The Hardy-Weinberg Law described a population that exists in genetic equilibrium.
Several factors can act to change fitness.
Viability and fertility are traits that are associated with fitness and are directly related to the ability of an individual to survive long enough to reproduce. By altering the fitness of an individual, the mating distribution will change. The distribution will change because genotypes in the subsequent generation will not appear in direct relationship to the gene frequencies of that population prior to the change. Consequently the gene frequencies will change and the population will evolve. The synthetic theory of evolution as described by Sewell Wright attempts to explain evolution in terms of changes in gene frequencies.