Drifts

Drifts are not really another species, not really, but are in fact variants on the human genome. 
They are still human at their core, with the same DNA structures, but the fall of humanity, access to genetic modification, and the subsequent isolation of many pockets of survivors was a rife situation for small groups to develop...differently. 
The Herma and the Pelagios are two prime examples of Drifts, showing the low and high end of changes that humanity has gone through to survive. One still looks largely human, just with different averages. The other could easily be confused with another species if their DNA wasn't the same.
Due to the isolation that caused their divergence, Drifts often have their own cultures, societies, and sometimes even languages than base humanity. This has caused a great deal of tension within the solar system, and the outlying colonies, though reintegration is proceeding steadily. Drifts do have a tendency to remain close to or on their own homes, as those are the environments they are most comfortable with. It seems in their adaptation they sacrificed some of humanity's innate flexibility. 
Strangely enough, M's can also fall under the umbrella of Drifts. Some models, produced on the respective planets, come out with the same changes made to them as the local Drifts. Others undergo operations that change them to look the part, in an effort to blend in, usually. There is some evidence to suggest that M's can naturally come into that state as well, with their internal nanites reacting to their environment and causing the process to happen much faster than it would in a human.