Microbial populations are microbes that are all the same species living in a given area. In an attempt to connect this to the high-throughput sequencing stuff we are going over, I suppose that all the microbes in a given population will have the same 16S sequences (and therefore the same V4 hyper-variable region). Microbial populations differ from microbial communities in that communities are whatever microbes, no matter the species, that live in a given area. The community is impacted by how the microbes all interact with each other. In a microbial community, if you sequenced the 16S genes (like we plan to in lab), then you would get all sorts of different sequences (I think).