What factors could lead to the rise of a new species?

Rise in new species can happen if two groups of the same species are somehow prevented from interbreeding for several generations. This can happen because of geographical isolation or because of some genetic drifts.Geographical isolation of a population caused by various barriers (such as mountain ranges, rivers and sea) which leads to reproductive isolation due to which there is no flow of genes between separated groups of population.Also the Genetic drift caused by drastic changes in the frequencies of particular genes alone can also be the reason.