How does natural selection drive evolution?
Natural selection drives evolution by preserving favorable variations and causing the extinction of unfavorable variations.
The main force or drive of natural selection is extinction. Populations always over populate more organisms are formed than the environment can support. Natural selection will "select" variations in the population for extinction that are poorly adapted to the present environment.
Adaptive evolution can be observed in the preservation of well adapted variations. The Cheetah of East Africa is very well adapted to its environment. It is so well adapted that only one variation of Cheetah now exists, all other variations have become extinct. Sadly a change in the environment would likely cause the extinction of all Cheetahs as there are no other variations left that might be adapted to a new environment.
Natural selection does not create new variations. Natural selection is limited to eliminating existing variations. Because of this natural selection can only explain adaptive evolution. Descent with modification or Darwinian evolution can not be explained by natural selection. A source of new variations must be invoked as the driver of Darwinian Evolution.