Chapter fifteen: Human Genetic Diversity

Chapter Review

Human Variation

Humans are a remarkably diverse species, exhibiting a high degree of phenotypic variation (Fig. 15.1). Individuals have a wide range of physical and behavioral characteristics and preferences. This becomes especially apparent when groups are examined cross-culturally. What might be a constructive way to examine this amount of variability?

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Figure 15.1 World map of human genetic similarity.
Image Credit: L. L. Cavalli-Sforza, P. Menozzi, and A. Piazza, The History and Geography of Human Genes, Princeton University Press.

One way researchers break down variation is by separating genetic and environmental components. By dividing the amount of influence each type of variation has on the phenotype of an individual, researchers attempt to quantify the genetic basis of physical and/or behavioral characteristics. An accurate measure of the exact impact of environmental versus genetic factors, however, is difficult (if not impossible), particularly because human beings are constantly exposed to both factors simultaneously during the course of their entire lifetimes.

Genetic Variation

Genetic variation is measurable in two general ways: by comparing within group variation (the difference between members of a single population) and by comparing between group variation (the genetic difference between two different populations of individuals). The question, however, is why would genetic variation be retained in either case?

Variation can be maintained within a population in several instances, such as the following:

Heterozygotes have a selective advantage.
The environment has recently changed, but the gene pool has not shifted accordingly.
More than one allele provides the same function with equal success.

Between group variation is often maintained through different selective environments associated with different regions. Characteristics that may be beneficial in a cold-weather environment may prove to be cumbersome and inefficient in hot climates, and vice versa. Also, variation may be a by-product of a strong founder effect, by which a population's genetic composition is significantly altered by breeding isolation and a small initial gene pool. Since mutation and drift will tend to make any two given populations increasingly different (just as gene flow will homogenize them), patterns of population differentiation become a historical source for demographics and population movement.

These patterns of genetic variation allow researchers to reconstruct migration events and population expansions associated with related groups of people. Various studies of mitochondrial DNA (mtDNA), as we learned in Chapter 14, have suggested a worldwide population expansion and migration approximately 100,000 years ago.

Understanding Human Variation

In the past, the study of human phenotypic variation has often been the victim of gross misinterpretation. The distinction made about differentiating within group variation from between group variation addresses this issue. When the subject of interest is a trait in a population, the fact that variation of that trait exists within a group does not say anything about the variation of that trait between groups, and vice versa.

Now, however, the study of human genotypic variation, and how such variation influences phenotypic variation, has significantly expanded our understanding of human variation as a whole. Some genetic diseases are maintained within groups because possessing one copy of a deleterious allele is actually advantageous. Sickle-cell anemia, for example, is a debilitating condition when a person carries both copies of the gene that misshapes their red blood cells, but carrying a single copy of the gene—the heterozygous condition—confers resistance to the most severe type of malaria, falciparum malaria. This example of a balanced polymorphism is joined by others (such as the possible resistance to tuberculosis granted by having one copy of the allele that causes the always fatal Tay-Sachs disease) in different regions with different selective pressures on human populations.

Other genes, such as the variants that cause lactose tolerance—typically primates lose their ability to digest lactose, a sugar found in mammalian milk soon after they are weaned—may be quite new in human populations. Joel Hirschhorn and his coworkers at the Harvard Medical School have gone deeper into the gene that controls the ability to digest milk. They found 101 nucleotide sites where the bases that make up the gene (single nucleotide polymorphisms) have the possibility to vary within different groups. Finding one of these variants actually allows researchers to predict the pattern of variation at the rest of the sites. This work corroborates earlier estimates hypothesizing that the ability to digest lactose among Middle Eastern and North African-descent peoples evolved as recently as 7,000 years ago, when these groups first domesticated livestock.

Race

The instance in which ideas about human variation are most abused is in the context of racial classification. Common racial classification apportions human variation in a flawed manner. Three of the more common misconceptions are these:

Human beings naturally divide into a small number of distinct races.
Races have certain inherent strengths and weaknesses, particularly in terms of intelligence, morality, and character.
Racial differences are due to definite and definable biological heritage.

The principal problem of this view from a scientific standpoint is that each statement is substantiated less by biological data and more by political and ideological perspectives and a lack of grounding in human cultural history. The amount of genetic variation between different groups of humans is very low overall, and a number of arbitrary groups can be created based on different criteria. It just so happens that certain phenotypic characteristics (for example, color of skin, shape of eyes, texture of hair) are emphasized in typecasting individuals along common views of race, a practice that actually provides little useful information. Moreover, these phenotypic traits do not map consistently to genetic profiles. In the past, anthropologists have divided the world's peoples into as few as three races and into as many as 20 or 30!

More recently, Brazilian researchers decided to test how well common racial classifications actually work. In Brazil (as in the United States!) people are generally confident that they can tell the difference between those with European ancestry and those with African ancestry. Flavia Parra and his coworkers at the Universidade Federal de Minas Gerais in Brazil examined this idea by taking blood samples from people who live on an island off the coast of Africa, close to the area where most Brazilians of African descent originated (Africans came to Brazil through the colonial slave trade). Then, they also took samples from a group of Portuguese people, the European ethnicity that was the origin of most Brazilians of European descent. Last, they took blood samples from Brazilians who appeared to have European descent, Brazilians who appeared to have African descent, and Brazilians who appeared to have “mixed” descent (“blacks,” “whites,” and “intermediates,” respectively).
What do you think they found? First, there was enough genetic difference between the African and Portuguese samples to compute an index of African ancestry. When they applied this index to genetic differences between Brazilians, people commonly classified as “white” and people classified as “black” were indistinguishable from each other genetically. In other words, the way people classify each other by physical looks has little to do with the ways in which they actually vary genetically.

When examining the biological basis of race and racial conflict, events in the past have had a clear cultural (that is, political or religious) basis. Having visible African ancestry in Brazil, for example, classifies one as “black,” and makes one subject to contemporary racism that stems from historical treatment of African slaves. However, people of the 16th and 17th centuries did not see race exactly the way we do today, and people in contemporary Brazil actually use more racial categories than do people in the United States. Race classification changes over time in different cultures and now, in different nations.

The major source of this difference is not biological in nature. This is not to say that there is no biological variation between different populations. Obviously we have reviewed a number of instances in which groups vary, both genotypically and phenotypically. Biological distinctions between people are much more opaque than implied by common notions of race. Often the reliance on race as a biological concept serves more of a political end—by creating an immutable difference between peoples that has never existed biologically.

To see some examples, and learn more about the detrimental effects of racism, visit the web site for the Institute for the Study of Academic Racism at http://www.ferris.edu/htmls/othersrv/isar/ .

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