5 THE GENETICS OF RACERead more at location 1476
Note: le differenze genetiche extracontinentali sono + accentuate rispetto a quelle continentali. frequenze + gruppi = cluster clusterizzando il genoma guardacaso escono i 5 continenti. clusterizzando oltre escono le nazioni le differenze genetiche tra razze si realizzano come differenze frequenziali negli alleli la genetica si correla ai linguaggi la genetica nn è lenta come pensavamo: i tibetani in 3000 anni si sono adattati malattie e razze polemiche intellettuali e argomenti contro la razza diamond e lewotin diamond: esistono diversi criteri per individuare potenziali razze. perchè nn considerare una razza composta da italuani greci e colombiani? si tratta di popolazioni resistenti alla malaria. ma diamond trascura il concetto di cluster nessuno assegna una razza ad un singolo tratto lewotin fa notare che l appartenenza ad una razza spiega una % bassa del differenziale nei genomi. manca xrò di far notare che il differenziale spiegato è sempre lo sresso e ciò rende la spiega altamente informativa. senza dire che, nel caso di altri animali, le % di cui parliamo vengono ritenute + che sufficienti ad introdurre subspecie Edit
One might expect that different races would have different genes, but they don’t. All humans, so far as is known, have the same set of genes.Read more at location 1484
the next expectation might be that races would be distinguished by having different alleles of various genes. But this too is not how the system works.Read more at location 1485
The genetic differences between human races turn out to be based largely in allele frequencies, meaning the percentages of each allele that occur in a given race.Read more at location 1487
everyone ends up in the cluster with which they share the most variation in common. These clusters always correspond to the five continental races in the first instance,Read more at location 1491
CA stands for the DNA unit known as a cytosine followed by adenine, so the DNA sequence CACACACA would be called a tandem CA repeat.Read more at location 1495
In other words, all the Africans had patterns of CA repeats that resembled one another, all the American Indians had a different pattern of repeats and so on. Altogether there were 5 principal clusters of CA repeats, formed by people living in each of the 5 continental regions of Africa, Europe, East Asia, the Americas and Australasia.2Read more at location 1502
“genetic differentiation is greatest when defined on a continental basis,” writes Neil Risch, a statistical geneticist at the University of California, San Francisco.Read more at location 1506
it’s possible to assign segments of an individual’s genome to different races if he or she has mixed ancestry. This is because each race or ethnicity has a characteristic number of repeats at each genomic site.Read more at location 1512
Within races, the Rosenberg-Feldman study showed that different ethnicities could be recognized.Read more at location 1526
Among Africans, it is easy to distinguish by their genomes the Yoruba of Nigeria, the San (a click-speaking people of southern Africa) and the Mbuti and Biaka pygmies.Read more at location 1527
A genetic gradient, or cline, is what some researchers prefer to think exists in place of races. “There are no races, there are only clines,” asserted the biological anthropologist Frank Livingstone.Read more at location 1535
Rosenberg and Feldman compared people’s genomes on the basis of DNA repeats. Another kind of DNA marker has since become available for global population comparison—the SNP, which is more useful for medical studies.Read more at location 1542
SNPs are arbitrarily defined as sites on the genome where at least 1% of the population has a DNA unit other than the standard one.Read more at location 1548
One of the new clusters is formed by the people of Central and South Asia, including India and Pakistan. The second is the Middle East, where there is considerable admixture between people from Europe and Africa.10 It might be reasonable to elevate the Indian and Middle Eastern groups to the level of major races, making seven in all.Read more at location 1554
Within Europe it distinguished French, Italians, Russians, Sardinians and Orcadians (people who live in the Orkney Islands, north of Scotland).Read more at location 1563
Genetics generally correlates with language family, except in the case of populations that have switched languages; the pygmies now speak Niger-Kordofanian languages, and the Luo of Kenya, whose genetics place them with Niger-Kordofanian speakers, now speak a Nilo-Saharan language.Read more at location 1583
On the basis of the new African and other genomic data, the origin of the modern human migration lies in southwestern Africa, near the border of Namibia and Angola,Read more at location 1591
Both repeated DNA units and SNPs, the two kinds of DNA marker used by the surveys described above, lie for the most part outside genes and have little or no effect on a person’s physical makeup.Read more at location 1595
Natural selection is the major shaper of differences, especially in large societies.Read more at location 1598
The evidence of natural selection at work on a gene is that the percentage of the population that carries the favored allele of the gene has increased.Read more at location 1609
Other researchers too have found that in doing genome scans for the fingerprints of natural selection, each major race or continental population has its own distinctive set of sites where selection has occurred.Read more at location 1624
The genes Pritchard identified as shaped by natural selection included genes for fertilization and reproduction, genes for skin color, genes for skeletal development and genes for brain function.Read more at location 1641
brain genes do not lie in some special category exempt from natural selection. They are as much under evolutionary pressure as any other category of gene.Read more at location 1644
Biologists have long had to depend on the evidence from fossils to judge the speed of evolution. But fossils capture just the bones of an animal. And since the skeletal structure of a species changes only slowly, evolution has long seemed a glacially slow and plodding process.Read more at location 1662
It’s now clear that evolution is no sluggard. There are already clear examples of human evolutionary change within the past few thousand years, such as the continued evolution of European skin, hair and eye color within the past 5,000 years.Read more at location 1665
The type of selection picked up by most genome scans is very recent selection, meaning within the past 5,000 to 30,000 years or so,Read more at location 1669
722 regions, containing some 2,465 genes, have been under recent pressure of natural selection, according to an estimate by Joshua M. Akey of the University of Washington. This amounts to at least 8% of the genome.15Read more at location 1673
some 80% of the 722 regions under selection are instances of local adaptation,Read more at location 1677
Changes in social behavior may well have been foremost, given that it is largely through their society that people interact with their environment.Read more at location 1689
“A plausible explanation is that humans experienced many novel selective pressures as they spread out of Africa into new habitats and cooler climates,”Read more at location 1695
populations began to grow after the beginning of agriculture some 10,000 years ago. Independently on all three continents, people’s social behaviors started to adapt to the requirements of living in settled societies that were larger and more complex than those of the hunter-gatherer band.Read more at location 1702
But given that all humans have the same set of genes and that there have been almost no full sweeps that push different alleles to dominance in different races, how have races come to differ from one another? The answer that has dawned on geneticists in the past few years is that you don’t always need a full sweep to change a trait.Read more at location 1727
Many traits, like skin color or height or intelligence, are controlled by a large number of different genes, each of which has alleles that individually make small contributions to the trait.Read more at location 1729
Of the traits specific to one race or another, a few are encoded in hard sweep alleles that have gone almost to fixation, such as the Duffy null allele or some of the alleles involved in shaping skin color, but many more are probably encoded in soft sweeps and hence in mere differences in the frequencyRead more at location 1765
genes often work in combination to specify a trait explains how there can be so much variation in the human populationRead more at location 1768
Given the importance of allele frequencies in shaping specific traits, it’s not surprising that they afford a means of identifying an individual’s race.Read more at location 1769
Excluding subjects of a different race is an essential procedure in surveys to detect the alleles that contribute to complex diseases like diabetes and cancer.Read more at location 1770
If so, the allele may be associated with the disease. But the statistics can be confounded if the population being surveyed includes people of more than one race.Read more at location 1772
Medical geneticists have therefore developed sets of test alleles that can be used to distinguish one race from another.Read more at location 1775
Some biologists insist that AIMs do not prove the existence of race and that they point instead to geographic origin. But geographic origin correlates very well with race, at least on the continental level.Read more at location 1784
A single AIM that occurs in 45% of East Asians and 65% of Europeans says that the carrier is a little more likely to be European, but is hardly definitive. When the results from a string of AIMs are combined, however, an answer with high statistical probability is obtained.Read more at location 1787
His principal argument for the nonexistence of race is that there are many different “equally valid procedures” for defining human races, but since all are incompatible, all are equally absurd. One such procedure, Diamond proposes, would be to put Italians, Greeks and Nigerians in one race, and Swedes and Xhosas (a southern African tribe) in another. His rationale is that members of the first group carry genes that confer resistance to malaria and those of the second do not. This is just as good a criterion as skin color, the usual way of classifying races,Read more at location 1815
The first flaw in the argument is the implied premise that people are conventionally assigned to races by the single criterion of skin color. In fact, skin color varies widely within continents.Read more at location 1820
Skin color is thus an ambiguous marker of race. People belong to a race not by virtue of any single trait but by a cluster of criteria that includes the color of skin and hair, and the shape of eyes, nose and skull.Read more at location 1822
The trait of resisting malaria is one that has been acquired secondarily to race, so obviously it is not an appropriate way of classifying the populations. A scholar’s duty is to clarify, but Diamond’s argument seems designed to distract and confuse.Read more at location 1828
Lewontin measured a property of 17 proteins from people of various different races and calculated a measure of variation known as Wright’s fixation index. The index is designed to measure how much of the variation in a population resides in the population as a whole and how much is due to differences between specific subpopulations.Read more at location 1831
Lewontin’s answer came out to 6.3%, meaning that of all the variations in the 17 kinds of protein he had looked at, only 6.3% lay between races, while a further 8.3% lay between ethnic groups within races.Read more at location 1834
“Of all human variation, 85% is between individual people within a nation or tribe,” Lewontin stated.Read more at location 1836
He went on to say that “Human racial classification is of no social value and is positively destructive of social and human relations.Read more at location 1838
Lewontin’s thesis immediately became the central genetic plank of those who believe that denying the existence of race is an effective way to combat racism. It is prominently cited in Man’s Most Dangerous Myth: The Fallacy of Race, an influential book written by the anthropologist Ashley MontaguRead more at location 1841
But despite all the weight that continues to be placed on it, Lewontin’s statement is incorrect. It’s not the basic finding that is wrong. Many other studies have confirmed that roughly 85% of human variation is among individualsRead more at location 1846
This is just what would be expected, given that each race has inherited its genetic patrimony from the same ancestral population that existed in the comparatively recent past.Read more at location 1847
What is in error is Lewontin’s assertion that the amount of variation between populations is so small as to be negligible. In fact it’s quite significant.Read more at location 1849
If differences of 10 to 15% were seen in any other than the human species they would be called subspecies,Read more at location 1852
Why should Wright’s judgment that a fixation index of 15% between races is significant be preferred over Lewontin’s assertion that it is negligible? Three reasons: (1) Wright was one of the three founders of population genetics, the relevant discipline; (2) Wright invented the fixation index, which is named after him; (3) Wright, unlike Lewontin, had no political stake in the issue.Read more at location 1854
Lewontin’s fallacy.30 The fallacy is to assume that the genetic differences between populations are uncorrelated with one another; if they are correlated, they become much more significant. As the geneticist A.W.F. Edwards wrote, “Most of the information that distinguishes populations is hidden in the correlation structure of the data.”Read more at location 1857
The 15% genetic difference between races, in other words, is not random noiseRead more at location 1860
True, races are not discrete entities and have no absolute boundaries, as already discussed, but that doesn’t mean they don’t exist.Read more at location 1870
The classification of humans into five continental based races is perfectly reasonable and is supported by genome clustering studies.Read more at location 1871
races are identified by clusters of traits, and to belong to a certain race, it’s not necessary to possess all of the identifying traits.Read more at location 1877
most East Asians have the sinodont form of dentition, but not all do. Most have the EDAR-V370A allele of the EDAR gene, but not all do. Most have the dry earwax allele of the ABCC11 gene, but not all do. Nonetheless, East Asian is a perfectly valid racial category, and most people in East Asia can be assigned to it.Read more at location 1878
Even when it is not immediately obvious what race a person belongs to from bodily appearance, as may often be the case with people of mixed-race ancestry, race can nonetheless be distinguished at the genomic level.Read more at location 1880
At least at the level of continental populations, races can be distinguished genetically, and this is sufficient to establish that they exist.Read more at location 1884
