Which statement regarding genes and evolution is true?
A. If an organism has a mutation, it will be less "fit" (less able to reproduce) than other organisms.
B. Today's plethora of microorganisms is the result of an extremely brief period of genetic recombination events.
C. Mutations function to prevent the offspring of an organism from evolving.
D. Random, heritable genetic mutations are a source of genetic variation. This variation is necessary for natural selection to occur.