DAVID: I accept common descent, but I remain doubtful about the importance of natural selection. It becomes active only when presented with alternative organisms, so it is basically passive in advancing evolution. Survival of the fittest is a tautology, and worthless.

dhw:So which part of Darwin’s thesis is not supported by genetic modification?

Darwin did not know about genetics! Neo-Darwinism incorporated genetics much later on, and we are still studying how genetic change results in new species.