Assertion: “The Biological Species” concept helps us to ask how species are formed.
Reason: The concept of biological species focuses our attention on the question of how reproductive isolation comes about.

Both Assertion (A) and reason (R) are correct and R is the correct explanation of A.

Ernst Mayr proposed the biological species concept, which defines species as groups of actually or potentially inter-breeding natural populations which are reproductively isolated from other such groups. Biological species concept emphasizes on species are a group of interbreeding populations that are isolated from each other. This is called speciation. Species are groups of individuals that breed together. Members breed with others that can give birth to potential organisms not according to their physical appearance.