SOC111INTRODUCTION TO ANTHROPOLOGY

Human Variation and Adaptation

Human Variation and Adaptation

• What is the race concept, and why

have anthropologists rejected it?

• How does natural selection

work on contemporary and

recent human populations?

• Does biological adaptation occur

during an individual’s lifetime?

Race: A Discredited

Concept in Biology

• Historically, scientists approached the

study of human biological diversity in two

ways:

– Racial classification, now largely rejected

– Explanatory approach that focuses on

understanding specific differences

Race: A Discredited

Concept in Biology

Racial classification is the attempt toassign humans to discrete categories

(purportedly) based on commonancestory.

Biological differences are real, importantand apparent. But not a source to

categorize people into race groups.

Race: A Discredited

Concept in Biology

• Race refers to a geographically

isolated subdivision of a species

–Human populations have not

been isolated enough from one

another to develop into discrete

groups

Race: A Discredited

Concept in Biology

• Phenotypic traits (skin color) have been used for racial classification

White

Black

Yellow

– This overly simplistic classification was compatible with the political use of race during the colonial period.

– Race kept white Europeans separate from African, Asian, and Native American subjects.

Races Are Not Biologically Distinct

• Problems with using a tripartite scheme

– “Color based” racial labels are not accurate.

• Caucasoid, Negroid, Mongoloid

– Many populations don’t fit neatly into any one

of the three “great races.”

– No single trait can be used as a basis for

racial classification.

– Phenotypic similarities and differences do not

necessarily have a genetic basis.

Genetic Markers Don’t Correlate with Phenotype

– The analysis of human DNA indicates that 94

% of human genetic variation occurs within

“races”.

– There is only 6 % variation between

conventional geographic “racial” groupings

(Africans, Asians and Europeans).

– There is much greater variation within each of

traditional “races” than between them.

Genetic Markers Don’t Correlate with Phenotype

– Phenotypical similarities and differences

are not precisely or necessarily

correlated with genetic relationships.

– Because of environment that affect

individuals during growth and

development, the range of phenotypes

characteristic of a population maychange without any genetic change

• Traditional racial classification assumes

biological characteristics are determined

by heredity and were stable over many

generations.

• Role of natural selection in

producing variation in skin

color illustrates an explanatory

approach to human biological diversity.

Explaining Skin Color

• Skin color biological trait is influenced by

several genes.

– Melanin: a natural sun screen produced

by skin cells responsible for pigmentation

– By screening out ultraviolet (UV) radiation

from sun, melanin offers protection

against a variety of maladies, including

sunburn and skin cancer.

How can we explain the geographic distribution of skin color?

• Prior to the16th century, very dark skinned

populations lived in the tropics: a belt

extending about 23 degrees north

and south of the equator.

– Outside the tropics, skin

color tends to be lighter.

– Melanin confers a selective

advantage on darker-skinned

people living in the tropics.

Explaining Skin Color

• Geographic distribution of skin color

involved effects of UV on folate.

• Folate is needed for cell division and

the production of new DNA.

• Folate deficiency can cause male

sterility.

• The discussion of skin color shows that, common ancestry is not the only reason for biological similarities.

• Natural selection makes a major contribution to variations in human skin color, as well as to many other human biological differences and similarities.

Human Biological Adaptation

• Abundant evidence exists for human

genetic adaptation and evolution

through selection working in specific

environments

• With thousands of human genes

known, new genetic traits are being

discovered every day.

Genes and Disease

• According to the World Health Report,

tropical diseases affect more than 10

percent of the world’s population.

– Malaria: 350 million to 500 million

people

– Schistosomiasis (snail fever): more than

200 million

– Filariasis: 120 million

Genes and Disease

• Microbes were the major selective agent

for humans, particularly before

the arrival of modern medicine.

– After food production emerged

10,000 years ago, infectious diseases posed

a mounting risk and became the foremost

cause of human mortality.

– ABO blood groups vary in their resistance to

disease.

Genes and Disease

• Smallpox had been a major threat to humans and a determinant of blood frequencies until 1977.

• The A B 0 blood groups have figured in human resistance to smallpox.

• People with A or AB blood type are more susceptible to smallpox than are people with type B or type 0.

Genes and Disease

• Associations between A B 0 blood types and noninfectious disorders also have been noted.

• Type 0 – duodenal and gastric ulcers.

• Type A – stomach and cervical cancer and ovarian tumors.

• However, since these noninfectious disorders tend to occur after reproduction has ended, their relevance to adaptation and evolution through natural selection is doubtful.

Facial Features

• Natural selection also

affects facial features.

– Long noses seem to be adaptive

in arid areas and cold environments.

– Thomson’s Nose Rule: There is an

association between nose form and

temperature for those who have lived for

many generations in the areas they now

inhabit.

Size and Body Build

• Bergmann’s rule: The smaller of two

bodies similar in shape has more surface

area per unit of weight.

Within the same species of warm-blooded

animals, populations having smaller

individuals are found more in warm climates.

• Allen’s rule: Relative sizes of protruding

body parts increase with temperature.

Phenotype Differences

• Tropical climates- slender bodies with long limbs are advantageous.

• North polars – Eskimos with short limbs and stocky bodies.

• Altitudes also affects phenotypes.

Lactose Tolerance

• Another difference between human groups.

• Lactose tolerence is important factor for survival when other foods are scarce and milk is available.

• Lactose tolerance appears to be one of many aspects of human biology governed both by genes and by phenotypical adaptation to environmental conditions.