Final answer:
Mutation is the ultimate source of genetic variation that introduces new alleles into a population, crucial for the process of evolution.
Step-by-step explanation:
The ultimate source of all genetic variation, providing the raw material for other evolutionary forces like natural selection and genetic drift to act upon, is mutation. A mutation is described as a permanent change in the DNA sequence. This process can introduce new alleles (variations of a gene) into a population, thereby generating new genetic diversity. Mutations may have various effects on an organism's phenotype ranging from deleterious, beneficial, to neutral, influencing the evolutionary fitness of the organism in different ways. While other mechanisms such as gene flow and genetic recombination also contribute to genetic variation within a population, they do not create new genetic information but rather redistribute existing genetic variants. Therefore, when considering factors that introduce new genetic variation, mutation holds the primary role.