Understand that this polymorphism can evolve more readily if habitat selection occurs, i.e., if each genotype can choose its preferred niche. Subdivided populations require special population genetic principles. Understand why this must be so, at least when the subdivisions do not all share the same gene frequency distribution. Understand the significance of this effect for sampling populations to determine if they are in Hardy-Weinberg equilibrium. Understand the significance of this effect when previously separate populations merge.
Migration acts to unify gene frequencies between populations.
Simply appreciate that non-selective migration must act in this way. The convergence of gene frequencies by gene flow is illustrated by the human population of the United States. This example makes some unstated assumptions, which you might wish to consider and criticize. A balance of selection and migration can maintain genetic differences between subpopulations. Understand that selection-migration balance can maintain a polymorphism which would be eliminated by either selection or migration acting alone.