6 Human Variation and Adaptation

Anthropology:Appreciating Human Diversity

14th Edition

Conrad Phillip Kottak

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Human Variation and Adaptation

• Race: A Discredited Concept in Biology

• Human Biological Adaptation

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Human Variation and Adaptation

• How does natural selection work on contemporary and recent human populations?

• Does biological adaptation occur during an individual’s lifetime?

• What is the race concept, and why have anthropologists rejected it?

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Race: A Discredited Concept in Biology

– Racial classification, now largely rejected

– Explanatory approach that focuses on understanding specific differences

• Historically, scientists approached the study of human biological diversity in two ways:

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Race: A Discredited Concept in Biology

Racial classification is the attempt to assign humans to discrete categories (purportedly) based on common ancestory.

Biological differences are real, important and apparent. But not a source to categorize people into race groups.

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Race: A Discredited Concept in Biology

– 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

• Race refers to a geographically isolated subdivision of a species

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• Clines are gradual genetic shifts and they are not compatible with discrete and separate races.

• Phenotype-based racial classifications raise the problem of deciding which traits should be primary.

height, weight, body shape, skull form, skin color?

Race: A Discredited Concept in Biology

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Race: A Discredited Concept in Biology

– 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.

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

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Races Are Not Biologically Distinct

– “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.

• Problems with using a tripartite scheme

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• The number of combinations is very large– Skin color, stature, skull form, nose form,

eye shape, lip thickness don’t go together as a unit

• The amount that heredity (versus environment) contributes to phenotypical traits is unclear.

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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.

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– Although long-term genetic markers do exist they don’t correlate neatly 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 may change without any genetic change.

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Genetic Markers Don’t Correlate with Phenotype

– Humans are more alike genetically than other hominoids.

– Long-term genetic markers exist, but they don’t correlate neatly with phenotype.

– Change in height and weight due to changes in dietary practices in a few generations (not race or genetics!)

• Conventional geographic “racial” groupings have about a 6% variation in genes

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Explaining Skin Color

– Role of natural selection in producing variation in skin color illustrates an explanatory approach to human biological diversity.

• Traditional racial classification assumes biological characteristics are determined

by heredity and were stable.

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• Skin color biological trait is influenced by several genes.

Explaining Skin Color

– 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.

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• Prior to the16th century, very dark skinned populations lived in the tropics: a belt extending about 23 degrees north

and south of the equator.

Explaining Skin Color

– Outside the tropics, skin color tends to be lighter.

– Melanin confers a selective advantage on darker-skinned people living in the tropics.

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• Loomis: focused on role of UV radiation in stimulating vitamin D

Explaining Skin Color

• Jablonski and Chaplin: explained how 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

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Recap 6.1: Advantages and Disadvantages of Dark and Light Skin Color

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• Jablonski: “Loking at Alaska, one would think that the native people should be pale as ghosts”

• Why are not they?– Haven’t inhabited the region very long in

geological time.– Their traditional diet supplies sufficient

vitamin D.

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Human Biological Adaptation

• With thousands of human genes known, new genetic traits are being discovered every day.

• Abundant evidence exists for human genetic adaptation and evolution through selection working in specific environments.

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Genes and Disease

– Malaria: 350 million to 500 million people– Schistosomiasis: more than 200 million– Filariasis: 120 million

• According to the World Health Report, tropical diseases affect more than 10 percent of the world’s population.

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Genes and Disease

– 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.

• Microbes were the major selective agent for humans, particularly before the arrival of modern medicine.

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Genes and Disease

– There is probably genetic variation in people’s susceptibility to HIV.

– AIDS could cause large shifts in human gene frequencies.

• In diseases for which there are no effective drugs, genetic resistance maintains significance.

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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.

• Natural selection also affects facial features.

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Size and Body Build

– 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.

• Bergmann’s rule: The smaller of two bodies similar in shape has more surface area per unit of weight.

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• Human populations use different, but equally effective, biological means of adapting to environmental stresses associated with high altitudes.

Size and Body Build

– Andeans– Tibetans– Ethiopians

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Lactose Tolerance

– Genes and phenotypic adaptation produce a biochemical difference between human groups in their ability to digest large amounts of milk.

– There is an adaptive advantage when other foods are scarce but milk is available.

• Phenotypic adaptation: adaptive changes that occur during an individual’s lifetime