Clines and Continuous Variation
Clines and Continuous Variation
Frank Livingstone, a specialist in genetic anthropology, has written that “there are no races, only clines” (Living-stone 1962, p. 279). For centuries, both everyday folk beliefs and the sciences presumed that “races” were separated by genetic boundaries, with a high degree of biological similarity among the members of each group. This was based on thinking in terms of a discrete distribution of traits. It was believed, for example, that all subSaharan Africans had black skin, all Europeans were white, and all Asians were yellow. Thinking in terms of homogeneous populations with discrete traits and boundary lines was supported by the selective perception that certain external physical traits fit stereotypical traditions. In the twentieth century, however, thinking in terms of continuous variation, also called clines, came to provide a more useful and precise way to analyze human variation, making the concept of “race” obsolete. Traits that were assumed to be unique to each race are in fact distributed continuously. For example, skin color, based mostly on the frequency of pigment (melanin), is darker near the equator and becomes lighter as one moves in a northern direction, reaching its lowest frequency in northern latitudes among populations that have resided in those areas for thousands of years.
The concept of cline was first proposed by the British biologist Julian Huxley in 1938. He derived the name from Greek word klinein, meaning “to lean.” He defined cline as a “gradation in measurable characters” (Huxley 1938, p. 219). A cline can be based on either directly observable external biological traits, also called phenotypes (e.g., hair color, skin pigmentation, stature), or it can be derived from genes (e.g., ABO blood type, sickle-cell hemoglobin) and referred to as a genotype. Clines may be continuous and vary gradually over a region, or they may vary abruptly. There may be steep clines or gradual clines as well as sudden midcline reversals. Clinal maps of England show areas where 15
percent of the population have red hair adjacent to areas where less than 5 percent have red hair. Variation may not be due to absolute barriers, but may instead be influenced by partly passable mountain ranges, deserts, and bodies of water. Even before the time of Columbus, clines were created or disrupted by the movement of peoples, a trend that intensified after 1492 with the enslavement and forced emigration of millions of Africans and the migration of Europeans into North and South America. The result resembles a weather map on which lines separate temperature variations. On a biological cline map, the lines separating phenotypical traits are called isophenes. Lines referring to genotype frequencies are isogenes. Similar illustrations of gradients are seen in maps of elevations of land contours above sea level, in this sense the word cline is related to incline and decline in altitude.
Together with his coauthor A. C. Haddon, Huxley presented the evidence for clines in 1936 in a pioneering map (see Figure 1) that showed the decrease of B-type hemoglobin in Europe and its increase into western Asia. Haddon and Huxley concluded that the evidence of clines invalidated the race concept’s assertions of racial homogeneity and boundary lines making for discrete races. Later, computers would make possible the analysis of more complete data into interval maps showing other clinal patterns. The exact numerical value varies, but any cline can be represented by a set of intervals. In this sense, a cline refers to both the concept of continual variation and a method of measuring and depicting variation in the frequency of any physical feature or gene frequency over a geographic area.
The pioneering efforts of Huxley and Haddon did not receive immediate acceptance. The idea of “race” was too strongly established in Western folk beliefs and scientific tradition. But newer research studies would provide a catalyst for change. Among the first was Livingstone’s 1958 study of sickle-cell anemia, which showed that it was more frequent in malarial areas. Prior to this it was believed by some that genes for sickle-cell anemia were a discrete racial trait of black Africans. Livingstone was able to show that the alleles for sickling (Hbs) are most frequent in populations in West Africa but decline in frequency in areas to the north and east, and are still less frequent around the Mediterranean and throughout South Asia. This is because another mutation, for hemoglobin E, also resists malaria in areas where the intensity of agriculture affects the frequency of mosquitoes.
Malaria continues to kill millions of people, mostly children, each year. Inheriting an allele for sickling from each parent leads to extreme anemia, severely reducing the number of offspring and the percent of sickling alleles in the population. Those inheriting normal hemoglobin—that is, without any sickling blood—contract malaria and have a significant death rate and a reduced number of offspring. Yet inheriting one such allele confers a resistance to the symptoms of malaria. Frequency of survival and reproduction with one sickle-cell allele is relatively greatest in areas where there is more agriculture being practiced, for the clearing of the land produces standing water where mosquitoes can breed. Therefore, the continuous variation over geographic regions is not due to biological race but is produced by human cultural practices in malarial climates.
Livingstone’s data was reported in a list, but a map developed later depicts a graphic clinal pattern (Johnston 1982, Figure 2). It is clearly a clinal pattern distributed through malarial regions of Africa, Europe, and Asia. Livingstone’s data demonstrated that continuous clinal variation occurs
within populations and across their boundaries, in clear disproof of the validity of the idea of race.
Another influence on the cline concept was presented by C. Loring Brace in “A Nonracial Approach Towards the Understanding of Human Diversity” in The Concept of Race (1964). Brace’s nonracial approach was the use of clines, and he illustrated it with four clinal maps (derived from Biasutti 1941), covering skin color, hair form, facial form based on relative tooth size, and nose form. All of these are traditional observable physical features (phenotypes) that had been used to construct racial stereotypes. Each clinal pattern can be studied, and Brace showed that evolutionary hypotheses could be developed and tested regarding their origin and distribution. When the four clinal patterns are overlaid on each other, it clearly demonstrates that racial boundaries do not exist, because the clinal patterns are not congruent and do not covary. Instead, they are discordant; that is, their distribution does not correspond with racial boundary lines. Brace declared that it was “extremely difficult to say where one population ends and another begins” (Brace 1964, p. 104). Thinking in terms of clines in this way clarified that racial boundaries are arbitrary cultural errors. The discordance of clines was further presented to biologists by Paul Ehrlich and Richard W. Holm (1964). The biologists Edward O. Wilson and W. L. Brown (1953) used clinal data as a basis for rejecting the concept of “subspecies,” in the sense of race.
Beginning in 1938–1939, and again in 1952–1954, genetic anthropologist Joseph B. Birdsell measured Australian Aborigines for a number of traits. Using this data, Birdsell constructed numerous clinal maps. He viewed the data in the context of the concept of race up to the early 1970s, but in 1975 he wrote, in Human Evolution, that “The use of the term race has been discontinued because it is scientifically undefinable and carries social implications that are harmful and disruptive” (p. 505). In 1993 he published Microevolutionary Patterns in Aboriginal Australia, A Gradient Analysis of Clines. It contains a large number of clinal maps showing lack of covariation, contrary to the Western image of there being one stereotypical image of Australian Aborigines. In 1994, the geneticists L. Luca Cavalli-Sforza, Paola Menozzi, and Alberto Piazza published a worldwide analysis using a database of 76,676 gene frequencies from aboriginal ethnic groups that were believed to be in the same location at the time of the study as they were at the end of the fifteenth century, although the gene pool and ethnic identity of each group had likely altered. They published more than 500 clinal maps, which were condensed into worldwide summary maps using 128 gene variants (alleles). The result did not correspond to racial boundary lines; and the coauthors rejected the race concept as a scientific failure and race classification as a futile exercise.
Acceptance of clines as a basis for rejecting the race concept was resisted by some anthropologists, especially by forensic anthropologists who asserted that they could identify an individual’s race by examining his or her skull. In doing so they ignored the fact that while crania might have some feature attributed to a person of one race, a particular skull could be that of a very light-skinned person who could be identified either as black or white. In addition, cranial features vary clinally within populations and change over time. Outspoken in defense of race was the forensic anthropologist Alice Brues in People and Races (1977). Brues wrote that clines were sometimes the appropriate concept to use, while at other times race was both a necessary and valid concept. Brues pointed out the apparent differences between races with a scenario of flying from a Scandinavian city and landing in Nairobi, Kenya. Brace replied that walking or bicycling between these two areas and progressing southward along the Nile, one would view a gradual change in physical features.
Acceptance of the new clinal concept and data on continuous variation became widespread beginning in the 1970s in anthropology, although the concept was less often explicitly stated than was the underlying and crucial fact of continuous variation. There continues to be reluctance among some scientists to relinquish race as the traditional and convenient way of extending to human populations the classification system of the Swedish botanist Carolus Linnaeus (1707–1778).
Thinking that uses the race concept assumes a high degree of uniformity of each trait, as well as the association of these traits within a population. Brace pointed out that this association “obscures the factors influencing the occurrence and distribution of any single trait. The most important thing for the analyses of human variation is the appreciation of the selective pressures which have operated to influence the expression of each trait separately” (Brace 1964, p. 107; italics in original). One example, as described above, is Livingstone’s explanation of the cline for sickle-cell allele in relation to the frequency of malaria, which in turn is affected by the intensity of agriculture. Brace proposed explanations for clinal distribution of nose form, hair form, skin color, and relative tooth size affecting face profile. Skin pigment is a protective response to ultraviolet radiation, which causes skin cancer. However, there is some uncertainty about the frequency of skin cancer as an influence on natural selection (through differential fertility), because the cancer develops after the years when reproduction is most likely. A stronger explanation for increased melanin is found in the effect of ultraviolet rays in reducing folic acid (folate) in the body. Low levels of folic acid result in a defect in the neural tube (spina bifida) of the developing fetus, and they may also affect the production of sperm (Jablonski 2004). The clinal pattern in melanin arises as the intensity of ultraviolet exposure decreased away from the equator. The presence of populations with lesser amounts of melanin as one proceeds north occurs because the reduced degree of ultra-violet intensity allows for the persistence of adequate folic acid, coupled with the need to generate more vitamin D for normal bone growth and the possibility of resistance of lighter skin to frostbite.
The covariation of hair form and skin color are an exception to the pattern of clinal discordance. Hair on the head varies, and for a biological reason—spiral and wooly hair insulates the head from ultraviolet radiation. Clinal patterns tend toward smaller teeth in areas with longer histories of food production from agriculture, while larger teeth occur in areas of hunting and gathering. Dental reduction began in the northern latitudes when cooking and the use of pottery for more liquid foods began, reaching equatorial areas later. As food became more tender, natural selection did not require large teeth and mutations for smaller teeth could accumulate. Stature, meanwhile, varied in response to climate. In cold climates, body temperature is conserved by stocky bodies and short arms and legs. In hot, dry areas, a more linear body with long arms and legs dissipates heat more efficiently. The small stature of pygmies is an exception to the linear pattern, but they live in a hot, moist rainforest, along with other species that are smaller than closely related species living in the open savannas.
Particular genetic conditions, such as Tay-Sachs disease or sickle-cell anemia, have mistakenly been viewed as identifying particular races. Tay-Sachs is a condition in which inheriting two recessive genes is lethal. It has been attributed to Jews and explained by the possibility that the presence of one gene conferred a resistance to tuberculosis among the Ashkenazic Jews of eastern Europe who lived in crowded ghettos. The condition is also found in other populations but at a lower frequency, and a slightly different mutation also causes Tay-Sachs among French Canadians of Quebec. Racial stereotypes attribute other features to one or another particular race, such as uniform epicanthic folds over the eyes, prominent cheekbones, or thick lips. However, these vary by degree in a clinal pattern. Explanations for them as advantageous adaptations have not been established. They may have originated in one small population of related families and dispersed with population expansion, becoming more varied due to mating with members of other populations. Clinal variations in physical features are most commonly explained as advantageous for survival in different and sometimes extreme geographic locations. These biological features, mislabeled in the past as racial markers, did not necessarily make migration into those areas possible, but they may have evolved in gradations after movement into those areas. The spread of humans throughout the globe occurred because humans had the potential to live in many different areas, from the Arctic Circle to the semi-arid, near deserts of southwestern Africa. It has been suggested that races varied in their achievements because of their hereditary intelligence, but no proven method of measurement free of the cultural variation in IQ tests has been devised. Genes relating to intelligence have not been found, although many different negative mutations may reduce the functioning intelligence of an individual. The kind of achievements of various populations is best viewed not as the result of biological differences, but rather as a result of human flexibility for problem solving expressed in diverse cultures.
The availability of clinal data was necessary to bring about thinking without the idea of biological races, and an awareness of continuous variation has made racist stereotypes more difficult to use. Clinal thinking has become standard among anthropologists, and it is increasing among biologists.
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