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169 Chapter 6 Answers: Human Variation
6.2 Genetic Variation: Review Questions and Answers
Compare and contrast the significance of genetic variation at the individual and population levels. At the individual level, most human genetic variation is not very important biologically because it has no apparent adaptive significance. It neither enhances nor detracts from individual fitness. At the population level, genetic variation is crucial for evolution to occur. Genetically-based differences in fitness among individuals are the key to evolution by natural selection.
Describe genetic variation within and between human populations on different continents. Any two randomly selected individuals differ in only about 0.1 per cent of their DNA base pairs. Of this genetic variation, about 90 per cent occurs between individuals within continental populations, and only about 10 per cent occurs between individuals from different continents.
Explain why allele frequencies for the Duffy gene may be used as a genetic marker for African ancestry. Allele frequencies for the Duffy gene differ greatly between African (and African-derived) populations and other human populations. The allele for no Duffy antigen is very high in African populations (and relatively high in African Americans) but virtually absent from non-African populations. Therefore, allele frequencies for this gene may be used as a genetic marker for African ancestry.
Identify factors that increase the level of genetic variation within populations. Factors that tend to increase genetic variation within a population include its age and size. You would expect an older, larger population to have more genetic variation. The older a population is, the longer it has been accumulating mutations. The larger a population is, the more people there are in which mutations can occur.
Discuss genetic evidence that supports the out-of-Africa hypothesis of modern human origins. Most studies of human genetic variation find greater genetic diversity in African than in non-African populations. This is consistent with the older age of the African population proposed by the out-of-Africa hypothesis. In addition, most of the genetic variation in non-African populations is a subset of the variation in African populations. This is consistent with the idea that part of the African population left Africa and migrated to other places in the Old World.
What evidence suggests that modern humans interbred with archaic human populations after modern humans left Africa? Recent comparisons of modern human and archaic human DNA show that interbreeding occurred between their populations to differing degrees. The comparisons reveal greater admixture with archaic humans in modern European, Asian, and Oceanic populations than in modern African populations. Populations with the greatest admixture are those in Melanesia. About eight per cent of their DNA came from archaic Denisovans in East Asia.
How do population size reductions and gene flow impact the genetic variation of populations?
Describe the role of genetic variation in human disease. Going through a dramatic reduction in size reduces a population’s genetic variation. A high rate of migration between populations may lead to gene flow, which decreases inter-population and increases intra-population variation. Gene flow primarily between nearby populations may contribute to the formation of clines in allele frequencies.
Explain the reasons why variation in a DNA sequence can have no effect on the fitness of an individual. Variation in a DNA sequence can have no effect on fitness for a number of reasons. First, the variation may not occur in a coding or regulatory region of DNA, and therefore would not affect phenotype. Even if it did occur in a coding region of DNA, it may not affect phenotype because it might not change the amino acid sequence of the encoded protein or it might not affect how the protein functions even if it does change the amino acid sequence. If a genetic variation does not affect the phenotype, it cannot affect fitness. Finally, even if it does affect the phenotype, it does not necessarily mean that it affects fitness — i.e., it could be a neutral phenotypic change.
Explain why migration between populations decreases inter-population genetic variation. How does this relate to the development of clines in allele frequency? Migration between populations decreases inter-population (between population) genetic variation because when individuals move between populations, their different alleles are included in the gene pool of the population that they move to. Interbreeding will often also occur between individuals who were originally from different populations. For these reasons, there will be fewer genetic differences between these populations if individuals are moving between them. Migration relates to the development of clines (i.e. gradual differences) in allele frequency because it causes gene flow between adjacent populations. If there was no gene flow, you would expect to see discrete areas of more significant differences in allele frequency.
The amount of mixing of modern human DNA and archaic human DNA is an example of admixture.
6.3 Classifying Human Variation: Review Questions and Answers
Name the 18th century taxonomist that classified virtually all known living things. Carl Linnaeus
Describe the typological approach to classifying human variation. The typological approach to classifying human variation involves creating a typology, which is a system of discrete types, or categories, such as races. People are sorted into these categories based on a few readily observable phenotypic traits, such as skin colour, hair texture, facial features, and body build.
Discuss why typological classifications of Homo sapiens are associated with racism. Typological classifications of Homo sapiens are associated with racism because unrelated and often negative traits are attributed to certain racial categories. This may lead to prejudice and discrimination against people based only on how they look. Race and racism are deeply ingrained in our history and culture.
Why is the breeding population considered to be the most meaningful biological group? The breeding population is considered to be the most meaningful biological group because it is the unit of evolution. It includes people who have mated and produced offspring together for many generations. As a result members of the same breeding population should share many genetic traits.
Explain why it is generally unrealistic to apply a populational approach to classifying the human species. It is generally unrealistic to apply a populational approach to classifying the human species because most human populations are not closed breeding populations. There has been and continues to be too much gene flow between populations.
What does a clinal map show? A clinal map shows the geographic distribution of a trait or allele frequency.
Explain how gene flow and natural selection can result in a gradual change in the frequency of a trait over geographic space. Gene flow tends to vary with distance between populations. Closer populations are likely to exchange genes more often than populations that are farther apart. Such differences in gene flow could produce a gradual change in the frequency of a trait over geographic space. An environmental stressor may vary gradually over space, creating a geographic continuum of selective pressure. Variation in selective pressure may produce corresponding variation in a trait over space.
Most human traits vary on a continuum. Explain why this presents a problem for the typological classification approach. Since most human traits vary on a continuum, it is difficult to create a sharp dividing line between categories of people, as is done in the typological classification approach.
6.4 Human Responses to Environmental Stress: Review Questions and Answers
List four different types of responses that humans may make to cope with environmental stress. Four different types of responses that humans may make to cope with environmental stress are adaptation, developmental adjustment, acclimatization, and cultural responses.
Define adaptation. An adaptation is a genetically based trait that has evolved by natural selection because it helps living things survive and reproduce in a given environment.
Explain how natural selection may have resulted in most human populations having people who can and people who cannot taste PTC. PTC is an artificial, harmless, bitter-tasting compound similar to toxic bitter compounds found in plants. The ability to taste PTC may have been selected for because it helped people identify bitter-tasting toxic plants so they could avoid eating them. Nontasters are hypothesized to be able to taste a different, yet-to-be-identified bitter compound. The gene for PTC tasting has two alleles, T for tasting PTC and t for nontasting PTC (or for tasting some other bitter compound). People who have both alleles (Tt) should be able to taste the widest range of bitter compounds, so they would be the most fit and favored by natural selection. This would result in both alleles, as well as both taster and nontaster phenotypes, being maintained in populations.
What is a developmental adjustment? A developmental adjustment is a type of nongenetic response to environmental stress. It consists of a phenotypic change that occurs during development in infancy or childhood and that may persist into adulthood. This type of change may be irreversible.
Define phenotypic plasticity. Phenotypic plasticity is the ability to change the phenotype in response to changes in the environment, allowing individuals to respond to changes that occur during their lifetime.
Explain why phenotypic plasticity may be particularly important in a species with a long generation time. Phenotypic plasticity may be particularly important in a species with a long generation time because in such species the evolution of genetic adaptations may occur too slowly to keep up with changing environmental stresses.
Why may stunting of growth occur in children who have an inadequate diet? Why is stunting preferable to the alternative? Stunting of growth may occur in children who have an inadequate diet because they do not take in enough nutrients and calories to fuel both growth and basic metabolic processes. The nutrients and calories are shunted away from growth and toward maintaining life, allowing children to survive at the expense of increased body size. The alternative would be to use nutrients and calories for growth at the expense of life processes, which could possibly result in death.
What is acclimatization? Acclimatization is the development of reversible changes to environmental stress that generally occur over a relatively short period of time. When the stress is no longer present, the acclimatized state declines, and the body returns to its normal baseline state.
How does acclimatization to heat come about, and what are two physiological changes that occur in heat acclimatization? Acclimatization to heat occurs when one gradually works out harder and longer at high temperatures. It may take up to two weeks to attain maximum heat acclimatization. Two physiological changes that occur in heat acclimatization are increased sweat output and earlier onset of sweat production. These changes help the body lose heat through the evaporation of sweat, which is called evaporative cooling.
Give an example of a cultural response to heat stress. An example of a cultural response to heat stress is the use of air conditioning to maintain a cool environment.
Which is more likely to be reversible — a change due to acclimatization, or a change due to developmental adjustment? Explain your answer. A change due to acclimatization is more likely to be reversible than a change due to developmental adjustment. This is because in acclimatization, the phenotype reverts back to the baseline state once the stressor is gone. In developmental adjustment, the changes that occur during development may or may not be permanent, depending on the circumstances.
6.5 Variation in Blood Types: Review Questions and Answers
Define blood type and blood group system. Blood type is a genetic characteristic associated with the presence or absence of antigens on the surface of the red blood cell. Blood group system refers to all of the genes, alleles, and possible genotypes and phenotypes that exist for a particular set of blood type antigens.
Explain the relationship between antigens and antibodies. Antigens are molecules that the immune system identifies as either self or nonself. If antigens are identified as nonself, the immune system responds by forming antibodies that are specific to the nonself antigens. Antibodies are large, Y-shaped proteins produced by the immune system that recognize and bind to nonself antigens. They fit together like a lock and key. When antibodies bind to antigens, it marks them for destruction by other immune system cells.
Identify the alleles, genotypes, and phenotypes in the ABO blood group system. The ABO blood group system is controlled by one gene with three common alleles, represented by A, B, and O. There are six possible genotypes with three alleles: AA, AB, BB, BO, AO, and OO. Because A and B are codominant and both are dominant to O, there are four possible phenotypes: type A (AA, AO), type B (BB, BO), type AB (AB), and type O (OO).
Discuss the medical significance of the ABO blood group system. The ABO blood group system is the most important blood group system in blood transfusions. If red blood cells containing a particular ABO antigen are transfused into a person who lacks that antigen, the person’s immune system will recognize the antigen on the red blood cells as nonself and attack them, causing agglutination.
Compare the relative worldwide frequencies of the three ABO alleles. The ABO allele for antigen B is the least common worldwide. The allele for antigen A is more common than the allele for antigen B but less common than the allele for antigen O, which is the most common ABO allele.
Give examples of how different ABO blood types vary in their susceptibility to diseases.Answers may vary. Sample answer: People with type O blood may be more susceptible to cholera, plague, and gastrointestinal ulcers; but they may be less susceptible to malaria. People with type A blood may be more susceptible to smallpox and certain cancers.
Describe the Rhesus blood group system. The Rhesus blood group system is controlled by two linked genes with many alleles on chromosome 1. There are five main Rhesus antigens: D, C, c, E, and e. The major antigen is D, which is either present (Rh+) or absent (Rh-).
Relate Rhesus blood groups to blood transfusions. People with Rh+ blood can safely receive a blood transfusion of Rh+ or Rh- blood. People with Rh- blood can safely receive a blood transfusion only of Rh- blood.
What causes hemolytic disease of the newborn? Hemolytic disease of the newborn is caused by an Rh- mother producing antibodies to the D antigen in the blood of an Rh+ fetus. The maternal antibodies may destroy fetal red blood cells, causing anemia.
Describe how toxoplasmosis may explain the persistence of the Rh- blood type in human populations. Toxoplasmosis is a common parasitic disease that may have lasting neurological effects such as delayed reaction times, which can lead to an increase in traffic accidents and possibly other accidental injuries. People who are heterozygous for the Rhesus D antigen appear to be less likely to develop these lasting neurological effects, so they might be selected for by natural selection. If so, this could explain why both Rh+ and Rh- phenotypes persist in human populations.
A woman is blood type O and Rh-, and her husband is blood type AB and Rh+. Answer the following questions about this couple and their offspring.
What are the possible genotypes of their offspring in terms of ABO blood group? AO or BO
What are the possible phenotypes of their offspring in terms of ABO blood group? A or B
Can the woman donate blood to her husband? Explain your answer. Yes, because O is the universal donor since it has no A or B antigens, and in any case, AB is the universal recipient since it has both antigens. Also, since she is Rh-, she can donate to an Rh+ person.
Can the man donate blood to his wife? Explain your answer. No, because he is AB which contains the antigens for both A and B, and since she is type O she has antibodies against A and B. Also, because he is Rh+ and she is Rh-, her body will create antibodies against his D antigen as well.
Type O blood is characterized by the presence of O antigens — explain why this statement is false. This statement is false because the O allele actually codes for the absence of an antigen, which means there is no “O” allele, just the absence of an antigen.
Explain why newborn hemolytic disease may be more likely to occur in a second pregnancy than in a first Hemolytic disease of the newborn may be more likely to occur in a second pregnancy than in a first, because the generation of anti-D antibodies usually requires exposure to Rh+ blood in an Rh- person. This exposure may happen during an Rh- mother’s first birth to an Rh+ baby, and then in a subsequent pregnancy, the fetus is at risk of HDN because the anti-D antibodies are already present.
6.6 Human Responses to High Altitude: Review Questions and Answers
Define hypoxia. Hypoxia is a lack of oxygen.
Why does hypoxia occur at high altitudes? Hypoxia occurs at high altitudes because the atmosphere is less dense at high altitudes, so there is less oxygen in each breath and lower air pressure to move air from the lungs across cell membranes into the blood.
Describe the body’s immediate response to hypoxia at high altitude. The body’s immediate response to hypoxia at high altitude is an increase in the rate of breathing (hyperventilation) and elevation of the heart rate.
What is high altitude sickness, and what are its symptoms? High altitude sickness is a collection of symptoms that occur in response to the hypoxia at high altitude in a person who is not acclimated or adapted to this stress. It includes symptoms such as fatigue, shortness of breath, loss of appetite, headache, dizziness, and vomiting.
What changes occur during acclimatization to high altitude? During acclimatization to high altitude, additional red blood cells are produced, capillaries become more numerous in muscle tissues, the lungs increase slightly in size, and there is a small increase in the size of the right ventricle of the heart, which is the heart chamber that pumps blood to the lungs.
Where would you expect to find populations with genetic adaptations to high altitude? You would expect to find populations with genetic adaptations to high altitude in high altitude areas above 2,500 metres where people have been living continuously for thousands of years, including the Andes Mountains, Himalaya Mountains, Tibetan Plateau, and Ethiopian Highlands.
Discuss variation in adaptations to high altitude in different high altitude regions. Different high altitude populations have evolved different adaptations to the same hypoxic stress. Tibetan highlanders, for example, have a faster rate of breathing and large arteries, whereas Peruvian highlanders have larger red blood cells and a greater concentration of the oxygen-carrying protein hemoglobin.
Why do you think that adaptations to living at high altitude are different in different regions of the world?
Using human responses to high altitude as an example, explain the difference between acclimatization and adaptation.
Why are most humans not well-adapted to living at high altitudes?
If a person that normally lives at sea level wants to climb a very high mountain, do you think it is better for them to move to higher elevations gradually or more rapidly? Explain your answer.
6.7 Human Responses to Extreme Climates: Review Questions and Answers
Compare and contrast hypothermia and hyperthermia. Both hypothermia and hyperthermia are dangerous responses to temperature extremes. Hypothermia is a decrease in core body temperature that occurs in the cold. Hyperthermia is an in increase in core body temperature that occurs in the heat.
State Bergmann’s and Allen’s rules. Bergmann’s rule states that populations or species have larger body size in colder climates, and vice versa. Allen’s rule states that populations or species have longer extremities in warmer environments, and vice versa.
How do the Maasai and Inuit match the predictions based on Bergmann’s and Allen’s rules? The Maasai, who live in the tropics, have long, linear bodies with very long legs, so they have a heat-adapted body build as predicted by Bergmann’s and Allen’s rules. The Inuit, who live in the Arctic, have stocky bodies with relatively short limbs, so they have a cold-adapted body build as predicted by Bergmann’s and Allen’s rules.
Explain how sweating cools the body. Sweating coats the skin with moisture. When the moisture evaporates, it requires heat. The heat comes from the surface of the body, which cools the body.
What is the heat index? The heat index is a number that combines air temperature and relative humidity to indicate how hot the air feels due to the humidity.
Relate the heat index to evaporative cooling of the body. When the heat index is high for a given air temperature, it means that the relative humidity is high. With high humidity, sweat will not evaporate readily from the body, and evaporative cooling will not be very effective.
Identify three heat-related illnesses, from least to most serious. Three heat-related illnesses from least to most serious are heat cramps, heat exhaustion, and heat stroke.
How does heat acclimatization occur? Heat acclimatization occurs by gradually increasing the duration and intensity of working out at high temperatures. Maximum acclimatization may take up to 14 days. Changes that occur with acclimatization include greater sweat production, decreased salt concentration in sweat, earlier onset of sweating, and vasodilation near the skin so blood can bring heat to the surface of the body from the body core.
State two major ways the human body can respond to the cold, and give an example of each. Two major ways the body can respond to cold are by generating more heat (for example, by shivering) and by conserving more body heat (for example, by vasoconstriction).
Explain how and why the hunting response occurs. The hunting response occurs as a response to cold. It involves alternating vasoconstriction and vasodilation in the extremities. This helps conserve body heat while preventing cold injury to the extremities.
Define basal metabolic rate. Basal metabolic rate is the amount of energy a person needs to keep the body functioning at rest.
How does a high-fat diet help prevent hypothermia? A high fat diet helps prevent hypothermia by increasing the basal metabolic rate so the body generates more heat.
Explain why frostbite most commonly occurs in the extremities, such as the fingers and toes. Frostbite most commonly occurs in the extremities, such as the fingers and toes, because one of the body’s responses to cold is vasoconstriction, which moves blood away from the extremities to protect the internal organs in the body’s core. This leaves the extremities more vulnerable to cold and frostbite.
6.8 Nutritional Adaptation: Review Questions and Answers
Distinguish between the terms lactose and lactase. Lactose is a disaccharide found in milk. Lactase is an enzyme that is needed to digest lactose by breaking it down into its two component sugars.
What is lactose intolerance, and what percentage of all people have it? Lactose intolerance is the inability to synthesize lactase and digest lactose after early childhood, leading to symptoms such as bloating and diarrhea if milk is consumed. Lactose intolerance occurs in about 60 per cent of all people.
Where and why did lactase persistence evolve? Lactase persistence evolved in populations that herded milking animals for thousands of years. People who were able to synthesize lactase and digest lactose throughout life were strongly favored by natural selection.
What is the thrifty gene hypothesis? The thrifty gene hypothesis posits that “thrifty genes” were selected for because they allowed people to use calories efficiently and store body fat when food was plentiful so they had a reserve to use when food was scarce. Thrifty genes become detrimental and lead to obesity and diabetes when food is plentiful all of the time.
How well is the thrifty gene hypothesis supported by evidence? Several assumptions underlying the thrifty gene hypothesis have been called into question, and genetic research has been unable to actually identify thrifty genes.
Describe an alternative hypothesis to the thrifty gene hypothesis. Several alternative hypotheses to the thrifty gene hypothesis have been proposed, so answers may vary. Sample answer: The drifty gene hypothesis explains variation in the tendency to become obese and develop diabetes by genetic drift on neutral genes.
Do you think that a lack of exposure to dairy products might affect a person’s lactase level? Why or why not? Answers may vary. Sample answer:I think that a lack of exposure to dairy products might affect a person’s lactase level, because production of lactase may not just depend on genes—it also may depend on exposure to lactose.
Describe an experiment you would want to do or data you would want to analyze that would help to test the thrifty phenotype hypothesis. Remember, you are studying people, so be sure it is ethical! Discuss possible confounding factors that you should control for, or that might affect the interpretation of your results. Answers will vary. Sample answer: To test the thrifty phenotype hypothesis, I would examine data on the rates of type II diabetes in adulthood for people whose mother was pregnant with them during times and regions of famine. Times of famine might have additional factors, such as types of food available, extreme maternal stress, or other environmental conditions that could also affect the development of diabetes, other than overall lack of food. Also, you may not be able to determine whether an individual’s mother personally experienced famine, or to what extent. It may be completely unknown or you may need to rely on self-reporting of events that happened many years ago.
Explain the relationship between insulin, blood glucose, and type II diabetes. Diabetes is a disease that occurs when there are problems with the pancreatic hormone insulin, which normally helps cells take up glucose from the blood and controls blood glucose levels. In type II diabetes, body cells become relatively resistant to insulin, leading to high blood glucose.
Chapter 6 Case Study Conclusion: Review Questions and Answers
Explain why an evolutionarily older population is likely to have more genetic variation than a similarly-sized younger population. The older a population is, the longer it has been accumulating mutations, so therefore an older population is likely to have more genetic variation than a similarly-sized younger population.
The genetic difference between any two people on Earth is only about 0.1 per cent. Based on our evolutionary history, describe one reason why humans are relatively homogeneous genetically. Answers may vary, but can include: modern humans’ relatively recent evolution (less than a quarter million years ago), which is a relatively short period of time for mutations to accumulate; the relatively small human population size (possibly around 10,000 adults) in the past, which also limited genetic variability.
What aspect(s) of human skin colour are due to adaptation? Be sure to define adaptation in your answer. What aspect(s) of human skin colour are due to acclimatization? Be sure to define acclimatization in your answer. Adaptation is a genetic change that occurs through natural selection. Adaptations that influence skin colour in humans include the type and amount of melanin produced by the skin. Acclimatization is a temporary physiological change in response to environmental stress. The ability of the skin to become darker, or tan, when exposed to UV radiation is a type of acclimatization that influences skin colour.
For each of the following human responses to the environment, list whether it can be best described as an example of adaptation, acclimatization, or developmental adjustment:
Reduction in height due to lack of food in childhood Developmental adjustment.
Resistance to malaria Adaptation.
Shivering in the cold Acclimatization.
Changes in body size and dimensions to better tolerate heat or cold Adaptation.
Give an example of a human response to environmental stress that involves changes in behavior, instead of changes in physiology. Answers will vary but may include: the creation of shelters, clothing, and technology such as air conditioning.
What are two types of environmental stresses that caused genetic changes related to hemoglobin in some populations of humans? Malaria and high altitude
The ability of an organism to change their phenotype in response to the environment is called phenotypicplasticity.
List three natural selection pressures that differ geographically across the world and contributed to the evolution of human genetic variation in different regions. Answers may vary. Sample answer: Altitude; climate; UV levels; presence of endemic malaria.
You may have noticed that when a sudden hot day occurs during a cool period, it can feel even more uncomfortable than higher temperatures during a hot period — even with the same humidity levels. Using what you learned in this chapter, explain why you think that happens.Answers may vary. Sample answer:I think that a sudden hot day during a cool period feels particularly uncomfortable because your body has not yet acclimated to the heat. Full heat acclimatization can take weeks. During longer periods of heat, your body acclimatizes physiologically to cool you more effectively.
Out of all mammals, why are humans the only ones that evolved lactase persistence? Humans are the only mammals that evolved lactase persistence, because humans are the only mammals to consume milk in adulthood, due to our domestication of other species for food. It is energetically costly to produce an enzyme that is not needed, so that is probably why other mammals stop making it after the weaning period.
If the Inuit people who live in the Arctic were not able to get enough vitamin D from their diet, what do you think might happen to their skin colour over a long period of time? Explain your answer.Answers may vary. Sample answer: I think that if the Inuit people were not able to get enough vitamin D from their diet, over a long period of time their skin colour may become lighter. This is because vitamin D, which is important for health, can be synthesized by the skin from UV light exposure. UV light penetrates lighter skin better than darker skin, so people with lighter skin will produce vitamin D more easily. In the absence of sufficient vitamin D in the diet in the Arctic where UV levels are low, people with lighter skin may have an evolutionary advantage due to better health. Over a long period of time, that may lead to the population as a whole having lighter skin.
Explain why malaria has been such a strong force of natural selection on human populations. Answers may vary. Sample answer: Malaria has been such a strong force of natural selection on human populations for several reasons. First, it is widespread in areas consistently inhabited by large numbers of humans, particularly after the advent of agriculture, because of the nature of malaria life cycle. Second, malaria has been around for a long period of human history, and natural selection causes evolutionary changes only over many generations. Third, malaria is often deadly, particularly to young children and infants, and can cause miscarriages and stillbirths. Because it affects the reproductive rate in this manner, malaria is a strong force of natural selection, dramatically reducing the fitness of individuals that are susceptible to it.
Give one example of “heterozygote advantage” (i.e. when the heterozygous genotype has higher relative fitness than the dominant or recessive homozygous genotype) in humans. Answers will vary but may include: hemoglobin adaptations in response to malaria; the Rhesus D antigen; the taster/nontaster alleles.
What is one way in which humans have evolved genetic adaptations in response to their food sources? Answers will vary but may include: lactose persistence; taster genes; the ability to survive on lower amounts of food.
Do you think adaptation to high altitude evolved once or several times? Explain your reasoning. Answers may vary. Sample answer: Adaptations to high altitude probably evolved independently several times because the specific adaptations are different in different regions. If it had evolved once, you would expect the adaptation to be the same in different populations.