The last two are the answers
Natural selection has everything to do with environment and how the populations adapt to the environment. For example there could be a rocky environment, and in that rocky environment there are grey mice and there are white mice. The grey mice are able to blend into the color of the rocks, but the white mice stick out like a sore thumb and end up being easy pray. The white mice population will end up going down, while the grey mice population goes up due their color being able to match the environment making them more hidden to predators.
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