Ronald Fisher
His book The Genetical Theory of Natural Selection was started in 1928 and published in 1930, with a second edition in 1950. He developed ideas on sexual selection, mimicry and the evolution of dominance. He showed that the probability of a mutation increasing the fitness of an organism decreases proportionately with the magnitude of the mutation. He also proved that larger populations carry more variation and have a larger chance of survival. He set forth the foundations of what was to become known as population genetics.