In Hawi'i near a volcano that hasn't been active in hundreds of years, lives a population of mice. The mice have developed an adaptation of green fur in order to camouflage into their surroundings on the ground. The bright tropical bird is unable to see the population of mice.
The volcano, that hasn't been active for hundreds of years, had awoken. It spewed lava and the population of green mice experienced a shift in their environment that would change them forever.
The lava cooled and covered the environment in a layer of igneous rock. The environment no longer had lush green grass which was a selective pressure for the green mice. Their fur no longer helped them and became the main prey for the birds that lived there. Variation became present in the population due to overproduction of the mice, where now there were black mice. The black mice did not get eaten by the predators.
The genetics of the population of mice shifted to the majority of the population having black fur due to the environment being almost entirely dark. However, the aftermath of the volcano would change the environment again.
The volcano erupted for a very long time which produced the most ash that anyone had ever seen. So much so, that it had blocked out the sun. The tropical climate of Hawai'i and the planet had reverted to Arctic temperatures. The tropical birds died due to their inability to adapt and the population of mice was now almost entirely white mice.
More variation came to the environment: polar bears and penguins moved into their new Arctic home.