Cultural interactions result in both progressive and aggressive interactions due to the evolution of those cultures being uninfluenced by one another. What may be considered good etiquette in one culture may be considered an offensive gesture in another. As this occurs constantly, cultures push each other to change.The biological variations between humans are summarized in the ideas of natural selection and evolution. Human variation is based on the principle that there is variation in traits that result for recombination of genes from sexual reproduction. These traits are variable and can be passed down generation to generation. It also relies on differential reproduction, the idea that the environment can't support unlimited population growth because not all individuals get to reproduce to their full potential.
An example of human variation can be found with a cline. A cline is a genetic variation between populations of species that are isolated in their reproduction (such as skin color variation in humans). Human skin color variation is a selective adaptation that relates to the populations' proximity to the equator. Because of pigmentation characteristics within the human population, a system and term emerged to categorize the differing variations. This category is recognized as race. Populations of humans in equatorial regions have selective advantages as a result of their darker skin pigmentation, whereas populations in more northern environments have less selective pressure to evolve darker pigmentation and have lighter skin. Other clines include differences in stature and hair type.