- [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.