Answer: Natural Selection
Explanation: Individuals in a species have variation or mutations.
These differences may either be helpful, such as a giraffe with a long neck, or unhelpful, such as a colour variance in a moth that makes them stand out to predators.
The individuals that are more suited to their environment will survive and breed, passing on their genes.
Those that are not suited will die and their genes will not be in the ‘gene pool’.
This is the survival of the fittest!