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Good afternoon ladies and gentlemen. Today professor Hemmings will explain how
to estimate the African slave ancestry of White Americans, and the European colonial
ancestry of Black Americans. Professor Hemmings: That is correct Karen.
The New World has been a mixing bowl since Columbus's time, and today we shall see how
to measure the extent of the genetic mixing in the United States.
Karen: I am looking forward to it. The new world was colonized during the ice
ages. When the ice melted 16 thousand years ago,
sea level rose, genetically isolating the New World from the old.
Then, for thousands of years, diseases swept the Old World, forcing its inhabitants to
develop immunities. Native Americans never developed similar immunities.
Within two generations of Columbus's landing, 95 percent of the Native American population
was wiped out by Old World diseases to which Europeans, Asians, and Africans were immune.
Some people refer to this misfortune as genocide. But it was not intentional.
The new-comers had no idea that their mere presence would kill so many innocents.
Over the next few centuries, the decimated population of the New World was replenished
by millions of people from the Old, carried across the Atlantic in sailing ships.
Eleven million came from Africa. Most of these were taken to the New World
against their will as slaves, although some became indentured.
Three million people came from Europe. Most came as indentured laborers, although
some became slaves. Incidentally, the difference between indenture
and slavery was that indentured forced labor was for seven years and not hereditary,
while slavery was life-long and hereditary. To historians of the New World, of earth's
many human populations, three large groups are of special interest:
First, those populations who were already in the New World when Europeans and Africans
arrived. Second, those who came from Europe.
And third, those who came from Africa. Autosomal DNA markers measure the average
continent-of-ancestry admixture in your genome. The technique was devised for New World inhabitants,
to measure what fraction of their ancestry came from each of the three demographic sources:
Africa, Europe, and Native America. The two scientists who pioneered autosomal
DNA mapping are Mark Shriver of Penn State University, on left.
And Rick Kittles, now at the University of Chicago, on right.
Dr. Kittles was with Howard University at the time of the original research.
This is one of the scatter diagrams that they made after measuring admixture in several
hundred subjects. Each dot represents an individual.
Each is plotted left-to-right depending on the ratio of European versus African admixture.
People of 100 percent European DNA are on the left edge, those of 100 percent African
DNA are on the right, and everyone else is proportionally in between.
This particular chart was meant to see if admixture matched skin tone, so each dot was
plotted higher or lower, representing darker or paler skin.
The trend is upwards and to the right. This shows that, although you can see hundreds
of exceptions on the chart, in general the more African admixture you have, the darker
your skin tone. Of interest to the historian is that each
subject was also asked his or her racial self-identity. Those who considered themselves White were
plotted with tiny diamond shapes. Self-identified African Americans were plotted
as tiny circles. With that in mind, you can see that self-identified
White Americans range from zero to 25 percent African admixture inherited from slave ancestors.
On the other hand, African Americans span the entire range.
And there is a lot of overlap in the range from 15 to 25 percent African admixture.
This graph is from a different study. It also matches the ancestral African versus
European DNA admixture of several hundred individuals with their ethnic self-identity.
Dots are plotted with subjects of 100 percent European admixture at the top, and individuals
of 100 percent African admixture at the bottom. They are plotted in four groups.
From left to right, the groups are: 147 White Americans, 264 African Americans, 135 inhabitants
of Zaire (formerly Congo), and 159 inhabitants of Nigeria.
As in the prior chart, there is much overlap between White and Black Americans in the range
of zero to 30 percent African admixture. Some White Americans have over twenty percent
African genetic admixture, and some Black Americans have little or none.
Here are two important things to notice about the two leftmost clusters.
First, the Black and White groups are not symmetrical.
The mean admixture among White Americans is about 0.7 percent African and 99.3 percent
European. Only about one-third of White Americans have
detectable African admixture (averaging about 2.1 percent),
but this varies regionally, from zero in the north, to 5 percent in the Cumberland plateau.
The second important point is that virtually all Black Americans have some European admixture.
They average about 83 percent African and 17 percent European admixture.
Again, this varies regionally, from 25 percent European in Philadelphia, to zero among the
Geechee Gullah people of the Sea Islands of South Carolina and Georgia.
Interestingly, the United States is the only nation in the western hemisphere with two
clusters: two genetically distinct populations. Every other country in the hemisphere has
a single scattering of dots centered on the admixture ratio that matches its colonial
populations of Europeans and Africans: Argentina's cluster would be at the top of
the chart, Haiti's at the bottom, and Puerto Rico's in the middle.
There is sometimes a mismatch between a person's autosomal DNA markers and his or her external
appearance. Only about a dozen genes determine visible
traits, such as skin tone, hair texture, features, compared to hundreds of markers that track
ancestral colonial source populations. Since heredity at each generation is random
between parents, it is not unknown for people to inherit European appearance while carrying
African markers or vice-versa. For example, the man on the left has about
eleven percent African DNA admixture, and the man on the right has nearly 30 percent
European. Why is there such a discrepancy between measured
genetic admixture and physical appearance? There is an immediate answer to this question,
and a deeper answer. The immediate answer is that many different
invisible genes identify continent of ancestry. As of the summer of 2010, the HapMap project
has identified hundreds of single nucleotide polymorphisms (markers) that reflect a person's
ancestral continents of origin. On the other hand only a handful of genes
encode for the few superficial, externally visible features, such as skin color, hair
curliness, etc., that Americans see as "racially" significant.
Parental genes are randomly recombined with each passing generation.
And so, it can happen through sheer chance, that an individual like Dr. Shriver, can inherit
many invisible African DNA markers, but few or none of the handful of gene variants that
encode for African "racial" appearance. Alternatively, a person like Dr. Kittles,
can inherit those few gene variants that encode for visible African appearance but otherwise
inherit many invisible but ancestrally informative European markers.
The deeper answer becomes evident if you plot each of the two U.S. endogamous groups from
the skin tone diagram separately. This graph shows just one portion of the population
of the earlier chart--those who self-identified as African Americans.
You can see a positive correlation, R squared of 0.211, between skin tone and African admixture.
On average, if you are considered an African American, the more African admixture you have,
the higher your melanin index. There are many exceptions of course, like
Dr. Kittles for instance, but on average this is the case.
What is more important is that the range of skin reflectance as measured by the melanin
index of African Americans is very large, spanning a vertical distance of fifty points
from 30 to 80. African Americans span a huge range of skin
tones. This graph shows the other U.S. endogamous
group from the earlier chart--those who self-identify as White.
In this diagram, you can see that there is no correlation either way (R squared = 0.001),
between skin tone and African admixture. Clearly, if you are considered a White American,
your melanin index is unrelated to your degree of African admixture.
More importantly, the range of skin reflectance as measured by the melanin index of White
Americans is very narrow. It spans a vertical distance of only twenty
points from 20 to 40. This is less than half of the range of skin-tone
variation found among Black Americans. The combination of narrow phenotype variation
(skin tone), along with a wide range of genotype variation (Afro-European admixture), among
White Americans, shows that a selection process has taken place.
Few human populations display such a clear mark of selection.
Narrow phenotype variation alone does not necessarily indicate selection.
Northern Europeans display little skin tone variation, but they lack a wide range of African
admixture. Broad genotype variation alone does not necessarily
indicate selection. Dominicans average 50 50 Afro-European admixture,
but they also display a wide range of skin tones.
The late Stephen J. Gould, Harvard biology professor and columnist for Natural History
magazine, explained this principle with a baseball analogy.
Plot a scatter diagram of the batting averages of a thousand amateur or minor-league players
and you will find a very large range of variation. A few such players are bad, a few are good,
but most spread across the entire range in-between. Now plot the batting averages of professional
athletes in the major leagues. All are very good indeed.
More importantly, the range of batting averages among them is tiny.
The difference between an outstanding star of the game and a rookie is a matter of mere
hundredths of a percentage point. The reason, of course, is because you cannot
get into the majors unless you are very good at it.
Similarly, wild cows vary greatly in the amount of milk that they produce.
Cows in a dairy farm produce more milk on average but also their milk production varies
very little among themselves. Those cows who do not make the cut become
hamburger. And so, why do few if any White Americans
display a strongly African appearance (have a high melanin index) despite having detectable
African admixture? Because those Americans who "look Black" are
assigned involuntarily to the Black endogamous group, whatever their genetic admixture.
The scatter diagrams are not symmetrical because the selection process acts only upon the White
group. As revealed in court records, discussed in
other sessions, a person of mixed ancestry who "looks European" (like Dr. Shriver) in
practice has the option of either adopting a White self-identity or a Black self-identity.
But a person of mixed ancestry who "looks African" lacks such a choice.
U.S. society assigns such a person to membership in the Black endogamous group, like it or
not. U.S. society has unwittingly applied selection
pressure to the color line. The only American families accepted into the
White endogamous group have been those whose African admixture just happened not to include
the half-dozen gene variants for dark skin (or the other physical traits associated with
"race"). Since those particular gene variants were
sifted out of the portion of the White population that originated in biracial families,
the relative percentage of the remaining, invisible, African gene variants in this population
cannot affect skin tone. In conclusion. The very fact that skin tone
does not vary with African genetic admixture among White Americans, despite their recent
African admixture, confirms that physical appearance has been
an important endogamous group membership criterion throughout U.S. history.
It has resulted in genetic selection of the White U.S. population for a European "racial"
appearance, regardless of their underlying continent-of-ancestry admixture ratio.
And, with that, I conclude today's presentation. Thank you for listening.
Karen: Thank you, professor Hemmings. Well, that is our time for today, ladies and
gentlemen. This is Karen Sharpe
Professor Hemmings: And Randolph Hemmings Karen: Signing off until next time.