Bullet points: Many things and activities (such as xenophobia) may be natural, without being fair or ethical, and some forces (such as natural selection) are literally natural, due to nature and not human choices. A species is a total collection of individuals (e.g., animals) that can interbreed. Sex between species normally does not result in viable offspring. There are exceptions, such as a horse and donkey producing a mule, but then the offspring is usually (again, with exceptions) infertile. The fact that interracial sex is just as fertile as intraracial sex indicates clearly that human beings form one species, with a wide range of genetic traits such as skin color, eye color, hair color, even height. Height is an example of a parameter that is partially genetic and partially due to environment (e.g., nutrition). Evolution acts on populations, not individuals. Natural selection causes evolution over a relatively long time (many generations) and sexual selection causes faster evolution (few generations). Natural selection favors the traits that allow the individual to survive long enough to procreate, not necessarily to live to an old age well past fertility. A perverse example is the prevalence of sickle-cell anemia in African populations, since the sickle-cell trait helps resist malaria long enough to reach fertility, but kills the subject in adulthood. Sexual selection is the choice of individuals as to who they wish to have children with. In humans, this is a reasonably intelligent choice, but in lower animals it is more instinctive. Choices are made for appearance, size, behavior, and other parameters, not all of which are genetic traits. If a population of a given species has a finite geographical extent, that population may evolve so that over time the traits of individuals in that population are a better fit to the environment. In antiquity, this produced populations isolated from each other that had different characteristic sets of traits. The classic example is skin color: nearer the equator, dark skin protects against sunburn and skin cancer; farther north, light skin is more efficient to make Vitamin D from less sunlight. This evolution is due to natural selection over a long time frame. Some other features of ancient isolated populations may have had accidental origins, reinforced through sexual selection. I do not know of any inherent survival advantages to the Asian eye shape, for example. Thus far, we have seen how different geographic areas could have human populations that evolved into recognizably different appearances (mostly skin color, but other traits as well). Should two such groups encounter each other in warfare, the difference is reinforced since each group recognizes members of the enemy group by skin color. Also, should two or more such groups mingle with each other in a single geographical area through migration, they could interbreed promiscuously, producing a population that combines the original traits, or they could segregate themselves into separate breeding communities. Often, this choice depends on the recent history of warfare between the groups. Now, in modern times, individuals are free to move all around the world, far from the lands of their ancestors. The original groupings may persist: for example, in the US with its history of black slavery, the social forces towards segregation may have been natural without being ethical. In other countries, a large group of mulattos resulted from interbreeding. Does any of this still matter? Now we are out of evolutionary biology and genetics into sociology. If the groups maintain their separation, then they probably have different social norms. More important, due to xenophobia, they may also be hostile to other groups. A classical liberal prescription for all this is that if the economy improves and good education is forced on everyone, that the hostility between groups can decreasethis seems to work somewhat. A forced interbreeding program would definitely work, but that is not a liberal suggestion. Forced segregation is inherently unfair: separate is intrinsically unequal, as stated in Brown v. Board of Education. Genocide is a horrible crime, and a waste of human potential.