Anthropology Discussion 6

Lecture #21—Understanding Modern Human Variation

Moving Past Eugenics The Emerging Evolutionary Perspective • WWII happened

– The consequences of eugenics-based policies.

• Darwin + Modes of Inheritance = Adaptation to Local Conditions – Our knowledge of evolution and biology was

expanding dramatically

• Humans are polytypic – local populations that vary in the expression of more

than one trait – even within human populations there is a great deal

of variation in the expression of traits

The Modern Perspective

• Genetics – Emphasis on phenotype too superficial

– Races are not fixed biological entities with all individuals conforming to a certain type

– Significant variation within populations

• Phenotypic variation exists and roughly corresponds to geographical distribution

• But, need to ask some important questionswhat does this variation mean? what is its origin? – Adaptive significance? Genetic drift? Gene flow between

populations?

The Modern Perspective

• There is more variation within populations of humans than there is between populations of humans

• Gene flow—there has always been interbreeding

• Modern humans are less genetically variable than all other species

– Recent African origins

• Traits used to define race are polygenic

– Continuous range of variation

Moving from Race to Adaptive Significance of Variation

• Human variation is the result of adaptations to environmental conditions and gene flow

• As human populations migrated to settle different parts of the world, they adapted to local conditions but gene flow is always a part of human populations

The Adaptive Significance of Skin Color

The Adaptive Significance of Skin Color

• Until 500 ya, skin color in populations followed a geographical distribution.

– One of the most superficial and rapidly evolving traits

• Populations with the greatest amount of pigmentation are found in the tropics.

• Populations with lighter skin color are associated with more northern latitudes.

Skin Color

• Skin color is influenced by 3 substances:

– Hemoglobin

– Carotene

– Melanin

• Polygenic trait—influenced by the actions of more than one gene

– There are at least 6 genes that influence skin color and each of them have at least 2 alleles

Folate-Vitamin D Hypothesis

Folate-Vitamin D Hypothesis

• Folate and Vitamin D Hypotheses work together to explain skin color variation evolutionarily

• Humans synthesize vitamin D through the interaction of UV rays and cholesterol in skin cells – Vitamin D is essential to bone growth, not enough

leads to ricketsneed sun exposure

• Folate is an essential vitamin that is destroyed by too much exposure to UV rays

• Too much exposure to UV rays also causes skin cancer

Folate-Vitamin D Hypothesis

• Folate is an essential vitamin that is destroyed by too much exposure to UV rays

– Too much exposure to UV rays also causes skin cancer

– Melanin assists in the absorption of UV rays, it makes them not as harmful

• The conflict—enough UV exposure for vitamin D, not so much to destroy folate

Folate-Vitamin D Hypothesis

• Natural selection favors dark skinned people in areas with high sun exposure

– Assists in the preservation of folate during pregnancy

• Natural selection favors lighter skinned people in areas with less sun exposure

– Synthesize Vitamin D

Body Proportions and Adaptation to Local Conditions

Figure 12.13 in Jurmain et al.

Body Proportions and Adaptation to Local Conditions

• Bergmann’s and Allen’s Rules

– Remember those from Neanderthal discussion?

– General rule that applies to animal bodies.

Biocultural Evolution and Lactose (In)Tolerance

Biocultural Evolution and Lactose (In)Tolerance

• Lactose Intolerance – Lactase is the enzyme that digests lactose, the sugar in milks – Most human adults, until recently, were lactose intolerant – But, if lactose continues to be ingested, the action of lactase

may not be switched off

• Lactose Tolerance – Biocultural Evolution

• Many Asian, African, and Native American populations – Traditionally have not relied heavily on dairy products – Those that have, like the Massai, are not lactose intolerant

• Europeans and many Middle Easterners relied heavily on domesticated animals and their products, including milk, over the last 10ky

Sickle-Cell Anemia and Disease as Selection Pressure

Sickle-Cell Anemia and Disease as Selection Pressure

• Think all the way back to the beginning of the course…

• Sickle-cell anemia is more common among Sub-Saharan African, South Asian, and Amazon Basin populations

– About exposure to malaria-carrying mosquitoes

• Modern human variation is best understood as a result of adaptation to local conditions!!