If you give a cat a girlfriend, he'll probably ask to get married.

If you let the cats get married, they'll probably want to have kittens.

If you let the cats have kittens, then the kittens will inherit a mix of the parents' genes.

If the kittens inherit a mix of the parents' genes, there might be a mutation.

If there is a mutation, then the kittens will have different traits.

If the kittens have different traits, then there will be more variation in the population.

If there is more variation, then some of them will be better suited for their environment than others.

If some of them are better suited, then they will be more likely to escape their predators.

If they escape their predators, then they won't become prey.

If they don't become prey, they will survive.

If they survive, then they can reproduce and pass on their traits.

If they pass on their traits, they'll teach their kittens to be nocturnal and hunt at night.

If they hunt at night, then they will need to compete with the owls for mice.