It was worked out in the 1920s and 1930s by the population geneticists R A Fisher, J B S Haldane and Sewall Wright, and later consolidated into the synthesis of the 1940s known as Neodarwinism. In practice genes in the gene pool spend their time either sitting in individual bodies which they helped to build, or travelling from body to body via sperm or egg in the process of sexual reproduction. Sexual reproduction keeps the genes shuffled, and it is in this sense that the long-term habitat of a gene is the gene pool. Any given gene originates in the gene pool as a result of a mutation, a random error in the gene-copying process. Once a new mutation has been formed, it can spread through the gene pool by means of sexual mixing.
Mutation is the ultimate origin of genetic variation.
Sexual reproduction, and genetic recombination due to crossing over see to it that genetic variation is rapidly distributed and recombined in the gene pool. Any given gene in a gene pool is likely to exist in the form of several duplicate copies, either all descended from the same original mutant, or descended from independent parallel mutants. Some genes, such as the albino gene, are rare in the gene pool, others are common. At the genetic level, evolution may be defined as the process by which gene-frequencies change in gene pools. Immigration, emigration, and random drift are not of much interest from the point of view of adaptation, although they may be quite important in practice.