The Hardy-Weinberg law is key to understand how evolution works on populations. British scientist G.H. Hardy and German doctor Wilhelm Weinberg independently proposed this theory in 1908. The Hardy-Weinberg law states that genes remain mathematically constant in a randomly mating, closed population from one generation to the next, unless certain disturbances occur. These disturbances include non-random mating, selection, mutations, gene flow (immigration or emigration), genetic drift, and bottlenecks (a population falling below a minimum size threshold). Any population experiencing one or more of these pressures will show a change in the frequency of one or more alleles. Over time these changes will cause the population to evolve.
The Hardy-Weinberg law also includes mathematical formulas to help predict the allele frequencies from one generation to the next. If you need to learn or practice the calculations, check out the second link below.