| cline | A gradual shift in gene frequencies between neighboring populations.
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| descent | Rule assigning social identity on the basis of some aspect of one’s ancestry.
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| hypervitaminosis D | Condition caused by an excess of vitamin D; calcium deposits build up in the body’s soft tissues, and the kidneys may fail; symptoms include gallstones and joint and circulation problems; may affect unprotected light-skinned individuals in the tropics.
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| hypodescent | Rule that automatically places the children of a union or mating between members of different socioeconomic groups in the less-privileged group.
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| melanin | Substance manufactured in specialized cells in the lower layers of the epidermis (outer skin layer); melanin cells in dark skin produce more melanin than do those in light skin.
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| natural selection | As formulated by Charles Darwin and Alfred Russel Wallace, the process by which nature selects the forms most fit to survive and reproduce in a given environment.
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| phenotype | An organism’s evident traits, its “manifest biology”—anatomy and physiology.
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| rickets | Nutritional disease caused by a shortage of vitamin D; interferes with the absorption of calcium and causes softening and deformation of the bones.
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| social race | A group assumed to have a biological basis but actually perceived and defined in a social context, by a particular culture rather than by scientific criteria.
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| tropics | Geographic belt extending about 23 degrees north and south of the equator, between the Tropic of Cancer (north) and the Tropic of Capricorn (south).
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