Hardy-Weinberg equation states that the frequency of occurrence of alleles of a gene is constant. The gene pool remains constant. The total sum of allelic frequencies is 1. Let the individual frequencies be p, q of alleles A and a respectively. Then the frequency of AA is square of p. Similarly, for aa it is square of q and for Aa it is 2pq. Hence p$$^2$$ + 2pq + q$$^2$$ = 1. It is the binomial expansion for (p+q)$$^2$$ = 1. Any change in this equation signifies evolution.