What was the evolutionary synthesis?

There has not been another scientific revolution that caused as much turmoil and dissension as the darwinian one. For almost 80 years it was again and again pronounced to be a failure and to be totally refuted, and at least three major alternatives were proposed to replace it. Yet, in the 1930s-1940s the opposing views were quickly and decisively refuted and a largely unified evolutionary theory emerged. Why and how this happened, however, is still rather controversial.