Human Variation and Adaptation

2. Human Variation and Adaptation • Race: A Discredited Concept in Biology • Human Biological Adaptation

3. 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?

4. 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:

5. 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.

6. 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

7. Race: A Discredited Concept in Biology • 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?

8. 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

9. 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.

10. 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.

11. 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.

12. Althoughlong-termgeneticmarkers do existtheydon’tcorrelateneatlywithphenotype. • Phenotypicalsimilaritiesanddifferencesare not preciselyornecessarilycorrelatedwithgeneticrelationships. • Because of environmentthataffectindividualsduringgrowthanddevelopment, therange of phenotypescharacteristic of a populationmaychangewithoutanygeneticchange.

13. 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

14. 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.

15. 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. • Skin color biological trait is influenced by several genes.

16. 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. • Prior to the16th century, very dark skinned populations lived in the tropics: a belt extending about 23 degrees north and south of the equator.

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

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

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. Size and Body Build • Human populations use different, but equally effective, biological means of adapting to environmental stresses associated with high altitudes. • Andeans • Tibetans • Ethiopians

27. 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