John Burdon Sanderson Haldane (1892-1964) was a brilliant polymath in the sense that was only possible in the early 20th century. He was one of three founders of modern evolutionary biology and made important contributions to enzymatic biochemistry. In population genetics, he is best known for Haldane's Rule and the mutation load caused by deleterious alleles in mutation-selection balance. Haldane arguably led the most colourful academic life relative to his rather proper and respectible British peers; he was a dedicated Marxist and wrote several books on the relationship between science and society, as well as a book for children.
If Fisher, Wright and Haldane were the Beatles, Haldane would be George.