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Chapter 12. Human Variation and Adaptation. How do you define “race” and do you think it’s a useful concept in understanding variation in our species?
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Chapter 12 Human Variation and Adaptation
How do you define “race” and do you think it’s a useful concept in understanding variation in our species? • Are observable characteristics (i.e. skin color and color of eyes) as important as the fundamental differences among humans that are defined by evolutionary processes?
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. • Polytypic species
Intelligence • Genetic and environmental factors contribute to intelligence. • IQ scores change during a person’s lifetime • Cognitive abilities are polygenic -- measured by many genes • Nature and Nurture!
Human Polymorphisms • Genetic traits with different phenotypic expressions • A cline is a gradual change in the frequency of a trait or allele in populations dispersed over geographical space. • Clinal distributions are thought to reflect natural selection and/or gene flow.
Polymorphisms at the DNA Level • 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. • Lactose intolerance…
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.
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. • 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. Acclimatization • 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.
Ultraviolet Rays • Ultraviolet Rays penetrate the skin and can eventually damage DNA within skin cells. • The three major types of cells that can be affected are squamous cells, basal cells, and melanocytes.
UV Radiation • 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. • Since this is how we evolved, when don’t get enough sun, we may develop…
Rickets • Insufficient amounts of vitamin D during childhood result in rickets, a condition that often leads to bowing of the long bones of the legs and deformation of the pelvis.
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. • This is due to both Culture….and Biology! • Biology part (next slide)
Human Response to Heat • Long-term adaptations to heat evolved in our ancestors: • Sweat Glands • Vasodilation – capillaries near skin’s surface widen
Human Response to Cold • Short-term responses to cold: • Metabolic rate and shivering • Vasoconstriction – to reduce blood flow to the skin
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) • Bergmann’s and Allen’s rules...
Bergmann’s Rule • Body size tends to be greater in populations that live in cold environments. • 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.
