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.

Additional recommended knowledge

How to quickly check pipettes?

Essential Laboratory Skills Guide

Daily Sensitivity Test

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. 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, mostly in the media, 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 the cause of the disparity between levels of technological achievement etween the diverse cultures that exist on this planet. 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.^{[citation needed]}

Race and Behaviour

Carolus Linnaeus was a pioneer in defining the concept of race in humans. He considered that each race had certain characteristics that described the 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 descriptions of different races were clearly skewed in favour of Europeans.^{[citation needed]} These notions survive today in stereotypes about the behaviour of persons from different races. This is an example of scientific racism.

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] The dominance of African Americans in some American sports has been the subject of longstanding controversy. While blacks make up the majority of players in both the NBA and the NFL, with 76% and 69% respectively in 2005, 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 implies that athletes of African descent are intellectually and morally inferior, and that it is 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 belief has been used in association with the overly simplifed idea that a single gene can be mapped to a single phenotype, so as to imply a single gene for intelligence, or a gene for homosexuality, or a gene for aggressive behavior, but genetic determinism also refers to complex genetic theories .

On the other hand social determinism is the hypothesis that social interactions and social constructs are responsible for influencing individual behavior, with minimal influence from inheritance. 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. Viewpoints range from one extreme to the other, but it is also common to hold the view that both nature and nurture have important roles.

Early history

Blood groups

  Prior to the discovery of DNA as the hereditary material, scientists 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 still suffered 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 could be explained in terms of the different blood groups of the ABO system.   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 regions.

When they compared the results, they 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. 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.^[10]

Blood proteins and molecular evolution

Further information: 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 for hemoglobin for 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 animal species were, the more similar their amino acid sequences were. For example the human and gorilla sequences differed in two places, while those for the human and the horse differed in 15 places. This suggested the novel idea that, one could calculate the length of time since two different species last shared a common ancestor, by counting the number of differences in an amino acid sequence, while also considering the rate at which such differences came into genetic records. That is, differences in the amino acid sequences of proteins could serve as a molecular clock indicating when evolutionary separations between species had taken place. A phylogenetic tree could then be built that would portray the evolutionary relatedness of various species. ^[10] 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 genetic 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

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 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 the 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.^[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 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.^[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

  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

  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

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 genetic loci involved in skin color determination it is possible for someone to have 15-20% African admixture and not possess any of 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 selective 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.

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

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

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.

"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)

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

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)

 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, Hazaraof 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.^[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 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

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 possibly 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, Germs, and Steel. 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 societies, racialism 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.

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 racism 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 have failed to find any relationship between these genes and intelligence or brain size.^[49][50]

See also

• Human mitochondrial DNA haplogroups
• Genetic history of Europe

References

• Bauchet, M., Brian McEvoy, Laurel N. Pearson, Ellen E. Quillen, Tamara Sarkisian, Kristine Hovhannesyan, Ranjan Deka, Daniel G. Bradley, Mark D. Shriver. (2007) Measuring European Population Stratification using Microarray Genotype Data. Am J Hum Genet. 80(5):948-56. Abstract
• Edwards AW (2003). "Human genetic diversity: Lewontin's fallacy". Bioessays 25 (8): 798–801. doi:10.1002/bies.10315. PMID 12879450.
• Gitschier J (2005) The Whole Side of It—An Interview with Neil Risch. PLoS Genet 1(1): e14 doi:10.1371/journal.pgen.0010014
• Helms, J. E., Jernigan, M. and Mascher, J. (Jan 2005). "The Meaning of Race in Psychology and How to Change It: A Methodological Perspective". American Psychologist 60 (1): 27-36. doi:10.1037/0003-066x.60.1.27.
• Jackson, F. L. C. (2004). Book chapter: Human genetic variation and health: new assessment approaches based on ethnogenetic layering British Medical Bulletin 2004; 69: 215–235 DOI: 10.1093/bmb/ldh012. Retrieved 29 December 2006.
• Keita, S. O. Y. (1993) The subspecies concept in zoology and anthropology, a brief historical review and test of a classification scheme. Journal of Black Studies 23:416-445.
• Keita, S. O. Y., Kittles, R. A., Royal, C. D. M., Bonney, G. E., Furbert-Harris, P., Dunston, D. M., and Rotimi, C. M. (2004). Conceptualizing human variation: Nature Genetics 36, S17 - S20 (2004) doi:10.1038/ng1455
• Kittles and Weiss RACE, ANCESTRY, AND GENES: Implications for Defining Disease Risk Annu. Rev. Genomics Hum. Genet. 2003. 4:33–67 doi:10.1146/annurev.genom.4.070802.110356
• Lewonin, R. C. (2005). Confusions About Human Races from Race and Genomics, Social Sciences Research Council. Retrieved 28 December 2006.
• Leroy, A. M. (2005). A Family Tree in Every Gene. Published: March 14, 2005, The New York Times, p. A23. [15]. Retrieved 8 January 2006.
• Long and Kittles (2003). Human genetic variation and the nonexistence of human races: Human Biology, V. 75, no. 4, pp. 449-471. PDF. Retrieved 10 January 2007.
• Miththapala, S., Seidensticker, J., O’Brien, S.J. (1996). "Phylogeographic Subspecies Recognition in Leopards (Panthera pardus)": Molecular Genetic Variation. Conservation Biology 10:1115-1132.
• Ossario and Duster (2005). Race and Genetics: Controversies in Biomedical, Behavioral, and Forensic Sciences. American Psychologist 60(1):115–128. doi:10.1037/0003-066X.60.1.115
• Parra, Kittles and Shriver. (2004) Implicatins of correlations between skin color and genetic ancestry for biomedical research'Nature Genetics Supplement 36: 11 S54-S60 doi:10.1038/ng1440
• Pigliucci, Kaplan On the Concept of Biological Race and Its Applicability to Humans [16]
• Risch, N., Burchard, E., Ziv, E. and Tang, H. (2002). "Categorization of humans in biomedical research: genes, race and disease". Genome Biology 3 (7): comment2007.2001 - comment2007.2012.
• Rohde, Olson and Chang (2004) Modelling the recent common ancestry of all living humans Nature 431: 562-566 doi:10.1038/nature02842. Retrieved 5 March 2007.
• Rosenberg NA, Pritchard JK, Weber JL, Cann HM, Kidd KK, (2002) Genetic structure of human populations. Science 298: 2381–2385.Abstract
• Rosenberg1, N. A., Mahajan, s., Ramachandran, S., Zhao, C., Pritchard, J. K., Feldman, M. W. (2005) Clines, Clusters, and the Effect of Study Design on the Inference of Human Population Structure. PLoS Genet 1(6): e70: doi:10.1371/journal.pgen.0010070
• Seldin, Michael F., Russell Shigeta, Pablo Villoslada, Carlo Selmi, Jaakko Tuomilehto, Gabriel Silva, John W. Belmont, Lars Klareskog, Peter K. Gregersen (2006): European Population Substructure: Clustering of Northern and Southern Populations. PLoS Genetics 2 (9): e143
• Serre, D and Pääbö, S. (2004) Evidence for Gradients of Human Genetic Diversity Within and Among Continents Genome Research 14:1679-1685, 2004 doi:10.1101/gr.2529604. Retrieved 8 January 2006.
• Smedley, A. and Smedley, B. D. (Jan 2005). "Race as Biology Is Fiction, Racism as a Social Problem Is Real: Anthropological and Historical Perspectives on the Social Construction of Race". American Psychologist 60 (1): 16-26. doi:10.1037/0003-066x.60.1.16.
• Tang, Hua., Tom Quertermous, Beatriz Rodriguez, Sharon L. R. Kardia, Xiaofeng Zhu, Andrew Brown, James S. Pankow, Michael A. Province, Steven C. Hunt,

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

• Witherspoon DJ, Wooding S, Rogers AR, Marchani EE, Watkins WS, Batzer MA, Jorde LB. (2007) Genetic similarities within and between human populations. Genetics. 176(1):351-9. Full Text