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Clines and Folk TaxonomyScience, particularly biology and physical anthropology, did try at one time to discover how many races existed in the human species, and how they could each be defined. While humans vary in many traits, only a few were arbitrarily selected, such as skin color and hair color and hair structure. Depending on how many traits were selected, how they were defined, and which traits were used, classifiers could come up with three races, four races, six races, twelve races, or even several hundred races. This in itself was good evidence that such groups were cultural inventions. Clearly, it is as cultural inventions or classifications that we use the word race in our culture. That is what is meant by calling race a folk taxonomy (also referred to as a social construct; see previous lesson on Race and Folk Taxonomy.) As a folk taxonomy, as previously noted, our notions of race change through time. The U.S. Census has added categories and subtracted categories many times over the years for what originally was a question on race, and now is a question on ethnicity (a cultural concept.) Most recently, it has added a category called "mixed" to go along with another category called "other". Hopefully nobody would defend these as biological categories. Below is a picture of two people. Are they the same or a different "race"? If we were to take the most recent country of origin (before the United States) of the ancestors of the woman, in terms of "percentages" she is Chinese (highest percentage), English, German, Hawaiian, and Scots-Irish. The man, using the same approach, is Polish and French. He is also a Jew. That fact alone, according to some, makes the man a different race. Some would claim the woman is Chinese, and hence a different race than her mother. Of course if she is not Chinese, than she would be a different race than her father. There can be no biological reality to ANY approach which puts into two different groups those (parents and children) who share the most genes. This is the complexity of a folk taxonomy. It is very real however. The man lost kin in Europe during the WW II Holocaust because they were members of the Jewish "race" according to the folk taxonomy then in use. And of course because many humans, including Hitler, so fervently wanted to believe that one "race" is superior to another.
My Oldest Daughter, while still in high school, with my now Son-in-Law ClinesHumans are physically diverse, but if you look at how they vary, it is in a continuous fashion across geographical space. This continuous variation can be graphically depicted as a cline. Below is a cline for skin color. Since skin color is a trait which various continuously (meaning there are not just four skin colors, as there are blood types), how a clinal map looks depends on exactly how the trait is defined. The map below used a series of continuously varying colored chips, numbered from 1-30, to decide the color of a person's skin. The map lumped skin colors into eight different categories.
Geographical Distribution of Skin Color Prior to 1500 A.D. (from Essentials of Physical Anthropology, Jurmain, Nelson, Kilgore, and Trevathan, 4th Ed.) In the above distribution, the most accurate available of indigenous populations prior to 1500 AD, one could say that there are eight skin colors (a simplification since each color on the map represents a range of actual skin colors). It would be a bit silly to claim that everyone from areas labeled 24-26 above, for example, are members of the same group or race, as that skin color range is found on two different continents, and in widely separated areas. With the exception of Australia, all continents had at least two different "colors"; most have three or four, and Africa had at least six. The lines that exist are arbitrary, and would be different depending on exactly what criteria and definitions we used for our map. What is useful about clines however, is that they can provide clues as to how such variation evolved. The primary skin pigment is melanin, produced in the epidermis of the skin by specialized cells call melanocytes. All humans have approximately the same number of melanocytes in their skin, but the amount of melanin produced varies. In albinos, a mutation has occurred that completely prevents melanocytes from producing any melanin. The amount of melanin produced is determined, remember, by as many as six different genes (with different alleles) on six different pairs of chromosomes, and is hence a polygenic trait. Melanin absorbs potentially dangerous ultraviolet rays that are present in sunlight. In part, a relaxed selection for melanin production as Homo sapiens moved into northerly environments may have occurred. In addition, since vitamin D is produced in the body in part due to ultraviolet radiation, a person who produced a large amount of melanin may have been more likely to suffer from vitamin D deficiency in northern environments with less direct sunlight. Finally, more melanin production has been associated with a greater risk of frostbite in colder climates. The studies on the adaptive value of melanin are far from conclusive however. That natural selection, mutation, and probably genetic drift were at work in the distribution of skin color shown above is almost certain. That much of the current variation of skin color is due to sexual selection (which is what Diamond claims in the previous article Race Without Color), is also highly possible. Sickle Cell AlleleAnother example of a cline, related to human genetic variation, is one you are already familiar with. The frequency of the allele for sickle cell anemia, discussed in unit 1, is expressed in the cline below.
Cline for Sickle Cell Allele
Area with Malaria Parasite The distribution of the sickle cell allele, which in the heterozygous individual protects from malaria, coincides almost exactly with the distribution of the malaria parasite. Malaria itself is a disease of horticulture, appearing as a health problem in certain areas when humans began to clear land to plant, altering drainage patterns and creating areas where mosquitoes could easily breed. Lactose IntoleranceAnother example of both genetic variation, as well as the influence of culture on natural selection, has to do with the trait of lactose intolerance. All young and infant mammals can digest milk. A major ingredient of milk is lactose, a sugar, which is broken down in the body to the enzyme lactase. In most mammals, including humans, the gene that codes for lactase production is turned off (by another gene) by early adolescence. If an adult human drinks much milk, it will ferment in the large intestine, causing cramping and diarrhea. Most adult humans in most areas are therefore termed lactose-intolerant. The environment plays some role: it is possible for a person to become more lactose-tolerant, though what they are really doing is increasing the bacteria in the intestine that act as a buffer. However, various groups of people are lactose-tolerant as adults, apparently due to a mutation which prevents the lactase production gene from turning off. The information on to what extent various groups are lactose intolerant is presented in the chart below, rather than in a cline.
Horticulture spread to Europe from Southwest Asia, in part via actual migrations of people who brought with them not only wheat but also cattle, sheep and goats. Milk was consumed from all these animals. In such an environment, any mutation to decrease lactose intolerance would be selected for, and would likely increase in the population. The same was true of African groups like the Fulani, who were also cattle herders who like to drink fresh milk. In other areas, animals that could be domesticated for milk were either unavailable, or the milk was turned into more digestible forms by using bacteria, such as cheese and yogurt. In those areas, any mutation to decrease lactose intolerance was of no advantage. ABO Blood SystemAs a final example, the distribution of blood types O, A and B vary in a clinal fashion. Study the three charts below, which are an attempt to reconstruct the distribution of human blood types at approximately 1500 AD. (Remember O is the world's most common blood type.)
Percent of Population with Blood Type O in 1500
Percent of Population with Blood Type A in 1500.
Percent of Population with Blood Type B in 1500. What explains variation in human blood type? For one thing, all three alleles have probably been with us for over six million years, as the the African apes also have these three blood types. The variation in human groups is certainly in part a result of genetic drift, specifically founder's effect: the earliest migrants into the Americas for the most part lacked the B allele. It is also possible that natural selection was in operation, as some scientists have tried to establish a relationship between blood type and resistance to certain diseases, such as plague. It would seem unlikely that sexual selection played a major role (though in some modern cultures, such as Korea, blood type and personality are believed to be linked, so that people are advised to marry blood types that have compatible personalitites to their own.) Like so much of human variation, the full answers are not as yet known, but it should be clear from the maps above that blood type, like any other genetic trait you might chose, does not divide us into discrete "races." One major problem with racial classification, and one point of Diamond's article Race Without Color, is that the selection of traits to use is arbitrary. The ABO system could be used instead of skin color, since clearly certain blood types are found more frequently in certain areas of the world than in other areas. However, as these clines and Diamond's article make clear, skin color and blood type (or ANY other gnetic trait chosen) do not vary in a concordant, or similar fashion. They are in fact discordant, and therefore physical traits can not be used to divide people into discrete groups. That is perhaps the major point of Diamond's article. Reread it! Conclusion
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