Final answer:
Mutation is the ultimate source of all genetic variation in and among populations, whereas other mechanisms such as genetic drift and gene flow can influence the distribution but do not create this variation.
Step-by-step explanation:
The ultimate source of all variations in and among populations is mutation. Mutation introduces new genetic variation that can lead to different traits within a population. Genetic drift, gene flow, and natural selection are all mechanisms of evolution that can act on genetic variation but do not introduce new genetic variation themselves. Genetic drift influences the allele frequencies in small populations by chance events, whereas gene flow introduces new alleles by migration of individuals between populations. However, it is mutation that creates this variation in the first place by changing DNA sequences.