Genetic views on race
Encyclopedia
The relationship between race and genetics has relevance for the ongoing controversies regarding race.
Ongoing genetic research have investigated how ancestral human populations migrated in the ancestral geographic environment into different geographic areas. Today it is possible to determine, by genetic analysis, from which ancestral geographic regions a person originates and to what degree from each region. Also other factors such as endogamy
for cultural reasons may give rise to genetically differentiated populations.
Critics have made a number of arguments against this. There were often no clear distinctions between the ancestral populations with transitions being gradual. Genetic variations in such groups were larger than genetic variations between groups. How a person is identified racially may be different from geographic ancestry. Especially for groups with mixed ancestry self-identified race is often uncertain and culturally dependent.
s about 5–7 million years ago. The genus Homo
evolved by about 2.3 to 2.4 million years ago from Australopithecine
s. Several species and subspecies of Homo evolved and are now extinct
. These include Homo erectus
, which inhabited
Asia, and Homo sapiens neanderthalensis, which inhabited West Eurasia. Archaic Homo sapiens
evolved between 400,000 and 250,000 years ago.
The dominant view among scientists concerning the origin of anatomically modern humans
is the "Out of Africa"
or recent African origin hypothesis, which argues that Homo sapiens arose in Africa and migrated out of the continent around 50,000 to 100,000 years ago, replacing populations of Homo erectus in Asia and Homo neanderthalensis in Europe. An alternative multiregional hypothesis argue that Homo sapiens evolved as geographically separate but interbreeding populations stemming from a worldwide migration of Homo erectus out of Africa nearly 2.5 million years ago. This theory has been contradicted by recent evidence, although it has been suggested that non Homo sapiens Neanderthal genomes may have contributed about 4% of non-African heredity, and the recently discovered Denisova
hominin may have contributed 6% of the genome
of Melanesians
.
s in genetic material, migration between populations (gene flow
), and the reshuffling of genes through sexual reproduction
. The two main mechanisms that produce evolution
are natural selection
and genetic drift
. A special case of genetic drift is the founder effect
. Epigenetic inheritance are heritable
changes in phenotype
(appearance) or gene expression
caused by mechanisms other than changes in the underlying DNA
sequence.
Many human phenotypes are polygenic, meaning that they depend on the interaction among many genes. Polygeneity makes the study of individual phenotypic differences more difficult. Additionally, phenotypes may be influenced by environment as well as by genetics. The measure of the genetic role in phenotypes is heritability
.
Nucleotide diversity
is based on single mutations called single nucleotide polymorphisms (SNPs). The nucleotide diversity between humans is about 0.1%, which is 1 difference per 1,000 nucleotide
s between two humans chosen at random. This amounts to approximately 3 million SNPs since the human genome
has about 3 billion nucleotides. It is estimated that a total of 10 million SNPs exist in the human population.
Recent analysis has shown that non-SNP variation accounts for much more human genetic variation than single nucleotide diversity. This non-SNP variation includes copy number variation and results from deletions, inversions
, insertions and duplications
. It is estimated that approximately 0.4% of the genomes of unrelated people typically differ with respect to copy number. When copy number variation is included, human to human genetic variation is estimated to be at least 0.5%.
. This is argued to have caused problems for early anthropologists
whose simplistic approach was inadequate for classifying race based on visible traits.
Prior to the discovery of DNA as the hereditary material, scientists used blood proteins (the human blood group systems
) to study human genetic variation. Research by Ludwik and Hanka Herschfeld during World War I
found that the frequencies of blood groups A and B differed greatly from region to region. For example, among Europeans, 15% were group B and 40% were group A. Eastern Europeans and Russians had higher frequencies of group B, with people from India having the highest proportion. The Herschfelds concluded that humans were made of two different "biochemical races," each with its own origin. It was hypothesized that these two pure races later became mixed, resulting in the complex pattern of groups A and B. This was one of the first theories of racial differences to include the idea that visible human variation did not necessarily correlate with invisible genetic variation. It was expected that groups that had similar proportions of the blood groups would be more closely related in racial terms, but instead it was often found that groups separated by large distances, such as those from Madagascar and Russia, had similar frequencies. This confounded scientists who were attempting to learn more about human evolutionary history.
Today researchers often use direct genetic testing
. Unlike earlier research using one or a few traits or proteins, today this often involve the simultaneous study of hundreds or thousands of genetic marker
s or even the whole genome
.
s from many individuals are examined simultaneously in order to find the population genetic structure. The basic idea is that while such subgroups are not distinct and overlap if looking at the distribution of the variants of one marker only, when many markers are examined simultaneously, then the different subgroups have distinctly different average genetic structure. An individual need not have exactly this average genetic structure and may be described as belonging, to varying degrees, to several subgroups. Such subgroups may be more or less distinct depending on how close a subgroup distribution
is to the average genetic structure of the subgroup and how much overlap there are with the distributions of different subgroups. One such mathematical method is cluster analysis. Another is principal components analysis
. The population genetic structure found is often similar.
In cluster analysis the number of clusters to search for ("K") is determined in advance; how distinct these clusters are from one another vary. The results obtained by clustering analyses are dependent on several factors:
refers to the genetic divergence between species or between populations within a species. Smaller genetic distances indicate a close genetic relationship whereas large genetic distances indicate a more distant genetic relationship. Genetic distance can be used to compare the genetic similarity between different species, such as humans and chimpanzees. Within a species genetic distance can be used to measure the divergence between different subgroups.
Genetic distance significantly correlates to geographic distance between populations, a phenomenon referred to as "isolation by distance
". Genetic distance can also be the result of physical boundaries which naturally restrict gene flow, such as islands, deserts, mountain ranges or dense forests.
Genetic distance is often measured by Fixation index (FST). FST is simply the correlation of randomly chosen allele
s within the same sub-population relative to that found in the entire population. It is often expressed as the proportion of genetic diversity due to allele frequency differences among populations. This comparison of genetic variability within and between populations is frequently used in the field of population genetics
. The values range from 0 to 1. A zero value implies complete panmixis, that the two populations are interbreeding freely. A value of one would imply the two populations are completely separate.
Geographic analyses attempt to identify the places of origin, relative importance, and the possible causes involved in the spread of genetic variation over an area. The results can be presented as map
s showing how genes vary between populations. Cavalli-Sforza and colleagues have argued that if variations in many genes between populations are investigated simultaneously, they often correspond to population migrations due to, for example, new sources of food, improved transportation, or shifts in political power. For example, in Europe the single most significant direction of genetic variation corresponds to the spread of farming from the Middle East to Europe between 10,000 and 6,000 years ago. Such geographic analysis works best when describing the situation before recent large scale and fast migrations with intermixing of many populations far from their ancestral homes.
Historic analyses use differences in genetic variation, genetic distance being one way to measure this, as a molecular clock
indicating the evolutionary relatedness of various species or groups. This method can be used to create evolutionary trees which attempt to reconstruct population separations over time,
or archeology.
Cavalli-Sforza and colleagues have argued that there is a strong correspondence between the language families found in linguistic research and the populations and the tree they found in their 1994 study. As a general rule, there is shorter genetic distances between populations using languages from the same language family. The notable exceptions to this rule are Sami
, Tibetans, and Ethiopians, who are genetically associated with populations which speak languages belonging to different language families. For example, the Sami speak an Uralic language
yet are according to the genetic analysis mainly Europeans. This is argued to have resulted from migration and interbreeding with Europeans while retaining the original language. There are similar explanations for the other exceptions. There is also a high agreement between dates from research done in archeology and as calculated using genetic distance.
et al. evaluated the genetic distances between 42 native populations from around the world based on 120 blood polymorphisms. These 42 populations can be grouped into 9 main clusters, which Cavalli-Sforza termed African (sub-Saharan), Caucasoid (European), Caucasoid (extra-European), Northern Mongoloid (excluding Arctic populations), Northeast Asian Arctic, Southern Mongoloid (mainland and insular Southeast Asia), Pacific Islander, New Guinean and Australian, and American (Amerindian). Though the clusters evidence varying degrees of homogeneity, the 9-cluster model represents a majority (80 out of 120) of single-trait trees and is useful in demonstrating the historic phylogenetic relationship between these populations.
The largest genetic distance between any two continents is between Africa and Oceania
at 0.2470. Based on physical appearance this may be counterintuitive, since Indigenous Australians and New Guineans resemble Africans with dark skin and sometimes frizzy hair. 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 0.2260. This is expected since the longest geographic distance by land is between Africa and South America. The shortest genetic distance at 0.0155 is between European Caucasoids and Non-European Caucasoids. Africa is the most genetically divergent continent, with all other groups being more related to each other than to Sub-Saharan Africans. This is expected in accordance with the recent single-origin hypothesis. Europe has a genetic variation in general about three times less than that of other continents, and the genetic contribution of Asia and Africa to Europe is thought to be 2/3 and 1/3 respectively.
Many more recent worldwide studies have also been published. Often they use an increasing number of genetic markers. Many studies have also been done on more limited regions, (one example being studies on the genetic history of Europe
), or on individual nations (one example being studies on the genetic history of Italy
), or on specific groups (one example being genetic studies on Jews
).
there is controversy and many proposed definitions which is sometimes referred to as the "species problem
". Since the 1960'es a majority of scientists have come to understand the concept of race as a social construct mapped on to phenotypes in different culturally determined ways, and not as a biological concept. A 2000 study by Celera Genomics found that human beings largely have similar genetic input, and that citizens of any given village in the world, whether in Scotland or Tanzania, hold 90 percent of the genetic variability that humanity has to offer. The study found only .01% of genes account for a person's external appearance. Biological adaptation also plays a role in phenotype of bodily features and skin type. According to Luigi Luca Cavalli-Sforza
, "Skin color and body size are less subject to genetic influence since they are also affected by exposure to the sun and diet, but there is always a hereditary component that can be quite important."
than other populations which may be a problem to seeing them as a single race.
performed a FST statistical analysis using 17 markers including blood group proteins. His results were that the majority of genetic differences between humans, 85.4%, were found within a population, 8.3% of genetic differences were found between populations within a race, and only 6.3% was found to differentiate races which in the study were Caucasian, African, Mongoloid, South Asian Aborigines, Amerinds, Oceanians, and Australian Aborigines. Since then, other analyses have found FST values of 6%-10% between continental human groups, 5-15% between different populations occupying the same continent, and 75-85% within populations. Lewontin's argument led a number of authors publishing in the 1990s and 2000s to follow Lewontin's verdict that race is biologically a meaningless concept.
While acknowledging the correctness of Lewontin's observation that racial groups are genetically homogeneous, geneticist A. W. F. Edwards
in the paper "Human Genetic Diversity: Lewontin's Fallacy
" (2003) argued that the conclusion that racial groups can not be genetically distinguished from each other is incorrect. Edwards argued that when multiple allelles are taken into account genetic differences do tend to cluster in geographic patterns roughly corresponding to the groups commonly defined as races. This is because most of the information that distinguishes populations from each other is hidden in the correlation structure of allele frequencies, making it possible to highly reliably classify individuals using the mathematical techniques described above. Edwards argued that, even if the probability of misclassifying an individual based on a single genetic marker is as high as 30% (as Lewontin reported in 1972), the misclassification probability becomes close to zero if enough genetic markers are studied simultaneously. Edwards saw Lewontin's argument as being based mostly in a political stance that denies the existence biological difference in order to argue for social equality.
Richard Dawkins
(2005) agreed with Edwards' view, summarizing the argument against Lewontin as being, "However small the racial partition of the total variation may be, if such racial characteristics as there are highly correlate with other racial characteristics, they are by definition informative, and therefore of taxonomic significance."
Alan Templeton
(2003) argued that in the nonhuman literature an FST of at least 25%-30% is a standard criterion for the identification of a subspecies.
Henry Harpending
(2002) has argued that the magnitude of human FST values imply that "kinship between two individuals of the same human population is equivalent to kinship between grandparent and grandchild or between half siblings. The widespread assertion that this is small and insignificant should be reexamined."
Sarich and Miele (2004) have argued that estimates of genetic difference between individuals of different populations fail to take into account human diploidity
. "The point is that we are diploid organisms, getting one set of chromosomes from one parent and a second from the other. To the extent that your mother and father are not especially closely related, then, those two sets of chromosomes will come close to being a random sample of the chromosomes in your population. And the sets present in some randomly chosen member of yours will also be about as different from your two sets as they are from one another. So how much of the variability will be distributed where? First is the 15 percent that is interpopulational. The other 85 percent will then split half and half (42.5 percent) between the intra- and interindividual within-population comparisons. The increase in variability in between-population comparisons is thus 15 percent against the 42.5 percent that is between-individual within-population. Thus, 15/42.5 is 32.5 percent, a much more impressive and, more important, more legitimate value than 15 percent."
Anthropologists such as C. Loring Brace
and Jonathan Kaplan and geneticist Joseph Graves, have argued that while there it is certainly possible to find biological and genetic variation that corresponds roughly to the groupings normally defined as races, this is true for almost all geographically distinct populations. The cluster structure of the genetic data is therefore dependent on the initial hypotheses of the researcher and the populations sampled. When one samples continental groups the clusters become continental, if one had chosen other sampling patterns the clusters would be different. Weiss and Fullerton have noted that if one sampled only Icelanders, Mayans and Maoris, three distinct clusters would form and all other populations could be described as being composed of admixtures of Maori, Icelandic and Mayan genetic materials. Kaplan therefore argues that seen in this way both Lewontin and Edwards are right in their arguments. He concludes that while racial groups are characterized by different allele frequencies, this does not mean that racial classification is a natural taxonomy of the human species, because multiple other genetic patterns can be found in human populations that crosscut racial distinctions. In this view racial groupings are social constructions that also have biological reality which is largely an artefact of how the category has been constructed.
A study by Tang et al. in 2005 used 326 genetic markers in order to determine genetic clusters. The 3,636 subjects involved in the study, from the United States
and Taiwan
, self-identified as belonging to white, African American, East Asian, or Hispanic (=self-identified race/ethnic group (SIRE)). The study found "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%."
Paschou et al. (2010) found "essentially perfect" agreement between 51 self-reported populations of origin and the population genetic structure found using 650,000 genetic markers. Selecting for especially informative genetic makers allowed a reduction to less than 650 while still retaining close to 100% accuracy.
That there is correspondence between genetic clusters in a current population, such as the current US population, and self-identified race/ethnic groups does not necessarily mean that such a cluster/group corresponds to only one ancestral origin/population. African Americans have an estimated 10%–20% European admixture
. The Hispanic group have European, Native American, and African ancestries. In Brazil, there has been extensive admixture between Europeans, Amerindians, and Africans resulting in no clear discontinuities in skin color in the population and relatively weak associations between between self-reported race (called Color in Brazil probably because it captures the continuous aspects) and African ancestry as well as between objectively measured skin color and African ancestry.
One argument is that genetic distances on average increase in a continuous manner with geographic distance, which causes any threshold or dividing line to 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. This has been argued to explain why some studies on population genetic structure have yielded varying results depending on the methodology used.
Ring species
show that also a continuous change in genetic variation can produce very large differences between different populations in a species.
Rosenberg et al. (2005) have argued, based on cluster analysis, that populations do not always vary continuously and that the population genetic structure is consistent if enough genetic markers and subjects are included. "Examination of the relationship between genetic and geographic distance supports a view in which the clusters arise not as an artifact of the sampling scheme, but from small discontinuous jumps in genetic distance for most population pairs on opposite sides of geographic barriers, in comparison with genetic distance for pairs on the same side. Thus, analysis of the 993-locus dataset corroborates our earlier results: if enough markers are used with a sufficiently large worldwide sample, individuals can be partitioned into genetic clusters that match major geographic subdivisions of the globe, with some individuals from intermediate geographic locations having mixed membership in the clusters that correspond to neighboring regions." They also wrote, regarding a model with five clusters corresponding to Africa, Eurasia (Europe, Middle East, and Central/South Asia), East Asia, Oceania, and the Americas, that "For population pairs from the same cluster, as geographic distance increases, genetic distance increases in a linear manner, consistent with a clinal population structure. However, for pairs from different clusters, genetic distance is generally larger than that between intracluster pairs that have the same geographic distance. For example, genetic distances for population pairs with one population in Eurasia and the other in East Asia are greater than those for pairs at equivalent geographic distance within Eurasia or within East Asia. Loosely speaking, it is these small discontinuous jumps in genetic distance—across ocean
s, the Himalayas, and the Sahara
—that provide the basis for the ability of STRUCTURE to identify clusters that correspond to geographic regions."
The above discussion applies to populations in their ancestral homes when migrations and gene flow
were slow. Recent large and fast migrations due to changed technology have changed this. Thus, regarding the situation today in the United States, Tang et al. (2004) write that "we detected only modest genetic differentiation between different current geographic locales within each race/ethnicity group. Thus, ancient geographic ancestry, which is highly correlated with self-identified race/ethnicity—as opposed to current residence—is the major determinant of genetic structure in the U.S. population."
Witherspoon et al. (2007) have argued that even when individuals can be reliably assigned to specific population groups, it may still be possible for two randomly chosen individuals from different populations/clusters to be more similar to each other than to a randomly chosen member of their own cluster. They found that many thousands of genetic markers had to be used in order for the answer to the question "How often is a pair of individuals from one population genetically more dissimilar than two individuals chosen from two different populations?" to be "never". This assumed three population groups separated by large geographic ranges (European, African and East Asian). The entire world population is much more complex and studying an increasing number of groups would require an increasing number of markers for the same answer. Witherspoon et al. conclude that "caution should be used when using geographic or genetic ancestry to make inferences about individual phenotypes."
vary for different populations. The leading explanation is that skin colour adapts to sunlight intensities which produce vitamin D
deficiency or ultraviolet
light damage to folic acid
. Other hypotheses include protection from ambient temperature, infections, skin cancer
or frostbite
, an alteration in food, and sexual selection
. The gene that causes light skin color in Europeans is different from the gene that causes light skin in East Asians. Europeans have a different version of the SLC24A5
than East Asians possibly indicating that they evolved light skin independently.
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, some argue that many of these traits are non-concordant in that they are not necessarily expressed together. For example, skin color and hair texture vary independently. Some examples of non-concordance include:
Others argue that this is just an example of Lewontin's Fallacy
. On the contrary, if several traits are looked at the same time, then today forensic anthropologists
can classify a person's race with an accuracy close to 100% based on only skeletal remains.
A 2010 examination of 18 widely used English anatomy
textbooks found that every one relied on the race concept. The study gives examples of how the textbooks claim that anatomical features vary between races.
states that numerous studies over past decades have documented biological differences among the races with regard to susceptibility and natural history of chronic diseases. Genes may be under strong selection in response to local diseases. For example, people who are duffy negative
tend to have higher resistance to malaria. Most Africans are duffy negative and most non-Africans are duffy positive. A number of genetic diseases more prevalent in malaria-afflicted areas may provide some genetic resistance to malaria
including sickle cell disease, thalassaemias, glucose-6-phosphate dehydrogenase
, and possibly others. Cystic fibrosis
is the most common life-limiting autosomal recessive disease among people of European heritage. Numerous hypotheses have suggested that it provides a heterozygote advantage
by giving resistance to diseases earlier common in Europe.
Information about a person's population of origin may in some situations help making a diagnosis
and adverse drug responses may vary between such groups. Because of the correlation between self-identified race and genetic clusters, medical treatments whose results are influenced by genetics often have varying rates of success between self-defined racial groups. For this reason, some doctors consider a patient’s race while attempting to identify the most effective possible treatment, and some drugs are marketed with race-specific instructions. Jorde and Wooding (2004) have argued that, because of the genetic variation within racial groups, when "it finally becomes feasible and available, individual genetic assessment of relevant genes will probably prove more useful than race in medical decision making." Even so, race will continue to be important when looking at groups instead of individuals such as in epidemiologic research.
differ with geographic ancestry in part due to genetic factors. Lactose tolerance appears to be an evolutionarily recent adaptation to dairy
consumption, and has occurred independently in both northern Europe and east Africa in populations with a historically pastoral
lifestyle.
s has led some to question whether there is a genetic component giving certain races a competitive advantage. Others point out that such overrepresentations are not necessarily due to genetic causes. Such views differ between nations. Among Chinese, the proposition that there are genetic differences affecting sports performance is a widely accepted. A 1994 examination of 32 English sport/exercise science textbooks found that 7 (21.9%) claimed that there are biophysical differences due to race that might explain differences in sports performance, 24 (75%) did not mention nor refute the concept, and 1 (3.12%) expressed caution with the idea.
. Some are agnostic about the causes, while others argue that environmental factors explain all of the differences, or that both genetics and environmental factors are important.
Regional: Archaeogenetics
|url=http://www.nytimes.com/2005/03/14/opinion/14leroi.html |accessdate=}}
Ongoing genetic research have investigated how ancestral human populations migrated in the ancestral geographic environment into different geographic areas. Today it is possible to determine, by genetic analysis, from which ancestral geographic regions a person originates and to what degree from each region. Also other factors such as endogamy
Endogamy
Endogamy is the practice of marrying within a specific ethnic group, class, or social group, rejecting others on such basis as being unsuitable for marriage or other close personal relationships. A Greek Orthodox Christian endogamist, for example, would require that a marriage be only with another...
for cultural reasons may give rise to genetically differentiated populations.
Critics have made a number of arguments against this. There were often no clear distinctions between the ancestral populations with transitions being gradual. Genetic variations in such groups were larger than genetic variations between groups. How a person is identified racially may be different from geographic ancestry. Especially for groups with mixed ancestry self-identified race is often uncertain and culturally dependent.
Human evolution
The human lineage diverged from the common ancestor with chimpanzeeChimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
s about 5–7 million years ago. The genus Homo
Homo (genus)
Homo is the genus that includes modern humans and species closely related to them. The genus is estimated to be about 2.3 to 2.4 million years old, evolving from australopithecine ancestors with the appearance of Homo habilis....
evolved by about 2.3 to 2.4 million years ago from Australopithecine
Australopithecine
The term australopithecine refers generally to any species in the related genera Australopithecus or Paranthropus. These species occurred in the Plio-Pleistocene era, and were bipedal and dentally similar to humans, but with a brain size not much larger than modern apes, lacking the...
s. Several species and subspecies of Homo evolved and are now extinct
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
. These include Homo erectus
Homo erectus
Homo erectus is an extinct species of hominid that lived from the end of the Pliocene epoch to the later Pleistocene, about . The species originated in Africa and spread as far as India, China and Java. There is still disagreement on the subject of the classification, ancestry, and progeny of H...
, which inhabited
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
Asia, and Homo sapiens neanderthalensis, which inhabited West Eurasia. Archaic Homo sapiens
Archaic Homo sapiens
Archaic Homo sapiens is a loosely defined term used to describe a number of varieties of Homo, as opposed to anatomically modern humans , in the period beginning 500,000 years ago....
evolved between 400,000 and 250,000 years ago.
The dominant view among scientists concerning the origin of anatomically modern humans
Anatomically modern humans
The term anatomically modern humans in paleoanthropology refers to early individuals of Homo sapiens with an appearance consistent with the range of phenotypes in modern humans....
is the "Out of Africa"
Recent African origin of modern humans
In paleoanthropology, the recent African origin of modern humans is the most widely accepted model describing the origin and early dispersal of anatomically modern humans...
or recent African origin hypothesis, which argues that Homo sapiens arose in Africa and migrated out of the continent around 50,000 to 100,000 years ago, replacing populations of Homo erectus in Asia and Homo neanderthalensis in Europe. An alternative multiregional hypothesis argue that Homo sapiens evolved as geographically separate but interbreeding populations stemming from a worldwide migration of Homo erectus out of Africa nearly 2.5 million years ago. This theory has been contradicted by recent evidence, although it has been suggested that non Homo sapiens Neanderthal genomes may have contributed about 4% of non-African heredity, and the recently discovered Denisova
Denisova hominin
Denisova hominins , or Denisovans, are Paleolithic-Era members of the genus Homo that may belong to a previously unknown species. In , scientists announced the discovery of a finger bone fragment of a juvenile female that lived about 41,000 years ago, found in Denisova Cave in Altai Krai, Russia, a...
hominin may have contributed 6% of the genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
of Melanesians
Melanesians
Melanesians are an ethnic group in Melanesia. The original inhabitants of the group of islands now named Melanesia were likely the ancestors of the present-day Papuan-speaking people...
.
Genetic variation
Genetic variation comes from mutationMutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
s in genetic material, migration between populations (gene flow
Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
), and the reshuffling of genes through sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
. The two main mechanisms that produce evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
are natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
and genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
. A special case of genetic drift is the founder effect
Founder effect
In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using existing theoretical work by those such as Sewall...
. Epigenetic inheritance are heritable
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...
changes in phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
(appearance) or gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
caused by mechanisms other than changes in the underlying DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
sequence.
Many human phenotypes are polygenic, meaning that they depend on the interaction among many genes. Polygeneity makes the study of individual phenotypic differences more difficult. Additionally, phenotypes may be influenced by environment as well as by genetics. The measure of the genetic role in phenotypes is heritability
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
.
Nucleotide diversity
Nucleotide diversity
Nucleotide diversity is a concept in molecular genetics which is used to measure the degree of polymorphism within a population.One commonly used measure of nucleotide diversity was first introduced by Nei and Li in 1979...
is based on single mutations called single nucleotide polymorphisms (SNPs). The nucleotide diversity between humans is about 0.1%, which is 1 difference per 1,000 nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
s between two humans chosen at random. This amounts to approximately 3 million SNPs since the human genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
has about 3 billion nucleotides. It is estimated that a total of 10 million SNPs exist in the human population.
Recent analysis has shown that non-SNP variation accounts for much more human genetic variation than single nucleotide diversity. This non-SNP variation includes copy number variation and results from deletions, inversions
Chromosomal inversion
An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.Paracentric inversions do not include the...
, insertions and duplications
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
. It is estimated that approximately 0.4% of the genomes of unrelated people typically differ with respect to copy number. When copy number variation is included, human to human genetic variation is estimated to be at least 0.5%.
Visible traits, proteins, and genes studied
The earliest classification attempts were done using surface traits such as done in anthropometryAnthropometry
Anthropometry refers to the measurement of the human individual...
. This is argued to have caused problems for early anthropologists
Anthropology
Anthropology is the study of humanity. It has origins in the humanities, the natural sciences, and the social sciences. The term "anthropology" is from the Greek anthrōpos , "man", understood to mean mankind or humanity, and -logia , "discourse" or "study", and was first used in 1501 by German...
whose simplistic approach was inadequate for classifying race based on visible traits.
Prior to the discovery of DNA as the hereditary material, scientists used blood proteins (the human blood group systems
Human blood group systems
The International Society of Blood Transfusion currently recognises 30 major blood group systems . Thus, in addition to the ABO antigens and Rhesus antigens, many other antigens are expressed on the red blood cell surface membrane...
) to study human genetic variation. Research by Ludwik and Hanka Herschfeld during World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
found that the frequencies of blood groups A and B differed greatly from region to region. For example, among Europeans, 15% were group B and 40% were group A. Eastern Europeans and Russians had higher frequencies of group B, with people from India having the highest proportion. The Herschfelds concluded that humans were made of two different "biochemical races," each with its own origin. It was hypothesized that these two pure races later became mixed, resulting in the complex pattern of groups A and B. This was one of the first theories of racial differences to include the idea that visible human variation did not necessarily correlate with invisible genetic variation. It was expected that groups that had similar proportions of the blood groups would be more closely related in racial terms, but instead it was often found that groups separated by large distances, such as those from Madagascar and Russia, had similar frequencies. This confounded scientists who were attempting to learn more about human evolutionary history.
Today researchers often use direct genetic testing
Genetic testing
Genetic testing is among the newest and most sophisticated of techniques used to test for genetic disorders which involves direct examination of the DNA molecule itself. Other genetic tests include biochemical tests for such gene products as enzymes and other proteins and for microscopic...
. Unlike earlier research using one or a few traits or proteins, today this often involve the simultaneous study of hundreds or thousands of genetic marker
Genetic marker
A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify cells, individuals or species. It can be described as a variation that can be observed...
s or even the whole genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
.
Population genetic structure
There are several mathematical methods for examining if a population have more or less distinct genetic subgroups and to quantify this. Many genetic markerGenetic marker
A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify cells, individuals or species. It can be described as a variation that can be observed...
s from many individuals are examined simultaneously in order to find the population genetic structure. The basic idea is that while such subgroups are not distinct and overlap if looking at the distribution of the variants of one marker only, when many markers are examined simultaneously, then the different subgroups have distinctly different average genetic structure. An individual need not have exactly this average genetic structure and may be described as belonging, to varying degrees, to several subgroups. Such subgroups may be more or less distinct depending on how close a subgroup distribution
Probability distribution
In probability theory, a probability mass, probability density, or probability distribution is a function that describes the probability of a random variable taking certain values....
is to the average genetic structure of the subgroup and how much overlap there are with the distributions of different subgroups. One such mathematical method is cluster analysis. Another is principal components analysis
Principal components analysis
Principal component analysis is a mathematical procedure that uses an orthogonal transformation to convert a set of observations of possibly correlated variables into a set of values of uncorrelated variables called principal components. The number of principal components is less than or equal to...
. The population genetic structure found is often similar.
In cluster analysis the number of clusters to search for ("K") is determined in advance; how distinct these clusters are from one another vary. The results obtained by clustering analyses are dependent on several factors:
- More genetic markers studied at the same time makes it easier to find distinct clusters.
- Certain genetic markers vary more than others which means fewer are required to find distinct clusters. Ancestry-informative markerAncestry-informative markerAn ancestry-informative marker is a set of polymorphisms for a locus, generally from humans, which exhibits substantially different frequencies between populations from different geographical regions....
s exhibits substantially different frequencies between populations from different geographical regions. Using AIMs, scientists can determine a person's ancestral continent of origin based solely on their DNA. AIMs can also be used to determine someone's admixture proportions. - The more individuals studied, the easier it becomes to detect distinct clusters, as statistical noise is reduced.
- Low genetic variation makes it more difficult to find distinct clusters. Larger geographic distances generally increases genetic variation which makes identifying clusters easier.
- A similar cluster structure is seen even if using different genetic markers, when the number of genetic markers included is sufficiently high. The clustering structure obtained with different statistical techniques is quite similar. A similar cluster structure is found in the original sample and if using a subsample of the original sample.
Genetic distance
Genetic distanceGenetic distance
Genetic distance refers to the genetic divergence between species or between populations within a species. It is measured by a variety of parameters. Smaller genetic distances indicate a close genetic relationship whereas large genetic distances indicate a more distant genetic relationship...
refers to the genetic divergence between species or between populations within a species. Smaller genetic distances indicate a close genetic relationship whereas large genetic distances indicate a more distant genetic relationship. Genetic distance can be used to compare the genetic similarity between different species, such as humans and chimpanzees. Within a species genetic distance can be used to measure the divergence between different subgroups.
Genetic distance significantly correlates to geographic distance between populations, a phenomenon referred to as "isolation by distance
Isolation by distance
Isolation by distance describes the tendency of individuals to find mates from nearby populations rather than distant populations. As a result of this tendency, populations that live near each other are genetically more similar than populations that live further apart. Isolation by distance results...
". Genetic distance can also be the result of physical boundaries which naturally restrict gene flow, such as islands, deserts, mountain ranges or dense forests.
Genetic distance is often measured by Fixation index (FST). FST is simply the correlation of randomly chosen allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s within the same sub-population relative to that found in the entire population. It is often expressed as the proportion of genetic diversity due to allele frequency differences among populations. This comparison of genetic variability within and between populations is frequently used in the field of population genetics
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
. The values range from 0 to 1. A zero value implies complete panmixis, that the two populations are interbreeding freely. A value of one would imply the two populations are completely separate.
Historic and geographic analysis of ancestry
Cavalli-Sforza has described two major methods of ancestry analysis. Note that current population genetic structure does not necessarily imply that the different current clusters/components found correspond to only one ancestral home per group. One example being a genetic cluster in the US corresponding to Hispanics who have European, Native American, and African ancestries.Geographic analyses attempt to identify the places of origin, relative importance, and the possible causes involved in the spread of genetic variation over an area. The results can be presented as map
Map
A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....
s showing how genes vary between populations. Cavalli-Sforza and colleagues have argued that if variations in many genes between populations are investigated simultaneously, they often correspond to population migrations due to, for example, new sources of food, improved transportation, or shifts in political power. For example, in Europe the single most significant direction of genetic variation corresponds to the spread of farming from the Middle East to Europe between 10,000 and 6,000 years ago. Such geographic analysis works best when describing the situation before recent large scale and fast migrations with intermixing of many populations far from their ancestral homes.
Historic analyses use differences in genetic variation, genetic distance being one way to measure this, as a molecular clock
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
indicating the evolutionary relatedness of various species or groups. This method can be used to create evolutionary trees which attempt to reconstruct population separations over time,
Validating the genetic ancestry research
The results from the genetic ancestry research are argued to be supported if they agree with the results from other research such as from linguisticsLinguistics
Linguistics is the scientific study of human language. Linguistics can be broadly broken into three categories or subfields of study: language form, language meaning, and language in context....
or archeology.
Cavalli-Sforza and colleagues have argued that there is a strong correspondence between the language families found in linguistic research and the populations and the tree they found in their 1994 study. As a general rule, there is shorter genetic distances between populations using languages from the same language family. The notable exceptions to this rule are Sami
Sami people
The Sami people, also spelled Sámi, or Saami, are the arctic indigenous people inhabiting Sápmi, which today encompasses parts of far northern Sweden, Norway, Finland, the Kola Peninsula of Russia, and the border area between south and middle Sweden and Norway. The Sámi are Europe’s northernmost...
, Tibetans, and Ethiopians, who are genetically associated with populations which speak languages belonging to different language families. For example, the Sami speak an Uralic language
Uralic languages
The Uralic languages constitute a language family of some three dozen languages spoken by approximately 25 million people. The healthiest Uralic languages in terms of the number of native speakers are Hungarian, Finnish, Estonian, Mari and Udmurt...
yet are according to the genetic analysis mainly Europeans. This is argued to have resulted from migration and interbreeding with Europeans while retaining the original language. There are similar explanations for the other exceptions. There is also a high agreement between dates from research done in archeology and as calculated using genetic distance.
Ancestral populations
A widely cited 1994 study by Cavalli-SforzaLuigi Luca Cavalli-Sforza
Luigi Luca Cavalli-Sforza is an Italian population geneticist born in Genoa, who has been a professor at Stanford University since 1970 .-Books:...
et al. evaluated the genetic distances between 42 native populations from around the world based on 120 blood polymorphisms. These 42 populations can be grouped into 9 main clusters, which Cavalli-Sforza termed African (sub-Saharan), Caucasoid (European), Caucasoid (extra-European), Northern Mongoloid (excluding Arctic populations), Northeast Asian Arctic, Southern Mongoloid (mainland and insular Southeast Asia), Pacific Islander, New Guinean and Australian, and American (Amerindian). Though the clusters evidence varying degrees of homogeneity, the 9-cluster model represents a majority (80 out of 120) of single-trait trees and is useful in demonstrating the historic phylogenetic relationship between these populations.
The largest genetic distance between any two continents is between Africa and Oceania
Oceania
Oceania is a region centered on the islands of the tropical Pacific Ocean. Conceptions of what constitutes Oceania range from the coral atolls and volcanic islands of the South Pacific to the entire insular region between Asia and the Americas, including Australasia and the Malay Archipelago...
at 0.2470. Based on physical appearance this may be counterintuitive, since Indigenous Australians and New Guineans resemble Africans with dark skin and sometimes frizzy hair. This large figure for genetic distance reflects the relatively long isolation of Australia and New Guinea since the end of the last glacial maximum
Last Glacial Maximum
The Last Glacial Maximum refers to a period in the Earth's climate history when ice sheets were at their maximum extension, between 26,500 and 19,000–20,000 years ago, marking the peak of the last glacial period. During this time, vast ice sheets covered much of North America, northern Europe and...
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 0.2260. This is expected since the longest geographic distance by land is between Africa and South America. The shortest genetic distance at 0.0155 is between European Caucasoids and Non-European Caucasoids. Africa is the most genetically divergent continent, with all other groups being more related to each other than to Sub-Saharan Africans. This is expected in accordance with the recent single-origin hypothesis. Europe has a genetic variation in general about three times less than that of other continents, and the genetic contribution of Asia and Africa to Europe is thought to be 2/3 and 1/3 respectively.
Many more recent worldwide studies have also been published. Often they use an increasing number of genetic markers. Many studies have also been done on more limited regions, (one example being studies on the genetic history of Europe
Genetic history of Europe
The genetic history of Europe can be inferred from the patterns of genetic diversity across continents and time. The primary data to develop historical scenarios coming from sequences of mitochondrial, Y-chromosome and autosomal DNA from modern populations and if available from ancient DNA...
), or on individual nations (one example being studies on the genetic history of Italy
Genetic history of Italy
During prehistory Italy was populated by different but very similar Indo-European groups, later collectively listed amongst the Ancient peoples of Italy, of whom the Italic one was predominant....
), or on specific groups (one example being genetic studies on Jews
Genetic studies on Jews
Genetic studies on the Jews are part of population genetics. This discipline is used to better understand the chronology of migration and thus complements the results provided by history, archeology, language or paleontology. The interest of these studies is to investigate the origins of various...
).
Race and population genetic structure
As described in the article on race (classification of humans), several different definitions of race have been proposed. Even for speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
there is controversy and many proposed definitions which is sometimes referred to as the "species problem
Species problem
The species problem or species concept is a mixture of difficult, related questions that often come up when biologists identify species and when they define the word "species"....
". Since the 1960'es a majority of scientists have come to understand the concept of race as a social construct mapped on to phenotypes in different culturally determined ways, and not as a biological concept. A 2000 study by Celera Genomics found that human beings largely have similar genetic input, and that citizens of any given village in the world, whether in Scotland or Tanzania, hold 90 percent of the genetic variability that humanity has to offer. The study found only .01% of genes account for a person's external appearance. Biological adaptation also plays a role in phenotype of bodily features and skin type. According to Luigi Luca Cavalli-Sforza
Luigi Luca Cavalli-Sforza
Luigi Luca Cavalli-Sforza is an Italian population geneticist born in Genoa, who has been a professor at Stanford University since 1970 .-Books:...
, "Skin color and body size are less subject to genetic influence since they are also affected by exposure to the sun and diet, but there is always a hereditary component that can be quite important."
Size of group
The research techniques can be used to detect subtle genetic population differences if enough genetic markers are used. One example being that the East Asian populations Japanese and Chinese can be identified if enough markers are used. Sub-Saharan Africans have higher genetic diversityGenetic diversity
Genetic diversity, the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary....
than other populations which may be a problem to seeing them as a single race.
Lewontin's argument and criticism
In 1972 Richard LewontinRichard Lewontin
Richard Charles "Dick" Lewontin is an American evolutionary biologist, geneticist and social commentator. A leader in developing the mathematical basis of population genetics and evolutionary theory, he pioneered the notion of using techniques from molecular biology such as gel electrophoresis to...
performed a FST statistical analysis using 17 markers including blood group proteins. His results were that the majority of genetic differences between humans, 85.4%, were found within a population, 8.3% of genetic differences were found between populations within a race, and only 6.3% was found to differentiate races which in the study were Caucasian, African, Mongoloid, South Asian Aborigines, Amerinds, Oceanians, and Australian Aborigines. Since then, other analyses have found FST values of 6%-10% between continental human groups, 5-15% between different populations occupying the same continent, and 75-85% within populations. Lewontin's argument led a number of authors publishing in the 1990s and 2000s to follow Lewontin's verdict that race is biologically a meaningless concept.
While acknowledging the correctness of Lewontin's observation that racial groups are genetically homogeneous, geneticist A. W. F. Edwards
A. W. F. Edwards
Anthony William Fairbank Edwards is a British statistician, geneticist, and evolutionary biologist, sometimes called Fisher's Edwards. He is a Life Fellow of Gonville and Caius College and retired Professor of Biometry at the University of Cambridge, and holds both the ScD and LittD degrees. A...
in the paper "Human Genetic Diversity: Lewontin's Fallacy
Lewontin's Fallacy
Human genetic diversity: Lewontin's fallacy is a 2003 paper by A. W. F. Edwards that refers to an argument first made by Richard Lewontin in his 1972 article The apportionment of human diversity, which argued that race for humans is not a valid taxonomic construct. Edwards' paper criticized and...
" (2003) argued that the conclusion that racial groups can not be genetically distinguished from each other is incorrect. Edwards argued that when multiple allelles are taken into account genetic differences do tend to cluster in geographic patterns roughly corresponding to the groups commonly defined as races. This is because most of the information that distinguishes populations from each other is hidden in the correlation structure of allele frequencies, making it possible to highly reliably classify individuals using the mathematical techniques described above. Edwards argued that, even if the probability of misclassifying an individual based on a single genetic marker is as high as 30% (as Lewontin reported in 1972), the misclassification probability becomes close to zero if enough genetic markers are studied simultaneously. Edwards saw Lewontin's argument as being based mostly in a political stance that denies the existence biological difference in order to argue for social equality.
Richard Dawkins
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
(2005) agreed with Edwards' view, summarizing the argument against Lewontin as being, "However small the racial partition of the total variation may be, if such racial characteristics as there are highly correlate with other racial characteristics, they are by definition informative, and therefore of taxonomic significance."
Alan Templeton
Alan Templeton
Alan R. Templeton is an American geneticist and statistician from Washington University in St. Louis, where he holds the Charles Rebstock professorship in biology. He is known for his work demonstrating the lack of genetic differences between humans of different races...
(2003) argued that in the nonhuman literature an FST of at least 25%-30% is a standard criterion for the identification of a subspecies.
Henry Harpending
Henry Harpending
Henry C. Harpending is an anthropologist and population geneticist at the University of Utah, where he is a distinguished professor...
(2002) has argued that the magnitude of human FST values imply that "kinship between two individuals of the same human population is equivalent to kinship between grandparent and grandchild or between half siblings. The widespread assertion that this is small and insignificant should be reexamined."
Sarich and Miele (2004) have argued that estimates of genetic difference between individuals of different populations fail to take into account human diploidity
Ploidy
Ploidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
. "The point is that we are diploid organisms, getting one set of chromosomes from one parent and a second from the other. To the extent that your mother and father are not especially closely related, then, those two sets of chromosomes will come close to being a random sample of the chromosomes in your population. And the sets present in some randomly chosen member of yours will also be about as different from your two sets as they are from one another. So how much of the variability will be distributed where? First is the 15 percent that is interpopulational. The other 85 percent will then split half and half (42.5 percent) between the intra- and interindividual within-population comparisons. The increase in variability in between-population comparisons is thus 15 percent against the 42.5 percent that is between-individual within-population. Thus, 15/42.5 is 32.5 percent, a much more impressive and, more important, more legitimate value than 15 percent."
Anthropologists such as C. Loring Brace
C. Loring Brace
C. Loring Brace is an anthropologist at the University of Michigan. He considers the attempt "to introduce a Darwinian outlook into biological anthropology" to be his greatest contribution to the field of anthropology.-Life and work:...
and Jonathan Kaplan and geneticist Joseph Graves, have argued that while there it is certainly possible to find biological and genetic variation that corresponds roughly to the groupings normally defined as races, this is true for almost all geographically distinct populations. The cluster structure of the genetic data is therefore dependent on the initial hypotheses of the researcher and the populations sampled. When one samples continental groups the clusters become continental, if one had chosen other sampling patterns the clusters would be different. Weiss and Fullerton have noted that if one sampled only Icelanders, Mayans and Maoris, three distinct clusters would form and all other populations could be described as being composed of admixtures of Maori, Icelandic and Mayan genetic materials. Kaplan therefore argues that seen in this way both Lewontin and Edwards are right in their arguments. He concludes that while racial groups are characterized by different allele frequencies, this does not mean that racial classification is a natural taxonomy of the human species, because multiple other genetic patterns can be found in human populations that crosscut racial distinctions. In this view racial groupings are social constructions that also have biological reality which is largely an artefact of how the category has been constructed.
Self-identified race/ethnic group
Jorde and Wooding (2004) wrote that some studies have argued that clusters from genetic markers did not correspond well to the subjects' self-identified race/ethnic group. These studies, however, were based on only several dozen or fewer genetic markers, and such a number, unless carefully selected, are argued to not be sufficient. In contrast, studies based on more genetic markers have found high agreements.A study by Tang et al. in 2005 used 326 genetic markers in order to determine genetic clusters. The 3,636 subjects involved in the study, from the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
, self-identified as belonging to white, African American, East Asian, or Hispanic (=self-identified race/ethnic group (SIRE)). The study found "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%."
Paschou et al. (2010) found "essentially perfect" agreement between 51 self-reported populations of origin and the population genetic structure found using 650,000 genetic markers. Selecting for especially informative genetic makers allowed a reduction to less than 650 while still retaining close to 100% accuracy.
That there is correspondence between genetic clusters in a current population, such as the current US population, and self-identified race/ethnic groups does not necessarily mean that such a cluster/group corresponds to only one ancestral origin/population. African Americans have an estimated 10%–20% European admixture
Genetic admixture
Genetic admixture occurs when individuals from two or more previously separated populations begin interbreeding. Admixture results in the introduction of new genetic lineages into a population. It has been known to slow local adaptation by introducing foreign, unadapted genotypes...
. The Hispanic group have European, Native American, and African ancestries. In Brazil, there has been extensive admixture between Europeans, Amerindians, and Africans resulting in no clear discontinuities in skin color in the population and relatively weak associations between between self-reported race (called Color in Brazil probably because it captures the continuous aspects) and African ancestry as well as between objectively measured skin color and African ancestry.
Continuous or discontinuous increase in genetic distance
Ring species
Ring species
In biology, a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene...
show that also a continuous change in genetic variation can produce very large differences between different populations in a species.
Rosenberg et al. (2005) have argued, based on cluster analysis, that populations do not always vary continuously and that the population genetic structure is consistent if enough genetic markers and subjects are included. "Examination of the relationship between genetic and geographic distance supports a view in which the clusters arise not as an artifact of the sampling scheme, but from small discontinuous jumps in genetic distance for most population pairs on opposite sides of geographic barriers, in comparison with genetic distance for pairs on the same side. Thus, analysis of the 993-locus dataset corroborates our earlier results: if enough markers are used with a sufficiently large worldwide sample, individuals can be partitioned into genetic clusters that match major geographic subdivisions of the globe, with some individuals from intermediate geographic locations having mixed membership in the clusters that correspond to neighboring regions." They also wrote, regarding a model with five clusters corresponding to Africa, Eurasia (Europe, Middle East, and Central/South Asia), East Asia, Oceania, and the Americas, that "For population pairs from the same cluster, as geographic distance increases, genetic distance increases in a linear manner, consistent with a clinal population structure. However, for pairs from different clusters, genetic distance is generally larger than that between intracluster pairs that have the same geographic distance. For example, genetic distances for population pairs with one population in Eurasia and the other in East Asia are greater than those for pairs at equivalent geographic distance within Eurasia or within East Asia. Loosely speaking, it is these small discontinuous jumps in genetic distance—across ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s, the Himalayas, and the Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
—that provide the basis for the ability of STRUCTURE to identify clusters that correspond to geographic regions."
The above discussion applies to populations in their ancestral homes when migrations and gene flow
Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
were slow. Recent large and fast migrations due to changed technology have changed this. Thus, regarding the situation today in the United States, Tang et al. (2004) write that "we detected only modest genetic differentiation between different current geographic locales within each race/ethnicity group. Thus, ancient geographic ancestry, which is highly correlated with self-identified race/ethnicity—as opposed to current residence—is the major determinant of genetic structure in the U.S. population."
Number of clusters
Cluster analysis has been criticized for that number of clusters to search for are decided in advance with many different values possible although with varying probability. Principal components analysis does not decide the numbers of components to search for in advance. An increasing number of studies have used it in recent years.Utility
While knowing a persons race can be helpful in some situations in medicine, it has been argued that this is of limited value since also persons from the same race vary from one another.Witherspoon et al. (2007) have argued that even when individuals can be reliably assigned to specific population groups, it may still be possible for two randomly chosen individuals from different populations/clusters to be more similar to each other than to a randomly chosen member of their own cluster. They found that many thousands of genetic markers had to be used in order for the answer to the question "How often is a pair of individuals from one population genetically more dissimilar than two individuals chosen from two different populations?" to be "never". This assumed three population groups separated by large geographic ranges (European, African and East Asian). The entire world population is much more complex and studying an increasing number of groups would require an increasing number of markers for the same answer. Witherspoon et al. conclude that "caution should be used when using geographic or genetic ancestry to make inferences about individual phenotypes."
Race and physical characteristics
Human skin colorHuman skin color
Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...
vary for different populations. The leading explanation is that skin colour adapts to sunlight intensities which produce vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....
deficiency or ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light damage to folic acid
Folic acid
Folic acid and folate , as well as pteroyl-L-glutamic acid, pteroyl-L-glutamate, and pteroylmonoglutamic acid are forms of the water-soluble vitamin B9...
. Other hypotheses include protection from ambient temperature, infections, skin cancer
Skin cancer
Skin neoplasms are skin growths with differing causes and varying degrees of malignancy. The three most common malignant skin cancers are basal cell cancer, squamous cell cancer, and melanoma, each of which is named after the type of skin cell from which it arises...
or frostbite
Frostbite
Frostbite is the medical condition where localized damage is caused to skin and other tissues due to extreme cold. Frostbite is most likely to happen in body parts farthest from the heart and those with large exposed areas...
, an alteration in food, and sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
. The gene that causes light skin color in Europeans is different from the gene that causes light skin in East Asians. Europeans have a different version of the SLC24A5
SLC24A5
Sodium/potassium/calcium exchanger 5 also known as solute carrier family 24 member 5 is a protein that in humans is encoded by the SLC24A5 gene that has a major influence on natural skin colour variation. The NCKX5 protein is a member of the potassium-dependent sodium/calcium exchanger family...
than East Asians possibly indicating that they evolved light skin independently.
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, some argue that many of these traits are non-concordant in that they are not necessarily expressed together. For example, skin color and hair texture vary independently. Some examples of non-concordance include:
- Skin color varies all over the world in different populations.
- Epicanthal foldEpicanthal foldAn epicanthic fold, epicanthal fold, or epicanthus is a skin fold of the upper eyelid, covering the inner corner of the eye....
are typically associated with East Asian populations but are found in populations all over the world, including many Native AmericansIndigenous peoples of the AmericasThe indigenous peoples of the Americas are the pre-Columbian inhabitants of North and South America, their descendants and other ethnic groups who are identified with those peoples. Indigenous peoples are known in Canada as Aboriginal peoples, and in the United States as Native Americans...
, the KhoisanKhoisanKhoisan is a unifying name for two ethnic groups of Southern Africa, who share physical and putative linguistic characteristics distinct from the Bantu majority of the region. Culturally, the Khoisan are divided into the foraging San and the pastoral Khoi...
, the SamiSami peopleThe Sami people, also spelled Sámi, or Saami, are the arctic indigenous people inhabiting Sápmi, which today encompasses parts of far northern Sweden, Norway, Finland, the Kola Peninsula of Russia, and the border area between south and middle Sweden and Norway. The Sámi are Europe’s northernmost...
, and even amongst some isolated groups such as the AndamaneseAndamaneseThe Andamanese people are the various aboriginal inhabitants of the Andaman Islands, which is the northern district of the Andaman and Nicobar Islands union territory of India, located in the southeastern part of the Bay of Bengal. They include the Great Andamanese, Jarawa, Onge, Sentinelese, and...
. - Lighter hair colorHair colorHair color is the pigmentation of hair follicles due to two types of melanin: eumelanin and pheomelanin. Generally, if more melanin is present, the color of the hair is darker; if less melanin is present, the hair is lighter...
s are typically associated with Europeans, especially Northern Europeans, but blondBlondBlond or blonde or fair-hair is a hair color characterized by low levels of the dark pigment eumelanin. The resultant visible hue depends on various factors, but always has some sort of yellowish color...
hair is found amongst a limited, small number of the dark skinned populations of the south pacific, particularly the Solomon IslandsSolomon IslandsSolomon Islands is a sovereign state in Oceania, east of Papua New Guinea, consisting of nearly one thousand islands. It covers a land mass of . The capital, Honiara, is located on the island of Guadalcanal...
and VanuatuVanuatuVanuatu , officially the Republic of Vanuatu , is an island nation located in the South Pacific Ocean. The archipelago, which is of volcanic origin, is some east of northern Australia, northeast of New Caledonia, west of Fiji, and southeast of the Solomon Islands, near New Guinea.Vanuatu was...
.
Others argue that this is just an example of Lewontin's Fallacy
Lewontin's Fallacy
Human genetic diversity: Lewontin's fallacy is a 2003 paper by A. W. F. Edwards that refers to an argument first made by Richard Lewontin in his 1972 article The apportionment of human diversity, which argued that race for humans is not a valid taxonomic construct. Edwards' paper criticized and...
. On the contrary, if several traits are looked at the same time, then today forensic anthropologists
Forensic anthropology
Forensic anthropology is the application of the science of physical anthropology and human osteology in a legal setting, most often in criminal cases where the victim's remains are in the advanced stages of decomposition. A forensic anthropologist can assist in the identification of deceased...
can classify a person's race with an accuracy close to 100% based on only skeletal remains.
A 2010 examination of 18 widely used English anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
textbooks found that every one relied on the race concept. The study gives examples of how the textbooks claim that anatomical features vary between races.
Race and medicine
Neil RischNeil Risch
Neil Risch is an American human geneticist and professor at the University of California, San Francisco . Risch is the Lamond Family Foundation Distinguished Professor in Human Genetics and Director of the Institute for Human Genetics and Professor of Epidemiology and Biostatistics at UCSF.Known...
states that numerous studies over past decades have documented biological differences among the races with regard to susceptibility and natural history of chronic diseases. Genes may be under strong selection in response to local diseases. For example, people who are duffy negative
Duffy antigen
Duffy antigen/chemokine receptor also known as Fy glycoprotein or CD234 is a protein that in humans is encoded by the DARC gene....
tend to have higher resistance to malaria. Most Africans are duffy negative and most non-Africans are duffy positive. A number of genetic diseases more prevalent in malaria-afflicted areas may provide some genetic resistance to malaria
Genetic resistance to malaria
Genetic resistance to malaria occurs through both modifications of the immune system that enhance immunity to this infection and also by changes in human red blood cells that hinder the malaria parasite's ability to invade and replicate within these cells...
including sickle cell disease, thalassaemias, glucose-6-phosphate dehydrogenase
Glucose-6-phosphate dehydrogenase
Glucose-6-phosphate dehydrogenase is a cytosolic enzyme in the pentose phosphate pathway , a metabolic pathway that supplies reducing energy to cells by maintaining the level of the co-enzyme nicotinamide adenine dinucleotide phosphate...
, and possibly others. Cystic fibrosis
Cystic fibrosis
Cystic fibrosis is a recessive genetic disease affecting most critically the lungs, and also the pancreas, liver, and intestine...
is the most common life-limiting autosomal recessive disease among people of European heritage. Numerous hypotheses have suggested that it provides a heterozygote advantage
Heterozygote advantage
A heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. The specific case of heterozygote advantage is due to a single locus known as overdominance...
by giving resistance to diseases earlier common in Europe.
Information about a person's population of origin may in some situations help making a diagnosis
Diagnosis
Diagnosis is the identification of the nature and cause of anything. Diagnosis is used in many different disciplines with variations in the use of logics, analytics, and experience to determine the cause and effect relationships...
and adverse drug responses may vary between such groups. Because of the correlation between self-identified race and genetic clusters, medical treatments whose results are influenced by genetics often have varying rates of success between self-defined racial groups. For this reason, some doctors consider a patient’s race while attempting to identify the most effective possible treatment, and some drugs are marketed with race-specific instructions. Jorde and Wooding (2004) have argued that, because of the genetic variation within racial groups, when "it finally becomes feasible and available, individual genetic assessment of relevant genes will probably prove more useful than race in medical decision making." Even so, race will continue to be important when looking at groups instead of individuals such as in epidemiologic research.
Race and food tolerance
Lactose tolerance and alcohol toleranceAlcohol tolerance
Alcohol tolerance refers to the bodily responses to the functional effects of ethanol in alcoholic beverages. This includes direct tolerance, speed of recovery from insobriety and resistance to the development of alcoholism.-Consumption-induced tolerance:...
differ with geographic ancestry in part due to genetic factors. Lactose tolerance appears to be an evolutionarily recent adaptation to dairy
Dairy
A dairy is a business enterprise established for the harvesting of animal milk—mostly from cows or goats, but also from buffalo, sheep, horses or camels —for human consumption. A dairy is typically located on a dedicated dairy farm or section of a multi-purpose farm that is concerned...
consumption, and has occurred independently in both northern Europe and east Africa in populations with a historically pastoral
Pastoral
The adjective pastoral refers to the lifestyle of pastoralists, such as shepherds herding livestock around open areas of land according to seasons and the changing availability of water and pasturage. It also refers to a genre in literature, art or music that depicts such shepherd life in an...
lifestyle.
Race and sports
The overrepresentation of certain ethnicities with respect to certain sportSport
A Sport is all forms of physical activity which, through casual or organised participation, aim to use, maintain or improve physical fitness and provide entertainment to participants. Sport may be competitive, where a winner or winners can be identified by objective means, and may require a degree...
s has led some to question whether there is a genetic component giving certain races a competitive advantage. Others point out that such overrepresentations are not necessarily due to genetic causes. Such views differ between nations. Among Chinese, the proposition that there are genetic differences affecting sports performance is a widely accepted. A 1994 examination of 32 English sport/exercise science textbooks found that 7 (21.9%) claimed that there are biophysical differences due to race that might explain differences in sports performance, 24 (75%) did not mention nor refute the concept, and 1 (3.12%) expressed caution with the idea.
Race and intelligence
There is an ongoing scientific controversy regarding the role of genetics in explaining racial differences in IQ and other measures of intelligenceIntelligence
Intelligence has been defined in different ways, including the abilities for abstract thought, understanding, communication, reasoning, learning, planning, emotional intelligence and problem solving....
. Some are agnostic about the causes, while others argue that environmental factors explain all of the differences, or that both genetics and environmental factors are important.
See also
- Y-chromosome haplogroups by populationsY-chromosome haplogroups by populationsThe following articles are lists of human Y-chromosome DNA haplogroups found in populations and various ethnic groups by regions or continents around the world based on relevant studies, and the samples have been taken from individuals identified by linguistic designation.General*Y-DNA haplogroups...
Regional: Archaeogenetics
Archaeogenetics
Archaeogenetics, a term coined by Colin Renfrew, refers to the application of the techniques of molecular population genetics to the study of the human past. This can involve:*the analysis of DNA recovered from archaeological remains, i.e...
- Archaeogenetics of the Near EastArchaeogenetics of the Near EastThe archaeogenetics of the Near East involves the study of aDNA or ancient DNA, identifying haplogroups and haplotypes of ancient skeletal remains from both YDNA and mtDNA for populations of the Ancient Near East The archaeogenetics of the Near East involves the study of aDNA or ancient DNA,...
- Genetics and archaeogenetics of South AsiaGenetics and archaeogenetics of South AsiaThe study of the genetics and archaeogenetics of the ethnic groups of South Asia aims at uncovering these groups' genetic history. The geographic position of India makes Indian populations important for the study of the early dispersal of all human populations on the Eurasian continent.The Indian...
- Genetic history of Africa
- Genetic history of EuropeGenetic history of EuropeThe genetic history of Europe can be inferred from the patterns of genetic diversity across continents and time. The primary data to develop historical scenarios coming from sequences of mitochondrial, Y-chromosome and autosomal DNA from modern populations and if available from ancient DNA...
- Genetic history of ItalyGenetic history of ItalyDuring prehistory Italy was populated by different but very similar Indo-European groups, later collectively listed amongst the Ancient peoples of Italy, of whom the Italic one was predominant....
- Genetic history of North AfricaGenetic history of North AfricaThe population genetics of North Africans has been heavily influenced by geography.The Sahara desert to the south and the Mediterranean Sea to the North were important barriers to gene flow in prehistoric times. However Eurasia and Africa form a single land mass at the Suez. At the Straits of...
- Genetic history of indigenous peoples of the Americas
- Genetic history of the Iberian PeninsulaGenetic history of the Iberian PeninsulaThe ancestry of modern Iberians is consistent with the geographical situation of the Iberian Peninsula in the south-west of Europe: there are strong connections with both Mediterranean Europe and Atlantic Europe....
- Genetic history of the British IslesGenetic history of the British IslesThe genetic history of the British Isles is the subject of research within the larger field of human population genetics. It has developed in parallel with DNA testing technologies capable of identifying genetic similarities and differences between populations...
Further reading
|title=A Family Tree in Every Gene |work=The New York TimesThe New York Times
The New York Times is an American daily newspaper founded and continuously published in New York City since 1851. The New York Times has won 106 Pulitzer Prizes, the most of any news organization...
|url=http://www.nytimes.com/2005/03/14/opinion/14leroi.html |accessdate=}}
External links
- The Race FAQ by John Goodrum
- Minorities, Race, and Genomics via Human Genome ProjectHuman Genome ProjectThe Human Genome Project is an international scientific research project with a primary goal of determining the sequence of chemical base pairs which make up DNA, and of identifying and mapping the approximately 20,000–25,000 genes of the human genome from both a physical and functional...
information. - Race and human evolution by Alan TempletonAlan TempletonAlan R. Templeton is an American geneticist and statistician from Washington University in St. Louis, where he holds the Charles Rebstock professorship in biology. He is known for his work demonstrating the lack of genetic differences between humans of different races...