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I. Biology and Society: Mosquitoes, Microbes, and Malaria 1. In the 1960s, the World Health Organization (WHO) launched a campaign to eradicate the mosquitoes that transmit malaria. 2. It used DDT, to which some mosquitoes have evolved resistance. 3. The evolution of pesticide-resistant insects is just one of the ways that evolution affects our lives. 4. An understanding of evolution informs every field of biology, for example, a. medicine, b. agriculture, c. biotechnology, and d. conservation biology. II. Charles Darwin and The Origin of Species 1. Biology came of age on November 24, 1859. Charles Darwin published On the Origin of Species by Means of Natural Selection, an assemblage of facts about the natural world. 2. Darwin made three observations from these facts. a. Life shows rich diversity. b. There are similarities in life that allow the classification of organisms into groups nested within broader groups. c. Organisms display many striking ways in which they are suited for their environments. 3. In The Origin of Species, Darwin a. proposed a hypothesis, a scientific explanation for his observations, b. used hundreds of pages in his book to describe the evidence supporting his hypothesis, c. made testable PREDICTIONS, and d. reported the results of numerous experiments he had performed. 4. Darwin hypothesized that a. present-day species are the descendants of ancient ancestors that they still resemble in some ways and b. change occurs as a result of “descent with modification,” with natural selection as the mechanism. 5. Natural selection is a process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are individuals with other characteristics. 6. As a result of natural selection, a population, a group of individuals of the same species living in the same place at the same time, changes over generations. 7. Natural selection leads to evolutionary adaptation, a population’s increase in the frequency of traits suited to the environment. 8. Natural selection thus leads to evolution, seen either as a. a change in the genetic composition of a population over time or b. on a grander scale, the entire biological history, from the earliest microbes to the enormous diversity of organisms that live on Earth today. 9. Natural selection leads to a. a population (a group of individuals of the same species living in the same place at the same time) changing over generations and b. evolutionary adaptation. 10. In one modern definition of evolution, the genetic composition of a population changes over time. A. Darwin’s Cultural and Scientific Context 1. The Origin of Species was fundamentally different from the prevailing scientific and cultural views of Darwin’s time. 2. Let’s place Darwin’s ideas in their historical context. B. The Idea of Fixed Species 1. The Greek philosopher Aristotle held the belief that species are fixed and do not evolve. 2. The Judeo-Christian culture fortified this idea with a. a literal interpretation of the biblical book of Genesis and b. the suggestion that Earth may only be 6,000 years old. 3. At the same time, naturalists were grappling with the interpretation of fossils, imprints or remains of organisms that lived in the past. 4. Lamarck and Evolutionary Adaptations 5. Naturalists compared fossil forms with living species and noted patterns of similarities and differences. 6. In the early 1800s, French naturalist Jean Baptiste Lamarck suggested that life evolves and explained this evolution as the refinement of traits that equip organisms to perform successfully in their environment. 7. Lamarck suggested a mechanism that we now know is wrong. 8. Lamarck proposed that by using or not using its body parts, an individual may develop certain TRAITS that it passes on to its offspring, thus, acquired traits are inherited. 9. Lamarck helped set the stage for Darwin by proposing that species evolve as a result of interactions between organisms and their environment. 10. The Voyage of the Beagle 11. Darwin was born on February 12, 1809, the same day that Abraham Lincoln was born. 12. In December 1831, Darwin left Great Britain on the HMS Beagle on a five-year voyage around the world. 13. On his journey on the Beagle, Darwin a. collected thousands of specimens and b. observed various adaptations in organisms. 14. Darwin was intrigued by a. the geographic distribution of organisms on the Galápagos Islands and b. similarities between organisms in the Galápagos and those in South America. 15. Darwin was strongly influenced by the writings of geologist Charles Lyell. 16. Lyell suggested that Earth a. is very old and b. was sculpted by gradual geological processes that continue today. 17. Darwin reasoned that the extended time scale would allow for gradual changes to occur a. in species and b. in geologic features. C. Descent with Modification 1. Darwin made two main points in The Origin of Species. a. Organisms inhabiting Earth today descended from ancestral species. b. Natural selection is the mechanism for descent with modification. D. Evidence of Evolution 1. Evolution leaves observable signs. 2. We will examine five of the many lines of evidence in support of evolution: a. the fossil record, b. biogeography, c. comparative anatomy, d. comparative embryology, and e. molecular biology. E. The Fossil Record 1. Fossils are a. imprints or remains of organisms that lived in the past b. often found in sedimentary rocks. 2. The fossil record a. is the ordered sequence of fossils as they appear in rock layers, b. reveals the appearance of organisms in a historical sequence, and c. fits with the molecular and cellular evidence that prokaryotes are the ancestors of all life. 3. Paleontologists (scientists who study fossils) have discovered many transitional forms that link past and present. 4. Transitional fossils include evidence that a. birds descended from one branch of dinosaurs and b. whales descended from four-legged land mammals. F. Biogeography 1. Biogeography, the study of the geographic distribution of species, first suggested to Darwin that today’s organisms evolved from ancestral forms. 2. Darwin noted that Galápagos animals resembled species of the South American mainland more than they resembled animals on similar but distant islands. 3. Many examples from biogeography would be difficult to understand, except from an evolutionary perspective. 4. One example is the distribution of marsupial mammals in Australia. G. Comparative Anatomy 1. Comparative anatomy a. is the comparison of body structure between different species and b. attests that evolution is a remodeling process in which ancestral structures become modified as they take on new functions. 2. Homology is a. the similarity in structures due to common ancestry and b. illustrated by the remodeling of the pattern of bones forming the forelimbs of mammals for different functions. 3. Vestigial structures a. are remnants of features that served important functions in an organism’s ancestors and b. now have only marginal, if any, importance. H. Comparative Embryology 1. Early stages of development in different animal species reveal additional homologous relationships. a. For example, pharyngeal pouches appear on the side of the embryo’s throat, which i. develop into gill structures in fish and ii. form parts of the ear and throat in humans. b. Comparative embryology of vertebrates supports evolutionary theory. I. Molecular Biology 1. The hereditary background of an organism is documented in a. its DNA and b. the proteins encoded by the DNA. 2. Evolutionary relationships among species can be determined by comparing a. genes and b. proteins of different organisms. III. Natural Selection 1. Darwin noted the close relationship between adaptation to the environment and the origin of new species. 2. The evolution of finches on the Galápagos Islands is an excellent example. 3. Darwin’s Theory of Natural Selection 4. Darwin based his theory of natural selection on two key observations. a. All species tend to produce excessive numbers of offspring. b. Organisms vary, and much of this variation is heritable. 5. Observation 1: Overproduction and competition a. All species have the potential to produce many more offspring than the environment can support. b. This leads to inevitable competition among individuals. 6. Observation 2: Individual variation a. Variation exists among individuals in a population. b. Much of this variation is heritable. 7. Inference: Unequal reproductive success (natural selection) a. Those individuals with traits best suited to the local environment generally leave a larger share of surviving, fertile offspring. A. Natural Selection in Action 1. Examples of natural selection include a. pesticide-resistant insects, b. antibiotic-resistant bacteria, and c. drug-resistant strains of HIV. B. The Process of Science: Does Predation Drive the Evolution of Lizard Horn Length? 1. Observation: Flat-tailed horned lizards defend against attack by a. thrusting their heads backward and b. stabbing a shrike with the spiked horns on the rear of their skull. 2. Question: Are longer horn length and spread a survival advantage? 3. Hypothesis: Longer horn length and spread are a survival advantage. 4. Prediction: Live horned lizards have longer and more widely spread horns than dead ones. 5. Experiment: Measure the horn lengths and the tip-to-tip spread distance of side horns from the skulls of a. 29 killed and b. 155 living lizards. 6. Results: The average horn length and spread of live lizards is about 10% greater than that of killed lizards. C. Evolutionary Trees 1. Darwin saw the history of life as analogous to a tree. a. The first forms of life on Earth form the common trunk. b. At each fork is the last common ancestor to all the branches extending from that fork. c. The tips of millions of twigs represent the species living today. IV. The Modern Synthesis: Darwinism Meets Genetics 1. The modern synthesis is the fusion of a. genetics with b. evolutionary biology. A. Populations as the Units of Evolution 1. A population is a. a group of individuals of the same species, living in the same place at the same time and b. the smallest biological unit that can evolve. 2. The total collection of alleles in a population at any one time is the gene pool. 3. When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale. B. Genetic Variation in Populations 1. Individual variation abounds in all species. a. Not all variation in a population is heritable. b. Only the genetic component of variation is relevant to natural selection. 2. Variable traits in a population may be a. polygenic, resulting from the combined effects of several genes, or b. determined by a single gene. 3. Polygenic traits tend to produce phenotypes that vary more or less continuously. 4. Single-gene traits tend to produce only a few distinct phenotypes. C. Sources of Genetic Variation 1. Genetic variation results from processes that both involve randomness: a. mutations, changes in the nucleotide sequence of DNA, and b. sexual recombination, the shuffling of alleles during meiosis. 2. For any given gene locus, mutation alone has little effect on a large population in a single generation. 3. Organisms with very short generation spans, such as bacteria, can evolve rapidly with mutation as the only source of genetic variation. D. Analyzing Gene Pools 1. A gene pool a. consists of all the alleles in a population at any one time and b. is a reservoir from which the next generation draws its alleles. 2. Alleles in a gene pool occur in certain frequencies. 3. Alleles can be symbolized by a. p for the relative frequency of the dominant allele in the population, b. q for the frequency of the recessive allele in the population, and c. p + q = 1. 4. Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele. 5. Genotype frequencies can be calculated from allele frequencies (if the gene pool is stable = not evolving). 6. The Hardy-Weinberg formula a. p2 + 2pq + q2 = 1 b. can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles. E. Population Genetics and Health Science 1. The Hardy-Weinberg formula can be used to calculate the percentage of a human population that carries the allele for a particular inherited disease. 2. PKU a. is a recessive allele that prevents the breakdown of the amino acid phenylalanine and b. occurs in about one out of every 10,000 babies born in the United States. 3. People with PKU must strictly regulate their dietary intake of the amino acid phenylalanine. F. Microevolution as Change in a Gene Pool 1. How can we tell if a population is evolving? 2. A non-evolving population is in genetic equilibrium, a. called the Hardy-Weinberg equilibrium, meaning the population’s gene pool is constant over time. 3. From a genetic perspective, evolution can be defined as a generation-to-generation change in a population’s frequencies of alleles, sometimes called microevolution. G. Mechanisms of Evolution 1. The main causes of evolutionary change are a. genetic drift, b. gene flow, and c. natural selection. 2. Natural selection is the most important, because it is the only process that promotes adaptation. H. Genetic Drift 1. Genetic drift is a change in the gene pool of a small population due to chance. I. The Bottleneck Effect 1. The bottleneck effect a. is an example of genetic drift and b. results from a drastic reduction in population size. 2. Passing through a “bottleneck,” a severe reduction in population size, a. decreases the overall genetic variability in a population because at least some alleles are lost from the gene pool, and b. results in a loss of individual variation and hence adaptability. 3. Cheetahs appear to have experienced at least two genetic bottlenecks: a. during the last ice age, about 10,000 years ago, and b. during the 1800s, when farmers hunted the animals to near extinction. 4. With so little variability, cheetahs today have a reduced capacity to adapt to environmental challenges. J. The Founder Effect 1. The founder effect is likely when a few individuals colonize an isolated habitat. 2. This represents genetic drift in a new colony. 3. The founder effect explains the relatively high frequency of certain inherited disorders in some small human populations. K. Gene Flow 1. Gene flow a. is another source of evolutionary change, b. is separate from genetic drift, c. is genetic exchange with another population, d. may result in the gain or loss of alleles, and e. tends to reduce genetic differences between populations. L. Natural Selection: A Closer Look 1. Of all causes of microevolution, only natural selection promotes adaptation. 2. Evolutionary adaptation results from a. chance, in the random generation of genetic variability, and b. sorting, in the unequal reproductive success among the varying individuals. M. Darwinian Fitness 1. Relative fitness is a. the contribution an individual makes to the gene pool of the next generation b. relative to the contributions of other individuals. N. Three General Outcomes of Natural Selection 1. If we graph the coat color of a population of mice, we get a bell-shaped curve. 2. If natural selection favors certain fur-color phenotypes, a. the populations of mice will change over the generations and b. three general outcomes are possible. 3. Directional selection shifts the overall makeup of a population by selecting in favor of one extreme phenotype. 4. Disruptive selection can lead to a balance between two or more contrasting phenotypic forms in a population. 5. Stabilizing selection favors intermediate phenotypes, occurs in relatively stable environments, and is the most common. O. Sexual Selection 1. Sexual selection is a form of natural selection in which individuals with certain traits are more likely than other individuals to obtain mates. 2. Sexual dimorphism is a distinction in appearance between males and females not directly associated with reproduction or survival. P. Evolution Connection: An Evolutionary Response to Malaria 1. We can see the results of past natural selection in present-day humans. 2. Malaria first emerged as a serious threat to people in Africa just 10,000 years ago, a. long after humans had established populations around the globe, b. therefore only producing evolutionary responses in malarial regions. 3. Sickle hemoglobin a. is a mutation that denies the malarial parasite essential access to human hemoglobin and b. distorts the shape of red blood cells. 4. Individuals with one copy of this sickle allele (heterozygotes) are relatively resistant to malaria. a. Individuals with two copies (homozygotes) are usually fatally ill. 5. In the African tropics, a. malaria is most common and b. the frequency of the sickle-cell allele is highest.