Natural Selection occurs through reproduction of an organism over a long period or by mutations that help or hurt the organisms population. These two mice have decided to reproduce and have a litter of babies.
The mice have seven babies with all different variations of genes and one of them seems to have had a mutation at birth giving him red fur and there may be another secret about him.
The group of mice decide to go downstairs to scavenge for food when they come across rat poison and everyone dies except the red furred mouse. This is because this mouse has a mutation that makes him immune to rat poison.
This lonely mouse finds a female new comer and decides to mate with her.
The couple has four babies and two have the same genes as dad while the other two do not allowing the rat poison to kill them. The gene that makes the red mice immune to the rat poison is heritable and offspring can find themselves with the same genetic mutation.
This gene that protects the mice gives them a reproductive advantage as being immune to rat poison allows them to survive in this house. Over generations of offspring, most of the mice will have this gene and the rat poison will be useless for the family of humans that live here. This is a great example that shows how natural selection can help a population survive.