Hardy-Weinberg maintains gene frequencies at an equilibrium in the absence of other forces. The four parameters change gene frequencies and typically perturb Hardy-Weinberg equilibrium. Components of this lecture are available as a simulation exercise. The equations used to simulate selection can be readily derived in using algebra very similar to that used for the H-W equilibrium.
Heterozygotes are a large reservoir of genetic variability.
Play with strength of selection. Overdominance occurs when the heterozygote at a locus has a higher fitness than either homozygote. One the best known examples of overdominance is sickle cell hemoglobin in humans. The HbA allele is the normal allele, HbS is the sickle cell allele. Individuals who are homozygous for the HbA allele are susceptible to malaria in West-central Africa.