Ultimate source of variation is

The ultimate source of variation is Mutation.
Genetic variation is the difference in DNA among individuals or the differences between populations. Mutations are the ultimate sources of genetic variation, but other mechanisms, such as genetic drift, contribute to it, as well.
A mutation is a change in the DNA sequence of an organism. Mutations can result from errors in DNA replication during cell division, exposure to mutagens, or a viral infection. It is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA.