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to the differential survival and reproduction they cause. This is an example of how new variation that is produced by mutation may be selected for by natural selection. 3.Mutations in human populations — Humans have acquired mutations leading to inherited disorders or traits (for example, sickle cell anemia, lactose intolerance, albinism) and, unfortunately, mutations are the cause of many of the types of cancers found today. Isn’t evolution “just a theory?” “Theory” in science has a very different meaning than in popular use. In everyday conversation, when someone says “I have a theory,” they are guessing as to an answer about something, which, in science, is a hypothesis. In the sciences, a theory is a broad explanation of a natural phenomenon that is widely supported by many experiments. While many theories are long-standing and consistently upheld, they are still subject to being modified or disproved over time in the face of new evidence. Examples of theories that explain natural phenomena include the theory of gravity, the theory of atoms, the theory of plate tectonics, and the theory of evolution. What is genetic variation and why is it important? Variation serves as the vital “fuel” for evolution and must be present for natural selection to occur. Individuals within species may vary in such traits as visual acuity, wing shape, tooth enamel thickness, odor detection, leg length, canine size, fur thickness, disease immunity, and so on. This variation enables some individuals to have higher reproductive success than others, and makes it more likely that at least some individuals of a species will survive to reproduce in the face of continuous environmental changes. If biologists cannot agree on evolution, how can I accept it? Scientists do not disagree about whether or not evolution occurs, but about some of the specifics of how it occurs. Their arguments are typically about the importance of each evolutionary force in causing allele frequency change, or the rates of evolutionary change for various genes in different species. While scientists may debate specifics of evolutionary processes and patterns, the existence and actions of the five evolutionary forces are agreed upon by all. If humans evolved from apes, why do apes still exist? Humans did not evolve from modern apes. The fossil record and genetic evidence indicates that we shared a common ancestor with the apes. This common ancestor would have been quite ape-like, however, and the earliest members of the human family do have many ape-like features such as smaller brain size and a forward-jutting jaw. These early human ancestors occurred in the fossil record soon after the split with the African apes. By the time we see recognizable modern chimps and humans in the fossil record, those earlier forms had become extinct. Why did the apes stop evolving (unlike the humans)? The simple answer to this is, “They didn’t.” Apes have by no means “stopped evolving.” They have their own unique evolutionary history, and since the split from our common ancestor, their environment and lifestyle have changed less than ours, resulting in their maintaining more similarities to the ancestral form. Evolution is not goal-directed, but is a continual process, affected by random events as well as environmental conditions. For more information, visit thelibrary.org/exploringhumanorigins. How do we know that species are related and share a common ancestor? The greater the number of features that are shared between taxonomic groups, the more closely related they are presumed to be, and the more recent in time their presumed shared ancestor lies. The fossil record represents numerous intermediate forms between extinct and living species that serve as “snapshots of time” and complement the evidence from genetics, comparative anatomy, embryology, and physiology. The more fossils we find, the more complete the record becomes and the more gaps we fill in. For example, it was long predicted that birds and reptiles shared a common ancestry because of their similar anatomical and physiological features. Scientists expected to eventually discover bird-like dinosaurs (reptile-like fossils with feathers, bird-like fossils with teeth, or bird-like fossils with long reptilian tails). In fact, the fossil record now includes numerous intermediate dinosaur-to-bird forms, including Archaeopteryx as well as Eoraptor, Compsognathus, Protarchaeopteryx, Caudipteryx and Velociraptor. We have also used new, sophisticated genetic evidence to support the relationship between reptiles and birds. For example, geneticists recently discovered that the very same genes producing scales in reptiles are responsible for feathers in birds. Scientists are able to draw conclusions and develop scientific theories from the observations and data available to us in the natural world. Those conclusions and theories continue to be questioned and evaluated by scientists with each new observation and data set in order to better understand the world in which we live. Springfield-Greene County Library District 4653 S. Campbell Ave • thelibrary.org THE SCIENCE OF EVOLUTION Prepared by Suzanne Walker-Pacheco, Ph.D., a professor in the Department of Sociology and Anthropology at Missouri State University; and Kevin Jansen, Ph.D., department chair and associate professor of biology at Drury University. In conjunction with the Smithsonian Institution traveling exhibit Exploring Human Origins: What Does It Mean To Be Human? May 9–June 2 at the Library Center, 4653 S. Campbell Ave. Exploring Human Origins: What Does It Mean To Be Human? was organized by the Smithsonian’s National Museum of Natural History in collaboration with the American Library Association. This project was made possible through the support of a grant from the John Templeton Foundation and support from the Peter Buck Human Origins Fund. Smithsonian Institution Questioning, probing, and critiquing are qualities in the nature of scientists, and we continue to engage these qualities to better provide explanations for how our natural world works. That makes this document a helpful supplement to the Smithsonian Institution traveling exhibit, Exploring Human Origins: What Does It Mean to Be Human? May 9–June 2 at the Library Center, 4653 S. Campbell Ave. The exhibit of panels, interactive kiosks, hands-on displays and videos explore milestones in the evolutionary journey of becoming human. See a complete list of programs accompanying the exhibit at thelibrary.org/exploringhumanorigins or in the Bookends calendar of events. What is Science? Science involves seeking out reliable explanations for what we see in the world around us, using three basic approaches: observation, experimentation, and logic. Scientists use a series of steps in the scientific method to: 1) observe some phenomenon empirically, using at least one of our five senses; 2) ask ourselves a question about the phenomenon — how/ what/why types of questions; 3) propose a hypothesis that is a testable statement to investigate the question; and 4) design an experiment or other alternate study to collect data that will test the reliability of our hypothesis. Similar conclusions to repeated testing of similar hypotheses can lead to the development of a theory. We derive benefits from the scientific method every day. Most household products we use, and all the medications we take, have been subject to rigorous testing as part of this process. What is Evolution? Biologists define evolution as a change in allele frequency in a population over time. A population is considered to be a breeding group, and alleles are simply variant forms of a gene. For example, the ABO blood type in humans is determined by one gene, three possible alleles (A, B, or O), and four possible phenotypes (types A, B, AB, and O). If a population experiences a change in the frequencies of the three allele variants, we conclude that evolution has occurred. We continually observe and document the occurrence of allele frequency change, and thus evolution, in populations all around us, both in nature and in the laboratory. How Does Evolution Work? Five main factors cause evolution to occur. These evolutionary forces are mutation, natural selection, sexual selection, gene flow, and genetic drift. • Mutations are heritable changes in the DNA, occurring randomly in the genome of all species. Mutation is the only evolutionary force to produce new variation. • Natural selection was proposed independently by both Charles Darwin and Alfred Russel Wallace to explain how evolution works to allow organisms to become better adapted to their environment. The steps are: 1) More individuals of each species are produced than survive; 2) variation exists among individuals such that some have inherited traits allowing them to be better suited to their environment than others; 3) those better suited to their environment are more likely to survive and produce more offspring than those with traits that make them less well-suited; and 4) alleles for the wellsuited traits will be found in more offspring in the next generation, causing a change in allele frequency, or evolution. • Sexual selection can be considered a type of natural selection that takes the form of choosing a mate or competing for mates. • Gene flow occurs when individuals move from one population to another, taking their alleles with them. This causes a change in allele frequency in both the population they leave and the one they go to. Cover image and above: Smithsonian’s Human Origins Program • Genetic drift is also referred to as “random genetic drift” because the “drift” (or change) in allele frequency that occurs from one generation to the next results from chance. The smaller the population, the greater the effect genetic drift will have. Remember Gilligan’s Island? A boat is lost at sea, leaving its passengers stranded on an island. The fateful trip of the SS Minnow resulted from a chance event, a storm. The gene pool of that new population of seven individuals obviously would have been a very limited one, without all the variation found in the mainland population from which they came. That random shift in allele frequency from their old population (on the mainland) to their new population (on the island) is genetic drift. The effects of evolutionary forces are often combined; mutation produces new genetic variation, and the other evolutionary forces redistribute or eliminate it within and among populations. Are there examples of evolution that affect humans? Yes, some common examples of evolution that directly affect humans include: 1. Changes in the influenza virus each year — Scientists found the rate of mutation in the influenza virus to be high enough that we must develop new flu vaccines each year. 2.Development of bacterial resistance to antibiotics — Some strains (MRSA, for example) of Staphylococcus aureus, a type of bacteria found normally on humans and often without ill effects, have developed resistance to at least four antibiotics, including penicillin, over the past several decades and currently have no known treatment. A particular mutation may allow some individuals to survive at least one round of antibiotics; when these individuals reproduce, they pass on those new alleles responsible for its hardy nature and confer the antibiotic resistance to a portion of the next generation. These new alleles will increase in frequency, due