How does the hardy Weinberg theory define evolution in a population?

Assuming that evolution is simply a change in frequencies of alleles in the gene pool of a population Godfrey Hardy and Wilhelm Weinberg went on to develop a simple equation that can be used to discover the probable genotype frequencies in a population and to track their changes from one generation to another.  This has become known as the Hardy-Weinberg equilibrium equation.  In this equation (p² + 2pq + q² = 1), p is defined as the frequency of the dominant allele and q as the frequency of the recessive allele for a trait controlled by a pair of alleles (A and a).

While mathematically p (or q) could vary right to 1 or 0 in real populations that would bring things to a halt because one of the alleles would be eliminated.