Answer:
Reproductive isolation prevents interbreeding and does not allow the gene flow between two species. If two organisms can interbreed, they belong to same species.
Step-by-step explanation:
A species is a group of individuals that can interbreed to produce fertile progeny. Reproductive isolation does not allow the members of the two species two interbreed.
Speciation is the formation of new species from existing one. Reproductive isolation does not allow the two populations of same species to interbreed. These populations accumulate more genetic variations and gradually evolve as two separate species.
Without reproductive isolation, populations would interbreed and would belong to the same species.