(a) Because of presence of same set of genes.
(b) Having different pair of genes.
© (i) Natural selection.
(ii) Geographical isolation, Reproductive isolation.
(iii) Genetic drift.
(iv) Accumulated variations causing changes in the DNA of germ cells to an extent, such as a change in the number of chromosomes. Due to this, the germ cells of the two groups cannot fuse with each other. This results in the emergence of a new species.