Patterns of Human Adaptation: Understanding the Adaptive Significance of Human Variation

1. Chapter 16 Modern Human Biology: Patterns of Adaptation

2. Chapter Outline • The Adaptive Significance of Human Variation • Infectious Disease • The Continuing Impact of Infectious Disease • New Frontiers in Research: Molecular Applications in Modern Human Biology

3. Adaptive Significance of Human Variation • Human variation is the result of adaptations to environmental conditions. • Physiological response to the environment operates at two levels: • Long-term evolutionary changes characterize all individuals within a population or species. • Short-term, temporary physiological response is called acclimatization.

4. Homeostasis • A condition of stability within a biological system, maintained by the interaction of physiological mechanisms that compensate for changes. • Stress is the body’s response to any factor that threatens its ability to maintain homeostasis.

5. Acclimatization • Physiological responses to changes in the environment. • Responses may be temporary or permanent, depending on the environmental change. • Because it is under genetic influence, acclimatization is subject to evolutionary factors such as natural selection or genetic drift.

6. Geographical Distribution of Skin Color in Indigenous Populations

7. Ultraviolet Light and Skin Cells

8. Skin Color • Influenced by three substances: • Hemoglobin, when it is carrying oxygen, gives a reddish tinge to the skin. • Carotene, a plant pigment which the body synthesizes into vitamin A, provides a yellowish cast. • Melanin, has the ability to absorb ultraviolet radiation preventing damage to DNA.

9. Three Points: Skin Color and Evolution • Early hominids lived in the tropics, where solar radiation is more intense than in temperate areas to the north and south. • Unlike modern city dwellers, early hominids spent their days outdoors. • Early hominids didn’t wear clothing that would have protected them from the sun.

10. Folate and UV Radiation • Folate is a B vitamin that isn’t stored in the body and must be replenished through dietary sources. • In pregnant women, insufficient levels of folate are associated with numerous fetal developmental disorders, including neural tube defects such as spina bifida.

11. Folate and UV Radiation • Studies have shown that UV radiation depletes folate serum levels both in laboratory experiments and in light-skinned individuals. • These findings have implications for pregnant women, for children, and for the evolution of dark skin in early hominids.

12. Question • Which of the following has not been linked with light skin color in humans? • cloudy skies • folate • all of these choices • clothing

13. Answer: c • None of the following has been linked with light skin color in humans: • cloudy skies • folate • clothing

14. Vitamin D Synthesis

15. Thermal Environment • Mammals and birds have evolved complex physiological mechanisms to maintain a constant body temperature. • Humans are found in a wide variety of thermal environments, ranging from 120° F to -60° F.

16. Human Response to Heat • Long-term adaptations to heat evolved in our ancestors: • Sweat Glands • Vasodilation • Bergmann's rule - body size tends to be greater in populations that live in cold environments.

17. Bergmann’s Rule • In mammalian species, body size tends to be greater in populations that live in colder climates. • As mass increases, the relative amount of surface area decreases proportionately. • Because heat is lost at the surface, it follows that increased mass allows for greater heat retention and reduced heat loss.

18. Allen’s Rule • In colder climates, shorter appendages, with increased mass-to-surface ratios, are adaptive because they are more effective at preventing heat loss. • Conversely, longer appendages, with increased surface area relative to mass, are more adaptive in warmer climates because they promote heat loss.

19. Examples of Bergmann’s andAllen’s Rules • (a) This African woman has the linear proportions characteristic of many inhabitants of sub-Saharan Africa. • (b) By comparison, the Inuit woman is short and stocky.

20. Human Response to Cold • Short-term responses to cold: • Metabolic rate and shivering • Narrowing of blood vessels to reduce blood flow from the skin, vasoconstriction. • Increases in metabolic rate to release energy in the form of heat.

21. Vasodilation • Expansion of blood vessels, permitting increased blood flow to the skin. • Vasodilation permits warming of the skin and facilitates radiation of warmth as a means of cooling. • Vasodilation is an involuntary response to warm temperatures, various drugs, and even emotional states (blushing).

22. Vasoconstriction • Narrowing of blood vessels to reduce blood flow to the skin. • Vasoconstriction is an involuntary response to cold and reduces heat loss at the skin’s surface.

23. High Altitude • Multiple factors produce stress on the human body at higher altitudes: • Hypoxia (reduced available oxygen) • Intense solar radiation • Cold • Low humidity • Wind (which amplifies cold stress)

24. Infectious Disease • Caused by invading organisms such as bacteria, viruses, or fungi. • Throughout evolution, disease has exerted selective pressures on human populations. • Disease influences the frequency of certain alleles that affect the immune response.

25. Impact of Infectious Disease • Before the 20th century, infectious disease was the number one limiting factor to human populations. • Since the 1940s, the use of antibiotics has reduced mortality resulting from infectious disease.

26. Impact of Infectious Disease • In the late 1960s, the surgeon general declared the war against infectious disease won. • Between 1980 and 1992 deaths from infectious disease increased by 58%. • Increases in the prevalence of infectious disease may be due to overuse of antibiotics.

27. Question • The number one cause of death among humans until the 20th Century was: • AIDS. • TB. • malaria. • infectious diseases.

28. Answer: d • The number one cause of death among humans until the 20th Century was infectious diseases.

29. Vectors • Agents that serve to transmit disease from one carrier to another. • Mosquitoes are vectors for malaria, just as fleas are vectors for bubonic plague.

30. Endemic • Continuously present in a population.

31. Zoonotic • Pertaining to a zoonosis, a disease that’s transmitted to humans through contact with nonhuman animals.

32. Antibodies • Proteins that are produced by immune cells and serve as major components of the immune system. • Antibodies recognize and attach to foreign antigens on bacteria, viruses, and other pathogens. • Pathogens are substances or microorganisms, such as bacteria, fungi, or viruses, that cause disease.

33. Environmental Factors • Global warming may expand the range of tropical diseases. • The spread of disease is associated with encountering people; this includes crossing borders and penetrating remote areas. • The increasingly large human population leads to overcrowding, unsanitary conditions and the spread of communicable disease.