- [Instructor] Hi, here's
a question for you.
Do you think humans evolved from apes?
If that's true, then why
are apes still around?
Why haven't all the apes
evolved into humans?
What do you think?
Well, the simple answer is, no.
Humans did not evolve from apes.
Instead, the right way to think about it
is that both humans and apes
evolved from a common ancestor.
These guys were neither apes nor humans
and this is just drawn by an artist, okay.
We don't know exactly what they look like.
But this can raise a very big question.
How do new species like these
evolve from an old species?
Well, I'm glad you asked that question
because that's exactly
what we're gonna talk about
in this video.
So, this process when new
species evolved from old ones,
we call that speciation.
Okay, so let me just write that down.
That's basically what I wanna talk about.
I'm gonna talk about
something called speciation.
Speciation and this is where
new species, new species,
new species form or evolve
from old ones or you
can say existing ones,
whatever you can say old ones now.
But before we continue, one
question you might be having is
hey, what's a species, right?
Well, in short we say two members
belong to the same species
if they can reproduce and
make a fertile offspring.
Fertile meaning even this offspring
is further able to reproduce.
So that therefore we will say these guys
belong to the same species.
They're all dogs.
On the other hand, if
two members are unable
to reproduce at all
or make a fertile offspring
then we will say they
belong to different species.
So, for example this
chimpanzee and this dog,
they belong different species
because they cannot reproduce at all
and of course if you want more details
then we have a dedicated
video called species
which explores this in great detail.
You feel free to go
back and check that out.
Anyways, coming back to our question,
how do new species get formed?
Well, the major requirement for this
is something called
reproductive isolation.
Reproductive isolation.
So, what does this fancy term mean?
Well, it basically means
that we need to make sure
that there are at least two
groups of the same species
that are not reproducing with each other.
That's the whole idea where
in reproductive isolation.
If you can maintain two groups
which do not reproduce with each other
then we will see as time
passes by they will both evolve
separately into two
different species, okay.
And now if you're wondering
how does that happen,
let's look at a particular example.
We will not look at humans and the apes,
that's basically because
it's very complicated.
Instead, we look at a
much simpler example.
Let me tell you a story.
This is a story of a group
of red colored beetles
living on a bush, okay.
Now, let's say some of these beetles
migrate to a different place altogether.
Now, this can happen in many ways.
Maybe a wind carries them away
or maybe they sit on in some animal
and that takes them to a different place
or maybe I don't know, they
sneak into an aeroplane
and that takes them or do to a
different country altogether.
Anything can happen.
So, let's say these beetles go to
a different place
altogether, very far away
and then they repopulate
and we have a new set of population.
Now, imagine that these two population
of the same species, right,
they're all red beetles, same species.
They cannot reproduce with each other
simply because they are too far away
to travel back and forth, all right.
Then we will say they are
reproductively isolated
because they cannot
reproduce with each other.
Now, we will see as time passes by,
as more and more generations come,
natural selection and genetic drift
will start working differently
in these different environments
and they will eventually grow
into two different species.
Okay, first let me tell
you how these two places
are very different, okay.
First of all, these bushes
are pretty much gray in color
because they are covered
with smoke let's say
and that could be because this is staying
this bush is very close
to a large factory nearby
which is producing a lot of smoke
so that makes this completely gray.
And secondly, in this environment,
let's say there are a lot of
crows that eat on the beetles
but in this environment let's
assume there are no crows
but instead there are lots of lizards
that eat on these beetles.
Now, with the state set,
let's see how the two
population evolve differently.
So, as these beetles are
continuously reproducing
and continuously dying,
let's say one fine day
some mutation happens
in the genes that's
responsible for color, okay.
Because of this some bugs are formed,
some beetles are having
blue color let's say,
some beetles are born
with gray color let's say
and some other beetles
are born with green color.
This this can happen to
to random changes in DNA
and that keeps on happening
all the time, all right.
And let's say for the sake of simplicity,
the exact same mutation
happens here also, all right.
So, again here also we have
some green, we have some blue
and we have some gray beetles.
So, if you come back over
here, what'll happen?
Well, the green ones have an advantage
because they are sitting on
a green bush, harder to see
and therefore more chances of survival,
more chances of reproducing.
And so as time passes by,
as generations goes by
we will now see the green beetles number
to start increasing, all right.
So, after many generations we might see
the majority of the
beetles are green in color.
We call this natural selection
and this also we have talked a lot about
in our previous videos
on natural selection.
So, the green color gets
naturally selected over here.
But what do you think happens
in this particular environment?
Well, over here green
doesn't have an advantage.
Instead, it's the gray one
that has an advantage, right
which means after a few
generations over here,
it's the gray one that
gets naturally selected.
All right, let's do one more mutation.
Let's say another mutation
causes some beetles
to be born with large wings.
Okay, I'm just making things up over here.
Beetles already have wings,
small wings with which they
can you know hop a little bit
but let's say these large
wings allow the beetles to fly
to large distances, okay.
Some beetles are born with
large wings, let's say
and maybe some beetles are born
with no wings at all, okay,
pure mutation this is all right.
And of course the same
thing happens here as well.
Some beetles are born with large wings,
some beetles are born with
no wings at all, okay.
Now, let's see what happens
in these two environments.
Let's come back over here now.
Over here don't you see that
this beetle has an advantage
like over these lizards
because if they can fly higher
that means lizards can't eat them, right.
That means again higher
chances of survival
therefore after a few generations
they will get naturally selected
and we will see most of
the beetles have wings now.
On the other hand, what
do you think happens here?
Well, here notice if
the beetles are flying,
they can be easily
picked up by these crows.
These crows are excellent.
They are also flying, right.
So, can you see flying beetles
are at a disadvantage over here.
In fact, the beetles which
have lost their wings
they have an advantage
because the ones that
have lost their wings
will maybe will start crawling
you know at the bottom
and maybe because of that
they will have a higher
chance of surviving.
So, over here these guys
will not be naturally picked.
Instead, the ones which have no wings
that get naturally picked
and after a few generations
you find here we have
beetles with no wings.
Since this is so much fun, let's
throw in one last mutation.
Let's say one last mutation
causes some beetles
to be born with horns.
Now, usually things don't
happen so drastically
but we are making up
an example, so why not?
Okay and let's imagine
that one day some fire
you know that the bush is under fire
because of that almost
all the beetles died
but luckily the ones that survived
we're mostly the horned ones, okay.
The horned ones let's say
had absolutely no advantage
but just by pure luck they
survived that accident
and after a few generations
now we find most of the beetles
are having horns.
This evolution by pure
luck we have seen before
is called a genetic drift, okay.
These horned ones were lucky
to survive that accident
and so they pass their genes.
Maybe on this side also some
beetles are born with horns
but maybe they were not so lucky.
Let's say the beetles
which are born with horns
immediately got eaten by this lizard
and they never got to pass their genes
and so there are no
horned beetles in this.
And this is where we we'll stop.
Now, look at these beetles,
how different they are.
They have different
colors, these ones fly,
these ones don't, they have horns,
so many different features
and we did this for a few generations.
Imagine this happening for
thousands of generations.
How different, how much different features
they would have accumulated
over the years, right.
They could be so different now that if now
one of this beetle tries
to reproduce with this one,
they will just not be able to do it.
The DNA of these beetles
might be so different
than these beetles that is
just impossible to form a baby.
In other words, we now have two
different species of beetles
and this is how one
species evolves into two
or more other new species, speciation.
And of course this was
an oversimplification
but you get the idea, right?
So, let's quickly write down what we saw.
The first thing we saw was
that the two population
got separated from each other.
This technically we call
geographical isolation.
Basically means the two
population are confined
to two separate locations,
geographically isolated.
That made sure that they couldn't
reproduce with each other.
That made sure of reproductive isolation.
Turns out there are other ways
to make sure of reproductive isolation
which we'll not talk about
but keeping them separate,
geographical isolation
is the best way to make sure
that they do not reproduce
with each other.
Okay, after that what happened?
Then we saw that in
both these environments
natural selection and genetic
drift worked differently.
Because of this as generations passed on,
their DNA started changing
and becoming more and more
different from each other
and eventually after a few thousand years
it became so different
that they were incompatible
to for fertilization and
so we had two new species.
And just to be super
clear, if you're wondering
why it's necessary that these population
do not reproduce with each other,
think about what would have happened
if they were constantly visiting
each other and reproducing
then whatever variations
were selected over here,
that variation could flow over here,
those genes could flow over here.
Similarly, whatever variations
were selecting over here,
those genes could flow over here.
This gene flow would
make sure that the DNA
of this population and
DNA of this population
stays pretty similar to each other, right.
Then we wouldn't have new species.
So, you see it is super
important that the genes
selected over here do not flow over here,
genes over here do not flow over here.
Gene flow shouldn't happen, right.
Only then as time passes by,
the DNA starts becoming
more and more different.
Only then we will have speciation
after lots of generations.
So, reproductive isolation
is super-important
without which it's almost
impossible to have speciations.
And so we can put all of this
together and we can now say
that these two species of beetles
evolved from one common
ancestor, the red beetles right.
And I'm pretty sure use now
agree that it's wrong to say
this evolved from this or
this evolved from this.
That's not that I will say that, right.
Both of these evolved from
one particular species.
In a similar manner, humans and apes
evolved from one ancestral species.
And of course things over
here we're definitely
way more complicated than I told you
but you get some idea now, right.
It turns out that this took about
seven to eight million years
and there may have been even
more species in between,
more speciation in between
which I have not drawn over here
but you get the overall picture
of how new species are formed now.
Okay, so that's pretty much it.
Let's quickly see if we can
recall what we learned here.
Can you recall what speciation is?
What do you think is the most important
requirement for speciation?
And now can you explain to
yourself or maybe to your friends
very briefly, how speciation occurs?
If you have difficulty in
answering any of these questions,
no worries you can always go back
and just revisit the video.
