[tsimionescu]

Yes, and for a fixed mutation chance, the bigger the population, the greater the likelihood that a mutation will happen. Say a mutation has a 1 in 1000 chance of happening. For a disease that infects 1000 people, you would expect 1 person to end up with some mutation. If the same disease infects 1,000,000 people, you would expect 1000 mutations to occur.

[krona]

Likelihood of mutation is not the dominant factor in determining how or why a variant becomes established in a population.

[tsimionescu]

Yes, the size and density of the population and the efforts to contain it are.