Encyclopedia
- For other uses, including athletics events and other speed competitions, see Race .
The term
race distinguishes one
population of an animal species from another of the same species. Many regard race as a social construct. The most widely used human racial categories are based on visible traits , genes, and self-identification. Conceptions of race, as well as specific racial groupings, vary by culture and over time and are often controversial, for scientific reasons as well as their impact on social identity and
identity politics.
Since the 1940s,
evolutionary scientists have rejected the view of race according to which any number of finite lists of essential characteristics could be used to determine a like number of races. For example, the convention of categorizing the human population based on
human skin colors was used, but hair colors, eye colors, nose sizes, lip sizes, and heights were not. Many evolutionary and social scientists think common race definitions, or any race definitions pertaining to humans, lack taxonomic rigour and validity. They argue that race definitions are imprecise, arbitrary, derived from custom, have many exceptions, have many gradations, and that the amounts of races observed vary according to the culture examined. They further maintain that "race" as such is best understood as a social construct, and conceptualize and analyze human genotypic and phenotypic variation in terms of populations and clines instead. Some scientists, however, have argued that this position is motivated more by political than scientific reasons. Some others also argue that categories of self-identified race/ethnicity or biogeographic ancestry are both valid and useful, that these categories correspond with clusters inferred from
multilocus genetic data, and that this correspondence implies that genetic factors might contribute to unexplained phenotypic variation between groups.
Current disagreement across disciplines
One result of debates over the meaning and validity of the concept "race" is that the current literature across different disciplines regarding human variation lacks consensus, though some fields, such as biology, have strong consensus. Some studies use the word race in its previously essentialist taxonomic sense. Many still use the term race, but use it to mean a population, clade, or
haplogroup. Others eschew the word race altogether, and use the word population as a less pejorative synonym.
In the 19th century, race was a central concept of
anthropology. In 1866, James Hunt, the founder of the Anthropological Society of London, declared that anthropology’s primary truth “is the existence of well-marked psychological and moral distinctions in the different races of men.” However, this view became marginalised in the 20th century. Since 1932,
college textbooks introducing physical anthropology have increasingly come to reject race as a valid concept: from 1932 to 1976, only seven out of thirty-two rejected race; from 1975 to 1984, thirteen out of thirty-three rejected race; from 1985 to 1993, thirteen out of nineteen rejected race. The American Anthropological Association, drawing on biological research, currently holds that "The concept of race is a social and cultural construction. . . . Race simply cannot be tested or proven scientifically," and that, "It is clear that human populations are not unambiguous, clearly demarcated, biologically distinct groups. The concept of 'race' has no validity . . . in the human species" Nevertheless, many scientists, including many anthropologists, reject this position.
In an ongoing debate, some geneticists argue race is neither a meaningful concept nor a useful heuristic device, and even that genetic differences between groups are biologically meaningless, on the basis of that more genetic variation exists within such races than between them, and that racial traits overlap without discrete boundaries.
Other geneticists, in contrast, argue that categories of self-identified race/ethnicity or biogeographic ancestry are both valid and useful. that these categories correspond with clusters
inferred from multilocus genetic data, and that this correspondence implies that genetic factors might contribute to unexplained phenotypic variation between groups.
Races of physical anthropology
see Races of physical anthropologyScientific support for the Caucasoid, Negroid, Mongoloid terminology of racial classification has fallen steadily over the past century. Where 78 percent of the articles in the 1931 Journal of Physical Anthropology employed these or similar synonymous terms reflecting a bio-race paradigm, only 36 percent did so in 1965, and just 28 percent did in 1996. In February, 2001, the editors of the medical journal
Archives of Pediatrics and Adolescent Medicine asked authors to no longer use "race" as explanatory variable nor to use obsolescent terms. Other prestigious peer-reviewed journals, such as the
New England Journal of Medicine and the
American Journal of Public Health, have done the same. Furthermore, the National Institutes of Health recently issued a program announcement for grant applications through February 1, 2006, specifically seeking researchers who can investigate and publicize among primary care physicians the detrimental effects on the nation's health of the practice of medical racial profiling using such terms. The program announcement quoted the editors of one journal as saying that, "analysis by race and ethnicity has become an analytical knee-jerk reflex."
The most recent survey, taken in 1985 , asked 1,200 scientists how many
disagree with the following proposition: "There are biological races in the species
Homo sapiens." The responses were:
The figure for physical anthropologists at PhD granting departments was slightly higher, rising from 41% to 42%, with 50% agreeing. This survey, however, did not specify any particular definition of race; it is impossible to say whether those who supported the statement thought of race in taxonomic or population terms.
Origins
History of the term
Given visual complex social relationships, humans presumably have always observed and speculated about the physical differences among individuals and groups. But different societies have attributed markedly different meanings to these distinctions. The division of humanity into distinct "races" can be traced as far back as the
Ancient Egyptian sacred text the
Book of Gates, which identifies four categories that are now conventionally labelled "Egyptians", "Asiatics", "Libyans", and "Nubians". However, such distinctions tended to merge differences defined by features such as skin color, with tribal and national identity. Classical civilizations from Rome to China tended to invest much more importance in family or tribal affiliations than in physical appearance . Ancient Greek and
Roman authors also attempted to explain and categorize visible biological differences between peoples known to them. Such categories often also included fantastical human-like beings that were supposed to exist in far-away lands. Some Roman writers adhered to an environmental determinism in which climate could affect the appearance and character of groups . But in many ancient civilizations, individuals with widely varying physical appearances could become full members of a society by growing up within that society or by adopting the society's
cultural norms .
Medieval models of race mixed Classical ideas with the notion that humanity as a whole was descended from Shem,
Ham and Japheth, the three
sons of Noah, producing distinct
Semitic , Hamitic , and
Japhetic peoples.
At the end of the
Reconquista was the process by which the Christian Kingdoms of northern Hispania [i] defeated ...
, the
Spanish Inquisition persecuted
Jews and
Muslims, theorizing a
limpieza de sangre doctrine. Furthermore, after the discovery of the
New World,
Bartolomé de Las Casas opposed the
conquistadores theories, upheld by Sepúlveda, on the pretended
Amerindians's absence of souls.
It wasn't until the 16th century that the word
race entered the
English language, from the
French race - "race, breed, lineage" . Meanings of the term in the 16th century included "
wines with a characteristic flavour", "people with common occupation", and "generation". The meaning "tribe" or "nation" emerged in the 17th century. The modern meaning, "one of the major divisions of mankind", dates to the late 18th century, but it never became exclusive . The ultimate origin of the word is unknown; suggestions include
Arabic ra'is meaning "head", but also "beginning" or "origin".
In
Society Must be Defended ,
Michel Foucault traced the "historical and political discourse" of "race struggle" to the 1688 "
Glorious Revolution" in England and
Louis XIV's reign in France, during which conflicting political values were ascribed to ancestral ethnicities . According to him, these debates initated a form of "popular history" based on ethnic identity, as opposed to the classical juridical and philosophical discourse of sovereignty. In England, it was used by
Edward Coke and
John Lilburne to demand "inalienable rights" and oppose the
monarchy. In France, Boulainvilliers, Nicolas Fréret, and then Sieyès, Augustin Thierry and
Cournot reappropriated this discourse.
During the 19th century, the discourse developed in two different directions. On the one hand, according to Foucault,
Marxists seized this historical and political discourse, replacing the essentialist notion of "race" with the historical and social concept of "class struggle." On the other hand, also according to Foucault, at the end of the 19th century, the notion of "race" was adopted by
racist biologists and
eugenicists, who gave it the modern sense of "biological race", which was then integrated to "state racism". This displacement of discourse constitutes one of the basis of Foucault's thought: discourse is not tied to the subject, rather the "subject" is a construction of discourse.
The English word "race", along with many of the ideas now associated with the term, were products of the European era of exploration . As Europeans encountered people from different parts of the world, they speculated about the physical, social, and cultural differences between human groups. The rise of the
African slave trade, which gradually displaced an earlier trade in slaves from throughout the world, created a further incentive to categorize human groups to justify the barbarous treatment of African slaves . Drawing on classical sources and on their own internal interactions — for example, the hostility between the English and Irish was a powerful influence on early thinking about the differences between people — Europeans began to sort themselves and others into groups associated with physical appearance and with deeply ingrained behaviors and capacities. A set of folk beliefs took hold that linked inherited physical differences between groups to inherited intellectual, behavioral, and moral qualities . Although similar ideas can be found in other cultures , they appear not to have had as much influence on social structures as they did in Europe and the parts of the world colonized by Europeans. However, often brutal conflicts between ethnic groups have existed throughout history and across the world, and racial prejudice against Africans also exists in non-colonised countries such as Japan and China.
History of race research
See From "racial theory" to "racism"The first scientific attempts to categorize race date from the 17th century, along with the development of European imperialism and colonization around the world. The first post-Classical published classification of humans into distinct races seems to be François Bernier's
Nouvelle division de la terre par les différents espèces ou races qui l'habitent , published in 1684.
17th and 18th century
In the 18th century, the differences between human groups became a focus of scientific investigation . Initially, scholars focused on cataloging and describing "The Natural Varieties of Mankind," as
Johann Friedrich Blumenbach entitled his 1775 text . From the 17th through the 19th centuries, the merging of folk beliefs about group differences with scientific explanations of those differences produced what one scholar has called an "ideology of race" . According to this ideology, races are primordial, natural, enduring, and distinct. Some groups might be the result of mixture between formerly distinct populations, but careful study can distinguish the ancestral races that had combined to produce admixed groups.
19th century
The 19th century saw attempts to change race from a taxonomic to a biological concept. In the 19th century a number of
natural scientists wrote on race:
Georges Cuvier,
Charles Darwin,
Alfred Wallace,
Francis Galton,
James Cowles Pritchard,
Louis Agassiz, Charles Pickering, and
Johann Friedrich Blumenbach. As the science of
anthropology took shape in the 19th century, European and American scientists increasingly sought explanations for the behavioral and cultural differences they attributed to groups . For example, using
anthropometrics, invented by
Francis Galton and
Alphonse Bertillon, they measured the shapes and sizes of skulls and related the results to group differences in intelligence or other attributes .
These scientists made three claims about race: first, that races are objective, naturally occurring divisions of humanity; second, that there is a strong relationship between biological races and other human phenomena , thus biologizing the notion of "race", as Foucault demonstrated in his historical analysis; third, that race is therefore a valid scientific category that can be used to explain and predict individual and group behavior. Races were distinguished by
skin color,
facial type,
cranial profile and size, texture and color of
hair. Moreover, races were almost universally considered to reflect group differences in moral character and intelligence.
The
eugenics movement of the late 19th and early 20th centuries, inspired by
Arthur Gobineau's
An Essay on the Inequality of the Human Races , Vacher de Lapouge's "anthroposociology" asserted as self-evident the biological inferiority of particular groups . In many parts of the world, the idea of race became a way of rigidly dividing groups by culture as well as physical appearances . Campaigns of oppression and genocide were often motivated by supposed racial differences .
In Charles Darwin's most controversial book
The Descent of Man is a book on evolution [i]ary theory by British [i] ...
, he made strong suggestions of racial differences and European superiority. In Darwin's view, stronger tribes of humans always replaced weaker tribes. As savage tribes came in conflict with civilized nations, such as England, the less advanced people were destroyed. Darwin also pointed out the arbitrary use of any number of categories to divide up the human species, which is a major problem of racial theories.
The destruction of the weaker peoples seemed desirable to many scientists at the time. It was thought that "fit" people would replace the "unfit" and human evolution would be accelerated.
20th century
At the beginning of the 20th century,
anthropologists questioned, and subsequently abandoned, the claim that biologically distinct races are isomorphic with distinct linguistic, cultural, and social groups. Then, the rise of population genetics led some mainstream evolutionary scientists in
anthropology and
biology to question the very validity of race as scientific concept describing an objectively real phenomenon. Those who came to reject the validity of the concept, race, did so for four reasons: empirical, definitional, the availability of alternative concepts, and ethical .
The first to challenge the concept of race on empirical grounds were
anthropologists Franz Boas, who demonstrated phenotypic plasticity due to environmental factors , and
Ashley Montagu , who relied on evidence from genetics.
Zoologists Edward O. Wilson and W. Brown then challenged the concept from the perspective of general systematics, and further rejected the claim that "races" were equivalent to "subspecies" .
Claude Lévi-Strauss's
Race and History enforced this cultural relativist thesis, by the famous metaphor of cultures as trains crossing each other in different directions, thus each one seeing the others as immobile while they themselves are progressing
One of the crucial innovations in reconceptualizing genotypic and phenotypic variation was anthropologist
C. Loring Brace's observation that such variations, insofar as they are affected by
natural selection,
migration, or genetic drift, are distributed along geographic gradations called "clines" . This point called attention to a problem common to phenotypic-based descriptions of races : they ignore a host of other similarities and difference that do not correlate highly with the markers for race. Thus, anthropologist Frank Livingstone's conclusion that, since clines cross racial boundaries, "there are no races, only clines" . In 1964, biologists
Paul Ehrlich and Holm pointed out cases where two or more clines are distributed discordantly—for example,
melanin is distributed in a decreasing pattern from the equator north and south; frequencies for the haplotype for beta-S
hemoglobin, on the other hand, radiate out of specific geographical points in Africa . As anthropologists Leonard Lieberman and Fatimah Linda Jackson observe, "Discordant patterns of heterogeneity falsify any description of a population as if it were genotypically or even phenotypically homogeneous" .
Finally, geneticist
Richard Lewontin, observing that 85 percent of human variation occurs within populations, and not between populations, argued that neither "race" nor "subspecies" was an appropriate or useful way to describe populations . This view is described by its opponents as
Lewontin's Fallacy. Some researchers report the variation between racial groups accounts for as little as 5% of human genetic variation
2. However, because of technical limitations of F
ST, many geneticists now believe that low F
ST values do not invalidate the suggestion that there might be different human races . Meanwhile, neo-Marxists such as David Harvey believe that race is a social construct that in reality does not exist, used instead to extenuate class differences.
These empirical challenges to the concept of race forced evolutionary sciences to reconsider their definition of race. Mid-century, anthropologist William Boyd defined race as:
- A population which differs significantly from other populations in regard to the frequency of one or more of the genes it possesses. It is an arbitrary matter which, and how many, gene loci we choose to consider as a significant "constellation" .
Lieberman and Jackson have pointed out that "the weakness of this statement is that if one gene can distinguish races then the number of races is as numerous as the number of human couples reproducing." Moreover, anthropologist Stephen Molnar has suggested that the discordance of clines inevitably results in a multiplication of races that renders the concept itself useless .
Alongside empirical and conceptual problems with "race" following the
Second World War, evolutionary and social scientists were acutely aware of how beliefs about race had been used to justify discrimination,
apartheid,
slavery, and genocide. This questioning gained momentum in the 1960s during the American Civil Rights Movement and the emergence of numerous anti-colonial movements worldwide.
In the face of these issues, some evolutionary scientists have simply abandoned the concept of race in favor of "population." What distinguishes population from previous groupings of humans by race is that it refers to a breeding population and not to a biological taxon. Other evolutionary scientists have abandoned the concept of race in favor of cline . The concepts of population and cline are not, however, mutually exclusive and both are used by many evolutionary scientists.
In the face of this rejection of race by some evolutionary scientists, many social scientists have replaced the word race with the word "ethnicity" to refer to self-identifying groups based on beliefs in shared
religion, nationality, or race. Moreover, they understood these shared beliefs to mean that religion, nationality, and race itself are social constructs and have no objective basis in the supernatural or natural realm . See also the American Anthropological Association's Statement on Race .
Summary of different definitions of race
Biological definitions of race et al.| Concept | Reference | Definition |
| Essentialist | Hooton | "A great division of mankind, characterized as a group by the sharing of a certain combination of features, which have been derived from their common descent, and constitute a vague physical background, usually more or less obscured by individual variations, and realized best in a composite picture." |
| Taxonomic | Mayr | "An aggregate of phenotypically similar populations of a species, inhabiting a geographic subdivision of the range of a species, and differing taxonomically from other populations of the species." |
| Population | Dobzhansky | "Races are genetically distinct Mendelian populations. They are neither individuals nor particular genotypes, they consist of individuals who differ genetically among themselves." |
| Lineage | Templeton | "A subspecies is a distinct evolutionary lineage within a species. This definition requires that a subspecies be genetically differentiated due to barriers to genetic exchange that have persisted for long periods of time; that is, the subspecies must have historical continuity in addition to current genetic differentiation." |
| Clade | Levin | Race "connotes geographic ancestry, by continent or large continental subregion" and "is used to denote continental or subcontinental clades". In "Cladistic taxonomy ... the basic taxon [is] the genealogical unit, ancestors-plus-line--of-descent, what according to the present analysis races are." |
The
United States government has provided definitions regarding race . Racial classification in the U.S. 2000 census was based solely on self-identification, did not pre-suppose disjointedness, and did not include a category "Hispanic," which is considered an ethnicity, rather than a race, by the U.S. Census. On the other hand, the EEOC explicitly defines Hispanics as a separate and distinct "race."
Human genetic variation
Origins of modern humans
- see also single-origin hypothesis, multiregional hypothesis.
Any biological model for race must account for the development of racial differences during human evolution. For much of the 20th century, however, anthropologists relied on an incomplete
fossil record for reconstructing human evolution. Their models seldom provided a firm basis for drawing inferences about the origin of races. Modern research in
molecular biology, however, has provided evolutionary scientists with a whole new kind of data, which adds considerably to the knowledge of our past.
There has been considerable debate among anthropologists as to the origins of
Homo sapiens. About a million years ago
Homo erectus migrated out of Africa and into Europe and Asia. The debate hinges on whether
Homo erectus evolved into
Homo sapiens more or less simultaneously in Africa, Europe, and Asia, or whether
Homo sapiens evolved only in Africa, and eventually supplanted
Homo erectus in Europe and Asia. Each model suggests different possible scenarios for the evolution of distinct races.
Multiregional hypothesis
Advocates of the first scenario , the
multiregional continuity evolution model, cite as evidence
anatomical continuity in the fossil record in South Central Europe , East Asia and Australia . They argue that very strong genetic similarities among all humans do not prove recent common ancestry, but rather reflect the interconnectedness of human populations around the world, resulting in relatively constant gene flow . They further argue that this model is consistent with clinal patterns .
The most important element of this model for theories of race is that it allows a million years for the evolution of
Homo sapiens around the world; this is more than enough time for the evolution of different races. Leiberman and Jackson , however, have noted that this model depends on several findings relevant to race: that marked morphological contrasts exist between individuals found at the center and at the perimeter of Middle Pleistocene range of the genus
Homo; that many features can be shown to emerge at the edge of that range before they develop at the center; and that these features exhibit great tenacity through time. Regional variations in these features can thus be taken as evidence for long term differences among genus Homo individuals that prefigure different races among present-day Homo sapiens individuals.
Out of Africa
Information about the history of our species comes from two main sources: the paleoanthropological record and historical inferences based on current genetic differences observed in humans. Although both sources of information are fragmentary, they have been converging in recent years on the same general story.
Since the 1990s, it has become common to use multilocus genotypes to distinguish different human groups and to allocate individuals to groups . These data have led to an examination of the biological validity of races as evolutionary lineages and the description of races in
cladistic terms. The technique of multilocus genotyping has been used to determine patterns of human demographic history. Thus, the concept of "race" afforded by these techniques is synonymous with ancestry, broadly understood.
Studies of human genetic variation imply that
Africa was the ancestral source of all modern humans, and that
Homo sapiens migrated out of Africa and displaced
Homo erectus between 140,000 and 290,000 years ago .
Indigenous Australians are believed to be an early out-group that remained isolated. Most other groups, including
Europeans,
Asians, and
Native Americans, were found to be a single related group resulting from a later out-migration from Africa, which could reasonably be divided into West and East Eurasian groups.
The existing fossil evidence suggests that anatomically modern humans evolved in Africa, within the last ~200,000 years, from a pre-existing population of humans . Although it is not easy to define "anatomically modern" in a way that encompasses all living humans and excludes all archaic humans , the generally agreed-upon physical characteristics of anatomical modernity include a high rounded skull, facial retraction, and a light and gracile, as opposed to heavy and robust, skeleton . Early fossils with these characteristics have been found in eastern Africa and have been dated to ~160,000–200,000 years ago . At that time, the population of anatomically modern humans appears to have been small and localized . Much larger populations of archaic humans lived elsewhere in the Old World, including the Neandertals in Europe and an earlier species of humans, Homo erectus, in Asia .
Fossils of the earliest anatomically modern humans found outside Africa are from two sites in the Middle East and date to a period of relative global warmth, ~100,000 years ago, though this region was reinhabited by Neandertals in later millennia as the climate in the northern hemisphere again cooled . Groups of anatomically modern humans appear to have moved outside Africa permanently sometime >60,000 years ago. One of the earliest modern skeletons found outside Africa is Mungo Man, from Australia, and has been dated to ~42,000 years ago , although studies of environmental changes in Australia argue for the presence of modern humans in Australia >55,000 years ago . To date, the earliest anatomically modern skeleton discovered from Europe comes from the Carpathian Mountains of Romania and is dated to 34,000–36,000 years ago .
Existing data on human genetic variation support and extend conclusions based on the fossil evidence. African populations exhibit greater genetic diversity than do populations in the rest of the world, implying that humans appeared first in Africa and later colonized Eurasia and the Americas . The genetic variation seen outside Africa is generally a subset of the variation within Africa, a pattern that would be produced if the migrants from Africa were limited in number and carried just part of African genetic variability with them . Patterns of genetic variation suggest an earlier population expansion in Africa followed by a subsequent expansion in non-African populations, and the dates calculated for the expansions generally coincide with the archaeological record .
Aspects of the relationship between anatomically modern and archaic humans remain contentious. Studies of mtDNA , the Y chromosome , portions of the X chromosome , and many autosomal regions support the "Out of Africa" account of human history, in which anatomically modern humans appeared first in eastern Africa and then migrated throughout Africa and into the rest of the world, with little or no interbreeding between modern humans and the archaic populations they gradually replaced . However, several groups of researchers cite fossil and genetic evidence to argue for a more complex account. They contend that humans bearing modern traits emerged several times from Africa, over an extended period, and mixed with archaic humans in various parts of the world . As a result, they say, autosomal DNA from archaic human populations living outside Africa persists in modern populations, and modern populations in various parts of the world still bear some physical resemblance to the archaic populations that inhabited those regions .
However, distinguishing possible contributions to the gene pool of modern humans from archaic humans outside Africa is difficult, especially since many autosomal loci coalesce at times preceding the separation of archaic human populations . In addition, studies of mtDNA from archaic and modern humans and extant Y chromosomes suggest that any surviving genetic contributions of archaic humans outside Africa must be small, if they exist at all . The observation that most genes studied to date coalesce in African populations points toward the importance of Africa as the source of most modern genetic variation, perhaps with some subdivision in the ancestral African population . Sequence data for hundreds of loci from widely distributed worldwide populations eventually may clarify the population processes associated with the appearance of anatomically modern humans , as well as the amount of gene flow among modern humans since then.
Cladistics
A
phylogenetic tree like the one shown above is usually derived from
DNA or
protein sequences from populations. Often mitochondrial DNA or Y chromosome sequences are used to study ancient human demographics. These single-locus sources of DNA do not
recombine and are almost always inherited from a single parent, with only one known exception in mtDNA . Individuals from the various continental groups tend to be more similar to one another than to people from other continents. The tree is rooted in the common ancestor of
chimpanzees and humans, which is believed to have originated in
Africa. Horizontal distance corresponds to two things:
- Genetic distance. Given below the diagram, the genetic difference between humans and chimps is roughly 2%, or 20 times larger than the variation among modern humans.
- Temporal remoteness of the most recent common ancestor. Rough estimates are given above the diagram, in millions of years. The mitochondrial most recent common ancestor of modern humans lived roughly 200,000 years ago, latest common ancestors of humans and chimps between four and seven million years ago.
Chimpanzees and humans belong to different genera, indicated in red. Formation of species and subspecies is also indicated, and the formation of "races" is indicated in the green rectangle to the right . Note that vertical distances are not meaningful in this representation.
Distribution of variation
A thorough description of the differences in patterns of genetic variation between humans and other species awaits additional genetic studies of human populations and nonhuman species. But the data gathered to date suggest that human variation exhibits several distinctive characteristics. First, compared with many other mammalian species, humans are genetically less diverse—a counterintuitive finding, given our large population and worldwide distribution . For example, the chimpanzee subspecies living just in central and western Africa have higher levels of diversity than do humans .
Two random humans are expected to differ at approximately 1 in 1000
nucleotides, whereas two random chimpanzees differ at 1 in 500 nucleotide pairs. However, with a genome of approximate 3 billion nucleotides, on average two humans differ at approximately 3 million nucleotides. Most of these single nucleotide polymorphisms are neutral, but some are functional and influence the phenotypic differences between humans. It is estimated that about 10 million SNPs exist in human populations, where the rarer SNP allele has a frequency of at least 1% .
The distribution of variants within and among human populations also differs from that of many other species. The details of this distribution are impossible to describe succinctly because of the difficulty of defining a "population," the clinal nature of variation, and heterogeneity across the genome . In general, however, 5%–15% of genetic variation occurs between large groups living on different continents, with the remaining majority of the variation occurring within such groups . This distribution of genetic variation differs from the pattern seen in many other mammalian species, for which existing data suggest greater differentiation between groups .
Our history as a species also has left genetic signals in regional populations. For example, in addition to having higher levels of genetic diversity, populations in Africa tend to have lower amounts of linkage disequilibrium than do populations outside Africa, partly because of the larger size of human populations in Africa over the course of human history and partly because the number of modern humans who left Africa to colonize the rest of the world appears to have been relatively low . In contrast, populations that have undergone dramatic size reductions or rapid expansions in the past and populations formed by the mixture of previously separate ancestral groups can have unusually high levels of linkage disequilibrium .
In the field of population genetics, it is believed that the distribution of neutral polymorphisms among contemporary humans reflects human demographic history. It is believed that humans passed through a
population bottleneck before a rapid expansion coinciding with migrations
out of Africa leading to an African-Eurasian divergence around 100,000 years ago , followed by a European-Asian divergence about 40,000 years ago .
The rapid expansion of a previously small population has two important effects on the distribution of genetic variation. First, the so-called
founder effect occurs when founder populations bring only a subset of the genetic variation from their ancestral population. Second, as founders become more geographically separated, the probability that two individuals from different founder populations will mate becomes smaller. The effect of this assortative mating is to reduce gene flow between geographical groups, and to increase the genetic distance between groups. The expansion of humans from Africa affected the distribution of genetic variation in two other ways. First, smaller populations experience greater genetic drift because of increased fluctuations in neutral polymorphisms. Second, new polymorphisms that arose in one group were less likely to be transmitted to other groups as gene flow was restricted.
Many other geographic, climatic, and historical factors have contributed to the patterns of human genetic variation seen in the world today. For example, population processes associated with colonization, periods of geographic isolation, socially reinforced endogamy, and natural selection all have affected allele frequencies in certain populations . In general, however, the recency of our common ancestry and continual gene flow among human groups have limited genetic differentiation in our species.
Substructure in the human population
New data on human genetic variation has reignited the debate surrounding race. Most of the controversy surrounds the question of how to interpret these new data, and whether conclusions based on existing data are sound. A large majority of researchers endorse the view that continental groups do not constitute different subspecies. However, other researchers still debate whether evolutionary lineages should rightly be called "races". These questions are particularly pressing for
biomedicine, where self-described race is often used as an indicator of ancestry .
Modern biological evidence from the anthropological textbook
Human Species contradicts earlier theories of which groups were more genetically related to other groups. Humans are all related. Humanity divided itself into the African and the Eurasian/Oceanic branch. The Eurasian and Oceanic branches are the products of this common origin. The Eurasian branch split into the Amerindian and major East Asian branch. The major East Asian branch divided itself into eastern Russian and the East Asian. The Oceanic branch divided itself into the Southeast Asians and Pacific Islanders. According to the
Human Species ,
East Asians generally are more genetically similar to the
South Asians than to
Southeast Asians, because the Far East and the Indian Subcontinent are members of the Eurasian branch while Southeast Asians are members or the Oceanic branch. More interestingly, Asians have very local genetic clusters inside these regions, implying different Asian ethnic groups have not historically interbred with each other. Examples of localized genetic clusters include Japan, Korea, Mongolia and China which form separate genetic clusters from each other.
Although the genetic differences among human groups are relatively small, these differences in certain genes such as duffy,
ABCC11, SLC24A5, called ancestry-informative markers nevertheless can be used to reliably situate many individuals within broad, geographically based groupings or self-identified race. For example, computer analyses of hundreds of polymorphic loci sampled in globally distributed populations have revealed the existence of genetic clustering that roughly is associated with groups that historically have occupied large continental and subcontinental regions .
Some commentators have argued that these patterns of variation provide a biological justification for the use of traditional racial categories. They argue that the continental clusterings correspond roughly with the division of human beings into sub-Saharan Africans; Europeans, western Asians, and northern Africans; eastern Asians; Polynesians and other inhabitants of Oceania; and Native Americans . Other observers disagree, saying that the same data undercut traditional notions of racial groups . They point out, for example, that major populations considered races or subgroups within races do not necessarily form their own clusters. Thus, samples taken from India and Pakistan affiliate with Europeans or eastern Asians rather than separating into a distinct cluster.
Furthermore, because human genetic variation is clinal, many individuals affiliate with two or more continental groups. Thus, the genetically based "biogeographical ancestry" assigned to any given person generally will be broadly distributed and will be accompanied by sizable uncertainties .
In many parts of the world, groups have mixed in such a way that many individuals have relatively recent ancestors from widely separated regions. Although genetic analyses of large numbers of loci can produce estimates of the percentage of a person's ancestors coming from various continental populations , these estimates may assume a false distinctiveness of the parental populations, since human groups have exchanged mates from local to continental scales throughout history . Even with large numbers of markers, information for estimating admixture proportions of individuals or groups is limited, and estimates typically will have wide CIs .
For the paternal and maternal genetic lineages of the world, see: and [https://www5.nationalgeographic.com/genographic/atlas.html]
Physical variation in humans
The distribution of many physical traits resembles the distribution of genetic variation within and between human populations . For example, ~90% of the variation in human head shapes occurs within every human group, and ~10% separates groups, with a greater variability of head shape among individuals with recent African ancestors .
A prominent exception to the common distribution of physical characteristics within and among groups is skin color. Approximately 10% of the variance in skin color occurs within groups, and ~90% occurs between groups . This distribution of skin color and its geographic patterning—with people whose ancestors lived predominantly near the equator having darker skin than those with ancestors who lived predominantly in higher latitudes—indicate that this attribute has been under strong selective pressure. Darker skin appears to be strongly selected for in equatorial regions to prevent sunburn, skin cancer, the photolysis of folate, and damage to sweat glands . A leading hypothesis for the selection of lighter skin in higher latitudes is that it enables the body to form greater amounts of vitamin D, which helps prevent rickets . Evidence for this includes the finding that
a substantial portion of the differences of skin color between Europeans and Africans resides in a single gene, SLC24A5 the threonine-111 allele of which was found in 98.7 to 100% among several European samples, while the alanine-111 form was found in 93 to 100% of samples of Africans, East Asians and Indigenous Americans . However, the vitamin D hypothesis is not universally accepted , and lighter skin in high latitudes may correspond simply to an absence of selection for dark skin .
Melanin which serves as the pigment, is located in the epidermis of the skin, and is based on hereditary gene expression.
Because skin color has been under strong selective pressure, similar skin colors can result from convergent adaptation rather than from genetic relatedness. Sub-Saharan Africans, tribal populations from southern India, and Indigenous Australians have similar skin pigmentation, but genetically they are no more similar than are other widely separated groups. Furthermore, in some parts of the world in which people from different regions have mixed extensively, the connection between skin color and ancestry has been substantially weakened . So, taking them in isolation would appear to get you nowhere. In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups .
Considerable speculation has surrounded the possible adaptive value of other physical features characteristic of groups, such as the constellation of facial features observed in many eastern and northeastern Asians . However, any given physical characteristic generally is found in multiple groups , and demonstrating that environmental selective pressures shaped specific physical features will be difficult, since such features may have resulted from sexual selection for individuals with certain appearances or from genetic drift .
Ancestry
An alternative to the use of racial or ethnic categories is to categorize individuals in terms of ancestry. Ancestry may be defined geographically , geopolitically , or culturally . The definition of ancestry may recognize a single predominant source or multiple sources. Ancestry can be ascribed to an individual by an observer, as was the case with the U.S. census prior to 1960; it can be identified by an individual from a list of possibilities or with use of terms drawn from that person's experience; or it can be calculated from genetic data by use of loci with allele frequencies that differ geographically, as described above. At least among those individuals who participate in biomedical research, genetic estimates of biogeographical ancestry generally agree with self-assessed ancestry , but in an unknown percentage of cases, they do not .
Genetic data can be used to infer population structure and assign individuals to groups that often correspond with their self-identified geographical ancestry. The inference of population structure from multilocus genotyping depends on the selection of a large number of informative genetic markers. These studies usually find that groups of humans living on the same continent are more similar to one another than to groups living on different continents. Many such studies are criticized for assigning group identity
a priori. However, even if group identity is stripped and group identity assigned
a posteriori using only genetic data, population structure can still be inferred. For example, using 993 markers, Rosenberg et al. were able to assign 1,048 individuals from 52 populations around the globe to one of six genetic clusters, which correspond to major geographic regions.
However, in analyses that assign individuals to group it becomes less apparent that self-described racial groups are reliable indicators of ancestry. One cause of the reduced power of the assignment of individuals to groups is . Some racial or ethnic groups, especially
Hispanic groups, do not have homogenous ancestry. For example, self-described
African Americans tend to have a mix of West African and European ancestry. Shriver et al. found that on average African Americans have ~80% African ancestry. Likewise, many white Americans have mixed European and African ancestry, where ~30% of whites have less than 90% European ancestry. In this context, it is becoming more commonplace to describe "race" as fractional ancestry. Without the use of genotyping, this has been approximated by the self-described ancestry of an individual's grand-parents.
Nevertheless, recent research indicates that self-described race is a near-perfect indicator of an individual's genetic profile, at least in the United States. Using 326 genetic markers, Tang et al. identified 4 genetic clusters among 3,636 individuals sampled from 15 locations in the United States, and were able to correctly assign individuals to groups that correspond with their self-described race for all but 5 individuals . They conclude that ancient ancestry, which correlates tightly with self-described race and not current residence, is the major determinant of genetic structure in the U.S. population.
Genetic techniques that distinguish ancestry between continents can also be used to describe ancestry within continents. However, the study of intra-continental ancestry may require a greater number of informative markers. Populations from neighboring geographic regions typically share more recent common ancestors. As a result, allele frequencies will be correlated between these groups. This phenomenon is often seen as a cline of allele frequencies. The existence of allelic clines has been offered as evidence that individuals cannot be allocated into
genetic clusters . However, others argue that low levels of differentiation between groups merely make the assignment to groups more difficult, not impossible .
Jensen's interpretations of race
According to
Arthur Jensen the traditional races of physical anthropology have been more or less confirmed by the research of Cavalli-Sforza.
On pgs 430-431 of
the g factor Jensen writes:
- Cavalli-Sforza et al. transformed the distance matrix to a correlation matrix consisting of 861 correlation coefficients among the forty-two populations, so they could apply principal components analysis on their genetic data...PC analysis is a wholly objective mathematical procedure. It requires no decisions or judgments on anyone's part and yields identical results for everyone who does the calculations correctly...The important point is that if various populations were fairly homogenous in genetic composition, differing no more genetically than could be attributable only to random variation, a PC analysis would not be able to cluster the populations into a number of groups according to their genetic propinquity. In fact, a PC analysis shows that most of the forty-two populations fall very distinctly into the quadrents formed by using the first and second principal component as axes...They form quite widely separated clusters of the various populations that resemble the "classic" major racial groups-Caucasoids in the upper right, Negroids in the lower right, North East Asians in the upper left, and South East Asians and Pacific I
