The industrial revolution was when humans started to invent and use machines. Many of the machines released lots of smoke into the air, which eventually landed on trees.

Before the industrial revolution, trees were clean and had lighter bark. After the industrial revolution, trees became covered in ash, which made the bark darker in color. Two types of peppered moths, black moths and white moths, lived on the trees. They were constantly hunted and eaten by birds. The dark moth and white moth populations evolved significantly after the industrial revolution.