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This is my fourth post explaining why races don't exist--the other three are Another Reason Races Don't Exist, The Main Reason Races Don't Exist, and Yet One More Reason Races Don't Exist. As before, I'll begin by pointing out the difference between what might be called social race--using the word race to classify people by what they look like, or by their ancestry, or by some other social criteria--and biological race. In contrast to biological races, social races do exist--though racial systems of classification differ from one culture to another--and form the basis for unequal treatment of differently categorized groups
In my last post on this topic, Yet One More Reason Races Don't Exist, I discussed reasons that the notion of "racial syndromes"--the necessary co-occurrence of supposedly racial features--is mistaken. In this related post I want to explain why it is that these so-called racial features are variable (e.g., skin color varies from dark to light) rather than fixed (e.g., the mistaken belief that one biological race has dark skin color and another has light skin color).
Suppose you have a breeding population (members of a species that breed among themselves more than they do with other populations) of humans that has a high percentage of a certain gene. Let's call them Population 1--think for example of a geographically isolated village before modern transportation. At some distance there is another human population that we will call Population 2. Since there is contact between the two populations, including sexual contact, over time some people in Population 2 will also have the gene. However, because members of Population 1 never breed with those in Population 2 as often as they do among themselves, the percentage of people with the gene in Population 2 will never be as high as in Population 1. At another distance even further away from Population1 is Population 3. Following the same logic, the gene will spread to Population 3, but its frequency will never be as high as in Population 2. And so on, for populations 4, 5, 6, and others--the further from Population 1, the lower the percentage of people with the gene.
This gradual geographical change in the frequency of a gene (or, for that matter, a visible trait like body shape or skin color) is known as a cline--think of words like decline or incline.
So the reason that so-called racial features are variable rather than fixed is that their distributions are clinal. (Clinal distribution also applies to traits determined by multiple genes--the further away, the fewer the genes, and, therefore, the less of the trait.)
As with topographical maps, where dark brown signifies great altitude, and dark green signifies sea level, it is possible to make maps of gene frequencies, or the frequencies of visible traits, where different colors represent different frequencies instead of different altitudes.
You can make a bunch of these maps of the planet, each for a different gene or trait; and if the maps are transparent, you can place them on top of one another. If races existed, then they would all pretty much coincide. But they don't. Different traits go off in different directions, and are found together or separated in different parts of the world.
Why is this? The reason is to be found in human history and pre-history. Unlike species, which can be roughly represented as a branching tree because they have been reproductively isolated for a long time, humans are a relatively new species, without a lot of genetic variability, and travel all over the place and mate with one another wherever they are. Rather than a branching tree, the history of human populations would more accurately be depicted as a tangled lattice. The hodgepodge of clines going every which way reflects the chaotic human past of groups spreading out to populate the planet, breaking apart, coming together, and having gene frequencies affected by marriage rules and sexual taboos, not to mention war, disease, and natural disasters.
In brief, human physical variation is clinal, and because the various clines do not coincide, it is not racial.
Image Source:
Complexion by Polylerus
http://en.wikipedia.org/wiki/File:Complexion.jpg
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