2. 6
C H A P T E R
HUMAN VARIATION AND ADAPTATION
6-2

3. HUMAN VARIATION AND ADAPTATION
Race: A Discredited Concept in Biology
Human Biological Adaptation 3

4. 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?

5. RACE: A DISCREDITED CONCEPT
Historically, scientists approached the study of human biological diversity in two ways:
Racial classification, now largely rejected
Explanatory approach

6. RACE: A DISCREDITED CONCEPT
Racial classification: attempt to assign humans to discrete categories (purportedly) based on common ancestry
Human biological variation distributed gradually between populations is called clines
Human populations have not been isolated enough from one another to develop into discrete groups

7. RACE: A DISCREDITED CONCEPT
Phenotypic traits (skin color) have been used for racial classification
Overly simplistic classification compatible with political use of race during the colonial period
Race kept white Europeans separate from African, Asian, and Native American subjects

8. RACES ARE NOT BIOLOGICALLY DISTINCT
Problems with using a tripartite scheme
“Color based” racial labels are not accurate
Many populations don’t fit neatly into any one of the three “great races”
No single trait can be used as basis for racial classification
Phenotypic similarities and differences do not necessarily have genetic basis

9. GENETIC MARKERS DON’T CORRELATE WITH PHENOTYPE
Conventional geographic “racial” groupings have about a 6% variation in genes
Humans are more alike genetically than other hominoids
Haplogroup: lineage or branch of a genetic tree marked by one or more specific genetic mutations
Long-term genetic markers exist, but they don’t correlate neatly with phenotype

10. EXPLAINING SKIN COLOR
Traditional racial classification assumes biological characteristics are determined by heredity and are stable
Role of natural selection in producing variation in skin color offers explanatory approach to human biological diversity

11. EXPLAINING SKIN COLOR
Skin color is a complex biological trait influenced by several genes
Melanin: a “natural sunscreen” 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

12. EXPLAINING SKIN COLOR
Before 16th century, very dark-skinned populations lived in 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

13. EXPLAINING SKIN COLOR
Loomis: focused on role of UV radiation in stimulating vitamin D
Jablonski and Chaplin: explained geographic distribution of skin color involved effects of UV on folate, used to manufacture folic acid
Variation in human skin color:
Protects against all UV hazards
Provides an adequate supply of vitamin D

14. RECAP 6.1: Advantages and Disadvantages (Depending on Environment) of Dark and Light Skin Color14

15. THE AAA RACE PROJECT AAA Race Project: “RACE Are We So Different?”
Race is a recent human invention
Race is about culture, not biology
Race and racism are embedded in institutions and everyday life

16. 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

17. GENES AND DISEASE
World Health Organization (WHO) report: about one billion people worldwide are affected by neglected tropical diseases
Malaria: 216 million
Schistosomiasis: more than 200 million
Filariasis: 120 million

18. GENES AND DISEASE
Microbes 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 foremost cause of human mortality
ABO blood groups vary in resistance to disease

19. GENES AND DISEASE
In diseases for which there are no effective drugs, genetic resistance maintains significance
There is genetic variation in people’s susceptibility to HIV

20. FACIAL FEATURES Natural selection affects facial features
Long noses seem to be adaptive in arid areas and cold environments
Thomson’s nose rule: association between nose form and temperature for those who have lived for many generations in areas they now inhabit

21. SIZE AND BODY BUILD
Bergmann’s rule: smaller of two bodies similar in shape has more surface area per unit of weight
Within warm-blooded animals, populations with smaller individuals in same species are more often found in warm climates
Allen’s rule: relative sizes of protruding body parts increase with temperature

22. SIZE AND BODY BUILD
Human populations use different biological means of adapting to environmental stresses associated with high altitudes
Andeans
Tibetans
Ethiopians

23. LACTOSE TOLERANCE
Phenotypic adaptation: adaptive changes that occur during an individual’s lifetime
Biochemical difference among human groups involves the ability to digest large amounts of milk
There is an adaptive advantage when foods are scarce but milk is available