The fruit fly groups do not successfully reproduce to make viable offspring. The biological species concept has to do with whether living offspring are produced from a mating attempt between two distinct groups. If hybrids form, then the two groups may not have fully diverged.
I think you're talking about histones. Histones are proteins found in eukayotic cell nuclei. Histones are what DNA wrap around so it fits inside the nucleus and helps with the formation of chromosomes.
