What is the working mathematical formula for evolution

"Let us assume that there is a trait that is determined by the inheritance of a gene with two alleles--B and b. If the parent generation has 92% B and 8% b and their offspring collectively have 90% B and 10% b, evolution has occurred between the generations. The entire population's gene pool has evolved in the direction of a higher frequency of the b allele--it was not just those individuals who inherited the b allele who evolved."

The Hardy-Weinberg Equilibrium equation is a useful null hypothesis to test because it assumes there is NO evolution occurring in a population. A violation of any one of the above conditions will result in a change in allele frequencies which rejects the null hypothesis and therefore demonstrates that the population has evolved. And it has been demonstrated literally THOUSANDS of times.

Google "population genetics" for a much more detailed look at the mathematical tools used by evolutionary biologists. In particular, the Berkeley site.