This thread seems to have stalled along with the entire Board. Obviously they do. Geographic isolation has already resulted in groups displaying size, pigment, hair texture and other commonly recognized characteristics. None of this has been due to enough variation to get called speciation, although eventually I imagine that it could.
Environmental stressors are an obvious cause with another being a new exploitable niche.
Of course, so far we have overcome problems and exploited whatever we wanted with our brains and tool building abilities, with evolution rendered unnecessary. Otherwise, although we don't look particularly specialized, our brainy manipulative abilities and our huge gene pool will make evolving tough, whatever stresses we confront. Now if we took a group of particularly intellectually challenged folks who were unable to design tool and manipulate their environment, or had a group philosophically committed to living without any sort of language or technology, we might see some evolving over time in response to changes in the environment. Stronger teeth and jaws, thicker pelts, sharper senses, whatever would be needed to survive an environment that we couldn't live with unless we were able to alter it. Hey, we would probably look more like another ape, but we would have evolved.