,
which isn't true, but you'll even hear people
quote that today.
But his drawings and his observations,
this is compelling evidence for life sharing
a common ancestry, coming from similar origins
that got more and more different over time through
the process of natural selection.
So everything I've talked about so far has been
kind of macro structure, things we can observe.
The next thing I'm gonna talk about is, you can
think about as micro structures or processes,
and this is microbiology.
Microbiology...
Biolo, biology...
Microbiology.
And the more we understand about microbiology
the more compelling case of evolution
because when we look at even one, all life forms
that we know, they involve DNA.
How the DNA gets replicated and translated
and transcribed is very similar from one
life form to another.
The idea of DNA going to,
DNA coding for proteins, proteins,
that are made up of amino acids is something
that we see throughout biology.
Amino acids, which once again hints
at a common ancestry.
And not only are those molecular and many
of the very proteins are very, very similar,
more similar than if you look at the macro level
or even at the structural level between different
species, and not just do they share these common
microstructures and processes, but the actual
information stored in things like DNA also are
very, very strong evidence for evolution.
So this is a picture, I got this from,
I got this from the site, I should give proper
credit, 23andme.com.
But this and you'll see other data like this
that's very similar to this, which is how much
genetic similarity do we have between different
species, and these numbers tell us how much genetic
similarity at a high level do we have with
chimpanzees, mice, fruit flies, yeast, and plants.
And the fact that we have 26% of our genes
in common with yeast is mind-blowing because
at a macro level it doesn't seem like there's
a lot in common with yeast, but when you get
at a microbiological level, there's a good bit
that's in common with yeast.
And chimpanzees, we do relate to them, their facial
expressions often feel eerily human, their behaviors
often feel eerily human, but their genes, so just
how close to human beings they actually are.
And this actually shows that even mice are way
closer, if you look at the entire tree of life
based on genetic evidence, things like mice
and even fruit flies are awfully close to human
beings, especially if you were to compare it
to bacteria or a plant.
But once again, you share all of these common
processes and the fact that we can now measure
how far things are away allows us to create
a very accurate tree of life, and especially
thinking about how far in the past we had
evolutionary common ancestors.
Now the last thing that I promised I would talk
about is direct evidence, direct evidence
of evolution, and I talk about this in the first
evolution video, but the direct evidence we see
all the time with things like bacteria where
you have bacteria, let's say growing around,
and we have antibiotics that we use in our body
to kill bacteria, but the reason why many
physicians and scientists will tell you "Don't
"overuse antibiotics" is because the more you use
it, it causes a very strong natural selection
process for bacteria that are going to be resistant
to that antibiotic.
So if you keep using an antibiotic and the bacterias
keep changing, there's more and more variation,
well you're gonna kill a lot of the bacteria
but if even one of them is resistant to that
antibiotic that you use, well then all of its
competition is gonna get killed and so that
drug-resistant superbug, it's often called, is going
to be able to go nuts and that antibiotic isn't
going to be able to do anything.
And if you look at science today or you look
at medicine today, this is kind of an arms race.
You have this constant need to create new
antibiotics because more and more bacteria are
becoming drug-resistant, they're becoming what's
often called superbugs, where they are resistant
to the existing antibiotics.
And this is evolution, natural selection,
happening on a human scale.
You could also think about the flu virus where every
year that vaccine for the flu virus, you gotta get
a new one every year because the virus is changing.
Your immune system's ability to recognize it can't
recognize the next year's because it's changed so much.