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Since the work of Charles Darwin and Gregor Mendel, scientists have striven to understand human genetic variation and its relationship to human evolution. Race and genetics is a broad multidisciplinary set of studies that attempt to use the sciences of human genetics and evolution to inform our understanding of race.

Contents

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.[1]

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.

Defining race

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.[2] This caused problems to early anthropologists who were attempting to classify race based on visible traits. Some examples of non-concordance include:

  • Skin color varies all over the world in different populations. People from the Indian subcontinent are classified as Caucasian although most 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, the Khoisan, the Saami, and even amongst some isolated groups such as the Andamanese.
  • Lighter hair colors are typically 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.

Genetic distance

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. 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 FST 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.[3][4][5][6] 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.[4]

"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 FST 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 FST value is too small to have taxonomic significance under the traditional subspecies definition."[4]

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. He continues, "but that doesn't preclude you from using it or the fact that it has utility".[7]

Clusters controversy

Infobox

Multi Locus Allele Clusters

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.[8] 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)

Race-based medicine

Because of the correlation between self-identified race and genetic clusters, medical treatments whose results are influenced by genetics often have varying rates of success between self-defined racial groups.[9] For this reason, some doctors consider a patient’s race while attempting to identify the most effective possible treatment,[10] and some drugs are marketed with race-specific instructions.[11] However, because of the inexact nature of the correlation between self-defined races and genetic clusters, as well as because of the large amount of genetic variation within ethnic groups, the prevailing view among medical researchers is that when individual assessment of the relevant genes becomes available, it will probably prove more useful than race in medical decision-making.[12]

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 Asia.

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.[13]

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.

Human genetic variation

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 from 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 those of African descent were different from the genes that 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.

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 and as a result, lighter skin color emerged in populations around the world. [14] 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. [15]

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.[16]

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.

Other factors include genetic drift and founder effect.

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. Of this 0.1% difference, 85% is found within any given population, 7% is found between populations within a continent and only 8% is found on average between the various continental populations. Based on this observation, evolutionary biologist Richard Lewontin has claimed that accurate racial classification of humans is impossible and can have no taxonomic utility. However, this view has been rejected by geneticist A. W. F. Edwards in his paper entitled Human Genetic Diversity: Lewontin's Fallacy (2003). Edwards argues that accurate classification of humans is possible because most of the data that distinguishes populations occurs in correlations between allele frequencies, although these classifications vary depending on a number of criteria, such as sampling strategy, type of locus, distribution of loci around the genome and number of loci. Nonetheless, Witherspoon et al. (2007) demonstrate that even when accurate classification of human populations is achieved, often individuals classified into different groups are more genetically similar to each other than to members of their own group. This seems to be due to the fact that multi-locus clustering does not take into account the genetic similarities between individuals, and only uses population level traits for comparison. They conclude that accurate classification of individuals drawn from a continuously varying human population may be impossible. Compared with most other species, the amount of genetic diversity among humans is relatively small. 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.[17]

Ancestry-informative markers (AIMs) are stretches of DNA which have several polymorphisms that exhibit substantially different frequencies between different populations. Using AIMs, scientists can determine a person's ancestral continent of origin based solely on their DNA. AIMs can also be used to determine someone's admixture proportions.[18]

f===Genetic distance===

There are several methods used to model human genetic variation. Genetic distance is a measure used to quantify the difference between two populations in relation to the frequency of a particular trait. It is based on the principle that trait frequency indicates relatedness, and is measured by the difference in frequencies of a particular trait between two populations. For example, the frequency of Rh(D) negative alleles is 50.4% among Basques and 41.2% among the French. Thus, the genetic distance between the Basques and the French in terms of the Rh(D) trait is calculated as 9.2%.

When the relative frequencies of any one trait are compared, the results often demonstrate no significant genetic difference between populations. For example, the frequency of the blood group B allele in Russia is the same as in Madagascar, yielding a 0% genetic distance. To offset these inexpressive results, average values of several polymorphic traits are compared together as clusters to estimate both genetic distances and phylogenetic relationships between populations.

Tree analysis

Linkage tree and distance matrix for 9 population clusters.

Tree analysis attempts to reconstruct population separations and movements over time through the comparison of genetic distances for one or more traits.[19] A landmark study by Cavalli-Sforza evaluated the genetic distances between 42 native populations from around the world based on 120 blood polymorphisms. These 42 populations can be grouped into 9 main clusters, which Cavalli-Sforza termed African (sub-Saharan), Caucasoid (European), Caucasoid (extra-European), Northern Mongoloid (excluding Arctic populations), Northeast Asian Arctic, Southern Mongoloid (mainland and insular Southeast Asia), New Guinean and Australian, and American (Amerindian). Though the clusters evidence varying degrees of homogeneity, the 9-cluster model represents a majority (80 out of 120) of single-trait trees and is useful in demonstrating the historic phylogenetic relationship between these populations.[20]

Geographic analysis

Geographic analysis attempts to identify the places of origin of specific mutations and the possible selective factors involved in their spread.[19] Genetic distance significantly correlates to geographic distance between populations, a phenomena referred to as "isolation by distance".[21] Genetic distance can also be the result of physical boundaries which naturally restrict gene flow, such as islands, deserts, mountain ranges or dense forests.

In Cavalli-Sforza's geographic analysis of the above mentioned 42 populations, some admixed populations such as those of North Africa and West Asia (Non-European Caucasoid) were omitted for the purpose of simplicity.

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 Indigenous 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.

Africa is the most genetically divergent continent, with all other groups being more related to each other than to Sub-Saharan Africans. This is expected in accordance with the Recent single-origin hypothesis. When the Non-European Caucasoids of Northern Africa and Western Asia are omitted from the analysis, Europe demonstrates the shortest genetic distance of all continents to Africa. However, this short distance is possibly the result of 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, and the genetic contribution of Asia and Africa to Europe is thought to be 2/3 and 1/3 respectively.[22]

Linguistic analysis

Linguistic analysis reveals a very strong correlation between populations and language families.[19] As a general rule, the degree of genetic similarity between populations which belong to the same linguistic family is high. The notable exceptions to this rule are Lapps, Ethiopians and Tibetans, who are genetically associated with populations which speak languages belonging to different linguistic families. For example, the Lapps speak a Uralic language yet are genetically associated with populations which speak Indo-European languages. This kind of situation is thought to be a result of hybridization.[23]

See also

Footnotes

  1. ^ Sykes, Bryan (2001). "From Blood Groups to Genes". The seven daughters of Eve. New York: Norton. pp. 32–51. ISBN 0-393-02018-5. 
  2. ^ RACE - The Power of an Illusion . Background Readings | PBS
  3. ^ Risch, Neil; Burchard, Esteban; Ziv, Elad; Tang, Hua (2002). Genome Biology 3: comment2007.1. doi:10.1186/gb-2002-3-7-comment2007. 
  4. ^ a b c Templeton, Alan R. (2003). "Human Races in the Context of Recent Human Evolution: A Molecular Genetic Perspective". in Goodman, Alan H.; Heath, Deborah; Lindee, M. Susan. Genetic nature/culture: anthropology and science beyond the two-culture divide. Berkeley: University of California Press. pp. 234–257. ISBN 0-520-23792-7. http://slava.parma.ru/Doc/Unsorted/New/BOOKS/Genetic.Nature.Culture.Anthropology.And.Science.Beyond.The.Two-Culture.Divide.eBook-LiB.pdf#page=254. 
  5. ^ Ossorio and Duster, 2005
  6. ^ Lewonin, R. C. (2005). Confusions About Human Races from Race and Genomics, Social Sciences Research Council. Retrieved 28 December 2006.
  7. ^ Risch N (July 2005). "The whole side of it--an interview with Neil Risch by Jane Gitschier". PLoS Genetics 1 (1): e14. doi:10.1371/journal.pgen.0010014. PMID 17411332. 
  8. ^ "Genetic Similarities Within and Between Human Populations" (2007) by D.J. Witherspoon, S. Wooding, A.R. Rogers, E.E. Marchani, W.S. Watkins, M.A. Batzer and L.B. Jorde. Genetics. 176(1): 351–359.
  9. ^ Racial Differences in the Response to Drugs — Pointers to Genetic Differences. New England Journal of Medicine, Volume 344:1393-1396, May 3, 2001.
  10. ^ Bloche, Gregg M. Race-Based Therapeutics. New England Journal of Medicine, Volume 351:2035-2037, November 11, 2004.
  11. ^ Drug information for the drug Crestor. Warnings for this drug state, "People of Asian descent may absorb rosuvastatin at a higher rate than other people. Make sure your doctor knows if you are Asian. You may need a lower than normal starting dose."
  12. ^ Jordge, Lynn B. and Stephen P. Wooding. "Genetic Variation, classification and 'race'". Nature, Vol. 36 Num. 11, November 2004.
  13. ^ Back with a Vengeance: the Reemergence of a Biological Conceptualization of Race in Research on Race/Ethnic Disparities in Health Reanne Frank
  14. ^ NICHOLAS WADE (August 19, 2003). "Why Humans and Their Fur Parted Ways". The New York Times. http://query.nytimes.com/gst/fullpage.html?res=9C03E0DE1030F93AA2575BC0A9659C8B63&sec=health&spon=&pagewanted=2%20why%20humans%20and%20their%20fur%20parted. Retrieved 2008-01-12. 
  15. ^ Hall, Stephen S. (October 2008). "Last of the Neanderthals". National Geographic Magazine 214: 50. 
  16. ^ "Malaria and the Red Cell". Harvard University. April 2, 2002. http://sickle.bwh.harvard.edu/malaria_sickle.html. Retrieved 2008-01-12. 
  17. ^ understanding human genetic variation
  18. ^ Lewontin, R.C.. "Confusions About Human Races". http://raceandgenomics.ssrc.org/Lewontin/. 
  19. ^ a b c Cavalli-Sforza (1997:7721).
  20. ^ Cavalli-Sforza (1994:80).
  21. ^ Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa
  22. ^ Cavalli-Sforza (1997:7720).
  23. ^ Cavalli-Sforza (1997:7722).

References

External links


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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.

Contents

Interest in race and genetics

There are several reasons why people today are interested in the genetics of race.

Ethnocentrism

Ethnocentrism often entails the belief that one's own race or ethnic group is the most important and that some or all aspects of its culture are superior to those of other groups. Claude Lévi-Strauss defined racism as the belief that one's race is biologically superior—that superior genes, chromosomes, DNA put it at an advantage over all others.[1] These beliefs have led some to use science to attempt to find the genetic basis for the superiority of their own race.

Today almost all sociologists define race and ethnicity as social constructs with no biological basis, but other fields of academia, such as anthropology, speifically biological or cultural anthropologists, especially (the study of humans as opposed to the study of society), may have a more pertinent opinion on the definition of race and ethnicity. Sociologists cite many cases of how castes and ethnic groups have been constructed from groups that are genetically indistinguishable since many of the conflicts today are internal they involve groups that are closely related; examples include the Hutu-Tutsi conflict, the Yugoslav wars[2] or the various conflicts in the middle east.[3]

Race and Intelligence

Main article: Race and Intelligence

There is considerable controversy over whether there are any differences in intelligence between the various populations. Much of this controversy centers around racial and ethnic differences in intelligence test scores.

Another area of contention is the question of why certain societies such as those associated with western Eurasia, have made large technological strides in recent times while other societies are still living in the stone age in the 21st century. On one hand there are some who believe that these differences arose due to inherently genetic factors. On the other hand there are those who argue that the reason why certain societies progressed was more a result of opportunity and necessity rather than any inherent genetic advantages in cognitive ability.

Race and Behaviour

Carolus Linnaeus was a pioneer in defining the concept of race in humans. Each race had certain characteristics that he considered endemic to individuals belonging to it. Native Americans were reddish, stubborn and easily angered. Africans were black, relaxed and negligent. Asians were sallow, avaricious and easily distracted. Europeans were white, gentle and inventive. Linnaeus's races were clearly skewed in favour of Europeans. The legacy of these notions survives today in the stereotypes about racial behaviour.

Race and physical ability

The apparent dominance of certain ethnicities with respect to certain sporting abilities has led some to question whether there is a genetic component predisposing certain races to different sports. According to an article in the May 2000 issue of Scientific American, "...scientists have identified physical attributes that are more common to West Africans and East Africans than to Europeans, ones that might provide an edge in sprint and endurance exercises.";[4] eitherway, the dominance of African Americans in some American sports has been the subject of a longtime controversy. While blacks make up the majority of players in both the NBA and the NFL, with 76% and 69% in 2005 respectively, they make up the minority (10%) of players in major league baseball.[5] The theory that blacks, especially those of West African descent, have an advantage with respect to certain types of athletics is dismissed by some as racist; these critics say that presumption also infers that athletes of African descent are intellectually and morally inferior and dismissive of the hard-work of blacks who excel in sports[6][7].[8][9]

Nature versus nurture

The nature versus nurture debates concern the relative importance of an individual's innate qualities ("nature") versus personal experiences ("nurture") in determining or causing individual differences in physical and behavioral traits. On the nature side is the philosophy of genetic determinism. This is the belief that genes determine physical and behavioral phenotypes and that the environment has little or no role in influencing phenotypes. This term is often applied to the mapping of a single gene to a single phenotype such as a gene for intelligence or a gene for homosexuality or a gene for aggressive behavior.

On the other hand social determinism the hypothesis that social interactions and social constructs alone are responsible for influencing individual behavior. Environmental determinism is the view that the physical environment rather than genes or social conditions determine the culture of a society.

Nature versus nurture controversies often arise when attempting to explain any racial disparity such as athletic success, test scores or health indicators.

Early history

Blood groups

geographic distribution of blood group A

Prior to the discovery of DNA as the hereditary material scientist used blood proteins to study human genetic variation. The first blood transfusions were recorded in the 15th century in Italy. Many people died from severe reactions and the practice was banned. The practice started again in the 19th century to combat fatal hemorrhages occurring from childbirth. However, many patients were still suffering the sometimes lethal consequences of reaction to the transfusion. In 1875, scientists noticed that this reaction was due to the blood cells clumping together and sometimes bursting open. In 1900, Karl Landsteiner discovered that the problem was different blood groups of the ABO system.

geographic distribution of blood group B

During the first world war demand for blood transfusions increased. One of the first papers written on blood groups was by Ludwik and Hanka Herschfeld who worked at a global blood testing laboratory for the Allied forces. As the allies drew forces from several nations the Herschfelds were able to collect and compile blood group profiles of several nations.

When the compared the results the found 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. Africans 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. These two pure races later became mixed resulting in the complex pattern of groups A and B. This was one of the first indications that visible human variation did not necessarily correlate with the invisible variation.

It was hoped that groups that had similar proportions of the blood groups would be more closely related but instead it was often found that groups separated by large distances such as Madagascar and Russia would have similar frequencies. This confounded scientists who were attempting to learn more about human evolutionary history. The next big break would come with the discovery of more blood groups and proteins.[10]

Blood proteins and molecular evolution

In 1957 Emile Zuckerkandl began studying the amino acid sequences of various blood proteins. Hemoglobin was a useful protein to study because it was found in the blood of every living mammal. When studying the amino acid sequences of various mammals Zuckerkandl found that the protein sequences were quite similar but he also noticed an interesting pattern. He found that the more closely related animals were the more similar their amino acid sequences were. For example the human and gorilla sequences differed in two places while the human and the horse differed in 15 places. This suggested that the proteins could serve as a molecular clock indicating when the two different species last shared a common ancestor by counting the number of different amino acids. A phylogenetic tree could then be built that portrays the evolutionary relatedness of various speciesCite error: Closing </ref> missing for <ref> tag When scientist began studying global mitochondrial DNA sequences they identified 33 mitochondrial DNA clans, 13 were from Africa. Though Africa had only 12% of the worlds population it had 40% of the maternal clans. As a rule of thumb for any species the region of greatest diversity is usually the region of origin.

Studies using Mitochondrial DNA have found that all humans today are descended from one woman, named Mitochondrial Eve, who may have lived in Africa some 150,000 years ago. Since Mitochondrial Eve 7500 generations have passed, and since the first split between Africans and non Africans, 2500 generations have passed. This would explain why human genetic diversity is relatively low compared to species that have existed for much longer.

Human genome

Main article: human genome

Though Watson and Crick discovered the structure of DNA in 1953, its use in studying Human genetic variation was fairly limited because the technology to decode its sequences was too slow. Polymerase chain reaction was invented in 1983 by Kary Mullis. This technique allowed for rapid sequencing of segments of DNA. The human genome project would then proceed to sequence a working draft of the human genome in 2000.

The human genome was found to contain 3.1 billion DNA base pairs. Initially scientists had expected to find a significant number of genes, around 100,000.[11] However scientists continued to revise down their estimates until finally arriving at a number between 20,000 and 25,000 genes. This low estimate surprised many scientists who viewed the number of genes as related to an organisms complexity. As a comparison much simpler organisms such as the roundworm have only 20,000 genes and certain plants species have more genes than humans.

Another surprise was that only about 3% of the genome was found to code for protein or had some regulatory purpose. The other 97% of the genome at present has no known function and has been labeled junk DNA.

The human genome is remarkably similar to that of the chimpanzee. Initially it was estimated that the human and chimpanzee genomes were 98.6 similar. When insertions and deletions in DNA sequences were later considered the figure was revised down to about 95%. Much of the difference is also junk DNA.[12]

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.[13] 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

Main article: Human genetic variation

The human genome project has found that human genetical variation is confined to approximately one-tenth of a percent of the total genetic composition. The most common polymorphisms (or genetic differences) in the human genome are single base-pair differences. Scientists call these differences SNPs, for single-nucleotide polymorphisms. About 99.9% of the human genome is identical in all humans. On average there is only 0.1% difference, which 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 500 DNA base pairs, equivalent to double the diversity amongst humans. This indicates that chimpanzees have existed as a species much longer than humans.[14]

Most of this genetic variation is found in the "junk DNA" . Scientists estimate that up to 97% of the human genome is junk DNA.[15] This entails that the actual genes, that do function, vary much less. The reason for this is that mutations that occur in the junk DNA have no effect and are referred to as selectively neutral, whereas mutations that occur in the actual genes are subject to the rigors of natural selection. If the mutation has a strong adverse effect it is quickly eliminated from the population, as the affected organism does not survive or does not reproduce. For example it has been estimated that 20% of all conceptions end in miscarriages in the few days following fertilization. This is because of mutations in the genes that are harmful to the foetus. The net effect is that these mutations in the actual genes are not passed on to subsequent generations. On the other hand mutations in the junk DNA are free to accumulate with time.

Since mutations in junk DNA occur much faster than in the genes, they accumulate much faster in local populations. This is useful to population geneticists who can use these SNPs to distinguish various populations. 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.[16]

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.[17]

When only one trait is consider 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 the few traits that are highly polymorphic genetic distances can be calculated and used to create phylogenetic relationships.

An Indigenous Australian , Melanesian, African and European. Though Oceanians resemble Africans they are the most genetically distant. Africans are more closely related to Europeans than any other group despite having different skin colors.
Historically people have chosen spouses from nearby villages. Hence genetic distance is largely related to geographic distance between populations.[18] 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.[19]

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 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.[20]

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 in indication 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.[21] 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 Europeans are duffy positive.[22]

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 resistance 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.[23]

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.

Controversially 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. 20% of North Africans have sub-saharan African mitochondrial DNA haplogroups. During the 8th century the Moors from North Africa conquered the Iberian peninsula, in the process they would have brought African admixture to Europe. Studies have shown about 4% of the population in Spain and Portugal have sub-Saharan mtDNA haplogroups. This is clinically distributed across Europe from southwest to North east with Northern Europe showing no presence.

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.[17][24]

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.[25] More recently fisherman from Makassar in Indonesia regularly made contact with Indigenous Australians from possibly as early as 1000 CE.

Sexual selection

Sexual selection is a controversial theory that competition for mates between individuals of the same sex drives the evolution of certain traits. Neoteny is a term that describes the retention of infant like characteristics through adulthood. Some scientists believe sexual selection for certain neotenous traits has been a driving force in differentiating various populations. These traits include less hairy skin, more delicate skin, thinness of skull bones and a gracile skeleton. The so named "Mongoloid" skeleton is the most gracile skeleton. Gracile is defined as low bone thinness relative to length and is contrasted with a robust skeleton. Worldwide the skeletons of all populations have undergone considerable gracilization in the last 10000 years.[26]

Recent Admixture

Carol Channing lived as white only revealing when she was 81 that her father was part African American

Miscegenation between two populations reduces the genetic distance between the populations. During the Age of Discovery which began in the early 15th century, European explorers sailed all across the globe reaching all the major continents. In the process they came into contact with many populations that had been isolated for thousands of years. It is generally accepted that the Tasmanian aboriginals were the most isolated group on the planet. They were driven to extinction by European explorers, however a number of their descendants survive today as a result of admixture with Europeans. This is an example of how modern migrations have began to reduce the genetic divergence of the human race.

The demographic composition of the old world has not changed significantly since the age of discovery. However, the new world demographics were radically changed within a short time following the voyage of Columbus. The colonization of Americas brought Native Americans into contact with the distant populations of Europe, Africa and Asia. As a result many countries in the Americas have significant and complex multiracial populations. Furthermore many who identify themselves by only one race still have multiracial ancestry.

Admixture in the United States

Admixture in European-American population
 % European Admixture Frequency
90-100 68%
80-89.9 22%
70-79.9 8%
60-69.9 < 1%
50-59.9 < 1%
40-49.9 < 1%
0-39.9 0

Today the vast majority of African-Americans possess varying degrees of European and Native American admixture; some estimates put average African-American possession of European Admixture at 25% with figures as high as 50% in the Northeast and less than 10% in the south. A recent study by Mark D. Shriver of a European-American sample found that the average admixture in the white population is 0.7% African and 3.2% Native American. However, 70% of the sample had no African Admixture. The other 30% had African Admixture ranging from 2% to 20% with an average of 2.3%. By extrapolating these figures to the whole population some scholars suggest that up to 74 million European-Americans may have African admixture in the same range (2-20%).

Dr Mark Shriver himself the team leader of the study found that he had 11% West African ancestry though he identifies as white. Studies based on skin reflectance have shown the color line in the US applied selective pressure on genes that code for skin color but did not apply any selective pressure on other invisible African genes. Since there are an estimated 6 alleles involved for skin color it is possible for a someone to have 15-20% African admixture and not possess any of the alleles that code for dark skin. This is the basis of the passing phenomena. Thus African admixture amongst white Americans can increase without any significant change in skin tone. Conversely amongst African-Americans, an amount of African Admixture is directly correlated with darker skin since no selectionary pressure is applied; as a result, African-Americans may have a much wider range of African admixture (>0-100%), where as European-Americans have a lower range (2-20%). A small overlap exists so that it is possible that someone who self identifies as white may have more African admixture than a person who self identifies as black[27][28]

A statistical analysis done in 1958 using historical census data and historical data on immigration and birth rates, concluded that 21 percent of the white population had black ancestors. The growth in the white population could not be attributed to births in the white population and immigration from Europe alone, but had received significant contribution from the American black population as well.[29] The author states in 1958:

The data presented in this study indicate that the popular belief in the non-African background of white persons is invalid. Over twenty-eight million white persons are descendants of persons of African origin. Furthermore, the majority of the persons with African ancestry are classified as white.

Admixture in Latin America

Background

Prior to the European conquest of the Americas the demographics of Latin America was naturally 100% Native American. Today those who identify themselves are small minorities in many countries. For example Argentina's native population is 0.9%, Brazil is 0.4%, and Uruguay is 0%.[30]

In addition many Africans were shipped to regions all over the Americas and were present in many of the early voyages of the conquistadors. Brazil has the largest population of African descendants outside of Africa. Other countries such as Cuba, Puerto Rico, Dominican Republic, Venezuela, Haiti, and Colombia still have sizeable populations identified as Black. However countries such as Argentina and Chile today do not have a visible African presences today. Census information from the early 19th century shows that people categorized as Black made up 30% of the population.[31] Though almost completely absent today, their contribution to Argentine culture is significant include the Tango the Milonga and the Zamba words of Bantu origin.[32]

The early conquest of Latin America was primarily carried out by male soldiers and sailors from Spain and Portugal. Since they carried very few European women on their journeys the new settlers married and fathered children with Amerindian women and also with women imported from Africa. This process of miscegenation was even encouraged by the Spanish monarchy and it led to the system of stratification known as the Casta. This system had Europeans (mainly Spaniards and Portuguese) at the top of the hierarchy followed by those of mixed race. Unmixed Blacks and Native Americans were at the bottom. A philosophy of whitening emerged in which Amerindian and African culture was stigmatized in favor of European values. Many Amerindian languages were lost as mixed race offspring adopted Spanish and Portuguese as their first languages. Only towards the end of the 19th Century and beginning of the twentieth century did large numbers of Europeans begin to migrate to South America and consequently altering its demographics.

Demographics of Brazil from 1835 to 2000[33]
Year White brown black
1835 24.4% 18.2% 51.4%
2000 53.7% 38.5% 6.2%

The ideology of whitening encouraged non whites to seek white or lighter skinned partners. This dilution of non-white admixture would be beneficial to their offspring as they would face less stigmatization and find it easier to assimilate into mainstream society. After successive generations of European gene flow, non-white admixture levels would drop below levels at which skin color or physical appearance is not affected thus allowing individuals to identify as white. Through this process and the high death rate of blacks from poverty the Afro Argentine population was all but eliminated.

In Argentina the process of Europeanization was most efficient since those who identify as white are 97% of the population.[34][35] Some have therefore accused former Latin American governments of secretly promoting white supremacist policies. They cite the fact that many Nazi war criminals, such as Adolf Eichmann who lived in Argentina, were given safe havens in Latin America after the end of the world war 2.

Historians and scientists are thus interested in tracing the fate of Native Americans and Africans from the past to the future. The questions remain about what proportion of these populations simply died out and what proportion still has descendants alive today including those who do not racially identify themselves as their ancestors would have. Admixture testing has thus become a useful objective tool in shedding light on the demographic history of Latin America.

Recent studies

Evidence for sex biased mating in the White population of some Latin American countries
Country Amerindian African
mtDNA Y-chromosome mtDNA y-chromosome
Brazil 33% 0% 29% 2%
Argentina 45% 9% ns ns
Chile 84% 22% ns ns
Colombia 90% 1% 8% 5%
Costa Rica 83% 6% ns 7%

Unlike in the United States there were no anti-miscegenation policies in Latin America. Though still a racially stratified society there were no significant barriers to gene flow between the three populations. As a result admixture profiles are a reflection of the colonial populations of Africans, Europeans and Amerindians. The pattern is also sex biased in that the African and Amerindian maternal lines are found in significantly higher proportions than African or Amerindian Y chromosomal lines. This is an indication that the primary mating pattern was that of European males with Amerindian or African females. For example a study of white Brazilians found 33% had Amerindian mtDNA and 29% had African mtDNA. However, only 2% had African y chromosomes and 0% Amerindian. According to the study more than half the white populations of the Latin American countries studied have either native American or African admixture. In countries such as Chile and Colombia almost the entire white population has non-white admixture.[36][37].[38][39] Following the dispersal of Humans from Africa 50,000 years ago South America was the last continent to be occupied by humans. Thus the largest geographic distance between continents is between Africa and South America. Since genetic distance increases with geographic distance the two most genetically divergent groups are Africans and Native South American Indians based on distance. The arrival of Africans in Brazil and subsequent mixing with native South Americans entails the creation of intermediate populations, such as the Zambo or Garifuna between the two divergent groups.

Defining race

Main article: 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 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 FST 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 FST 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 FST 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)

Clusters controversy

Infobox

Multi Locus Allele Clusters

In a haploidImage:Wp_globe_tiny.gif population, when a single locus is considered (blue), with two alleles, wild-type (+) and mutant (-) we can see a differential geographical distribution between Population I and Population II, but there is a 30% chance of wrongly assigning any individual to either population based on a single allele.
× + -
Population I 70% 30%
Population II 30% 70%

For three loci blue, red and green, it becomes apparent that there is a correlation between certain allele frequencies. In this example Population I displays a correlation between wild-type blue (+) 70%, mutant red (-) 70% and wild type green (+) 70%. Population II has a correlation between the -, + and - alleles, each having a 70% frequency in this population. The genetic variation remains the same in these populations, irresepctive of the allele examined, but using a three locus approach, there is a much reduced chance of wrongly assigning any individual to a given population.

× + - + - + -
Population I 70% 30% 30% 70% 70% 30%
Population II 30% 70% 70% 30% 30% 70%

For an organism of genotype +/-/+, for each locus the chance of missclassification is 0.3 (30%), but when all three loci are take into account, the organism can be assigned to Population I with a 0.3x0.3x0.3 chance of error, that is a 0.027 (2.7%) chance of error. The two populations still share exactly the same alleles, but the frequency of these alleles varies between the populations.

Using modern computer software and the abundance of genetic data now available, it is possible not only to distinguish such correlations for hundreds or even thousands of alleles, which form clusters, it is also possible to assign individuals to given populations with very little chance of error. It should be noted, however, that genes tend to vary clinallyImage:Wp_globe_tiny.gif, and there are likely to be intermediate populations that reside in the geographical areas between our sample populations (Population III, for example, may lie equidistantly from Population I and Population II). In this case it may well be that Population III may display characteristics of both population I and Population II. For example Population III may be defined thus:

× + - + - + -
Population III 50% 50% 50% 50% 50% 50%

In which case any individual from Population III is likely to be misclassified equally into either Population I or Population II.(Edwards (2003)Kittles and Weiss (2003))

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.[40] 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 A. 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 individuals continent of origin is not a new concept. Using the ABO, RH and MNS blood groups, scientist in the 1950's could already determine continent of origin based on known frequencies of these trait.

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 comprised solely of the Kalash of Pakistan. Several groups in the study also appeared in two races such as Ethiopians, Hazaraof Pakistan 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. Somalian or Yemenites may cluster both with Africa and Europe.

Others say 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.[41]

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 their genotype (genes), this is not an indication of phenotype. This is because the SNPs used in the clustering study are selectively neutral ie 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 be 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

Though a blueprint for the entire human genome was made available by the Human genome project, much of how the human genome works is still a mystery. Scientists are still grappling with conundrum of how as few as 20,000 genes are responsible for all the complexities of the human body.[42] Thus many scientists argue that until much more is learned about the human genome it will be premature to any assumptions about racial differences.

Heritability

Heritability is the degree to which a characteristic is determined by genetics. Mendelian traits are those that are controlled by a single gene. Examples include dimples, sickle cell disease and cystic fibrosis. These traits follow the basic rules of Mendelian inheritance. The heritability of Mendelian traits is very high. For these traits it is possible for scientists to identify and locate the exact gene responsible for trait and make accurate predictions about outcomes.

However many traits are polygenic in that they depend on many genes. In a population these traits will show a continuous distribution on a bell curve. Examples include height. If it were controlled by only one allele people would either be tall or short, instead we see a wide range of heights. Skin color is also polygenic.

Polygenic traits can be multifactorial meaning that they depend on a complex interplay with other genes and the environment. Examples include Cancer, the outcome of which is determined by the interplay between cancer causing genes, cancer suppressing genes and environmental factors such as pollution or smoking. Complex traits like behavior and intelligence are very likely multifactorial. The heritability of multifactorial traits is generally much lower than those of single gene traits. Some scientists prefer not to see these traits as genetic but instead refer to inheriting a predisposition to developing the trait.[43]

Genome and intelligence

It has been argued that in order to make a hypothesis for race and intelligence work the genes for intelligence need to be identified and the frequencies in the various races computed. However, recent studies attempting to find loci in the genome relating to intelligence have had little success. Using several hundreds of people a study of 1842 DNA markers from a high IQ group with an IQ of 160 and a control group with an IQ of 102. The study used a five step inspection process to eliminate false positives. By the fifth step the study could not find a single gene that was related to intelligence.[44] The failure to find a specific gene associated with intelligence indicates that cognitive abilities are very complex and are likely to involve several genes. Some estimate that as much as 40% of all genes may contribute to intelligence.[45] The more genes that contribute to a trait the less likely that a trait can be race specific since most genetic variation is found within a race. The more genes that contribute to a trait the more the trait will be continuous instead of discrete, with smaller differences.

In the US, critics of these studies say that as long as social and environmental disparities between the races exist it will be impossible to scientifically test whether there are any genetic differences in IQ between the various populations. They propose that if the historical effects of poverty and social bigotry were eliminated and differences in IQ between the races still persisted then there might be some utility in such research.

Genetic heterogeneity

Genetic heterogeneity is used to describe the presence of different genes that produce same trait. For example the gene that causes light skin 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 indicating that they evolved light skin independently.

In 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.

This concept indicates that in some cases having a different genotype does not necessarily mean having a different phenotype.

Epigenetic inheritance

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.

Modern civilization and genetics

The global rise of modern civilization and technology can largely be traced to recent advances that took place in Western Europe. Author Jared Diamond tackles the question of why Europeans colonized much of the world instead of the other way around in his book Guns. According to Diamond, in the centuries after 1500, when European explorers came into contact with peoples around the world, they became aware of wide differences in the use of technology, political organization, phenotypical expression ("manifest" biological expression or physical traits), among other facets of human expression / culture. They logically assumed, for their time, that those non-biological (non-racial) differences arose from differences in innate biological ability that was reflected by phenotypical expressions ("manifest" biological expressions or physical traits). Darwinian evolution viewed the "primitive societies" to be vestiges of human descent from apelike ancestors. Finally in the 20th century, with the rise of genetics, Europeans came to be viewed as genetically superior than Africans and Australian aboriginals.

In Western societies, and among most, if not all, other soieties, racism is publicly denounced but privately or subconsciously many still hold the view that the rise of Western (European) civilization was at least in part due to genetic advantages. Diamond controversially holds the view that the rise of Western (European) civilization is basically entirely linked to geography and environment. He argues that the presence of wild ancestors of wheat and barley, two of the most nutritious cereals, in Eurasia and the presence of 12 out of 14 of the worlds domesticable large mammals gave Eurasia a head start over the rest of the world. He argues that Eurasia is the largest continent and its East-west extent meant similar climatic conditions. This facilitated the easy exchange of crops, knowledge and technology. Although of course there are thousands of other exalted authors, researchers, scholars, experts, etc. that have done research, studied, documented, recorded, etc. on the rise and fall of civilizations, with many concentrating and specilalizing on Western civilization, and have similar or greatly differing highly-praised views on the subject matter of environmental determinism than Diamond.

Recently the New York Times reported the discovery of two genes, microcephalin and ASPM that are associated with brain size, people who lack functional copies of these genes are born microcephalic. A new version (allele) of the microcephalin gene is though to have arisen about 37,000 years ago, this new version of the gene is found in about 70% of Europeans and Asians but is rarer in Africans. A new ASPM allele is thought to have arisen about 5,800 years ago, this new allele occurs in about 50% of Middle Eastern and European people, but is rare in East Asia and Africa. The rapid spread of these new alleles may indicate positive selection. The new microcephalin allele coincides with Upper Paleolithic transitions in Europe and the ASPM allele is about concurrent with the start of agriculture, but the researchers claim there is no clear connection. It should be noted that the New York Times article also states
Even if the new alleles should be shown to improve brain function, that would not necessarily mean that the populations where they are common have any brain-related advantage over those where they are rare. Different populations often take advantage of different alleles, which occur at random, to respond to the same evolutionary pressure, as has happened in the emergence of genetic defenses against malaria, which are somewhat different in Mediterranean and African populations.[46]

Following the release of the study websites promoting racialism quickly seized on the evolutionary findings. One magazine called the discovery "the moment the antiracists and egalitarians have dreaded". In an article in the National Review Online, John Derbyshire wrote that the research implied that "our cherished national dream of a well-mixed and harmonious meritocracy may be unattainable."

Consequently the study by Bruce Lahn began to attract considerable controversy. Many scientists criticized Lahn stating that he overinterpreted and sensationalized his findings. One of the co-authors, distanced herself from the study saying that she was bothered how the paper drew a link between the genetic changes and the rise of civilization. She felt that it was too early to reach any conclusions about why the changes spread and said it is "very simplistic" to imagine that a single gene could have a major effect on complex cultural traits. Richard Lewontin stated that the two papers were egregious examples of going well beyond the data to try to make a splash. Lahn would later concede that there was no real evidence natural selection had acted on cognition or intelligence through these genes.[47][48] Subsequent studies by other scientist found no relationship between these genes and intelligence or brain size.[49][50]

See also

Footnotes

  1. ^ [1]
  2. ^ [Genes, Culture, and Human Evolution: A Synthesis page 156 ISBN 1405150890 ]
  3. ^ [2]
  4. ^ [3]
  5. ^ http://www.detnews.com/2005/specialreport/0504/10/A01B-145339.htm
  6. ^ [4]
  7. ^ [5]
  8. ^ Race, the power of an illusion
  9. ^ LETTER FROM ASIA; Racial 'Handicaps' and a Great Sprint Forward
  10. ^ The Seven Daughters of Eve By Sykes, Bryan Chapter 3 ISBN 0393020185
  11. ^ [6]
  12. ^ [7]
  13. ^ [8]
  14. ^ understanding human genetic variation
  15. ^ Human DNA, the Ultimate Spot for Secret Messages
  16. ^ Lewontin, R.C.. Confusions About Human Races.
  17. ^ a b [Genes, Peoples, and Languages By L. L. (Luigi Luca) Cavalli-Sforza ISBN 0520228731 ]
  18. ^ Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa
  19. ^ [9]
  20. ^ Genes, Peoples, and Languages by Luigi Luca Cavalli-Sforza, 1997
  21. ^ why humans and their fur parted
  22. ^ [10]
  23. ^ page 326
  24. ^ [11]
  25. ^ [12]
  26. ^ growing young Ashely Montagu
  27. ^ ISBN 0939479230 legal History of the Color Line: The Notion of Invisible Blackness, By Frank W. Sweet
  28. ^ afro European admixture
  29. ^ AFRICAN ANCESTRY OF THE WHITE AMERICAN POPULATION
  30. ^ cia factbook
  31. ^ African ancestry of the population of Buenos Aires
  32. ^ BLACKS IN ARGENTINA: DISAPPEARING ACTS
  33. ^ Skidmore, Thomas E. (April 1992). "Fact and Myth: Discovering a Racial Problem in Brazil". Working Paper 173. 
  34. ^ Race in Argentina
  35. ^ Argentina's forgotten people
  36. ^ [http://www.erin.utoronto.ca/~eparra/profile/PDF%20files/Martinez-Marignac%202004.pdf Characterization of Admixture in an Urban Sample from Buenos Aires, Argentina, Using Uniparentally and Biparentally Inherited Genetic Markers]
  37. ^ [13]
  38. ^ ancestry of Brazilian mtDNA lineages
  39. ^ The Evolution and Genetics of Latin American Populations By Maria Cátira Bortolini, Francisco M. Salzano ISBN 0521652758
  40. ^ "Genetic Similarities Within and Between Human Populations" (2007) by D.J. Witherspoon, S. Wooding, A.R. Rogers, E.E. Marchani, W.S. Watkins, M.A. Batzer and L.B. Jorde. Genetics. 176(1): 351–359.
  41. ^ [http://paa2006.princeton.edu/download.aspx?submissionId=61713 Back with a Vengeance: the Reemergence of a Biological Conceptualization of Race in Research on Race/Ethnic Disparities in Health Reanne Frank]
  42. ^ [14]
  43. ^ Understanding human genetic variation
  44. ^ A Genome-Wide Scan of 1842 DNA Markers for Allelic Associations With General Cognitive Ability: A Five-Stage Design Using DNA Pooling and Extreme Selected Groups
  45. ^ The race myth p178 ISBN 0452286581
  46. ^ Brain May Still Be Evolving, Studies Hint
  47. ^ scientists study of brain gene sparks a backlash
  48. ^ Brain Man Makes Waves With Claims of Recent Human Evolution
  49. ^ The ongoing adaptive evolution of microcephalin and ASPM is not explained by increased intelligence
  50. ^ Normal variants of Microcephalin and ASPM do not account for brain size variability

References

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  • Miththapala, S., Seidensticker, J., O’Brien, S.J. (1996). "Phylogeographic Subspecies Recognition in Leopards (Panthera pardus)": Molecular Genetic Variation. Conservation Biology 10:1115-1132.
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Eric Boerwinkle, Nicholas J. Schork, and Neil J. Risch. (2005) Genetic Structure, Self-Identified Race/Ethnicity, and Confounding in Case-Control Association Studies. Am. J. Hum. Genet. 76:268–275. PDF

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