Actualy, I'd like to say that single ecosystem planets are foolish and unrealistic. Mars, for example, has seasons and there's a noticeable difference in temperatures from more northern/southern locals and more central locals. The presence of nearby mountains also makes a difference.
Whatever eco-system they originally evolved in (For humans it was most likely flat savanna, we are terminators compared to a lot of other land species when it comes to endurance)... whatever Eco-system was originally their evolution site they would have, as they gained intelligence, spread to more and more hostile-to-their-lifestyle ecosystems on the planet and adapted to them too.
Because of this it's foolish to say a world is a desert or a savana world. That's like saying that europe is an old growth forrest, or like saying all of America is prairie. Or like saying england is one massive swamp, or Australia is just a giant desert or...
You get the point. What are the ecosystems that they're less adapted to and, more importantly, how do they change these ecosystems to adapt to them? Do they have large dehumidifiers that they run to make humidity more comfortable in wetter ecosystems of their planet? Heating elements for cooler ecosystems?
Are some of them simply adapted to certain environments the way different groups of people have different skin colors due to the normal sunlight level in the place the population spent the last few thousand years? Or the way that Caucasians are less likely to be lactose intolerant because milk was a staple foodstuff in Europe? Or the way people from places where malaria is endemic are more likely to carry one sickle cell gene one whole gene because it reduces the effects of malaria? Or the way Prima Indians are very prone to obesity because they spent a long long time in a desert environment and developed very effective fat creation and storing routines?
Keep in mind if constructing a racial system that in truth, racial differences are quite small too, but often are exaggerated by groups who want to prove superiority or inferiority of other groups. There's no such thing as superior or inferior in genetics, just which environment they're most adapted to.
So you should think about climate/ecological regions and the adaptions likely to occur in them. Keeping in mind that these adaptions would most likely be less different then the difference between two breeds of dog.