The best answer would most likely be genetics or genetic theory.
There were many doubts about Darwin's theory because Darwin could not yet describe the mechanism of inheritance. Mendel's work on inheritance was already published, 2 years earlier, but this eluded Darwin. It was only till after Darwin's death that the science of genetics came about.
This union brought about the "modern synthesis," a theory that proposes that both genetics and natural selection is a result of evolutionary changes.
