This is what a normal day looked like in the forest of Alaska. The mouse are white which helps them blend in with snow. Their camouflage gives them and advantage over other colored mice because it makes there predators(the snakes) harder for them to find them.
suddenly a heat wave hit, all the snow in there habitat melted! This caused the white mice to be very vulnerable to the snakes because they where no longer blending in with the snow. the mice started dying very quickly and not being able to reproduce with other mice because the snakes had very easy targets.
Oh look! A dark furred mouse. This mouse was created by a mutation. Mutations are random! In this case the mutation gave the mouse an advantage. Not all mutations give animals advantages. In some cases mutations give animals disadvantages.
This mutation caused the dark furred mice to have a great advantage over the light furred ones. The population of the dark furred drastically increased because they where able to live longer. Since the dark furred mice where able to live longer they are also able to reproduce more unlike the white mice.
Eventually, all the white mice died off because they where so vulnerable to the snakes. One thing you have to take into account is that all these events do not happen overnight it could take thousands and even millions of years for some of these slides to happen.
This is what a normal day in Alaska now looks like. THE END!