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AR103 - RN Ch#12 W7.pdf

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Jonathan Haxell

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AR103 - Readings pgs. 314-333 - Week 7 Notes - Oct. 30, 2011. Chapter Twelve: Human Variation and Adaption Pages # 314 - 333 Population Genetics - Physical anthropologists use the approach of population genetics to interpret microevolutionary patterns of human variation. - Population genetics is the study of the frequency of alleles, genotypes, and phenotypes in populations from a microevolutionary perspective. - Attempts to identify the various factors that cause allele frequencies to change over time. - Gene pool is the total complement of genes shared by the reproductive members of a population. - As a rule, a population is the group within which individuals are most likely to find mates. - In every generation, the genes in a gene pool are mixed by recombination and then reunited with their counterparts through mating. - What emerges in the next generation is a direct product of the genes going into the pool, which in turn is a product of who is mating with whom. - Factors that determine mate choice are geographical, ecological, and social. - Breeding isolates are populations that are clearly isolated geographically and/or socially from other breeding groups. - Geography plays a dominant role in producing these isolates by strictly determining the range of available mates. - Even within these limits, cultural rules can play a deciding role by prescribing who is most appropriate among those who are potentially available. - Inhabitants of large cities may appear to be members of a single population, but within the city, socioeconomic, ethnic, and religious boundaries crosscut in complex ways to form smaller population segments. - In addition to being members of these smaller local populations, we’re also members of overlapping gradations of larger populations: the immediate geographical region (a metropolitan area or a state), a section of a country, a nation, and ultimately the entire species. - To determine whether evolution is taking place at a given locus, population geneticists measure allele frequencies for specific traits and compare these observed frequencies with a set predicted by a mathematical model called the Hardy-Wienberg equilibrium equation. - Hardy-Weinberg Equilibrium is the mathematical relationships expressing, under conditions in which no evolution is occurring, the predicted distribution of alleles in populations; the central theorem of population genetics. - Provides a tool to establish whether allele frequencies in a population are indeed changing. - Factors that act to change allele frequencies: - 1. New variation (that is, new alleles produced by mutation) - 2. Redistributed variation (that is, gene flow or genetic drift) - 3. Selection of “advantageous” allele combinations that promote reproductive success (that is, natural selection) The Adaptive Significance of Human Variation - To survive, all organisms must maintain the normal functions of organs, tissues, and cells within the context of an ever-changing environment. - Physical activity also places stress on physiological mechanisms. 1 AR103 - Readings pgs. 314-333 - Week 7 Notes - Oct. 30, 2011. - Stress in a physiological context, any factor that acts to disrupt homeostasis; more precisely, the body’s response to any factor that threatens its ability to maintain homeostasis. (examples: temperature, wind, solar radiation, humidity, etc) - The body must accommodate these changes by compensating in some manner to maintain internal constancy, or homeostasis, and all life-forms have evolved physiological mechanisms that, within limits, achieve this goal. - Homeostasis is a condition balance, or stability, within a biological system, maintained by the interaction of physiological mechanisms that compensate for changes (both external and internal). - Physiological response to environmental change is influenced by genetic factors. - Examples of long-term adaptations in humans include physiological responses to heat (sweating) and excessive levels of ultraviolet light (deeply pigmented skin in tropical regions). - Acclimatization is physiological responses to changes in the environment that occur during an individual’s lifetime. Such responses may be temporary or permanent, depending on the duration of the environmental change and when in the individual’s life it occurs. The capacity for acclimatization may typify an entire species or population, and because it’s under genetic influence, it’s subject to evolutionary factors such as natural selection and genetic drift. - These responses to environmental factors are partially influenced by genes, but some can also be affected by the duration and severity of the exposure, technological buffers (such as shelter or clothing), and individual behaviour, weight, and overall body size. - The simplest type of acclimatization is a temporary and rapid adjustment to an environmental change (e.g. tanning). - Developmental acclimatization is irreversible and results from exposure to an environmental challenge during growth and development. Solar Radiation & Skin Colour - Skin colour is a commonly cited example of adaptation through natural selection in humans. - Populations with the greatest amount of pigmentation are found in the tropics, while light skin colour is associated with more northern latitudes, particularly the inhabitants of northwestern Europe. - Skin colour is mostly influenced by the pigment melanin, a granular substance produced by specialized cells (melanocytes) found in the epidermis. - All humans have approximately the same number of melanocytes. - It’s the amount of melanin and the size of the melanin granules that vary. - Melanin is important because it acts as a built-in sunscreen by absorbing potentially dangerous ultraviolet rays present in sunlight. - Melanin protects us from overexposure to UV radiation, which can cause genetic mutations in skin cells. - These mutations may lead to skin cancer. - Exposure to sunlight triggers a protective mechanisms in the form of tanning, the result of temporarily increased melanin production (acclimatization). - This is response occurs in all humans except albinos (genetically mutated). ▯ Dark Skin - Natural selection has favored dark skin in areas nearest the equator, where the sun’s rays are the most direct / UV is intense. - Cancer-causing effects of UV radiation from an evolutionary perspective, 3 points to keep in mind: 2 AR103 - Readings pgs. 314-333 - Week 7 Notes - Oct. 30, 2011. - 1. Early hominins lived in the tropics, where solar radiation is more intense than in temperature areas to the north and south. - 2. Unlike modern city dwellers, early hominins spent their days outdoors. - 3. Early hominins didn’t wear clothing that would have protected them from the sun. - Given these conditions, UV radiation was probably a powerful agent selecting for high levels of melanin production in early humans. - Folate isn’t stored in the body and therefore must be replenished through dietary sources. Folate deficiencies in pregnant women are associated with numerous complications, including maternal death, and neural tube defects. - Neural tube is in early embryonic development, the anatomical structure that develops to form the brain and spinal cord. - Consequences can include pain, infection, paralysis, and even death. - Some studies have shown that UV radiation rapidly depletes folate serum levels in fair- skinned individuals. - Implications for pregnant women and children and also for evolution of dark skin in hominins. - Jablonski ad Chaplin suggest that the earliest hominins may have had light body skin covered with dark hair, but a loss of body hair in hominins occurred, dark skin evolved rather quickly as a protective response to the damaging effects of the sun. - Skin cancer and the maintenance of sufficient levels of folate have no doubt been selective agents favoring dark skin in humans living where UV radiation is most intense. ▯ Lighter Skin - The populations as hominins migrated out of Africa and Asia into Europe, encountered cold temperatures and cloudy skies, frequently during the summer as well as in the winter. Winter also meant fewer hours of daylight, solar radiation is very indirect. - People in these areas wore animal skins and other types of clothing, which blocked the suns rays - Relaxed selection for dark skin due to reduced exposure to sunlight. - The need for adequate amounts of vitamin D. - The theory called the vitamin D hypothesis. - Vitamin D is produced in the body partly as a result of the interaction between UV radiation & a substance similar to cholesterol. Also found in some foods. - VItamin D is necessary for normal bone growth and mineralization, and some exposure to UV radiation is therefore essential. - An insufficient amount of vitamin D during childhood results in rickets, a condition that often leads to bowing of the long bones of the legs and deformation of the pelvis. - In addition to its role in bone mineralization, vitamin D is critical to numerous other biological processes. Through its conversion to 1,25D it can attach directly to DNA, regulating genes. - Jablonski and Chapin have also concluded the support that vitamin D hypothesis to the point of stating that the requirement for vitamin D synthesis in northern latitudes was as important to natural selection as the need for protection from UV radiation in the tropics. 3 AR103 - Readings pgs. 314-333 - Week 7 Notes - Oct. 30, 2011. - The fact that lighter skin developed in two hominin species, but through different mutations in the same gene, strongly reinforces the theory that there is a significant selective advantage to lighter skin in higher latitudes. - Except for a person’s sex, more social importance has been attached to variation in skin colour than to any other single human biological trait. The Thermal Environment - Mammals and birds have evolved complex mechanisms to maintain a constant internal body temperature. - While reptiles rely on exposure to external heat sources to raise body temperature and energy levels, mammals and birds have sources to raise body temperature and energy levels, mammals and birds have physiological mechanisms that, within certain limits,
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