[intro ].

Hi everyone! You got here just in time for a story!

It’s a great one — and it’s about something that really happened. This is the story of the Peppered Moth. A long time ago, most peppered moths had white or gray wings with little dots on them.

That’s why people called them “peppered,” because the dots kind of look like pepper. Instead of sleeping at night like we do, these moths are awake at night and sleep during the day. While they’re sleeping, they like to hang out on trees.

In some parts of the world, they spend lots of time resting on a type of tree called a birch tree, which normally has light-colored bark with dark spots on it. Something called lichen grew on the tree bark, which made it look even lighter. That way, with their light-colored wings, they could blend in with the tree while they were resting and it would be harder for other animals, like birds, to find and eat them.

When animals blend in like that, it’s called camouflage. But even though almost all peppered moths had light-colored wings, there were a few with dark-colored wings with little light spots on them. There weren’t very many of them because it was really easy for birds to spot them against the light birch trees.

But then something happened to make the light trees become dark. About two hundred years ago, people started building factories, places where all sorts of things are made. This was an exciting time because a lot of stuff was being made, and fast, too — everything from clothes to huge metal boats.

We actually have a name for this special time: the Industrial Revolution. But there was a big problem. The factories burned something called coal for energy to power their machines.

They burned so much coal that the smoke made the air all dirty. That made those birch trees darker, too. It was like sprinkling little tiny bits of coal all over them.4 And remember the lichen that grew on the trees?

The dirt in the air made the lichen die, so they weren’t there to brighten up the tree bark. The darker trees were a big problem for peppered moths. It used to be that the light-colored ones would blend in with the trees, but now they didn’t, and they really stuck out.

It was super easy for birds to find them and eat them, and soon there weren’t as many peppered moths as there used to be. But those few moths with darker wings were totally fine — now that the trees were darker, it was easier for them to hide. Suddenly, more darker-winged moths were able to live long enough to have moth babies, which also had darker wings.

And then those moth children grew up and had more moth children. So gradually, over many years, the moths with darker wings became way more common than the moths with lighter wings. Scientists call this process natural selection: when some living things have a trait that helps them survive, like the moths’ darker wings, there end up being more babies that also have that trait.

That’s how peppered moths changed from mostly having light wings to mostly having dark wings. But the story doesn’t end there, Squeaks. When people realized what the new factories were doing to the air, eventually they figured out new and better ways of making stuff without making the air as dirty.

Over time, the air got cleaner again, and guess what happened? The bark on the birch trees got lighter again, and the lichen started to grow back. Those moths with darker wings started to stick out again, and the moths with lighter wings had an easier time hiding from birds.

Natural selection happened all over again! And today, the peppered moths with lighter wings are much more common than the ones with darker wings. That’s where our story ends.

But this was just one example of how one type of animal changed over time because there was something that helped some of them survive. Natural selection happens with all types of living things, all the time. It just usually takes too long for us to notice it as it’s happening.

But it’s how, a long long time ago, humans became very good at running. The people who were good at running had an easier time finding food and running away from animals trying to eat them, so they had more babies, and pretty soon lots of humans could run. Natural selection is also how rats became so good at smelling things — they can smell animals that want to eat them from far away, so they have time to hide.

There are examples of this type of change everywhere we look. In a way, the story of the peppered moth is a lot like the story of how all life on earth has come to be. Thanks for joining us!

If you want to keep learning all sorts of amazing things about our incredible world and all the animals in it, hit the subscribe button, and be sure to check us out on the YouTube Kids app! We’ll see you next time, here at the Fort! [ outro ].