(a) Because of presence of same set of genes.
(b) Having different pair of genes.
(c) (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.