1) Natural Selection - This is a phenomenon by which organisms better adapted to the environment tend to survive and reproduce more.
2) Geographical barriers - This is a phenomenon by which geographical barriers like distance, mountains, rivers etc. separate the poplulations of a species. Due to this, the population adapt differently to their environment, accumulate different variations and get reproctively isolated which ultimately results in the formation of a new species.
3) Genetic Drift - Random changes in the frequency of alleles in a gene pool, usually in small populations, irrespective of it being beneficial or harmful.