In this video lecture we are going to
understand evolution by natural selection.
This concept was first written down
already more than 150 years ago by
Charles Darwin in his famous
book The Origin of Species.
Well, evolution by natural
selection is amazingly simple.
Everybody can understand it.
You just need three ingredients.
As soon as those three
ingredients come together,
evolution by natural selection
will automatically take place.
There is simply no way around it.
A characteristic, a trait,
will evolve by natural selection if
the following three conditions are met.
First, we need variation.
The trait we are interested in has to show
variation among individuals of a species.
Obviously, that's a lot.
For instance,
variation in body height in humans, or
slight variations in the wing
patterns of these moths.
Spring ushers, they are called,
photographed in Naturalis,
the natural history museum
in the Netherlands.
Or as an example, the length of
the neck in these imaginary giraffes.
There are giraffes with
slightly longer necks and
giraffes with slightly shorter necks.
The second ingredient
we need is selection.
How does that work?
Well, in principle, numbers of
animals can increase exponentially.
If we start with two giraffes,
a male and a female, and
we give them everything they need.
Then they get, let's say, four children.
Well, if these children also get four
children, and those children also, and so
on and so on, then sooner or
later we will be covered under
a thick layer of giraffes.
Obviously, that's not the case.
Not all giraffes survive.
Not all giraffes can reproduce.
There has to be some selection.
There is simply not enough food for
all those giraffes.
The resources are limited and
there's competition for those resources.
There is, as Darwin phrased it,
a struggle for life.
Well, giraffes,
they eat leaves from shrubs and trees.
It is very well possible that giraffes
with a longer neck are able to eat a bit
more leaves from the tall trees than
giraffes with the shorter necks.
And we can very well imagine that,
particularly if we make it as busy as on
the previous slide, that if not every
giraffe can survive, that those giraffes
with the slightly longer neck have a
better chance to survive and to reproduce.
That is what we call selection.
In the end, the important thing is
that the trait we are interested in
consistently leads to
a different number of offspring.
Now maybe this giraffe with the long neck
is a mother that can eat a bit more and
produce a bit more milk just
to raise one baby giraffe more
than the mothers with the shorter neck.
Well, that is already sufficient.
The crucial thing is that organisms
with an advantageous trait
will leave more offspring
to the next generation.
That is what the second
ingredient is about.
That is what we call selection.
And lastly, the trait has to be heritable.
In our example,
this seems very likely too.
Now it is easy to imagine that parents
with a long neck also raise offspring
with a long neck.
And now it becomes very clear.
If those organisms with an advantageous
trait get more offspring, and
if that trait is heritable,
well then it must be
that in the next generation there will
be more individuals with that trait.
On average, the length of
the giraffe's neck has increased.
The population has evolved.
And that's evolution,
evolution by means of natural selection.
There's nothing more to it.
Those three conditions are required,
but also sufficient for
evolution by natural selection.
Well, in the next video lecture,
Menno and I are going to have a look for
an example of natural
selection in our surroundings.
