Question

Why it is so that some of the animals bear the same number of chromosomes but they are not members of the same species?

Answer 1

The same number of chromosomes does not ensure that the same instructions are present on them. Moreover, the biological concet of species suggests that production of viable offspring should be possible. Chromosome number acts only at the level of postzygotic mechanisms, while there are multiple prezygotic isolating mechanisms. For example, sex organs could be of different size, making mating between two species with the same number of chromosomes impossible.