These are groups of reproducing populations that are isolated from other groups.

Species are groups of reproducing populations that are isolated from other groups.

Explanation:

All organisms that belong to the same species are able to mate with each other and produce viable and fertile offspring. However, in some case the species belonging to different species are also able to mate but they produce infertile offspring. These infertile offspring are healthy but they cannot reproduce. Thus, it is better that organisms belonging to one species are kept away from the organisms of other species.  

the supermassive black hole is in the middle of the galaxy.