NEO DARWINISM OR SYNTHETIC THEORY OF EVOLUTION -
Five factors have been identified as main bases. First is gene mutation, second is chromosomal changes and third is genetic recombination to cause genetic variability. These changes are then acted upon by natural selection and reproductive isolation which guide the population into adaptive channels.
There are also accessory factors like gene migration and hybridization to increase the genetic variability. Lastly the chance factors (genetic drift) in small population is also relevant in the evolution of the new species.