Part of Horiz. Spp Concept, plugs into population genetics framework. Species are groups of interbreeding natural populations that are reproductively isolated. If individuals interbreed, allele frequencies stay similar within population. If no interbreeding, alleles between populations vary. Can Explain similarities within species. Concerned with isolating mechanisms like Pre/Post-zygotic barriers. Probs with BSC is concerning asexual organisms like bacteria, fossils and poorly known species where interbreeding cannot be tested, also incomplete isolation in ring spp or hybrid zones.