Human Biological Variation
Human Biological Variation
To some people, “race” is a four-letter word associated with negative connotations, while for others it refers to actual biologically inherited traits. Skin color is the most readily visible signifier of race, and as such is the characteristic upon which most racial classifications are based. Historically, the ancient Egyptians were the first to classify humans on the basis of skin color. In 13# BCE Egyptians classified humans into four races: “red” for Egyptians, “yellow” for people living to the east of Egypt, “white” for people living north of Africa, and “black” for Africans from the south of Egypt. The ancient Greeks, on the other hand, referred to all Africans as “Ethiopians.” A major tenet of the biological concept of race is that the traits that identify a given race are unchangeable and have been fixed since the beginning of humankind. Since the early twentieth century, however, an evolutionary approach led by anthropologists and human biologists has emerged that calls into question the validity of the biological concept of race.
In the early 1900s, head shape was considered an innate “racial” trait that was inherited, with little environmental influence at work. This concept changed with the pioneering studies of Franz Boas (1858–1942). Boas demonstrated that the cephalic index (the ratio of head width to head length) of children born to immigrants to the United States changed because they grew up in a different environment than that of their parents. These and subsequent genetic studies have demonstrated that the biological features that distinguish racial groups are subject to environmental influence and are of recent origin. Furthermore, data and models from DNA studies suggest that common race definitions pertaining to humans have little taxonomic validity, because there is no correlation
between genetic markers such as blood type and markers for race such as skin color. For example, as shown in Figure 1, the Australian aborigines, East and West African populations, and native populations from India have a similar dark skin color. Based on this trait, they could all be assigned to an “African race.” However, with reference to frequencies of the B blood group and Rh blood genes C and E, the Australian aborigines are very different from the East African, West African, and Indian populations. In other words, there is no concordance between blood type and skin color. Likewise, the ABO blood type frequencies for natives of Taiwan and Greece are very similar (0 = 45.2 %, A = 32.6 %, B =18.0 %, AB = 3.4 %), but on the basis of geography and physical appearance these two populations clearly belong to different categories.
Likewise, the indigenous populations of sub-Saharan Africa, southern Europe, the Middle East, and India have similar frequencies of the sickle-cell trait (20 to 34 percent), yet they differ in skin color. The similarity of these populations in the frequency of sickle-cell trait is related to their common adaptation to malaria, not to a common racial origin. Similarly, lactose tolerance occurs both in European and African populations, not because they have the same racial origin, but because both were evolutionarily adapted to dairy products. In other words, the concept of “race” is both too broad and too narrow a definition of ancestry to be biologically useful. The reason that definitions of race lose their discriminating power for identifying races is due to the fact that humans share a common origin and have been constantly migrating throughout their evolutionary history. For example, the large-scale migrations between Africa and Europe, as well as the colonial expansion of European populations into Asia and the New World, have resulted in the mating of individuals from different continents and the concomitant mixture of genetic traits.
For these reasons, using the biological concept of race to describe biological diversity has largely been abandoned. Nevertheless, because the risks of some diseases have a genetic basis in some populations that may have originated in a geographic region that differs from their current area, there is still great interest in understanding how genetic diversity has been structured in the human species.
In the taxonomic literature, “race” is any distinguishable type within a species. Among researchers, however, “race” as a biological concept has had a variety of meanings. Some use frequency of genetic traits between and within groups as the point of reference, while others use geographical area.
Trait Frequency . Genetic studies demonstrate that about 85.4 percent of all the variation in the human species can be attributed to variation within populations and that there is only a 6.3 percent difference between “races,” with less than half of this value accounted for by known racial groupings (see Lewontin 1972; Barbujani, Magani, Minch, et al. 1997). In other words, there is much more genetic variation within local groups than there is among local groups or among races themselves. This genetic unity means, for instance, that any local group contains, on average, 85 percent of the genetic variation that exists in the entire human species. As a result, there is about 15 percent genetic variation between any two individuals. Therefore, a randomly selected white European, although ostensibly far removed from black Americans in phenotype, can easily be genetically closer to an African black than to another European white. As summarized by Jeffrey Long and Rick Kittles in a 2003 article, the patterns of genetic variation within and between groups are too intricate to be reduced to a single summary measure. In other words, identification of trait frequencies and statistical partitions of genetic variation do not provide accurate information to justify claims for the existence of “races.”
Geographical Race . Because some phenotypes, such as skin color, facial features, and hair form, differ between native inhabitants of different regions of the world, biological anthropologists and geneticists introduced the idea of geographical races (see Dobzhansky 1970, Brues 1977, Garn 1961, Mayr 2002). In this classificatory approach, each geographic region (e.g., South America, Australia, sub-Saharan Africa, East Asia, Polynesia) is associated with a race. According to these authors, “geographical races” refer to an aggregate of phenotypically similar populations of a species inhabiting a geographic subdivision. An underlying assumption of this approach is that in each geographical area there are clusters of genetic traits that, taken together, differentiate them from those of other geographic areas. Current evidence indicates that variability in the genotypic and phenotypic expression of genetic traits is affected by natural selection, migration, and genetic drift. As a result of these processes, genetic diversity follows a pattern characterized by gradients of allele frequencies that extend over the entire world. (Alleles are alternative versions of a particular gene.) In other words, when identified, the clustering of genetic traits in a given area reflects the demographic and evolutionary history of the population rather than a racial category. Therefore, there is no reason to assume that “races” represent any units of relevance for understanding human genetic history.
In summary, and as stated by the 1996 American Association of Physical Anthropologists’ “Statement on Biological Aspects of Race’’: (1) all human populations derive from a common ancestral group, (2) there is great genetic diversity within all human populations, and (3) the geographic pattern of variation is complex and presents no major discontinuity. In other words, race is a consequence of social history and any variation is therefore transitory. For these reasons, among biological anthropologists at least, the biological concept of race for describing biological diversity has largely been abandoned.
An illustration of the dangers of misusing information on the intelligence quotient (IQ) and heritability is found in studies of IQ and race. The IQ test was developed by the French psychologist Alfred Binet in the 1910s to identify children’s reading readiness. The IQ test was intended to measure “mental age” in various categories. Binet warned that the IQ test could not properly be used to measure intelligence “because intellectual qualities are not superposable, and therefore cannot be measured as linear surfaces are measured” (Binet and Simon 1916, p. 206). Intelligence was therefore not considered by Binet to be a fixed quantity, but rather one that could be increased through teaching. Yet in the United States, tests of IQ have been used to measure general intelligence.
As shown in Figure 2, the normal range for IQ for about 67 percent of the population falls between 85 to 115, while only 5 percent of the population attain IQ values greater than 140 and below 70. The use of IQ as a measure of an individual’s innate intelligence is not valid for two reasons. First, there are many kinds of intelligence. There are some people with outstanding memories, some with mathematical skills, some with musical talents, some good at seeing analogies, some good at synthesizing information, and some with manual and mechanical expertise. These different kinds of intelligence cannot be subsumed into an IQ score.
Second, there is no evidence that IQ is genetically determined. It is true that about 60 percent of the variability in IQ is inherited within family lines, but the fact that it is inherited does not mean that it is genetically determined. Discrete traits such as blood type that do not change through the life cycle are genetically determined and, therefore, have a high heritability, but continuous traits such as height, weight, or IQ are highly subject to environmental influence. Heritability is computed as the fraction of phenotypic variability due to genetic differences divided by total variability. It is expressed as h2 = G/P = G/(G + E), where G is variability in genotype, E is variability in environment, and P is variability in phenotype. Depending upon whether the environmental variance (E) is large or small, the phenotypic variance (P) can be either large or small, and the heritability (h2) can be either large or small. Measures of heritability, especially of continuous traits such as intelligence, indicate the joint influence of genetic and environmental factors. Twin and family studies have shown that shared environmental factors have an important effect on educational attainment (see Silventoinen et al. 2004). Shared environmental factors such as education have a greater impact on intelligence during childhood than in adulthood. In other words, heritability of intelligence (unlike genetic determination) can be very different in different populations, depending upon the environmental condition in which each population develops. Therefore, a low IQ score reflects the effects of poor education during childhood and negative environmental conditions.
Despite these pitfalls, some researchers have attempted to show that difference in IQ reflects difference in genetic capabilities. For example, Richard Herrnstein and Charles Murray, in their book The Bell Curve (1994), argue that differences in IQ between white and black Americans reflect differences in the genetic capability of intelligence in each race. They point out that the distribution of IQ scores in black Americans is shifted to the left, so that there are higher frequencies of low IQ scores and lower frequencies of high IQ scores when compared to white Americans. However, this difference is more a reflection of the different educational experiences of black and white Americans. For example, a study by Dickens and Flynn (2006) shows that “in nine standardization samples for four major tests of cognitive ability blacks gained four to seven IQ points on non-Hispanic whites between 1972 and 2002. Gains have been fairly uniform across the entire range of black cognitive ability.” Similarly, in the Barclay School of Baltimore, black children who previously scored at the 20th percentile were later attaining scores at the 85th percentile. These findings together indicate that the lower IQ scores associated with black samples is more a function of educational experiences than of genetic determinants.
It is evident that cultural environment is an important contributor to any measures of IQ. This inference can be illustrated by several examples. First, consider two hypothetical groups of eight-year-old children: one from a middle class U.S. school and one from a poor rural area in Guatemala. These children are asked the following question: “Suppose you have five eggs and you drop two, how many eggs do you have?”
The U.S. children will likely answer that they have three eggs left, but the rural children may answer that they have five eggs. Based on this result, one might conclude that the Guatemalan rural children do not know how to add or subtract. However, the rural children have been raised in an environment associated with food shortages, and they will likely believe that just because an egg has been dropped does not mean it cannot be eaten. Hence, for the Guatemalan rural children, there are still five eggs. The “correct” answer, therefore, depends on the children’s past experience. In another example, suppose that Australian aborigine trackers and Peruvian Andean weavers are asked to identify a series of drawings that will make a complete square as fast as possible. It is likely that the speed of the Peruvian Andean weavers at this task will be faster than that of Australian aborigine trackers. This difference is related to the fact that Australian aborigine trackers have had little or no contact with the concepts of two-dimensional geometry, whereas the Peruvian Andean weavers are in an occupation that involves experience with two-dimensional geometric designs. Thus, differences in responses may reflect an individual’s or population’s past experience.
IQ should be defined as a measure of an individual’s sum of cultural experience, rather than a measure of genetic difference. This does not mean that a person’s genetic makeup is not a significant factor in individual intelligence in particular areas. Without the proper environment, however, this trait may not be expressed.
It is evident that the biological concept of race is poorly defined and cannot be used as a surrogate for multiple environmental and genetic factors in disease causation. Recent genetic studies of DNA polymorphisms have suggested that human genetic diversity is organized in continental or geographical areas (see Serre and Pääbo 2004; Feldman, Lewontin, and King 2004). This conclusion suggests that geographic area, rather than race per se, has a valid role in biomedical research because many medically important genes vary in frequency between populations from different regions. If, for example, there are major differences in allele frequencies between geographic areas, individuals from different origins may be expected to respond differently to medical treatments. In this case, the identification of the origin of people in a geographic area does have some justification as a proxy for differences in environmental and other factors of relevance for public health.
However, the ability to place an individual within a geographic region and range of variation does not mean that this variation is best represented by the concept of race. For example, sickle-cell disease is a characteristic of ancient ancestry in a geographic region where malaria was endemic (e.g., Africa, the Mediterranean, and southern India), rather than a characteristic of a particular racial group. Therefore, a diagnostic approach toward sickle-cell disease must take into account the individual’s geographical ancestry. Similarly, populations who throughout their evolutionary history have developed an adaptive response to economize salt loss under the condition of tropic heat stress are more susceptible to developing high blood pressure than other populations when living in temperate climates. In other words, in biomedical research, it is not race that is relevant, but rather how the forces of evolution in a geographic area have shaped the individual’s genes. Thus, because an individual’s genes are grounded in his or her genealogy, identifying all contributions to a patient’s ancestry is useful in diagnosing and treating diseases with genetic influences.
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A. Roberto Frisancho