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Chapter 15

Chapter 15 How Humans Evolved - Bio 1M03

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James S Quinn

Bio 1M03 How Humans Evolved Part 4: Evolution And Modern Humans Chapter 15: Human Genetic Diversity Explaining Human Variation  Scientists distinguish two sources of human variation: genetic and environmental o Genetic Variation – differences between individuals that are caused by the genes they inherited from their parents o Environmental Variation – differences between individuals caused by environmental factors (climate, habitat, and competing species) of the organisms phenotypes  It is difficult to determine the relative importance of genetic and environmental influences for a particular phenotypic traits o Both genetic transmission and shared environments cause parents and offspring to be similar o Genetic variation is governed by the processes of organic evolution: mutation, drift, recombination and selection  It is important to distinguish variation within human groups from variation among human groups o Variation within groups – differences between individuals within a given group of people o Variation among groups – differences between entire groups of people Variation In Traits Influenced By Single Genes  By establishing the connection between particular DNA sequence and specific traits, scientists have shown that variation in some traits is genetic o Eg/ Sickle Cell Anemia – causes red blood cells to have a sickle shape instead of a rounded shape  Homozygous for a gene that codes for one variant of hemoglobin (protein that transports oxygen molecules in red blood cells)  Hemoglobin is made up of two different protein subunits: α and β  DNA sequence of the moth common hemoglobin allele, hemoglobin A, specifies the amino acid glutamate acid in the 6 position of the protein chain of the β subunit  Another hemoglobin allele, hemoglobin S, specifies the amino acid valine in the 6 position – people who suffer from sickle-cell anemia are homozygous for hemoglobin S  We can prove that traits are controlled by genes at a single genetic locus by showing that their patterns of inheritance conform to Mendel’s principles o When traits are affected by genes at a single genetic locus, we can make the distinction between genetic and environmental sources of variation using Mendel’s principles o Eg/ Specific Language Impairment (SLI) – difficulty learning to speak, small vocabulary, grammatical errors  Suggested dominant allele at a single genetic locus  KE family, 3 generations – Grandmother (blue circle) had SLI  SLI is rare in the population as a whole – almost all sufferers are heterozygotes  Anyone who does not have SLI must be homozygous for normal allele at this locus  Could potentially be environmental factors that cause SLI  SLI in KE is closely linked to a gene marker on chromosome 7 – controlled by a gene linked to this marker  FOXP2  Allele of a gene that was found in an unrelated person with SLI  Not found in people without SLI  Undergone strong directional selection since the divergence of humans and chimpanzees Bio 1M03  Damage to FOXP2 gene prevents normal development of some of the psychological machinery necessary for language o Not responsible for the entire psychological machinery of SLI  Codes for a transcription factor belonging to a gamily of genes that play an important role in regulating gene expression during development o Scientists search for two kinds of data to validate whether it is genetic or environmental  Collect data on more families  Search for genetic markers (genes whose location in the genome is known) that show the same pattern of inheritance  If every individual who has the disorder has a specific marker on a particular chromosome, can be confident that the gene that causes SLI lies close to that genetic marker Causes of Genetic Variation Within Groups  Mutation can maintain deleterious genes in populations, but only at a low frequency o Many diseases are caused by recessive genes – must be homozygous o Mutant genes that code for proteins that do not serve their normal function can produce server debilitation and sometimes death  Eg/ Sickle cell anemia, PKU, Tay-Sachs Disease, Cystic Fibrosis o Natural selection hasn’t eliminated these genes because:  Natural selection steadily removes such genes, but they are constantly being reintroduced by mutation  Low rates of mutation can maintain these deleterious genes because they are recessive traits  Most individuals who carry the gene are heterozygous for the deleterious gene and do not suffer from the affects  Selection-Mutation Balance – when selection eliminates the gene, but mutation introduces enough new mutants to maintain a constant frequency of the gene  Selection can maintain variation within populations if heterozygotes have higher fitness than either of the two homozygote o Some lethal genes are too common to be the result of selection-mutation balance o Heterozygote Advantage o Eg/ Sickling Allele, S vs Normal Allele A in West Africa  Heterozygotes, SA are more common than homozygotes AA or SS  SA – 15% more likely to reach adulthood because partially protected against falciparum malaria  SS – not selected toward, because causes sickle cell anemia  AA – not selected toward because doesn’t have added protection against malaria  Natural Selection maintains a balanced polymorphism 0 steady state in which both alleles persist in the population  Variation may exist because environments have recently changes and genes that were previously beneficial have not yet been eliminated o Eg/ Non Insulin-Dependent Diabetes (NIDD) – blood sugar levels rise above normal levels because the cells of the body do not respond properly to insulin in the blood  Insulin – protein that controls the uptake of blood sugar by cells  High blood sugar levels cause a number of problems – heart disease, kidney damage, impaired vision  NIDD is known to have a genetic basis  Insulin Dependent Diabetes – occurs because the insulin-producing cells in the pancreas have been destroyed by the body’s own immune system – unlikely that it was ever adaptive  NIDD may have been adaptive because it causes buildup of fat reserves  Populations may have had periods of plenty, in which fat reserves could be built up  During periods of famine, the fat reserves would help people survive Causes of Genetic Variation Among Groups  Selection that favors different genes in different environments creates and maintains variation among groups Bio 1M03 o Gene that controls the digestion of lactose  Lactose – a sugar found in mammalian milk; synthesized in the mammary glands and occurs in large amounts only in mammalian milk  Most mammals have the ability to digest lactose as infants but gradually lose this ability after they are weaned  Many people lack lactose persistence – if they drink more than ½ of a liter, they suffer gastric distress  Lactose Persistence – ability to digest lactose as adults  Northern Europeans and a number of North Africans and Arabian populations  LTC*P – dominant gene that controls the ability to digest lactose as an adult  People who have one copy of LTC*P continue to synthesize lactase as adults  LTC*R – people who are homozygous for LTC*R cannot synthesize lactose  Beja people; nomadic people who wander with their herds of camels and goats in the desert between the Nile and the Red Sea  During 9 month dry season – rely entirely on milk of camels and goats; 3 L of fresh milk/day, obtain protein, energy and water from milk  Must drink ilk fresh, high temperatures prevent storage of milk, nomadic lifestyle makes to difficult to make cheese and yogurt that lack lactose  Enhanced absorption of Vitamin D – lactose digestion is favored  When sunlight penetrates the fatty tissues of human skin – vitamin D is synthesized  In sunny environments, vitamin D deficiency is rare  In Northern Europe, dark, cloudy, long winters create a potential for vitamin D deficiency  Ability to digest lactose may have evolved since people began domesticating livestock in the Middle East and North Africa about 7 kya  Lactase gene has been subjected to recent strong selection
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