Selective pressure of a trait: Reproduction of pressure from the traits. The mice are on the ice, so their predators wouldn't be able to find them, which is because of their traits.
Mutation of a trait: A change in the DNA that affects its genes, which affects its traits. The rest of the mice are the same color, but the one mouse has a different color because of its traits from the parents.
Natural selection of a species: How animals adapt to their habitat. The dark mice have an easier time living in the habitat they are in because they can camouflage and survive.