The Mendelian and biometrician models were eventually reconciled with the development of population genetics. In a series of papers starting in 1918 and culminating in his 1930 book “The Genetical Theory of Natural Selection”, Fisher showed that the continuous variation measured by the biometricians could be produced by the combined action of many discrete genes, and that natural selection could change gene frequencies in a population, resulting in evolution. Another British geneticist, J.B.S. Haldane, applied statistical analysis to real-world examples of natural selection and showed that natural selection worked at an even faster rate than Fisher assumed.
The American biologist Sewall Wright, who had a background in animal breeding experiments, focused on combinations of interacting genes, and the effects of inbreeding on small, relatively isolated populations that exhibited genetic drift. In 1932, Wright introduced the concept of an adaptive landscape and argued that genetic drift and inbreeding could drive a small, isolated sub-population away from an adaptive peak, allowing natural selection to drive it towards different adaptive peaks. The work of Fisher, Haldane and Wright founded the discipline of population genetics. This integrated natural selection with Mendelian genetics, which was the critical first step in developing a unified theory of how evolution worked.