Natural selection is survival of the fittest.
What happens is:
- A mutation occurs
- If the mutation is beneficial, the animal will survive longer and reproduce more
- Some of its offspring will inherit the mutation
- These offspring will also have better chance of survival, meaning they live longer and reproduce more
- Over a long period of time this process is repeated and gradually the mutation becomes a common gene in a species and those with the mutation become the only ones, as those without cannot compete with those expressing the mutated gene