Anthropology 150
rfz94
lecture_9_text.pdf
Biological Variation in Modern Human Populations
After the extinction of the Neandertals, there was only a single type of hominid left on the
planet: Homo sapiens sapiens. After 25,000 years ago humans spread throughout the globe and
adapted, both culturally and biologically to their environments. As in earlier cases of adaptive
radiation, these differing adaptations brought resulted in biological diversity.
Unlike those earlier radiations, however, post-Neandertal diversification has not resulted
in speciation. Modern human diversity is not only very recent, it is quite small. Humans show
less biological variation than any other mammalian species, in part because the roots of diversity
are so shallow, and in part because no human population has been truly isolated from others for a
very long time. Even after post-Neandertal radiation took place, there was significant gene flow
between most populations.
No one can deny that there are physical difference among people from different parts of
the world. But let’s look at two questions: 1) What are the causes of those differences and 2)
How significant are they?
Causes of Modern Human Biological Diversity
The causes of modern diversity are the same as those we’ve analyzed in the past:
Adaptive Radiation and Natural Selection, combined with drift and things like the founder effect.
The founder effect refers to the fact that when a population splits and radiation takes place, the
genotype of the population isn’t distributed randomly. Family members, for instance, might be
expected to stick together when a group divides. If the new sub-population is comprised of
related people it is going to show less diversity than the original group did.
Many aspects of biological diversity can, however, be attributed to natural selection and
adaptation to the environment:
Environmental Factors in Natural Selection
A) body morphology: Remember the Neandertals. Bergman’s rule and Allen’s rule apply
to other groups as well. A great deal of diversity in physical appearance stem from these
principles. People from tropical regions tend to be taller and more slender than those from
temperate zones. Conversely, people from colder climates tend to 1) have greater overall body
mass and 2) have shorter extremities. Hence, they have the stocky build seen in Neandertals: see
the textbook section on the Inuit.
B) Selection for skin color: melanin: pale skin enables synthesis of Vitamin D from
sunlight, preventing rickets. There are clear selective pressure favoring pale skin in the upper
latitudes and dark skin tropical climes. See the presentation.
C) Nose size and shape: governed by Allen’s Rule
D) Head shape: ditto
Adaptation to Human-induced changes:
A) The sickle-cell gene:
The sickle-cell gene is the prime example of a selective response to environmental
change. In this case, however, the environmental change resulted from human activity.
When agricultural practices were adopted in west Africa, large tracts of forest were
cleared. Forest clearance resulted in soil erosion, which caused partial blockages of river
drainages. The blockages created swamps and marshes that were nearly ideal breeding grounds
for the anopheles mosquito, which carries the malarial parasite.
The sickle-cell trait is the result of a mutation that cases hemoglobin cells to assume an
abnormal shape. The mutation spread because in the heterozygous state, the trait confers
resistance to malaria. The allele for the sickle-cell trait is recessive, meaning it only causes
anemia when both alleles for the sickle-cell trait are present. Statistically, this means that 50% of
the people who have have two parents with the trait will be heterozygous, and actually benefit
from it, while 25% contract sickle-cell anemia, and 25% will not receive the allele. (If ‘A’ is the
non-sickle allele and ‘a’ is the sickle allele, the possible outcomes are AA, Aa, aA, and aa. The
states Aa and aA are heterozygous and do not express the trait. Only the “aa” state will sickle-cell
anemia be present).
From the viewpoint of the entire population, this is a good bet. 50% of the population
benefits from the sickle-cell allele, while 25% suffer from anemia. The remaining 25% suffer
higher risks of malaria and natural selection will mean that higher percentage of people without
the sickle-cell trait will die.
B) Adult lactose tolerance: ability to produce the enzyme lactase.
All mammals feed on mother’s milk in infancy. During infancy mammals synthesize the
enzyme lactase in order to digest the mother’s milk. Normally, lactase synthesis ceases after
weaning, so that adults do not produce lactase and hence are unable to digest milk and milk
products.
Prior to 10,000 years ago very few humans produced lactase in adulthood. That is, the
normal human adult condition was what we would today call “lactose intolerance.” After 10,000
years ago, however, many human groups domesticated animals. Still, not all groups did so, and
of those who did, only a few consumed the animal’s milk. It’s much more common to drink the
animal’s blood than to drink its milk.
In some circumstances, however, drinking an animal’s milk might make a huge
difference. In extremely cold climates, for example, there are few food resources available in
winter. If the winter is long and hard, you might run short of stored supplies. If you have the herd
animals, you might be tempted to eat their meat, but then you’d be in very bad trouble the next
year. In those circumstances, being able to drink the animal’s milk without becoming ill is a
huge benefit. It could make a difference between surviving and not surviving. Hence, there would
be strong selective pressures in favor of those adults able to synthesize lactase.
Lactose tolerance is actually very complicated. There is a genetic component, but there is
also some developmental plasticity, meaning that there is a non-genetic component as well.
Non-adaptive differences due to isolation of breeding populations
These factors include drift, founder effect, and other factors that are essentially random.
Fingerprints are a good example. Fingerprint patterns show strong regional trends in terms of the
classificatory schemes used by Interpol. No one would say, however that a particular pattern of
fingerprint yields a selective advantage.
Describing Human Variation
Now that we have some understanding of the causes of modern human diversity, the
question is how to describe it. Again, we must begin with the understanding that modern human
populations are polymorphic but not polytypic. The range of variation is small in the grand
scheme of an evolutionary perspective.
To say that human variation is not polytypic means that we cannot speak of distinct
“types” of humans. Certainly we can talk about populations, groups that breed more within
themselves than with individuals outside the group. But no human group has been completely
isolated from other human groups for more than a few millenia. For the most part, there has been
gene flow between populations. That means there are very few traits that can be analyzed on the
basis of presence or absence. Rather, what differs is allele frequencies across populations. In
short, there may be statistical differences between populations in the frequency of certain alleles,
but there are no huge qualitative differences between populations.
Clinal Variation
While there are clear explanations for why people in differing regions developed many
physical traits, those traits do not show rigid boundaries. To repeat, human variation is
polymorphic, not polytypic. There are not clear cleavages of types of people. Rather, there is
variation in allele frequencies across space.
The type of variation seen in human populations is called “clinal variation.” Often the
geographic variation observed within a species is gradual and continuous (for example biomass
and linear body measures). That type of gradual geographic variation in phenotypes is termed
clinal variation. A cline is largely synonymous with “population,” meaning that it refers to a
group that breeds within itself more than it breeds with outsiders (again, this a statistical
difference; there is nothing biological that prohibits members of a cline from mating with
outsiders). The causes for such variation can be either environmental differences, genotypic
differences, or both, as described above.
How not to describe human variation
Again, the approach that we should not use in describing modern human variation is one
in which we construct a typology of humanity. Racial classification is such typology. The
concept of race implies that large groups of people have existed in splendid isolation from other
large groups of people and that there has been no gene flow between them. That is not the case in
humans.
Race is a socially constructed category in search of a biological justification. I do not
mean to say that there are no biological variations in people across the world. Clearly there are.
What I do mean to say, though, is that racial classifications are not the way to deal with them.
What is a race ? How many are there?
A. Different cultures have different ways of carving up the human species. No one who
wants to define types of people can tell you how many types there are. Are there 3, 4, 20? Here
are a few examples of how different cultures have defined “races”.
Examples: Colonial Spanish America: Racial categories include Spanish, creole,
mulatto, indio, negro. Keep counting: Colonial Latin America had to up 16 distinct categories of
people, depending upon where your parents came from and where you were born.
19 century North Americans considered southern Europeans and eastern Europeans to beth
different races from themselves. Italians were defined as a racial group distinct from the English.
Of course, the Irish were also a distinct race in this view. So, you had up to 10 or 12 distinct
races just within Europe.
In North America today, we typically divide the world up into 4-5 categories of the type
you see on forms and polls all the time: Caucasian, Black (African American), Asian, Hispanic,
Native America, Pacific Islander. For the most part, you pick one group and join it for life.
In contrast, people in Brazil recognize up to 40 distinct racial categories that are “blends”
of European, African, and indigenous groups. In Brazil people are freer to decide which group
they choose to belong to.
According to the Hindu caste system, where you have 5 ranked groups, corresponding to
levels of ritual purity defined by Hindu scriptures. These 5 groups are also racial groups, they
practice what we call endogamy: they mate only within themselves, so that there are physical
differences between them. The origin of these groups may be traceable to the conquest of Indian
by outside groups in the middle of the first millenium B.C. Simply within India, then, you have 5
distinct biological populations. Are they races?
In Latin America today, it is popular to speak of La Raza: A new race created by the
interbreeding of Europeans and indigenous peoples.
If were look around the world today, everyone classifies people, but each culture does it
in a different way. What all of these racial categories do is to divide biological variation, using a
just a few traits, to construct social groups. Why do we do that?
We do it because it serves some social purpose. India is the classic example: ranked
social groups form classes defined by their relationship to the means of production. So you have
class differences mapped onto biological differences. That is what racial classification is all
about. It is a lot much more about class and culture than it is about biology.
Race and Racism
It would be foolish to deny that there are biological differences among people in various
parts of the world. We’ve just discussed that. Obviously there are genotypic and phenotypic
differences that correlate with spatial differences. Obviously you can distinguish someone of
northern European descent from someone of tropical African descent, from someone of East
Asian descent.
These differences arise from different gene frequencies within populations. Now if those
populations were neatly bounded and there was no interaction between them things might be
fairly straightforward. But it’s never that simple.
Let’s back up for a minute and consider the degree of diversity within modern
populations.
Who knows the definition of a species? A species is sometimes defined by the ability to
mate and produce fertile offspring.
By any biological definition, there is only one human species. We are all members of the
same species and sub-species Homo sapiens sapiens. Take a human male and a human female
from anywhere on the planet and they can produce a fertile offspring. So we are all capable of
sharing genetic material with people from the most distant parts of the earth. Let me say it one
more time: The human species is polymorphic, meaning that has differing phenotypic
expressions. The human species is not polytypic: there is only a single species and sub-species.
So the amount of genetic variation that separates all of us is far less than that which
separated homo habilis from the australopithecines. It’s even less than that which separated the
Neandertals from anatomically modern humans.
Now if you’re like me and you favor the out of Africa hypothesis, you have the position
that all modern genetic variation has developed in the last 150-200 K years. In evolutionary
terms, that’s not much. So our starting place in talking about race is that the biological
differences that separate all of us are fairly trivial.
Populations that are capable of sharing genetic material do so. So unless there is
complete isolation of one group from another, they are going to share genes. It’s just a question
of degree. For most genetic traits, the differences are of degree rather than kind. That is, most
differences between populations which are geographically defined are in the frequency of a trait,
rather than presence/absence.
So the task of analyzing genotypic variation within modern homo sapiens becomes quite
complex and requires some very sophisticated statistical methods. When you analyze genotypic
variation in that manner, the concept of race begins to break down.
An excellent, and still relevant, study of this type was done in 1972 by R.D.. Lewontin, a
geneticist at Harvard. Lewontin measure the degree of population differences in gene
frequencies for 17 polymorphic traits.
One of the first problems that Lewontin faced was how to define his groups. For a
number of reasons, Lewontin ended up defining 7 groups that in some ways correspond to
traditional racial classifications used by North Americans.
The seven groups that he defined are
1) Caucasians
2) Black Africans
3) Mongolids
4) South Asian Aborigines
5) Native Americans
6) Oceanians
7) Australian Aborigines
What Lewontin found was that only 6.3% of all variation can be accounted for at the level
of these geographic groups. So approximately 94% of all genetic variation is within groups.
Which is to say that differences within the geographically defined groups is about 15 times the
differences between them. In short, the “racial” groups show significantly variation within them,
and that variation exceeds the variation between “races.” Africans show the greatest variability,
as you might expect because Africa is where the species first developed. What the high diversity
means, however, is that two people randomly selected from the African pool may be no more
closely genetically than either one of them is to a European. The point of fact is that biological
variation does not correspond to the racial typologies typically used by most cultures, including
our own.
At a more refined level, we can analyze the history of populations, such as those in West
Africa with a high frequency of the sickle-cell trait, or Ashkenazi Jews, who have a higher
frequency of cancer than other populations. In such cases, however, we are analyzing clinal
variation, not racial variation. There can be significant variation between populations (clines)
but that does not translate into “race.”
Lewontin concluded his paper with a very strong statement:
“Human racial classification is of no social value and is positively destructive of social and
human relations. Since such racial classification is now seen to be of virtually no genetic or
taxonomic significance either, no justification can be offered for its continuance.
The Lazy Person’s Guide to Classification
The traits that are commonly associated with race: skin color, hair color, eye shape, are
only a very small part of our genetic make-up. They may be the most obvious ones to superficial
analysis, but all in all they are trivial.
Race is the quintessential example of what we in anthropology call a folk-classification.
That is, race is a culturally constructed taxonomy that has a great deal to say about the people
doing the classifying, and very little to say about the people or things being classified. Race may
have no biological validity, but it is quite real as a social construct.
The 6 Percent factor
But what about that 6% genetic variation that exists between geographic groups. Does it
have any meaning? Theoretically, you’d have to admit the possibility that maybe that 6% is
related to something important.
There are two reasons why we shouldn’t worry about it.
First, While that is a theoretical possibility that there are significant genetic differences
that could have behavioral significance, it is extremely unlikely given the amount of genetic
difference among us all.
Second, even if there were significant differences, those differences are so swamped by
social and cultural differences that we can’t tell. In other words, while the genetic differences
among modern peoples are small, social and cultural differences are enormous. Those social
differences overwhelm genetic factors to such an extent that genetics simply aren’t worth
worrying about.
An example of this is IQ testing. Much has been made of supposed racial differences in
intelligence, as manifested in IQ tests. Indeed, there are differences in the way that groups test.
Within the US people of Asian background test highest. People of European ancestry test about
5 points below that, and people of African descent test about 15 pts avg below Europeans.
Mean IQ scores for different groups in the US are:
asians 115
whites 110
blacks adopted by advantaged white families 106
black children as a whole 94
Question 1: What do intelligence tests measure? Whatever it is, it is not what most people
consider to be intelligence. The questions are largely biased toward the ideal behavior of upper
class whites. IQ tests largely measure how well you conform to a cultural ideal. Certainly IQ
tests can predict things like success in college, but that is not a truly objective standard for
“intelligence.”
Question 2: Can intelligence be reduced to a number? Psychologists ever since Francis Galton
have argued about that. Is there only one type of intelligence? What would an intelligence test
designed by jazz musicians look like? What would an intelligence test by a Maya shaman look
like?
Question 3: Can genetics be held constant? Probably not. “Intelligence” is formed by many
things: nutrition, opportunity, family situation. There are too many confounds to determine what
part of “intelligence” is based upon genetics. In the US today, you cannot separate genetics from
socio-economic standing, culture, childhood nutrition, and education opportunity. Social
inequality begins in the womb, with differences in prenatal nutrition that impact brain
development. Add drug use, alcoholism, with correlate with socio-economic standing, and you
have a situation wherein the genetic contribution to “intelligence” simply cannot be assessed.
