Racism has been a defining trait in how human civilizations have interacted and developed in regard to each other. It, however, is founded on differences which separate us visibly but not functionally. Can of worms; 'nuff said.
What if there were real differences? Presume a species with a baseline level of intelligence and interbreedability; assume them to even be humans for now. However, say there is a very distinct sliding scale of physical functionality which begins to emerge upon puberty: on one end, they are possessed of superhuman strength and endurance, but have post-puberty lifespans of only a few years before their bodies fail. On the other hand, they are practically biologically immortal, yet utterly frail and weak. Assume, again, a normal (bell-shaped) distribution along this scale.
Most importantly, there is a very visible external indication of this, whether it is body size or some other manner of odd phenomena (skin markings, hair color, eye size, whatever). How would society develop?
What if the distribution is very lopsided in one direction or another?