In 1938 the English biologist Julian Huxley proposed using the Greek-derived word cline to represent the gradual change of a single biological trait (e.g., skin color) in a given species over a geographical area. The gradient in the expression of any such trait represents a response to the graded change in theintensity of the selective force affecting the manifestation of the trait in question. Many human traits are clinally distributed, but they often require laboratory testing to discover their various manifestations. The most easily perceived trait that has a clinal distribution is skin color. Long-term residents of the tropics have a maximum amount of pigmentation in the skin, while long-term residents of arctic locations have the least amount of pigment in the skin. The transition of pigmentation from the tropics to northern parts of the globe follows an unbroken gradation, making the distribution of skin color a true cline.
The substance that accounts for pigment in the skin is called melanin. The more particles of melanin, the darker the skin. In areas of intense sunlight, such as the tropics, the penetration of ultraviolet B rays in the human skin can lead to cancer. Skin with a lot of melanin particles will block the penetration of ultraviolet rays and protect the person in question against the possibility of contracting skin cancer. In addition, melanin prevents ultraviolet radiation from breaking down folic acid in the body, thus helping to prevent neural tube defects in newborns (Jablonski 2004).
“Neutral theory” notes that where there is no selection maintaining a given trait, random genetic changes will not be selected against. Most such random changes interfere with the development of the structure coded by the genes that control its development. If there is no selection maintaining that structure, random mutations will decrease its manifestation through time—hence the reduction of pigmentation in northern populations.
Visual acuity and color perception are also clinally distributed, but the cline does not have the same distribution as the cline for skin color. As Richard H. Post documented in 1962, those populations that depended on hunting longer than others retained a degree of visual acuity that decreased in proportion to the length of time the other people in question had been practicing farming for their subsistence. Sub-Saharan African populations, for example, have fewer individuals in each population who show extremes of nearsightedness and far-sightedness, color blindness, and astigmatism than European populations. Among the latter, however, the inhabitants of the northwestern edge of Europe have fewer visual defects than people in the Middle East, where farming has been a way of life for longer than anywhere else in the world. In line with this picture, Hugh R. Taylor (1981) has shown that the greatest amount of visual acuity is found among Australian Aborigines, who were hunters up until a century or two ago.
Human tooth size is also clinally distributed, with those who depended on cooking for the longest having the smallest teeth in the world (Brace 2005). The people who inhabited Europe during the last glaciation were dependent on hunting for their subsistence, but they could hardly eat a whole Pleistocene cow at a single sitting. A day or so later it would have frozen solid. The pieces could be cooked, however, and that not only thawed the meat but reduced the amount of chewing necessary to get it to a swallowable consistency. The descendants of these people have the smallest teeth in the world. Sub-Saharan Africans did not have to thaw their food, but eventually they discovered that cooking counteracted the effects of decay. African teeth have been reducing in size, but not for as long as the teeth of the people further north. Cooking was introduced in Australia late in the eighteenth century after first contact with Europeans (later than in other places), and it spread slowly southward from the northern edge of the continent. There is a cline for tooth size in Australia that ranges from African-sized teeth in the north to larger teeth in the south, where the indigenous inhabitants have the biggest teeth in the world, being fully Neanderthal-sized. This is completely the reverse of the skin-color cline in Australia, which ranges from equatorial degrees of darkness in the north to a kind of medium brown in the south (where there is also a visible amount of juvenile blondism).
The ABO blood group genes also have clinal distributions, but it is not certain what the relevant selective forces were that produced those distributions. What is clear, however, is that the distributions have no relationship at all to the distributions of skin color, tooth size, or any other traits that have clinal distributions. The distribution of hemoglobin S, however, is well understood (Brace 2005, pp. 10-11). Hemoglobin is a protein molecule with 574 amino acid residues arranged in two alpha and two beta chains. Hemoglobin S differs from hemoglobin A by having avaline instead of a glutamate at position 6 of the beta chain.
An individual with two genes for hemoglobin S will have sickle-cell anemia and a shorter life expectancy, while an individual with normal hemoglobin (AA) is susceptible to a particular kind of malaria. The person with hemoglobin AS will not suffer from anemia under most circumstances and has the advantage that many of the infecting malaria parasites are removed from the bloodstream before they can reproduce. Where falciparum malaria is prevalent, the possession of a single S gene gives a person a survival edge over the person who only has A at that locus.
The completely independent and unrelated distribution of human traits that are adaptively advantageous is what led the anthropologist Frank Livingstone to declare, “There are no races, there are only clines” (Livingstone 1962, p. 279). This is true, however, only for traits that are under selective-force control, though not all traits under selective-force control are clinally distributed. Members of all human populations have to be able to learn their languages and have the wits to survive, and they all need hearts, kidneys, livers, and other organs that will last a normal lifetime. As a result, there is no demonstrable gradient in any population in either intellectual ability or in the functional capabilities of the internal organs. Many traits—such as eye shape, ear shape, and cheekbone shape—show recognizable regional similarities. Anything that clusters in regional fashion does so because of genealogical relationship and not as the result of natural selection. The regional clustering of such nonadaptive features has been referred to as “family resemblance writ large” (Brace 2005, p. 16).
Jablonski, Nina G. 2004. “The Evolution of Human Skin and Skin Color.” Annual Review of Anthropology 33: 585–623.
Livingstone, Frank B. 1962. “On the Non-Existence of Human Races.” Current Anthropology 3 (3): 279.
Post, Richard H. 1962. “Population Differences in Vision Acuity: A Review, with Speculative Notes on Selection Relaxation.” Eugenics Quarterly 9 (4): 189–212.
Taylor, Hugh R. 1981. “Racial Variations in Vision.” American Journal of Epidemiology 113 (1): 62–80.
C. Loring Brace