Parakeets are birds whose basic plumage color is green. However, parakeets exhibit a wide range of color both in the wild and in captivity. Which process is the source of the wide color variation observed in parakeet populations?

The questions have been asked in several important state and regional science exams in the United States. For example: TAKS-Texas Assessment of Knowledge and Skills, New York State Educational Department Regents exams, California Standards Test, etc.