The modern theory of evolution states that a basis for variation within species is provided by genetic mutations.

Here's why:

* Genetic mutations are changes in the DNA sequence. These changes can be small (a single nucleotide change) or large (a whole chromosome duplication).

* Mutations introduce new variations into the gene pool. They can be beneficial, harmful, or neutral in their effects.

* Natural selection acts on these variations. Individuals with beneficial mutations are more likely to survive and reproduce, passing on their genes to the next generation.

* Over time, the accumulation of beneficial mutations leads to the evolution of new traits and species.

So, genetic mutations are the ultimate source of variation that fuels the process of evolution.