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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
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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 • 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. • In any racial group, there will be individuals who fall into the normal range of variation for another group for one or several characteristics. • The characteristics used to define races are influenced by several genes and exhibit a continuous range of expression.
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.
Human Polymorphisms • Characteristics with different phenotypic expressions are called polymorphisms. • A genetic trait is polymorphic if the locus that governs it has two or more alleles. • Geneticists use polymorphisms as a tool to understand evolutionary processes in modern populations.
Clinal Distributions • A cline is a gradual change in the frequency of a trait or allele in populations dispersed over geographical space. • Example: The distribution of the A and B alleles in the Old World. • Clinal distributions are thought to reflect microevolutionary influences of natural selection and/or gene flow. • Consequently, clinal distributions are explained in evolutionary terms.
Polymorphisms at the DNA Level • Molecular biologists have recently uncovered DNA variability in various regions of the genome. • Scattered through the human genome are microsatellites, sites where DNA segments are repeated. • Each person has a unique arrangement that defines their distinctive “DNA fingerprint.”
Human Biocultural Evolution • Humans live in cultural environments that are continually modified by their activities. • Evolutionary processes can be understood only within this cultural context.
Human Biocultural Evolution • Example: Lactose intolerance • In all human populations, infants and young children are able to digest milk. • In most mammals, including humans, the gene that codes for lactase production “switches off” in adolescence. • The geographical distribution of lactose tolerance is related to a history of cultural dependence on fresh milk products.
Evolutionary Interactions Affecting the Frequency of the Sickle-cell Allele
Population Genetics • The study of the frequency of alleles, genotypes, and phenotypes in populations from a microevolutionary perspective. • A gene pool is the total complement of genes shared by the reproductive members of a population. • Breeding isolates are populations that are isolated geographically and/or socially from other breeding groups.
Hardy-Weinberg Equilibrium • The mathematical relationship expressing the predicted distribution of alleles in populations; the central theorem of population genetics. • Provides a tool to establish whether allele frequencies in a human population are changing.
Factors that Act to Change Allele Frequencies • New variation (i.e., mutation) • Redistributed variation (i.e., gene flow or genetic drift) • Selection of “advantageous” allele combinations that promote reproductive success (i.e., natural selection).
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.
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.
Homeostasis • Acclimatization is physiological responses to changes in the environment. • Such responses may be temporary or permanent, depending on the duration of the environmental change and when it occurs. • Because it is under genetic influence, acclimatization is subject to evolutionary factors such as natural selection or genetic drift.
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.
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.
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.
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.
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.
1. Which of the following is a true statement? • Polygenic traits are usually more straightforward than polymorphic traits. • Comparing allele frequencies between populations can tell us nothing about evolutionary events. • Distributions of alleles for a single genetic trait do not conclusively demonstrate genetic relationships between populations. • The best way to understand patterns of population relationships is to follow a single polymorphic trait.
Answer: c • The following is a true statement: • Distributions of alleles for a single genetic trait do not conclusively demonstrate genetic relationships between populations.
2. One of the results of mitochondrial DNA (mtDNA) research has been the discovery that: • the variation of mtDNA within Homo sapiens is much less than found in other species. • the variation of mtDNA within Homo sapiens is much more than found in other species. • chimpanzees have much less variation in their mtDNA than humans do. • the length of the mtDNA is as long as nuclear DNA, about 3 billion nucleotides.
Answer: a • One of the results of mitochondrial DNA (mtDNA) research has been the discovery that the variation of mtDNA within Homo sapiens is much less than found in other species.
3. The total complement of genes shared by reproductive members of a population, is that population's • gene flow. • gene drift. • gene pool. • bottleneck effect.
Answer: c • The total complement of genes shared by reproductive members of a population, is that population's gene pool.
4. The pigment which helps protect against ultraviolet radiation by absorbing it is • carotene. • lactose. • hemoglobin. • melanin.
Answer: d • The pigment which helps protect against ultraviolet radiation by absorbing it is melanin.
5. Inuits have a large "globular" body, while the body structure of the Kalahari !Kung is thin and linear. This is explained by • Bergmann's rule. • Allen's rule. • Gloger's rule. • Kleiber's rule.
Answer: a • Inuits have a large "globular" body, while the body structure of the Kalahari !Kung is thin and linear. This is explained by Bergmann's rule.
