Biological evolution is the change in the allele frequency of a gene in a population over time. That is to say some genetic change has happened in the population between generations. Only populations can evolve, not individuals. Individuals can not change their genetic makeup. Only between generations, is there the possibility for genetic changes due to the forces of evolution. These forces are natural selection, mutation, gene flow, nonrandom mating, and genetic drift. Evolution is a measure of a population, not of an individual. Genetic variation, genetic differences between individuals, must exist for evolution to occur.

Charles Darwin defined evolution as descent with modification. However, Darwin did not understand the genetic basis to evolution. Not until Gregor Mendel's work was rediscovered in 1900 could modification with descent be understood in terms of maintaining genetic variation. The mathematical proofs of Godfrey Harold Hardy and Wilhelm Weinberg, known as...