How exactly does natural selection know if a genetic change is successful? What feedback does it get? It has to make random changes to even start to evolve.
In order to improve the genetic structure, it has to make changes to the code itself. Mutations are either neutral or harmful - very rarely has one been shown to be even minorly beneficial, and none have shown the steps between species.
Diploid organisms (plants and animals) have two copies of each chromosome. Change just one gene and see what happens.