In the previous video lecture,
we have seen that evolution by
natural selection is very simple.
As soon as three ingredients come
together, it automatically occurs.
As soon as you have variation and
selection, and heritability, evolution by
natural selection automatically takes
place, but it's still a bit abstract.
It would be nice to see an example
of natural selection around us, and
that's why we are here,
together with Menno Schilthuizen,
our co-instructor here in
a forest close to Leiden.
>> That's right, and
what better animal to show evolution in
action than these pretty garden snails.
This is the common garden snail, Cepaea nemoralis.
And as you can see they
have very colorful shells.
They come in shells that can be pink or
yellow or brown or stripy.
>> So the first condition for
evolution by natural selection
actually seems to be met.
I mean,
there is variation among those animals.
>> Yep.
There is.
>> But is a variation also heritable?
>> Yes it is, yeah.
There are genes that determine
the color of the shells.
There's also genes that determine whether
or not they have these dark brown
bands and there's even genes that
can turn multiple bands on or off.
>> So, there's variation.
Variation is heritable.
But is there also selection?
Does the color of their shell somehow
determine the chance of the snail to
survive?
>> Yep, in two ways.
The first is overheating.
The snails in the summer, they sit on
the vegetation, often in the full sun and
they can get very hot,
which is not good for a snail.
But the yellow shells are better
protected against overheating.
Because the reflect more of the sun's
radiation than the brown ones.
>> And that can be the difference
between surviving or dying.
>> Yeah, can just make the difference.
>> But why are those brownish and
those pinkish shells still around?
I mean, why aren't all of them yellow?
>> That's a good question.
That's where the second type of selection
comes in, because there's also camouflage.
>> Is something eating these things?
>> Yep, there's a bird, the song thrush
which likes to eat these snails, and
they hunt by eye.
And some color morphs are better
camouflaged than others, but
it depends on the habitat.
So in this open sort of yellowish
vegetation with lot of stripy grass,
these yellow snails with a dark bands,
they're better camouflaged.
But over here in the forest,
these are more uniformed brown and
pink ones are better camouflaged.
>> So would you expect to find
more yellow ones in the grass and
more brown ones in the forest?
>> Yeah.
That's exactly what you expect to find.
>> But don't they crawl?
I mean, they can crawl from the one
habitat to the other right?
>> They crawl a bit but
they don't crawl very far.
Most snails in their whole life,
they just travel just a few meters so
selection can still, if it's strong
enough, can still produce different colors
of shells in this habitat compared to
this one which is just a few meters away
>> Okay, well let's check that.
>> Okay, let's do that.
So these are from the grassland.
>> And these from the forest.
Evolution in action.
>> So this shows us that evolution
can be observed here and now.
It does not need to
take millions of years.
And if these snails occur where you live,
you can even participate in a citizen
science project, the Evolution MegaLab.
And that allows you to observe
evolution in your own area.
>> So now, we've been in the field.
But in the next lecture, we are going
to use a completely different way to
understand evolution by natural selection.
Computer animations.
