Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Over several years, a scientist successfully isolates a species of flower for white color. This year, when the new generation of flowers blooms, one yellow flower is found in the population. Which is the best reason for the appearance of a new flower color within the species?
genetic mutation
Mutation.