Beginning in the 1930s, evolution became an experimental subject. New techniques, especially in Drosophila, made possible quantitative analysis of natural populations. In addition to a large number of studies on many species, there were four major controversies that dominated much of the discussion and experimentation. Some of the arguments were quite heated. These controversies were: Wright vs Fisher on Wright's shifting-balance theory; dominance vs overdominance as an explanation of heterosis; the classical vs balance hypothesis for genetic variability; the neutral theory of molecular evolution. Curiously, most of these issues were not really resolved. Rather they were abandoned in favor of more tractable studies made possible by the new molecular methods.