Brown and Armelagos
Apportionment
Genetic variability attributed to race = 2.8% - 52.7%, one analysis greater than 22%
Distribution of Genetic
Variation
0.1 - 0.5% difference in DNA
between any two people
>99.5% shared DNA between
any two people on the planet
Genetic structure of human
populations
Region is the analog for race in this study, variability attributed to race
is “Among regions”
Race and Genetic Variation
This is 5% of the <0.5% of
DNA that ~5%
varies—or
race
Between
Races
accounts for <0.025% of
human DNA differences!!!!
~5-8% Between
Populations
~85-90% Between Individuals
within populations
Racial Morphology
• Reproductive traits have priority in
defining a species
• There is no such guidance below the
species level (RACE)
– Easily observed characteristics, not
important for species,
species are used
– There is no evolutionary justification for
emphasizing easily observed
morphological traits
So how are human “racial”
markers distributed?
Hair, Head Shape, Nose Shape,
Eye Shape, Body Build, Skin color
• This is a function of the sensory
constraints of our own species
1
It is difficult to form any judgment as to how the hair on the head
became developed to its present great length in many races.
…Our semi-human progenitors were not furnished with long
tresses, which must therefore have been a late acquisition.
This is likewise indicated by the extraordinary difference in the
length of the hair in the different races; in the negro the hair forms
a mere curly mat; with us it is of great length, and with the
American natives it not rarely reaches to the ground.
Some [monkey] species have their heads covered with
moderately long hair, and this probably serves as an ornament
and was acquired through sexual selection.
selection
The same view may perhaps be extended to mankind, for we
know that long tresses are now and were formerly much admired,
as may be observed in the works of almost every poet; St. Paul
says, “if a woman have long hair, it is a glory to her”; and we
have seen that in North America a chief was elected solely from
the length of his hair. (Darwin, 1871)
Darwin on Hair Form
Eritrean
Maori Woman
Wooly
Wavy
Straight
American Indian
Hair!Kung
Form
Biasutti: Cephalic Index
The distribution of many “racial”
features is clearly determined by
various environmental factors
Size and shape of the nose
Size and shape of the skull
Body Size and Shape
Skin Color
Cephalic index: Cranial Breadth divided by Cranial Length
1, 2 = long heads
4, 5 = broad heads
Populations native to cold climates have larger and relatively broader skulls
to conserve heat by comparison to populations in warm climates
Nose Shape
Eye Shape
Eritrean
East Indian
BaTonga
Lower the value, the taller, more narrow the nose—adapted to dry environments
Higher the value, the shorter and broader the nose—adapted to warm, humid
• The almond-shaped eye, formed by the
epicanthic fold, is racially identified with
Asians
– There are several adaptive arguments for the
epicanthic fold
•
•
•
•
•
cold protection
glare reduction
protection from particulates
assisting in vision alignment
sexual selection
2
Distribution of the Epicanthic Fold
Distribution of the Epicanthic Fold
• Asia: North Asians, Central Asians, East
Asians, and Southeast Asians
• New World: Native Americans and Inuit
• Oceanic peoples: E.g., Tongans,
Samoans, Micronesians, and Hawaiians
• Africans: Khoisans, Dinka, and Nuer
• Europeans: eastern and northern
Europe
– E.g., Mongols, Chinese, Koreans,
Japanese, Vietnamese, Kazakhs, Hazaras,
Burmese, Bangladeshis, Filipinos,
Cambodians, Malays, Thais, Bhutanese,
northern Nepalis, Tibetans, Ladakhis,
North Eastern Indians (Mizo and Garo),
Stature
– E.g., Saami
Bergmann's Rule (1847)
• Within a polytypic warm-blooded species,
the body size of the sub-species usually
increases with decreasing mean
temperature of its habitat
– An increasing body size means several things
• There is an increase in the amount of metabolizing
tissue, therefore internal heat gain is greater
• There is an overall decrease in the surface area to
mass ratio, meaning that heat loss is reduced
Category 1 is pygmoid stature
The Italian Anthropologist, I. Cipriani standing with three adult
Vedda of pygmoid stature
Bergmann’s
Rule in
Humans
Allen's Rule (1877)
• In warm-blooded species, the relative size of
exposed portions of the body decreases with
decrease of mean temperature
– An increase in exposed portions of the body
(increasing surface area) also has several natural
concomitants
– Keeping mass constant, surface area is increased by assuming
a more linear form--taller, with long, slender arms and legs
• This is especially important for humans since heat loss from
evaporation of sweat is greater than in any other animal,
and evaporative loss is directly proportional to the amount
of exposed surface area
3
Body Build and Climate
Tall, slender,
Dinka from the
upper Nile, 10°
North
Allen’s
Rule in
Humans
Short, broad
Native American
from extreme
South America,
almost 60° South
Stature
Equalized
Skin
• The body’s largest organ
• Functions in many ways
– Thermoregulation
– Protection from physical and chemical
injury
– Protection from invasion by
microorganisms
– Manufactures essential nutrient
Genetics of Skin color
Skin Color
• As one of the most conspicuous human
polytypic variations, skin color has
probably attracted more scholarly
attention than any other aspect of
human variability
• Skin color has served as a primary
feature in most systems of racial
classification
Human Pigmentation Genes
• Skin color is a polygenic trait, meaning multiple
genetic loci are involved in determining skin
color
– Multiple genes working together produce a
continuous distribution in a “Bell Shape” curve of
degrees of light to dark.
• Recent work suggests many genes working together in
very complex, additive and non-additive combinations
that affect the synthesis of melanin
• Several genes have been identified, including
Melanocortin 1 receptor (MC1R) on chromosome 16
4
Measurement of Skin Color
• By the latter half of the nineteenth century,
while anthropologists still had no clear idea of
the underlying causes of pigmentation, they
began to devise measurement techniques to
use skin color in racial classification
• Broca established a 34 tone scale, which was
simplified by his student Topinard and used in
the video we saw
• These techniques were used into the 20th
century until the introduction of the
reflectance spectrophotometer in the early
1950s
Reflectance Spectrophotometry
Reflectance
Spectrophotometer
• A Reflectance Spectrophotometer
shines a light of a specific wave length,
using a filter, and measures the
intensity of light reflected by the skin
– The technique involves alcohol wash of the
skin on the inner upper arm
• allow time for local circulation to return to
normal
• shine light and measure reflectance
Skin color is Continuous:
Mean (dot) and s.d. (bar) of skin
color, for 22 populations
Where do you
draw the racial
boundaries?
Structure of the Epidermis
Melanin
• The primary determinant of variability in
human skin color is the amount, density,
and distribution of the pigment melanin
• Melanin varies in color depending on a
variety of genetic and environmental
factors that influence melanosome
formation in the melanocyte
5
The Melanosome Complex
MC1R
Receptor
Switching Melanin Synthesis
Protein
Melanocyte
cell wall
Red, Blonde,
Brown Hair
Biasutti: Skin Color
Black Hair
Biasutti’s Distribution of Skin Color
• Not an accident that there are 8 skin color
categories
– Biasutti preferred a 16 race system
Distribution of Skin Color, 2
Environmental Factors
• The clinal nature of skin color
distribution suggests an association with
environmental factors varying with
latitude
– Ultraviolet Radiation, in particular, the
quantity of UV rays striking the surface of
the earth from the sun
– Temperature
6
Selection Favoring Dark Skin
in Tropical Regions
• Selection favoring high levels of
melanin pigmentation in areas of high
Ultraviolet (UV) radiation may involve
several selective agents
– Sunburn can cause skin lesions and
infections, preventing some degree of heat
loss
– Skin cancer
– Secondary folic acid deficiency (Folate
Photolysis)
Folic Acid Deficiency
– UV light causes denaturization (a chemical
breakdown) of Folic Acid circulating in the
blood
• This can induce a deficiency even if the diet
supplies adequate folic acid
– Deficiency symptoms include anemia, infertility, and
birth defects, especially neural tube defects
– Reduces sperm production in men
• High melanin content in the epidermis can
protect circulating Folic Acid, thereby selecting
for dark skin in low latitude areas
Skin Cancer
• Skin cancer is found to be prevalent among
light skinned individuals in tropical latitudes
• In Nigeria and Tanzania no albino over the
age of 20 years was found to be free of
malignant or pre-malignant skin lesions
• In Tanzania chronic skin damage was found
in every albino infant by the end of the first
year of life
– This high rate and early evidence of skin damage
suggests that cancer may have been a strong
selective pressure in tropical areas
Selection favoring depigmentation
in higher latitudes
• As early members of the genus Homo left the
tropics and encountered new environments in
higher latitudes, depigmentation became
adaptive
• Selection favoring low levels of melanin
pigmentation in ecosystems where there are
low levels of UV radiation are thought to
primarily be affected by the regulation of
Vitamin D synthesis
Vitamin D Deficiency
Vitamin D Metabolism
• A deficiency of Vitamin D in infants and
children causes Rickets, in adults a deficiency
causes osteomalacia
– Rickets refers to a defect in the calcification of
growing bone so that the bones are structurally
weak and unable to withstand mechanical
pressure
– Symptoms include muscle weakness, deformity of
the long bones including bowed legs, knuckle-like
projections along the rib cage (rachitic rosary),
deformities of the pelvis that are often permanent
• Long bone deformity impairs locomotion
• Pelvic distortion can make childbearing dangerous-potentially killing mother and baby
7
Skin color and Vitamin D
Rickets
• Dark skin blocks UVB rays, preventing
photochemical conversion of 7Dehydrocholesterol into previtamin D3
– Prior to widespread Vitamin D
supplementation in the 1930s, Black
women in the U.S. showed nearly 8 times
greater pelvic deformity than White women
– Recent: High prevalence of rickets among
Muslim girls and women in the UK
• Dress codes and movement restrictions reduce
exposure to sunlight
Evolution and Vitamin D
• Assuming ancestral dark skin, as hominids
moved to higher latitudes selection would
have favored depigmentation to improve
Vitamin D synthesis
– Need to consider the effects of clothing on
northern latitude populations, where selection
against dark skin would be increased because of
less skin exposure
• Also, the animal organ and fish diets in high latitudes
may contribute more dietary Vitamin D
• Genome wide scan shows selection in
Europeans for SLC24A5 and SLC45A2 in
relation to the evolution of light skin color
Potential for synthesis of previtamin D3 in lightly
pigmented human skin based on annual average UV
Zone 1: adequate UV
radiation throughout the
year
Predicted shading of skin colors for indigenous humans
based on UV exposure
Zone3:
not2:sufficient
UV UV
Zone
not sufficient
radiation
on average
forleast
radiation
during at
the whole
year
one month
8
Gradation of skin colors for known
indigenous human populations
Predicted shading of skin colors for indigenous humans
Exceptions to predicted skin colors for
known indigenous human populations
Predicted shading of skin colors for indigenous humans
Exceptions to predicted skin colors for
known indigenous human populations
Races, Clines, or What?
• Highly visible characteristics like skin color,
hair form, nose shape, and body build show
strong clinal variation conditioned by natural
selection from the relevant environmental
factors
• All genetic analyses show race or region
accounts for very little global variability, with
most differences occurring between
individuals within populations
9