Human Variation and Adaptation Chapter 12 Human Variation and Adaptation

Chapter Outline Historical Views of Human Variation The Concept of Race Racism Intelligence Contemporary Interpretations of Human Variation

Chapter Outline Human Biocultural Evolution Population Genetics The Adaptive Significance of Human Variation The Continuing Impact of Infectious Disease

Historical Views of Human Variation Two schools of thought developed to explain diversity: Monogenists believed that all humans were descended from a original pair of humans. Polygenists believed that all humans were descended from a number of pairs of humans.

Historical Views of Human Variation Biological determinism - cultural and biological variations are inherited in the same way. Eugenics - "race improvement" through forced sterilization of members of some groups and encouraged reproduction among others.

Traditional Concept of Race Since the 1600s, race has been used to refer to culturally defined groups. Race is used as a biological term, but has enormous social significance. "Racial traits" are not the only phenotypic expressions that contribute to social identity: sex and age are also critically important. In the 1950's the use of the term "race" was replaced with "ethnicity "

Allele Frequencies Within and Between Populations After World War II, the study of human variation shifted to the study of differences in allele frequencies within and between populations. The application of evolutionary principles to human variation has replaced the view that was based solely on observed phenotype. Races are no longer viewed as fixed biological entities, composed of individuals fitting a particular type.

Groupings Used by Lewontin in Population Genetics Study (1972) Geographical Group Examples of Populations Included Caucasians Arabs, Armenians, Tristan da Cunhans Black Africans Bantu, San, U.S. blacks Asians Ainu, Chinese, Turks South Asians Andamanese, Tamils Amerinds Aleuts, Navaho, Yanomama Oceanians Easter Islanders, Micronesians Australians All treated as a single group

Racism Based on false belief that intellect and cultural factors are inherited with physical characteristics. Uses culturally defined variables to typify all members of particular populations. Assumes that one's own group is superior. A cultural phenomenon found worldwide.

Intelligence Genetic and environmental factors contribute to intelligence. Many psychologists say IQ scores measure life experience. Innate differences in abilities reflect variation within populations, not differences between groups. There is no convincing evidence that populations vary in regard to intelligence.

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 (genetic) evolutionary changes characterize all individuals within a population or species. Short-term, temporary physiological response is called acclimatization.

Pigmentation and Geographical Divisions Before 1500, skin color in populations followed a geographical distribution, particularly in the Old World. 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 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.

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.

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.

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.

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)

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.

Small Pox The only disease considered to be eliminated as a result of medical technology Smallpox has a higher incidence in those with type A or AB than in those type O blood. The immune systems of individuals with type A antigen may not recognize the small pox antigen as a threat.

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.

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.

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.