Race and genetics

Notions of race based on Human genetic variation have replaced historical approaches such as craniology with the advent of human genetics in the 20th century.

Early history

Blood groups

geographic distribution of blood group A

Prior to the discovery of DNA as the hereditary material, scientists used blood proteins to study human genetic variation. Research by Ludwik and Hanka Herschfeld during World War I found that the frequencies of blood groups A and B differed greatly from region to region. For example, among Europeans, 15% were group B and 40% were group A. Eastern Europeans and Russians had higher frequencies of group B, with people from India having the highest proportion. The Herschfelds concluded that humans were made of two different "biochemical races," each with its own origin. It was hypothesized that these two pure races later became mixed, resulting in the complex pattern of groups A and B. This was one of the first theories of racial differences to include the idea that visible human variation did not necessarily correlate with invisible genetic variation.

It was expected that groups that had similar proportions of the blood groups would be more closely related in racial terms, but instead it was often found that groups separated by large distances, such as those from Madagascar and Russia, had similar frequencies. This confounded scientists who were attempting to learn more about human evolutionary history. The next big advance in biological description of human variation would come with the discovery of more blood groups and proteins.

Blood proteins and molecular evolution

Techniques based on molecular evolution principles were used in early studies of presupposed racial differences. One major technique in the field is to use mutations in individual proteins or genetic sequences as a molecular clock indicating the evolutionary relatedness of various species or groups.

Luigi Luca Cavalli-Sforza and Anthony Edwards would then incorporate these techniques into the field of population genetics. Using computer based statistical analysis to average across the several blood group systems, they were able to produce a phylogenetic relationship of the various populations around the world.

In 1972 Richard Lewontin performed a statistical analysis of the data available on blood proteins. His results showed that the majority of genetic differences between humans, about 85%, were found within a population. 7% of genetic differences were found between populations within a race. Only 8% on average was found to differentiate the various races.

Non concordance

The most widely used human racial categories are based on various combinations of visible traits such as skin color, eye shape and hair texture. However, many of these traits are non-concordant in that they are not necessarily expressed together. For example skin color and hair texture vary independently. This caused problems to early anthropologists who were attempting to classify race based on visible traits. Some examples of non-concordance include:

• There are many people in Africa and all over the world affected by albinism who have very light skin.
• Skin color varies all over the world in different populations. People from the Indian subcontinent are classified as Caucasian but some have dark skin.
• Epicanthal fold are typically associated with East Asian populations but are found in populations all over the world, including many Native Americans, Southern Africans, the Saami, and even amongst some isolated groups such as the Andamanese, which can all be explained to genetically relate more closely to one another than to other populations, even neighboring ones, due to specific migrations.
• Lighter hair colors are associated with Europeans, especially Northern Europeans, but blond hair is found amongst a limited, small number of the dark skinned populations of the south pacific, particularly the Solomon Islands and Vanuatu.

Human genetic variation

Human to human total genetic variation is approximately 0.5%. Single-nucleotide polymorphisms (SNPs) are single base-pair DNA differences accounting for 0.1% variation. Based only on this SNP variation of 0.1%, this implies that the genomes of any two random humans are expected to differ by about 3 million base pairs. Of this 0.1% difference, 85% is found within any given population, 7% is found between populations within a race and only 8% is found on average between the various races. Thus there is a claim that there is more genetic diversity within a race than between various races. However, this view has been criticised by Cambridge scientist Edwards, and is known as Lewontin's Fallacy, because the type of genetic difference between phenotypes is not entirely the same as that within phenotypes. Compared with other species the amount of genetic diversity among humans is relatively small, but that is not to say that is not significant in terms of the effects of that difference. For example two random chimpanzee are expected to differ by about 1 in 500 DNA base pairs, equivalent to double the diversity amongst humans. This may indicate that chimpanzees have existed as a species much longer than humans.

Ancestry-informative marker are stretches of DNA which have several polymorphisms that exhibit substantially different frequencies between the different populations. Using these AIMs scientists can determine a person's continent of origin based solely on their DNA. AIMs can also be used to determine someone's admixture proportions.

Genetic variation is found also in genes, but at present this variation is poorly understood. Much of the variation is found the regions of the genome affected by the environment. A notable example is genes affecting physical appearance, in particular skin color. Many of the genes regulating physical appearance have yet to be discovered. Genes related to the immunity system also show great variability with geographic location as a result of positive selection from the effects of regional diseases.

Models of genetic variation

Percentage genetic distances among major continents based on 120 classical polymorphisms

	Africa	Oceania	East Asia	Europe
Oceania	24.7
East Asia	20.6	10
Europe	16.6	13.5	9.7
America	22.6	14.6	8.9	9.5

There are several methods used to model human genetic variation. Genetic distance is a measure used to quantify the genetic differences between two populations. It is based on the principle that two populations that share similar frequencies of a trait are more closely related than populations that have more divergent frequencies of a trait. In its simplest form it is the difference in frequencies of a particular trait between two populations. For example the frequency of RH negative individuals is 50.4% among Basques, is 41.2% in France and 41.1 in England. Thus the genetic difference between the Basques and French is 9.2% and the genetic difference between the French and the English is 0.1%for the RH negative trait.

When only one trait is considered it often results in two very distant populations having little or no genetic difference. For example the frequency of blood group B allele in Russia is the same as in Madagascar indicating zero value for genetic distance. To adjust for these instances it is thus necessary to average values over several genetic systems. As DNA of all humans is 99.9 percent the same the vast majority of traits show little genetic distance between the continents. However, for a few traits that are highly polymorphic genetic distances can be calculated and used to create phylogenetic relationships.

Historically people have chosen spouses from nearby villages. Hence genetic distance is largely related to geographic distance between populations. Genetic distance may also occur due to physical boundaries that restrict gene flow such as Islands cut off by rising seas.

A study by Cavalli-Sforza using 120 blood polymorphisms provides information on genetic distances of the various continents.For the purpose of simplicity, admixed populations such as those of North Africa and West Asia were omitted from the analysis.

The largest genetic distance between any two continents is between Africa and Oceania at 24.7. Based on physical appearance this may be counterintuitive, since Australians and New Guineans resemble Africans with dark skin and sometimes frizzy hair. This resemblance is probably an example of convergent evolution. This large figure for genetic distance reflects the relatively long Isolation of Australia and New Guinea since the end of the Last glacial maximum when the continent was further isolated from mainland Asia due to rising sea levels.

The next largest genetic distance is between Africa and the Americas at 22.6%. This is expected since the longest geographic distance by land is between Africa and South America. The shortest genetic distance at 8.9% is between Asia and the Americas indicating a more recent separation.

Africans are the most divergent continent with all other groups being more related to each other than to Africa. This is expected in accordance with the Recent single-origin hypothesis. The population most closely related to Africans are Europeans. However, this short distance indicates significant interaction and gene exchange between Africa and Europe in the not so distant past. Europe has a genetic variation in general about three times less than that of other continents. Even though Europeans are the non-African group closest to Africans, Europeans are most closely related to East Asians. As the genetic distance from Africa to Europe (16.6) is shorter than the genetic distance from Africa to East Asia (20.6) Cavalli-Sforza proposes that both Asian and African populations contributed to the settlement of Europe which began 40,000 years ago. The overall contributions from Asia and Africa were estimated to be around two-thirds and one-third, respectively. Europe has a genetic variation in general about three times less than that of other continents.

Factors influencing genetic diversity

Selection

Positive selection plays an important role in shaping genetic variation. Most notably is its role in influencing physical appearance. Dark skin appears to be under strong selection because the protein that causes it varies very little in African populations but is free to vary in populations found outside Africa. This indicates that dark skin was selected to protect against the harmful effects of UV radiation that cause birth defects due to destruction of vitamin b folate. UV radiation also causes sunburn and skin cancer. When people left the sun intensive regions of Africa the protein was free to vary as a result lighter skin color reemerged in populations around the world. Light skin color was probably an advantage in very cold and wet climates, for the manufacture of vitamin D by sun light, in the skin.

Immunoglobulins or antibodies are also under strong selection in response to local diseases. For example people who are duffy negative tend to have higher resistance to malaria. Most Africans are duffy negative and most non-Africans are duffy positive.

Native Americans are almost exclusively Blood group O at about 98%. Some scientists believe this widespread distribution indicates strong selection, possibly resistance to syphilis. During the European invasion of the Americas, millions of Native Americans were decimated because of diseases they were not immune to such as smallpox and influenza. Europeans had become resistant to these disease after suffering several series of deadly plagues (such as the Plague of Justinian and the Black death). In turn the Europeans contracted syphilis to which they had no immunity.

Genetic drift

Genetic drift is the random change in gene frequencies between generations. By chance, a few individuals may leave behind more descendants and thus genes than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals.

Founder effect

Simple illustration of founder effect. The original population is on the left with three possible founder populations on the right.

The founder effect is the establishment of a new population by a few original founders which carry only a small fraction of the total genetic variation of the parental population. As a result, the new population may be distinctively different, both genetically and phenotypically, from the parent population from which it is derived. Some scientists speculate that the ubiquity of Blood group O amongst native Americans is an example of a strong founder effect. They argue that a small band of Asian people who crossed the Bering strait into Alaska may have been predominantly Blood group O.

Founder effects are notable following the colonization of Islands. The crania of Indigenous Australians is one of the most differentiated from other populations and is the most easily identified due to more prominent brow ridges. Since the crania shows little variability amongst Australians some scientists believe it arose from a founding effect.

Gene flow between continents

Gene flow is the exchange of genes from one population to another. Gene flow has the effect of reducing the genetic distance between two populations. Since genes are exchanged between neighboring populations many traits are distributed along clines. The boundaries of the major continents may in some cases restrict gene flow, allowing for genetic differentiation.

However many of the political divisions of today are not naturally occurring and in the past have not restricted gene flow. Europe and Asia are in fact the single continent of Eurasia. This would explain the relatively small genetic distance of 9.7% as calculated by Cavalli-Sforza.

North Africa is sometimes included as part of Eurasia. Northeast Africa is adjacent to Saudi Arabia and thus Africans have a long history of interaction with the middle east. Populations in the horn of Africa have significant Arab admixture. African mitochondrial DNA haplotypes are also frequent in the Middle east. Across the Sahara from Sudan to Senegal interactions between blacks and Arabs have resulted in significant gene exchange between the populations. In North Africa, in a study by Rando et al. 1998, levels of sub-Saharan mtDNA (excluding L3 lineage, which may be of ancient introduction and so remains ambiguous) ran from 2% in Moroccan Berbers, 9% in Algerian Berbers and in non-Berber Moroccans to 40% in Saharans and Mauritanians, and 56% in Tuareg. During the 8th century the Moors from North Africa conquered the Iberian peninsula, in the process they would have brought African admixture to Europe. This is clinically distributed across Europe from southwest to North east with Northern Europe showing no presence. On the other hand, Northern Europe, especially some parts of Scandinavia and the Baltic states have the highest Asian lineage-related genetic markers in Europe, represented mainly by haplogroup N.

Africa is the most genetically divergent continent. However, the most closely related population to Africa based on genetic distance is Europe at 16.6%. This may be counterintuitive based on different skin colors. Independent evolution on the different continents would result in equal genetic distances between Africa and the other continents. However, this low figure of 16.6(relative to Australia 24.7, and America 22.6%) indicates that there has been substantial interaction and exchange of genes between Africa and Europe. Cavalli-Sforza estimates that Europeans are mixed race population, one third African and two thirds Asian.

Joseph Greenberg classified American languages into three large families. He proposed that these families represent three separate migrations that filled the Americas in the order they arrived. These separate migrations across the Bering strait would have continued to bring new genes from Asia thus reducing the genetic distance between Asia and America.

Australasia is largely considered to be the most isolated continent. It was occupied at least 40,000 years ago when sea levels were much lower and the shortest distance between Indonesia and Australia was a 90 km sea voyage. 20,000 years ago at the end of the last Glacial Maximum, sea levels rose due to melting ice sheets flooding much of Australia's coastline and increasing its geographic isolation from Asia. Tasmania was cut off from Australia 10,000 years ago making it the most isolated region. These obstacles significantly restricted gene flow to indigenous Australasians. Second to Africa, Australasia is the most genetically divergent continent by genetic distance; however evidence suggests that even with Australasia gene flow has been taking place. Fossils of the Dingo in Australia have been dated to only 3500 years ago indicating that it was recently introduced. The dingo is native to India. Some Y chromosomal studies indicate a recent influx of y chromosomes from the Indian subcontinent. More recently fisherman from Makassar in Indonesia regularly made contact with Indigenous Australians from possibly as early as 1000 CE.

Recent admixture

Genetic techniques have been used to study racial admixture in America.

Defining race

The 0.1% genetic difference that differentiates any two random humans is still the subject of much debate. The discovery that only 8% of this difference separates the major races led some scientists to proclaim that race is biologically meaningless. They argue that since genetic distance increases in a continuous manner any threshold or definitions would be arbitrary. Any two neighboring villages or towns will show some genetic differentiation from each other and thus could be defined as a race. Thus any attempt to classify races would be imposing an artificial discontinuity on what is otherwise a naturally occurring continuous phenomenon.

However, other scientists disagree by claiming that the assertion that race is biologically meaningless is politically motivated and that genetic differences are significant. Neil Risch states that numerous studies over past decades have documented biological differences among the races with regard to susceptibility and natural history of a chronic disease, though acknowledges that these differences do not constitute any major subdivisions of the human species: '...These conclusions seem consistent with the claim that "there is no biological basis for 'race'" and that "the myth of major genetic differences across 'races' is nonetheless worth dismissing with genetic evidence". Of course, the use of the term "major" leaves the door open for possible differences but a priori limits any potential significance of such differences.' Effectively Neil Risch is attempting to redefine "race" for human populations to represent that small proportion of variation that is known to vary between continental populations. It is well established, that the level of differentiation between the continental human groups, as measured by the statistic F_ST is about 0.06-0.1 (6-10%), with about 5-10% of variation at the population level (that is between different populations occupying the same continent) and about 75-85% of variation within populations.(Risch et al., 2002; Templeton, 1998; Ossorio and Duster, 2005; Lewontin, 2005). Tempeton (1998) states that in biology a level of 0.25-0.3 (20-30%) of differentiation normally accepted in biological literature for a population to be considered a race or subspecies.

"A standard criterion for a subspecies or race in the nonhuman literature under the traditional definition of a subspecies as a geographically circumbscribed, sharply differentiated population is to have F_ST values of at least 0.25 to 0.3 (Smith et al. 1997). Hence as judged by the criterion in the nonhuman literature, the human F_ST value is too small to have taxonomic significance under the traditional subspecies definition."(Templeton, 1998)

Indeed Neil Risch himself avoids defining race, when asked to respond to the comment "Genome variation research does not support the existence of human races.” he replied

What is your definition of races? If you define it a certain way, maybe that's a valid statement. There is obviously still disagreement....Scientists always disagree! A lot of the problem is terminology. I'm not even sure what race means, people use it in many different ways.(Gitschier, 2005)

Racial classification is a modern phenomenon dating back to the 15th century when Portuguese and Spanish sailors encountered sub-saharan Africans and referred to them as Negro (the color black). Literature from earlier Roman and Greek eras is noticeably lacking in racial references. Instead people were often described by their tribal origin or by their status as freemen or slaves.

Research published in July 2008 concludes that there is no race. A Scientific American article entitled "Traces of a Distant Past" by Gary Stix has the most interesting quote:

Genetic literacy will let a term like "Asian" or "Chinese" be replaced by more subtle classifications based on the differences in ancestral genetic makeup found in recent genome-wide scans, such as the distinction between China's southern and northern Han groups. "There is no race."Quintana-Murci says

Clusters controversy

A computer program called STRUCTURE is used by some scientists to determine clusters of human populations. It is a statistical program that works by placing individuals into one of two clusters based on their overall genetic similarity, many possible pairs of clusters are tested per individual to generate multiple clusters. These populations are based on multiple genetic markers that are often shared between different human populations even over large geographic ranges. The notion of a genetic cluster is that people within the cluster share on average similar allele frequencies to each other than to those in other clusters.(Edwards, 2003)

The results obtained by clustering analyses are dependent on several criteria:

• The clusters produced are relative clusters and not absolute clusters, each cluster is the product of comparisons between sets of data derived for the study, results are therefore highly influenced by sampling strategies. (Edwards, 2003)
• The geographic distribution of the populations sampled, because human genetic diversity is marked by isolation by distance, populations from geographically distant regions will form much more discrete clusters than those from geographically close regions. (Kittles and Weiss, 2003)
• The number of genes used. The more genes used in a study the greater the resolution produced and therefore the greater number of clusters that will be identified.(Tang, 2005)

A study by Noah Rosenberg and Jonathan K. Pritchard, geneticists from the laboratory of Marcus W. Feldman of Stanford University, assayed 377 polymorphisms (ie gene types) in more than 1,000 people from 52 ethnic groups in Africa, Asia, Europe and the Americas. They concluded that without using prior information about the origins of individuals, they were able to identify six main genetic clusters, five of which correspond to major geographic regions, and subclusters that often correspond to individual populations. The clusters corresponded to Africa, Europe and the part of Asia south and west of the Himalayas, East Asia, Oceania, the Kalash (of Pakistan) and the Americas. (Rosenberg, 2002 and Rosenberg, 2005)

Distribution of European clusters identified by Bauchet. When two clusters are identified there is a north-southeast cline that may be due to demic diffusion during the European Neolithic

Another study by Neil Risch in 2005 used 326 microsatellite markers and self-identified race/ethnic group (SIRE), white, African-American, Asian and Hispanic (individuals involved in the study had to choose from one of these categories), to representing discrete "populations", and showed distinct and non-overlapping clustering of the white, African-American and Asian samples. The results confirmed the integrity of self-described ancestry: "We have shown a nearly perfect correspondence between genetic cluster and SIRE for major ethnic groups living in the United States, with a discrepancy rate of only 0.14%." But also warned that: "This observation does not eliminate the potential for confounding in these populations. First, there may be subgroups within the larger population group that are too small to detect by cluster analysis. Second, there may not be discrete subgrouping but continuous ancestral variation that could lead to stratification bias. For example, African Americans have a continuous range of European ancestry that would not be detected by cluster analysis but could strongly confound genetic case-control studies." (Tang, 2005)

Additionally two studies of European population clusters have been produced. Seldin et al. (2006) identified three European clusters using 5,700 genome-wide polymorphisms. Bauchet et al. (2007) used 10,000 polymorphisms to identify five distinct clusters in the European population, consisting of a south-eastern European cluster (including samples from southern Italians, Armenian, Ashkenazi Jewish and Greek "populations"); a northern-European Cluster (including samples from German, eastern English, Polish and western Irish "populations"); a Basque cluster (including samples from Basque "populations"); a Finnish cluster (including samples from Finnish "populations") and a Spanish cluster (including samples from Spanish "populations"). Most "populations" contained individuals from clusters other than the dominant cluster for that population, there were also individuals with membership of several clusters. The results of this study are presented on a map of Europe. (Bauchet, 2007)

Criticism of the clusters study

Though the authors of the study do not equate the clusters with race there are some who view the studies on clusters as evidence of the existence of biological races. Hence these studies have attracted considerable controversy. Critics argue that using genetic information to determine an individual's continent of origin is not a new concept. Using the ABO, RH and MNS blood groups, scientists in the 1950s could already determine continent of origin based on known frequencies of these traits.

Critics argue that any attempt to divide humanity will always produce artificial results. They point to the fact that in the study when six clusters were used an additional cluster (race) appeared which consisted solely of the Kalash of Pakistan. Several groups in the study also appeared in two races such as Ethiopians, Hazara of Pakistan, and Uyghur from Pakistan and western China. Joseph Graves argues that in the study the people sampled were from regions separated by large distances such as South African Bantu and Russians. He argues that if more people came from the regions that bridge the continents results may have been different. Examples such as Armenians would cluster both with Asia and Europe. Somalians or Yemenites may cluster both with Africa and Europe.

Others say the bulk of human variation is continuously distributed and, as a result, any categorization schema attempting to meaningfully partition that variation will necessarily create artificial truncations. It is for this reason, they argue, that attempts to allocate individuals into ancestry groupings based on genetic information have yielded varying results that are highly dependent on methodological design.

Nicholas Wade, who often cites the work of clusters in articles for the New York Times, says that even if individuals can be assigned to continent of origin based on their genotype (genes), this is not an indication of phenotype. This is because the SNPs used in the clustering study are selectively neutral i.e. stretches of Junk DNA that have no known function. Since they do not code for any protein or have regulatory function, mutations can occur without interfering in normal cell function. Over time these mutations can accumulate much quicker in local populations and thus they can be used to identify continent of origin. Therefore these SNPS that can be used to differentiate continental populations are not known to influence intelligence, behavior, susceptibility to disease or ability in sports. Wade argues that it is possible that even though the sites used are nonworking sections of DNA, mutations in them may serve as a proxy for mutations in genes that influence intelligence and behaviour. However, he admits that at the moment there is no known relationship between mutations in junk DNA and mutations in genes.

Complexities of the human genome

Many human phenotypes are polygenic, meaning that they depend on the interaction among many genes. Polygeneity makes the study of individual phenotypic differences more difficult. Additionally, phenotypes may be influenced by environment as well as by genetics. The measure of the genetic role in phenotypes is heritability.

Different genes may also produce the same phenotype. For example the gene that causes light skin color in Europeans is different than the gene that causes light skin in East Asians. Europeans have a different version of the SLC24A5 than East Asians possibly indicating that they evolved light skin independently. A recent asthma study found that genes that defined susceptibility to asthma in blacks were different than the genes defined susceptibility in whites which were again different for the genes that defined susceptibility to asthma in Hispanics.

Epigenetic inheritance describes a phenomenon where traits are passed on to the next generation based on environmental effects or experience. These traits are inherited without being written into the DNA sequence. In some cases traits are passed on to the next generation by the switching off or on of various genes that are already present. The implication of this is that having the same genotype at a locus does not necessarily mean having the same phenotype.

Popular beliefs about race and genetics

In his book Guns, Germs, and Steel, Jared Diamond argues that European explorers believed that differences between European societies and American societies could be explained by differences in innate biological ability. Darwinian evolution viewed the "primitive societies" to be vestiges of human descent from apelike ancestors.