How do reproductive barriers relate to the biological species concept?

The effect of reproductive barriers on evolution, or, on , is usually causing isolation, and, in due to that, differentiation from a “matrix population” (the original population).
So, reproductive barriers are normally an important factor to the specialization process itself.
In the specialization process, there will be a point when a population is not able to reproduce with the individuals from its “matrix population”, but it is still morphologically similar to them. Then it will depend on your concept of to say if it is or if it is not the same species.