Darwin's theory of evolution is based upon the idea of natural selection.
Natural selection is what causes certain characteristics to be more prominent in humans in different parts of the world.
Traits that are neither necessary nor helpful for survival can disappear over long periods of time due to natural selection.
Due to the process of natural selection, prey that blend in to their environments are more likely to survive and pass on those genes that allowed survival.
The process of natural selection allows the fittest creatures to survive and continue to reproduce.