ANTH 1B Ch - Unit 1B of CSN Physical Anthropology 102 Notes Chapter 4; Dr. Levin - Our Origins PDF

Preview

Summary

Unit 1B of CSN Physical Anthropology 102 Notes Chapter 4; Dr. Levin...

Description

Chapter 4: Genes and Their Evolution - Popular Genetics Big Questions: 1) What causes evolutionary (genetic) change? 2) How is evolutionary (genetic) changed measured, and how is the cause determined? Gene frequencies tied to natural selection High population sickle cell along Kenya’s coast (southeast) and near Lake Victoria (southwest), a remarkably high 20%-30% of the population. The highlands (west) with no malaria, only 1% carried the gene. - Allison’s theory: he hypothesized that individuals with the sickle-cell allele were resistant to malaria and that natural selection was operating on the gene. But how? - His results showed that carriers of the gene are much more likely to survive malaria than are noncarriers. Natural selection was favoring the carriers. - “Disease is an agent of natural selection.” - Science of genetics helps biologists answer questions about biological differences

4.1 Demes, Reproductive Isolation, and Species Deme: members of a species that produce offspring; study genetics of individuals focusing on the reproductive population. Evolution is about groups of organisms that have the potential to reproduce. Gene pool: when physical anthropologists talk about populations; all the genetic material within a population. - The central definition of species is the concept of the breeding population. - A species is composed of all the populations (and their individual members) that are capable of breeding with each other and producing viable (fertile) offspring. - Reproductive isolation: members of one population cannot interbreed with members of the other; related to geographic isolation. If two populations of the same species become isolated, enough genetic differences could accumulate for two entirely different species to emerge (like a mountain range or a large body of water). Population genetics: the study of changes in genetic material- specifically, the change in frequency of alleles (genes). Genes are the records from which evolution is reconstructed, both over the course of a few generations (microevolution) and over many generations (macroevolution). - “Geneticists strive to document genetic change and to explain why it occurred. Such documentation and explanation are the central issues of evolutionary biology.” Equilibrium: frequency remains relatively constant Anthropology Matters: Got Milk? The LCT Phenotype Lactose Tolerance and Lactase Persistence Lactose is a complex sugar composed of the linked simple sugars glucose and galactose. Break down the lactose into two sugar components. Populations across Asia, Africa, Australia, southern Europe, and both North and South America experience a decline in lactase activity after weaning. People lose the ability to digest lactose.

-

-

-

Lactase persistence: inherited autosomal dominant gene LCT w/ three genotypes PP (homozygous for lactase persistence), RR (homozygous for lactase nonpersistence), and PR (homozygous for lactase persistence but having the intermediate output of the enzyme.) Located on chromosome 2. 77% of European Americans and only 14% of African Americans have the LCT gene, and most human populations approach the African American frequency. History shows elevated lactase activity as well as animal husbandry and milk consumption. The ability to digest lactose reflects peoples’ histories: Europe, West Africa, and a band extending from East Africa eastward across the Arabian Peninsula to Inda

4.2 Hardy-Weinberg Law: Testing the Conditions of Genetic Equilibrium Godfrey Hardy and Wilhelm Weinberg recognized that some alleles are in a state of equilibrium. - If no mutation or natural selection or gene flow occurs, if the population is large, if mating is random, and if all members of the population produce the same number of offspring, then genotype frequencies at a single gene locus will remain the same after one generation. Hardy-Weinberg law of equilibrium: the equilibrium frequencies will be a function of the allele frequencies at the locus. - A single locus has A (dominant) and a (recessive) alleles, with respective frequencies of p and q. - Prediction of genotype frequencies: p^2 for AA homozygous alleles, 2pq for Aa (heterozygous) alleles, q^2 for aa homozygous alleles - The total population (100%) should be the sum of the frequencies of three genotypes with this mathematical equation - The equilibrium worked out in a simple Punnett square, is used to assess whether gene frequencies have changed from one generation to the next. It can also be used to estimate the genotype frequencies of the subsequent generation - In the absence of evolution, the frequencies of the genotypes will, in theory, remain the same forever. In this way, the Hardy-Weinberg equilibrium hypothesizes that gene frequencies remain the same because no evolutionary change takes place - If the genotype frequencies change from one generation to the next, the population is not in equilibrium- it is evolving. 4.3 Mutation: The Only Source of New Alleles Mutation: the replication process produces an error or a collection of errors in the DNA code. The mutation can be any heritable change in the structure or amount of genetic material. For noncoding nonfunctional DNA, mutations do not affect the individual’s health, wellbeing, or survival. However, a new sequence of coding DNA that results from mutation may have profound consequences, positive or negative. - Mutations occur at random and can occur in any cell, but the ones with

consequences for future generations take place in gametes. - Gametes may transfer mutations to offspring, depending on what happens during meiosis - Base substitution: the simplest kind of mutation, occurs when a single nitrogen base is submitted by another base. - The mRNA strand receives complimentary guanine rather than the adenine that pairs with thymine. This further substitution changes the amino acid that is added to the polypeptide chain at the ribosomes. - Base insertion occurs when a nitrogen base is added to the DNA template. - The mRNA receives the appropriate complementary nitrogen base, uracil. When the mRNA reaches the ribosomes, the series of codons is changed because of the insertion. Instead of reading the correct AUU, the ribosomes read UAU, which codes for the amino acid tyrosine instead of isoleucine. Point mutations: mutations involving incorrect base pairing Synonymous point mutation: an altered triplet in the DNA is created, but the alteration carries with it the original amino acid. Because the amino acid is the same, the protein formed is the same. Nonsynonymous point mutation: a matchup that brings along a different amino acid. Such mutation can have dramatic results for the individual carrying it. Frameshift mutation: a result of the shifting base pairs caused by base insertion, the reading frame of the gene is altered or stopped entirely; this mutation produces a protein having no function. Such a mutation usually involves a small part of the DNA sequence, often just a base pair or a relatively limited number of base pairs. Transposable elements: (affects far more than the genome) genes that can copy themselves to entirely different places along the DNA sequence. If such a gene inserts itself into another gene, it can fundamentally alter the other gene, doing real damage. If the gene transposes itself to a noncoding area of the DNA sequence, little or no significant alteration will occur. Example of trisomies: Down syndrome; entire chromosomes are duplicated, extra chromosome 21 and Klinefelter’s syndrome, a common sex chromosome variant that appears in about 1 of 500-1,000 births, which men only receive. This extra chromosome usually leads to sterility and can produce a range of physical effects, from a youthful, lanky appearance to a rounded, generally feminized body with gynecomastia (additional breast tissue). Two types of mutations: 1) Spontaneous mutations: no known cause 2) Induced mutations: caused by specific environmental agents, usually associated with human activity - Mutagens: agents - Most mutations are spontaneous and some are DNA copying errors. - The human mutation rate is higher in male sex cell (sperm) than female eggs but is generally on the order one per million per nucleotide per generation.

-

The human genome includes about 3 million base pairs, about 1.5% of which code for proteins, so the average mutation rate in humans is .45 mutations in proteincoding genes per generation or about one new, potentially significant mutation in every other person born.

4.4 Natural Selection: Advantageous Characteristics, Survival, and Reproduction Fitness: natural selection focuses on reproductive success - Darwin’s conclusion that individuals with advantageous characteristics will survive in higher numbers and produce more offspring than members of a population lacking advantageous characteristics. - Defined in reference to individuals in a population or to specific genotypes; some genotypes have more (or less) fitness than other genotypes. Changes to allele frequencies. Patterns of Natural Selection 3 alternative patterns by which natural selection can act on a specific trait: 1) Directional selection: favors one extreme form of a trait- more children are produced by individuals who have that extreme trait, so selection moves in that direction. 2) Stabilizing selection: favors the average version of a trait. - E.g. living humans whose birth weights are in the middle of the range have a better chance of surviving and reproducing than do those born with the lowest and highest weights. 3) Disruptive selection: the pattern of variation is discontinuous. Individuals at both extreme ends of the range produce more offspring than does the remainder of the population. Natural Selection in Animals: The Case of the Peppered Moth and Industrial Melanism The best evidence ever documented of natural selection operating on a heritable trait concerns the peppered moth, Biston betularia, a species common throughout Great Britain. White-colored sprinkled with black. A new species name, Biston carbonaria, distinguished this melanic (dark) form from the nonmelanic (light) form. - The frequency of the melanic form remained relatively low for a couple of decades but climbed rapidly in the late nineteenth century. The 1950s, 90% of peppered moths were melanic. - The rapid increase in melanic frequency was a case of evolution profoundly changing phenotype. - Directional selection had favored the melanic form over the non melanic form, and the melanic form exhibited a greater fitness. But what was this form’s adaptive advantage? - The Industrial Revolution. The charcoal on air and trees changed the color surface of the trees from light-colored to black, greatly altering the peppered moth’s habitat. Nonmelanic died because they had no camouflage. - The pollution crisis provided a huge selective advantage for the melanic

moths - However, the Biston carbanaria dropped when stricter pollution laws were developed, affecting the moth population. 90% to 10% - This rapid evolutionary change reflected the return of the original coloration of the trees, which conferred a selective disadvantage- predation visibility on the melanic variety. - Example of positive selection Natural Selection in Humans: Abnormal Hemoglobins and Resistance to Malaria Positive selection: an organism’s biology is shaped by selection for beneficial traits. Sickle-cell anemia: targets of natural selection, the sickle-cell allele- the hemoglobin S (or simply S) allele. Hemolytic anemias: the destruction of red blood cells. A low number of red blood cells can produce health problems because of the resultant lack of hemoglobin, the chemical in red blood cells that carries oxygen to all the body tissues. - The S gene yields a specific kind of abnormal hemoglobin. G6pd Gene: one target of natural selection in humans is this gene, located on the X chromosome (glucose-6-phosphate dehydrogenase); a deficiency enzyme, without this enzyme, a person who takes sulfa-based antibiotics or eats fava beans risks the destruction of red blood cells, severe anemia, and occasionally death. “favism” Sickle-Cell Gene: Hemoglobin S appears on human chromosome 11 - Sickle-cell anemia begins with a single nitrogen base mutation- a base substitution. The abnormal hemoglobin that results is less efficient at binding oxygen and causes red blood cells to become sickle-shaped - Simple base-pair mutation - People who carry the sickle-cell allele on one of the two homologous chromosomes only are AS, and people who have the homozygous form of the disease are SS. AS individuals are for all practical purposes normal in their survival and reproduction rates. Capillaries: the narrow blood vessels that form networks throughout tissues. When the clogging of capillaries cuts off the oxygen supply in vital tissues, severe anemia and death can result. How sickle-cell appears: - 20%-30% had the S gene in equatorial Africa. - Discovery that high heterozygous (AS) frequencies appear in regions of Africa where malaria (a potentially lethal parasitic infection in which the parasite is introduced to a human host by a mosquito) is endemic - AS people (sickle-gene carriers) die of malaria more than AA carriers do. - Low-wet areas of Kenya, and higher in the highland areas Balanced polymorphism: suffer less because the frequencies are maintained Anthropogenic: of environmental pollution, originating in human activity Hemoglobinopathies: rare mutation due to environmental conditions Thalassemia: genetic anemia found in Europe (especially in Italy and Greece), Asia, and

the Pacific, reduces or eliminates hemoglobin synthesis. The presence of malaria makes a strong case for a selective advantage for heterozygous individuals, for whom the condition and malaria are not lethal. 4.5 Genetic Drift: Genetic Change Due to Chance One of the four forces of evolutionary genetic drift is a random change in allele frequency over time. Variations in human populations work in that the genetic drift operates over a period of time rather than at a single point. - The probability of an allele’s frequency changing in a relatively short period of time increases with decreasing population size. The larger the population, the less divergence from the original gene frequency over time - Endogamous: discouraging reproduction outside the group; genetic drift might occur in a small group - Exogamous: extends reproduction outside its community - A smaller population means greater chance for genetic drift Founder Effect: A Special Kind of Genetic Drift Founder Effect: one form of genetic drift, occurs when a small group (fewer than several hundred members) of a large parent population migrates to a new region and is reproductively isolated. Founder effect has also been documented in several genetic diseases that affect humans. Huntington’s chorea, genetic abnormality caused by an autosomal dominant gene (located on chromosome 4 at the locus that codes for the Huntingtin protein and a person needs only one allele from a parent to have the disease). - Huntington’s chorea appears in very high frequencies in communities around Lake Maracaibo, Venezuela- more than half the occupants of some villages have the disorder. - Nancy Wexler spent years tracking the genealogies of families living there finding that everyone with the disease was descended from one woman who had lived 200 years earlier and carried the allele. The original carrier was not representative of her parent population, which would have had a much lower frequency of the allele. - Whenever interbreeding occurs across population boundaries, gene flow occurs. 4.6 Gene Flow: Spread of Genes across Population Boundaries Gene flow: another force of evolution; the transfer of genes across population boundaries. The key determinant for the amount of gene flow is the accessibility to mates- the less the physical distance between populations, the greater the chance of gene flow. While mutation increases genetic variation between two populations over time, gene flow decreases such variation - Gene flow: new genetic material can be introduced into a population through gene

flow from another population. Say for example, that a population in one place has genes for only brown hair. Members of that population interbreed with an adjacent population, which has genes for only blond hair. After interbreeding for a generation, both populations have genes for blond and brown hair. As a result, when people from either population interbreed with yet another population, they may contribute alleles for brown, blond, or both. - Gene flow and genetic variation are highly influenced by social structure. Endogamous societies; Australian aborigines have relatively little genetic diversity because few individuals migrate into the community and thus little new genetic material is introduced. Exomagous societies have relatively high genetic diversity because proportionately more genetic material is brough into the gene pool. - Specific genetic markers in living populations, such as the ABO blood group system, provide evidence of gene flow across large regions. For example, the frequencies of type B blood change gradually from eastern Asia to farwestern Europe, This clinal—that is, sloping—trend was first noted in the early 1940s by the American geneticist Pompeo Candela, who made the case that the gradient from east to west reflects significant gene flow that occurred as Mongol populations migrated westward from AD 500 to 1500. - Subsequent data on blood groups have revealed sharp distinctions in frequencies between adjacent populations. These distinctions suggest that, in addition to gene flow, genetic drift within small, isolated groups contributed to the frequency variations. Agriculture and Origins of Modern Europeans 1925, the Australian archaeologist V. Gordon Childe proposed that Europe’s first farming communities were founded by Middle Eastern peoples who invaded Europe, replaced the hunter-gatherers there, and become the ancestors of today’s Europeans. In the 1970s, the Italian geneticist Luca Cavalli-Sforza and the American archaeologist Albert Ammerman countered Childe’s hypothesis with a model - Demic diffusion: framing the origin of agriculture as not invasion but rather gradual expansion and accompanying gene flow. - Middle Eastern peoples invented farming in their homelands of Western Asia, then expanded and moved, interacting with, interbreeding with, and exchanging ideas about food production with local European populations. Ideas can spread without gene flow, as ideas about communication do today; but Cavalli-Sforza and Ammerman proposed that the spread of agriculture as a cultural innovation involved population spread. - Numerous contradictions that suggested weaknesses in their hypothesis. The record showed evidence of gene flow but not of uniform westward expansion. - Anatolians brought their farming practices to Europe, a key subsistence change that put into motion the spread of farming across the continent....