Outline and Chapter notes to accompany chapter 7 HUMAN VARIATION Dec., 2003 A. THERE IS BIOLOGICAL VARIATION BOTH WITHIN AND BETWEEN HUMAN POPULATIONS In modern evolutionary theory, populations are defined in terms of their interbreeding, not in terms of any physical features. In most cases, people belonging to a population live in the same geographic area, but migrations make these situations more complex. CONTINUOUS AND DISCONTINUOUS VARIATION Most human variation occurs within populations. Variation occurs in continuously measured traits like height or pigmentation. Such variation is often shown as a frequency distribution or normal curve. Variation also occurs discontinuously in many genetically controlled traits. Examples include eye color, blood types, and many genetically controlled diseases. For such discontinuous traits, we can either show the population frequency of a trait (the percent of individuals in a population having the trait) or the frequency of an allele for that trait. VARIATION BETWEEN POPULATIONS Continuous traits vary geographically in their average (mean) values. Differences between populations are usually differences in their mean values, nearly always with considerable overlap. Discontinuous traits vary geographically in the frequencies of various blood groups, disease conditions, or genotypes. CONCEPTS OF RACE (in historical order): 1. SOCIALLY CONSTRUCTED RACES are recognized by a group of people in control as part of an effort to deny equal status to other people. Language, customs, and physical features are all used to distinguish "us" from "them" (racism), often resulting in groups that do not make sense biologically. Characteristics assigned to oppressed groups are viewed as marks of inferiority and are asserted to be inherited (hereditarian bias). 2. MORPHOLOGICALLY DEFINED RACES are distinguished by physical features only, not languages or customs. The extremes of variation are distinct biologically, but geographically distinct populations are often connected by a graded series of intermediate populations that make boundaries hard to draw. Historically, each morphologically defined race was thought to conform to a different Platonic type or to have originated in a different center of origin, concepts which biologists no longer accept. 3. GENETICALLY DEFINED RACES are based on the frequencies of alleles and blood groups. These frequencies are used to describe different groups of populations, while admitting that the boundaries between them are gradual. 4. THE NO-RACES CONCEPT uses the gradualness of boundaries, the imprecision of identifying membership in human populations, and the evils resulting from past and current racial conflicts as arguments against recognizing any racial groupings at all. Advocates of this position say that no racial groupings make sense biologically. THE STUDY OF HUMAN VARIATION The study of human variation is important in such fields as engin- eering, design, and architecture. Buildings, furniture, instrument panels, space capsules, fire fighting equipment, and safety equipment must all be designed with human dimensions, abilities, and limitations in mind. This is sometimes called "human factors engineering". B. POPULATION GENETICS CAN HELP US UNDERSTAND HUMAN VARIATION HUMAN BLOOD GROUPS AND GEOGRAPHY Many genetically determined traits (such as blood groups) vary geographically. Gradual geographic variations in allele frequencies are called CLINES. ABO blood groups: A and B are codominant alleles; o is recessive. Genotypes AA and Ao are both blood type A; they have type A molecules on the surface of their red blood cells. Genotypes BB and Bo are both blood type B; they have type B molecules on the surface of their red blood cells. Genotype AB is blood type AB, which has both type A and type B molecules on the surface of their red blood cells. Genotype oo is blood type O, which has neither type A nor type B molecules on the surface of their red blood cells. The frequencies of alleles A, B, and o vary geographically: A is more frequent in Western Europe than elsewhere, but is less frequent than o in nearly all populations. B shows its highest frequencies in East Asia and is absent among Native Americans. o shows its highest frequencies (close to 100%) among Native Americans. Rh blood groups (a combination of 3 closely linked genes) also vary geographically: Rh blood groups are important because Rh-negative women, when pregnant, make antibodies against any Rh-positive fetuses they may be carrying. These antibodies are a threat to any subsequent Rh-positive fetus carried by the same mother. Rh-negative (cde) has its highest frequencies in northern Spain (among Basques). It is the second most frequent combination of alleles in most of Europe and Africa, but is very rare or absent in all other populations. CDe is the most frequent allele combination in most populations, except in Africa, where cDe is most frequent. MN blood groups also vary geographically: Native American populations have high frequencies of M and almost no N; Australian Aborigines and some Pacific Island populations have high frequencies of N and almost no M; All other populations have both M and N alleles frequent. ISOLATED POPULATIONS AND GENETIC DRIFT: Genetic drift consists of random changes in genotype frequencies due to chance, especially in small populations. Populations that become isolated (for any reason) may undergo genetic drift, especially if the population is small. Populations descended from small numbers of individuals will reflect the allele frequencies of these founders (the founder effect). Examples that have been studied include the Dunkers of Pennsylvania (and neighboring states) and the Hutterites of the northern prairies. RECONSTRUCTING THE HISTORY OF HUMAN POPULATIONS Genotypes and RFLPs can now be used to measure the degree of relatedness of modern populations and to reconstruct their past histories. Results of such studies are consistent with population histories based on linguistic or other evidence. C. MALARIA AND OTHER DISEASES ARE AGENTS OF NATURAL SELECTION MALARIA Malaria causes more deaths world-wide than any other single infectious disease. Malaria is a parasitic disease caused by a protozoan called Plasmodium. Plasmodium is transmitted by mosquitoes when they bite. Many stages of the parasite's life cycle are carried out inside human red blood cells. The parasites can reproduce either sexually or asexually. Asexual reproduction can go on repeatedly within a human host. Parasites can be picked up by a female mosquito when she bites a human host. Sexual reproduction takes place inside the mosquito, where the early larval stages mature. Larval parasites later migrate to the mosquito's salivary glands and are injected into the next human. SICKLE-CELL ANEMIA AND RESISTANCE TO MALARIA: Originally described in the United States, then in Jamaica, sickle- cell anemia affects mostly people of African descent. It results from a change in one amino acid in the beta chain of the hemoglobin protein, a molecule in red blood cells which helps carry oxygen through blood. People homozygous for hemoglobin S develop sickle-cell anemia. Their red blood cells assume deformed and often sickle-like shapes, causing these cells to be destroyed. Other symptoms include an increase in bone marrow activity, enlargement of the spleen, "towering" of the skull, blockage of many small blood vessels, painful swelling of joints, rheumatism, and heart failure. The disease is fatal if untreated. People heterozygous for hemoglobin S are healthy under most conditions, but they could develop sick cell symptoms if they over-exert themselves. They are protected against malaria. We can now test for heterozygotes. People homozygous for hemoglobin A are more often bitten by Anopheles mosquitoes and more often die from malaria. Sickle-cell anemia persists in many African populations because hemoglobin S confers resistance to malaria, even in heterozygous form. Mosquitoes are less likely to bite, and, if they do bite, the parasite's life cycle is interrupted in the sickle cells and any illness is mild and brief. THALASSEMIA (Mediterranean anemia): Occurs all around the Mediterranean (including North Africa, Italy, Greece, Middle East), and eastward across Pakistan to populations in Cambodia and Thailand. The heterozygous form (thalassemia minor) is mild; the homozygous form (thalassemia major) can be fatal. Both can now be controlled with drugs if funds are available. All forms of thalassemia protect against malaria. Many forms of thalassemia are known; all are defects in one of the protein chains of hemoglobin. OTHER GENETIC TRAITS THAT PROTECT AGAINST MALARIA: Favism, or G6PD deficiency, occurs in many populations around the Eastern Mediterranean. Both homozygous and heterozygous individuals are more resistant to malaria. Adult persistence of fetal hemoglobin (APFE) occurs mostly in African populations and appears to protect against malaria. Ovalocytosis, a condition in which red blood cells are oval instead of circular, occurs in Southeast Asia and New Guinea, and is believed to offer resistance to malaria. POPULATION GENETICS OF MALARIA RESISTANCE: Hardy-Weinberg principle: allele frequencies tend to remain unchanged in large random-mating populations that do not experience selection, migration or unbalanced mutation. Frequencies of many alleles (including harmful ones) can always be maintained in a population (a balanced polymorphism) if the heterozygous genotypes show the highest fitness. The incidence of malaria varies geographically, in part because some habitats offer more opportunities for mosquitoes to breed. Geographic variations in the natural selection caused by malaria can explain variations in the gene frequencies for sickle-cell anemia, thalassemia, and the other traits listed above. OTHER DISEASES AS AGENTS OF SELECTION: Cystic fibrosis is a genetic disease whose highest frequencies are in Western Europe (especially Ireland); it is the most common genetic disease in the United States. It has been hypothesized that the gene for this condition persists because the heterozygotes are resistant to tuberculosis, a disease which caused high mortality in Europe prior to 1900. D. NATURAL SELECTION BY PHYSICAL FACTORS CAUSES MORE POPULATION VARIATION HUMAN VARIATIONS IN PHYSIOLOGY AND PHYSIQUE Bergmann's rule: Within a geographically variable species, larger body sizes are favored in colder climates because of their greater efficiency in conserving heat. (Larger bodies generally have smaller surface-to-volume ratios because surface area is proportional to the square of length dimensions, while volume is proportional to the cube of length.) Conversely, smaller body sizes are favored in warm climates. Allen's rule: Within a geographically variable species, protruding parts such as arms, legs, ears, and tails will be shorter and thicker in cold climates, and longer and thinner in warm climates, again because of differences in heat loss. In accordance with the above two rules, tropical people tend in most cases to be small and thin-legged, while Arctic people have thicker-set bodies and thicker limbs. During the Korean War (1950-1953), a disproportionate number of African-Americans (compared to Euro-Americans) lost toes and fingers to frostbite. Subsequent tests by the U.S. Army confirmed racial variation in resistance to environmental extremes. When temperatures drop, blood flow to limbs is reduced in all people. When temperature drops further or persists long enough to raise risks of frostbite, the blood vessels often dilate and send warm blood into the extremities, but not usually in people of African or African-American descent. "Thrifty genes" may account for the higher incidence of diabetes in certain Pacific Island and Native American populations; diet and exercise may also play a role. NATURAL SELECTION, SKIN COLOR, AND DISEASE RESISTANCE Gloger's rule: Within a geographically variable species, populations living in warm humid regions tend to be dark in color, those living in warm arid places tend toward tan and reddish hues, and those in cold places tend to be pale or white. Skin color variation in human populations generally follows Gloger's rule among continents and also more gradually within continents. (This comparison is based on places where populations lived prior to 1500, when long-distance migrations became more frequent.) Sunlight is most abundant in the tropics and decreases with latitude, especially in the UV portion of the spectrum. Europe receives the least sunlight of any populous region. People living far from the equator, especially in Europe, receive the least sunlight and are therefore at risk for vitamin D deficiency because the final step of vitamin D synthesis takes place under the skin with the aid of ultraviolet light. Deficiency of vitamin D can lead to rickets in children. Selection by vitamin D deficiency has resulted in pale skin colors over most of northern Europe. People living in the tropics receive the most sunlight and are therefore at risk for skin cancer. This risk is reduced if the skin is darkly pigmented, so most tropical people on all continents have darker skin colors. Inuit people (Eskimos) get their vitamin D in fully synthesized form from fish livers, so they do not need to expose their skin to the sun to get adequate amounts of vitamin D. ---------------------------------- Dec., 2003 PERMISSION IS HEREBY GRANTED to instructors who have adopted the book BIOLOGY TODAY for classroom use to download, modify, and use these notes as needed to aid them in in their teaching. Students of such instructors may likewise use and modify these notes as study aids.