Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Sexual selection wikipedia , lookup
Gene expression programming wikipedia , lookup
The Selfish Gene wikipedia , lookup
Catholic Church and evolution wikipedia , lookup
Hologenome theory of evolution wikipedia , lookup
Genetic drift wikipedia , lookup
State switching wikipedia , lookup
Inclusive fitness wikipedia , lookup
Natural selection wikipedia , lookup
Theistic evolution wikipedia , lookup
Koinophilia wikipedia , lookup
CHAPTER 3 EVOLUTION, GENETICS, AND HUMAN VARIATION CHAPTER OVERVIEW This chapter discusses evolution and how it relates to human evolution and human variation. Particular attention is given to mechanisms of genetic evolution. CHAPTER OBJECTIVES 1. Understand the differences between creationism and evolution. 2. Know the importance of Mendel’s experiments and their implications for evolution. 3. Understand what population genetics is and how it studies evolution. 4. Be able to discriminate between the key mechanisms of evolution and understand the role each mechanism plays in evolution. CHAPTER OUTLINE I. Creationism and Evolution A. Creationism and Catastrophism 1. Creationism accounts for biological diversity by referring to the divine act of Creation as described in Genesis. 2. The discovery of fossil remains of creatures clearly unknown to modern humans was not accountable within the terms of simple creationism. 3. Catastrophism is a modified version of creationism, which accounts for the fossil record by positing divinely authored worldwide disasters that wiped out the creatures represented in the fossil record, who were then supplanted by newer, created species. 4. Both versions of creationism describe the different species of plants and animals as essentially different, having distinct, separate moments of creation. B. Evolution 1. An alternative term for early evolutionism was “transformism.” 2. Darwin was influenced by the geological concept of uniformitarianism. a. Uniformitarianism states that past geological events can be best explained by observing the ongoing events of the present and generalizing backward through time. b. It further asserts that current geological structures are the result of long-term natural forces. 3. Transformism had posited the primordial relatedness of all life forms. 4. Darwin posited natural selection as the mechanism through which speciation takes shape (reaching this conclusion along with Alfred Russell Wallace). 5. Natural selection is the gradual process by which nature selects the forms most fit to survive and reproduce in a given environment. 6. For natural selection to work on a given population, there must be variety within that population and competition for strategic resources. 7. The concept of natural selection argues that organisms which have a better fit within their environmental niche will reproduce more frequently than those organisms that fit less well. 1 II. Genetics A. The science of genetics explains the origin of the variety upon which natural selection operates. 1. Mendelian genetics studies the ways in which chromosomes transmit genes across generations. 2. Biochemical genetics examines structure, function, and changes in DNA. 3. Population genetics investigates natural selection and other causes of genetic variation, stability, and change in breeding populations. B. Mendel’s Experiments 1. The study of hereditary traits was begun in 1856 by Gregor Mendel, an Austrian monk. 2. By experimenting with successive generations of pea plants, Mendel came to the conclusion that heredity is determined by discrete particles, the effects of which may disappear in one generation and reappear in the next. 3. Mendel determined that the traits he observed occurred in two basic forms: dominant and recessive. a. Dominant forms manifest themselves in each generation. b. Recessive forms are masked whenever they are paired with a dominant form of the same trait in a hybrid individual. c. It has since been demonstrated that some traits have more than these two forms--human blood type, for example, has several forms, some of which are codominant. 4. The traits Mendel identified occur on chromosomes. a. Humans have twenty-three matched pairs of chromosomes, with each parent contributing one chromosome to each pair. b. Chromosomes contain several genes, or genetic loci, which determine the nature of a particular trait. c. A trait may be determined by more than one gene. d. Alleles are the biochemically different forms which may occur at any given genetic locus. e. Chromosome pairs’ loci may be homozygous (identical alleles) or heterozygous (mixed). C. Independent Assortment and Recombination 1. Mendel also determined that traits are inherited independently of one another. 2. The fact that traits are transmitted independently of one another, and hence may occur in new combinations with other traits, is responsible for much of the variety upon which natural selection operates. 3. Mitosis is ordinary cell division, wherein one cell splits to form two identical cells. 4. Meiosis is the type of division particular to sex cells, wherein four cells are produced from one, each with half the genetic material of the original cell (i.e., twenty-three chromosomes instead of forty-six). 5. Fertilization allows the products of meiosis from one parent to recombine with those from the other parent. 6. Because genes sort independently during recombination, the number of possible combinations is exponentially high (223): a major source of variety. III. Population Genetics A. Population genetics looks at changes in gene frequencies at the level of the community or breeding population 1. Gene pool refers to all of the alleles and genotypes within a breeding population. 2. Genetic evolution is defined as change in the frequency of alleles in the breeding population from generation to generation. B. There are four basic mechanisms which produce changes in gene frequency in a population: natural selection, mutation, genetic drift, and gene flow. 2 IV. Mechanisms of Genetic Evolution A. Natural Selection 1. Genotype refers to the genetic makeup of an organism. 2. Phenotype is the expression of the genotype as it has been influenced through development by interacting with its environment. 3. Environmental influence in this interaction is extremely important, and lends great plasticity to human biology. 4. Natural selection acts upon phenotypes. B. Directional Selection 1. Natural selection affects gene frequencies within a population. 2. Adaptive genes are selected for (organisms containing them reproduce more frequently). 3. Maladaptive genes are selected against (organisms containing them reproduce less frequently). 4. When specific adaptive genes are selected for over a long time period, causing a major shift in gene frequency, this is called directional selection. 5. Directional selection continues until equilibrium is reached (due to the effects of contradictory selective forces, the base mutation rate, or both). 6. Directional selection, in favoring one gene, can reduce variation in a gene pool. C. Sickle-Cell Anemia 1. Just as directional selection can reduce variety, it can also maintain genetic variety by favoring a situation in which the frequency of certain alleles remains constant between generations. 2. Hemoglobin in Africa a. HbA and HbS are two alleles for a gene which largely determines hemoglobin production in humans. b. Homozygous HbA produces normal hemoglobin; homozygous HbS produces lethal sicklecell anemia; heterozygosity for this gene produces (in some circumstances) the deleterious but nonlethal sickle-cell syndrome. c. It was discovered in certain populations in Africa, India, and the Mediterranean that Hb S existed at surprisingly high frequencies. d. This is largely explained by the fact that the populations noted were in heavily malarial areas, and that the heterozygous form produced a phenotype that was resistant to malaria, and was thus the phenotype most fit for that environment. 3. It is important to note that traits that are maladaptive in one environment, such as the sickle cell would be in a malaria-free zone, can be adaptive in a different environment, and the reverse of this is also true. D. Mutation 1. Mutation introduces genetic variation into a breeding population. 2. Chemical alterations in genes may provide a population with entirely new phenotypes, with possible concomitant selective advantages. 3. The spread of HbS in heavily malarial environments is one example. E. Random Genetic Drift 1. Random genetic drift is the loss of alleles from a population's gene pool through chance. 2. There is no set form for this chance; it may simply occur through a statistical fluke in sexual reproduction patterns, or through the effects of a catastrophe on the population as a whole. 3 F. Gene Flow 1. Gene flow occurs through interbreeding: the transmission of genetic material from one population to another. 2. Gene flow inhibits speciation, the formation of new species. a. A species is an internally interbreeding population whose offspring can survive and are capable of reproduction. b. Speciation occurs when populations of the same species become isolated from each other (thus stopping gene flow) allowing natural selection and genetic drift gradually to produce gene pools that are different, to the extent that successful interbreeding is no longer impossible. V. Race: A Discredited Concept in Biology A. In biological terms, a race is a geographically isolated subdivision of a species that can reproduce with individuals from other subspecies of the same species, but does not because of its geographic isolation. 1. Human populations vary biologically, but there are no sharp breaks between populations. 2. Human biological variation is distributed gradually between populations along clines. B. Ethnicity and race are not synonymous, although American culture does not discriminate between the two terms. C. Races Are Not Biologically Distinct 1. Race is supposed to describe genetic variation, but racial categories (particularly early on) are based on phenotypes. a. Phenotypes are the product of genetic, developmental, and environmental factors. b. There is no clear logical hierarchy to phenotypic traits, thus it is difficult to demonstrate which should be a definitive racial feature. 2. The so-called three great races (white, black, and yellow) are more a reflection of European colonialist politics than an accurate representation of human biological diversity. 3. Even skin color-based race models that include more than three categories do not accurately represent the wide range of skin color diversity among human populations. 4. Fundamental Problems with Phenotype-Based Race. a. Populations grouped into one race based upon phenotypic similarity may be genetically distinct; such similarities may be the result of parallel evolution or other factors. b. Genetic traits occur together due to the selective forces of the environments in which they evolved, and therefore do not constitute an internally coherent “type.” D. Explaining Skin Color 1. Natural selection “is the process by which nature selects the forms most fit to survive and reproduce in a given environment.” 2. Variation in skin color is determined by the amount of melanin in the skin cells, which in turn isgenetically determined. 3. Prior to the sixteenth century, darker skinned populations were closest to the equator, while lighter skinned populations were closer to the poles. 4. Selective Advantages and Disadvantages. a. Light skin in the tropics is selected against because it burns more easily, thus subjecting light-skinned individuals to a greater likelihood of infection and disease. b. Sunburn impairs the body's ability to withstand heat by reducing the skin’s ability to sweat. c. Light skin is more susceptible to skin cancer. d. The effect of sunlight on vitamin D formation indicates how dark skin might have been selected for in tropical environments (protection against hypervitaminosis D), and against in lower-sunlight environments (protection against rickets); and it further indicates how light skin might have been selected for in low-sunlight environments, and against in the tropics. 4 E. Lactose Tolerance 1. The term phenotypic adaptation refers to changes which occur to an individual organism during it's lifetime which enhance its reproductive fitness. 2. Individuals from herding populations in northern Europe and parts of Africa maintain their ability to digest milk (continue to produce the enzyme, lactase) into adulthood, whereas people from other populations can digest milk (specifically, milk sugar, called lactose) only during childhood. 3. The fact that descendents of these herding populations who no longer herd continue to be lactose tolerant as adults indicates genetic adaptation to a milk-rich diet. 4. The fact that lactose intolerance can vary during an individual's adult life, depending on how much milk is consumed, indicates that some phenotypic adaptation also takes place. VI. Box: American Anthropological Association (AAA) Statement on “Race” A. Human populations are not unambiguous, clearly demarcated, biologically distinct groups. B. There is greater genetic variation within racial groups than between them. C. Physical variations are distributed gradually rather than abruptly through space. D. Physical variations in human populations have no meaning other than the social ones societies attribute to them. E. Historically, racial categories have been used to divide, rank, and control populations ethnically separate from Western Europe. 1. Some populations have been assigned to a perpetual low status (e.g., African-Americans). 2. Other populations have been assigned to a perpetual high status with access to privilege, power, and wealth. LECTURE TOPICS 1. Use the sources of diversity to explain the genetic mechanism and the relationship between genotype and phenotype. List the types of diversity and the sources of diversity. Explain the relationships among diversity, adaptation, and selection. 2. Explain how natural selection is blind, not working toward a predetermined end. Explain the relationships among differential reproduction, key resources, key hazards, and natural selection. 3. Challenges to the teaching of evolutionary theory have become increasingly numerous and threatening to free speech in the classroom. The crux of many of these challenges has to do with the claim that evolution is “just a theory.” Build on the discussion of the scientific method in Chapter 2 by using the theory of evolution as your case study. Give your students some indications about the necessary attributes of critical inquiry by telling them how to decide whether evidence supports or refutes a hypothesis. SUGGESTED FILMS DNA and the Evidence for Evolution 20 minutes This film presents the structure and replicating processes of DNA and the effect of genetic mutation. The film uses DNA and fossil evidence to demonstrate that the process of adaptation and the selection of adaptors rest on a wide range of genetic variability. From Films for the Humanities and Sciences. 5 Lifelines: Darwin and the Theory of Inheritance 1998 30 minutes In this film, experts in Darwinian evolution discuss three important researchers and their contributions to evolutionary genetics: Charles Darwin; Gregor Mendel; and Friedrich Meischer. From Films for the Humanities and Sciences. The Blind Watchmaker: The Evolutionary Ideas of Richard Dawkins 1987 49 minutes This film presents the views of both creationists and Darwinists regarding evolution. Based on and inspired by Richard Dawkins’ book, this film argues that the diversity and complexity we see in biological life can best be explained by cumulative natural selection over long periods of time. A BBC Production. Series: The Evolution of Darwin 6-part series 26 minutes each This series is an introduction to the evidence for and against the Darwinian view of the origin of species. The series includes The Origins of Darwin’s Theory; The Theory of Inheritance; The Evolution of Human Purpose; Darwin’s Theory Today; The Creationist Argument; The Evidence for Evolution. From Films for the Humanities and Sciences. Series: Evolution: The Evidence for Modern Ideas on Evolution 11-part series 20 minutes each This series represent a minicourse in evolution. Titles in the series: The Evolution of Man; The Record of the Rocks; Fossils: Plants and Tetrapods; Fossils: Reptiles and Mammals; Relationships: Structural Homologies and Coevolution; Behavior and the Protein Record; Selection in Action: Natural Selection; Selection and Adaptation; The Human Influence; Origins of Change: Heredity and Mutation; DNA and the Evidence for Evolution. From Films for the Humanities and Sciences. 6