Which mode of natural selection is most likely to contribute to speciation?

Natural selection leads to speciation because as populations in a species change due to mutations, the less fit organisms die out, while the fit organisms survive and pass on their genes. As this process continues, the evolved population will eventually accumulate enough genetic changes, such that the evolved population can no longer produce fertile offspring with the original population, hence it is said that speciation has occurred. 

Directional selection is a primary cause of phenotypic diversification