Some useful generalizations concerning mutation and migration . Migration and mutation are analogous. We obtain similar population genetic equilibrium solutions. If selection is acting on a locus, advantageous alleles will become fixed. Perhaps the selective environment is fluctuating.
Fixation is counteracted by mutation.
Fixation is counteracted by gene flow. Given similar parameters, different populations will converge on the same equilibrium solution. There may be probabilistic aspects to mutation, migration, and selection, but the outsome is predictable. Genetic drift is truly stochastic and works in conjuction with the other forces to determine a distribution of evolutionary outcomes. For example a weakly selected allele is primarily affected by selection if the population is very large .