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Principles of Evolution Chapter 6 • There is some evidence that wisdom teeth have gotten smaller over the last several thousands of years. Which of these statements about that trend would an evolutionary scientist agree with? Explain why. • Wisdom teeth are getting smaller because we don’t need them. • People can die from impacted, infected wisdom teeth, which takes them out of the gene pool. • We are using wisdom teeth less and less, and that causes them to get smaller and smaller. W O R K T O G E T H E R Evolution is: 1. How organisms get better and better over time. 2. Change in the genes of a population over generations. 3. Change to an individual during its lifetime. 4. How monkeys gave birth to humans and stuff like that. 94% 4% 1 0% 2 3 2% 4 What is Evolution? • Evolution is the change in inheritable traits in a population over generations. • Change in traits is caused by changes in the genes (in DNA) that code for those traits. • Natural selection tends to increase favorable traits in a population and decrease unfavorable traits. Remember what a Scientific Theory is: Predictions A scientific theory is a wellsupported explanation for a natural phenomenon. Scientific theories are built from abundant evidence. Theories (and Laws) are further tested by making predictions. Theories are NOT blind guesses or ideas waiting to be “proven.” What Evolution is NOT • Many people argue against evolution based on statements they have heard. Yet many of those statements are ideas that are not part of evolutionary theory. Evolution is NOT about things getting “better and better” or more and more complex. Selection is about survival and reproduction. Organisms with traits that help them survive long enough to reproduce will pass their genes on. Evolution is NOT about organisms “trying” to adapt. “Trying” does not change genes. Only inheritable traits already in a population can be passed on. Evolution does NOT give organisms what they “need.” If that were true, there would never be deaths or species extinctions. If individuals have traits that help them survive environmental change, they may pass those traits on while others die. Evolution does NOT mean that individuals in a population slowly change. Change happens to populations over generations as those with unfavorable traits are weeded out and those with favorable traits are more likely to reproduce. Evolution is NOT a theory of how life began. Evolution explains how living organisms change over generations. Which of these is true about evolution? 1. It’s a theory of how life began. 2. It’s about organisms trying to adapt. 3. It’s about change in inheritable traits in a population over generations. 4. It’s something Darwin made up and no one has proved it yet. 94% 0% 1 6% 2 0% 3 4 Is this evolution? 33% 33% 33% 1. Yes 2. No 3. I’m not sure 1 2 3 Is this evolution? – Mutation – Genetic drift – Selection 1. Yes 2. No 3. I’m not sure 33% 33% Genetic change Genetic change 33% 1 2 3 Is this evolution? 33% 33% 33% 1. Yes 2. No 3. I’m not sure 1 2 3 Evolution is a Scientific Theory • Theories are evidencesupported explanations for natural phenomenon. • To exist, theories must have abundant evidence from many well-tested hypotheses. • Theories grow from evidence! Theorizing without evidence is not science! Evolution is a theory. This means that it: 25% 25% 25% 25% 1. Is an unsupported hypothesis. 2. Is an educated guess. 3. Is a proven fact. 4. Is supported by welltested evidence. 1 2 3 4 Biologists accept evolutionary theory because: 25% 25% 25% 25% 1. Darwin said it’s true. 2. Evidence shows that species change over time. 3. It sounds logical. 4. They are still working to prove the theory. 1 2 3 4 Early Ideas • While many people associate evolution with Darwin, other scientists were talking about organic change long before Darwin published his theory of natural selection. Aristotle (384-322 B.C.) viewed the living world as fixed and unchanging. All living things could be arranged in a ladder from inferior to superior, with humans on top. Aristotle’s views influenced thought for over 2000 years. Curiosity about fossils around the 16th century led to new questions. Were fossils the remains of organisms? If they were, what did this say about species being unchanging? George Louis LeClerk, Compte de Buffon (1707 - 1788) sought a naturalistic explanation for fossils. Buffon devised one of the first theories of organic change. However, Buffon’s ideas were grounded in logic rather than in evidence from the natural world, as much science was at the time. Buffon suggested that organisms had a basic form, the “interior mold,” which could be shaped by the environment. Buffon thought that the “internal mold” was caused by particles in an organism that shaped the organism while it was an embryo. All animals, he said, had a basic form: horse-like, dog-like, cat-like, and so on. In Europe, horses are brown and horse-shaped because of the environment. Move the horses to Africa, and the environment rearranges particles in embryos when the horses are bred. Over many generations, the particles reshape horses into zebras. So… what happened to Buffon’s theory? • Discuss these questions with a neighbor: • Horses were introduced to South Africa in Buffon’s time. Are they changing into zebras? • Can the environment change genes? Does it change the genes in a way that always makes the organism better adapted to that environment? • Write a brief response to this question: Does our modern knowledge of heredity support or reject Buffons’s theory? W O R K T O G E T H E R James Hutton (1726 - 1797) Charles Lyell (1797 - 1875) These two naturalists are best known for the theory of Uniformitarianism, proposed by Hutton and expanded by Lyell. Uniformitarianism states that the physical laws that now shape the earth have always done so, and that the past can be understood by studying the present. Both suggested that the Earth is very old. Georges Cuvier (1769 - 1832) was a student of Buffon, but disagreed about organic change. Cuvier was an anatomist. He pointed out that animals had features that helped them survive. If those features changed, wouldn’t that be harmful? Cuvier studied fossils and identified several new animals based on fossils. However, he had his own explanation for fossils. Cuvier doubted Uniformitarianism and used Catastrophism to explain the fossils of the Paris basin. He proposed one natural origin of life, followed by unpredictable catastrophes -perhaps beyond known physical laws. After each catastrophe, organisms from other places moved in. So… what happened to Cuvier’s theory? W O R K • Cuvier stated that organisms are welladapted to their environment, and change would be fatal. Does he have a point? Are there conditions that would favor change? T O G E T H E R Jean-Baptiste de Lamarck (1744 1829) was also a student of Buffon. Like Buffon, Lamarck sought a naturalistic explanation for the diversity of modern organisms and the animals seen in the fossil record. Lamarck used the theory of Inheritance of Acquired Characteristics, a widely-held belief at the time, to explain change in organisms. Blacksmiths, for example, were thought to pass their well-developed right arms on to their sons. Lamarck’s Organic Theory of Development Organisms are shaped by their environment. Change is goal-directed -organisms have an internal drive to become more and more complex. Use and disuse of parts causes change that is passed on from generation to generation. So… what happened to Lamarck’s theory? • Discuss these questions with a neighbor: • A tattoo is an acquired characteristic. If you get a tattoo, could your children be born with a similar mark? Why or why not? • How do traits get passed on from generation to generation? Does the way we exercise or not exercise affect how our traits are passed on? • Write a brief response to this question: Does our modern knowledge of heredity support or reject Lamarck’s theory? W O R K T O G E T H E R Charles Darwin (1809 - 1882) and Alfred Russell Wallace (1823 - 1913) Darwin and Wallace, working separately, devised a model for organic change that was based on their observations of living and fossil organisms over many years. Natural Selection was the first evidence-based mechanism for evolution that was proposed. Darwin and Wallace observed that change did not happen equally to all members of a population, as Buffon and Lamarck thought. Instead, Darwin and Wallace proposed that selection happens each generation. Some individuals have traits that help them survive and reproduce, while others have traits that put them at a disadvantage. Over generations, the number of individuals with favorable traits increases while those with unfavorable traits decrease. • Discuss these questions with a neighbor: • If a horse is born with a genetic disorder that causes it to die before it reaches sexual maturity, will it pass that trait on to its offspring? • If a cougar is born with genes shaping his nervous system that make him a better tracker and hunter, will that cougar have an advantage over other cougars? Can this trait be passed on to the cougar’s offspring? • Write a brief response to this question: Does our modern knowledge of heredity support or reject Darwin’s theory of selection? W O R K T O G E T H E R Which historical scientist believed that organisms strive to improve, and that change in species is caused by use or disuse of parts? 25% 1. 2. 3. 4. 25% 25% 25% Darwin Cuvier Buffon Lamarck 1 2 3 4 Which historical scientist believed that organisms changed when the environment acted on internal particles to make the organism better adapted? 25% 1. 2. 3. 4. 25% 25% 25% Buffon Lamarck Cuvier Darwin 1 2 3 4 Which historical scientist believed that organisms were perfectly adapted, and any change would be fatal? 25% 1. 2. 3. 4. 25% 25% 25% Lamarck Buffon Cuvier Darwin 1 2 3 4 Which historical scientist believed that organisms changed when those that were best adapted were the ones that survived and reproduced? 25% 1. 2. 3. 4. 25% 25% 25% Buffon Lamarck Cuvier Darwin 1 2 3 4 Evidence for Evolution • Both Darwin and Wallace were convinced that evolution -- change in the traits of a population over time -- did happen. • Darwin spent over 20 years collecting evidence and reading research by others before he wrote On the Origin of Species. He found many features of organisms that suggested they were related by ancestry. Fossils Fossils showed that species existed in the past that no longer exist today, and species exist today that did not exist in the past. Fossils Smith • • • Rocks found in layers Fossils in layers are consistent Law of superposition Lyell • • • • Natural forces produce layers of rock Slow repeated process Uniformitarianism The Earth is very old Fossils - Slow natural geologic processes - Dating of rocks by elemental analysis Fossils In many groups of organisms, fossils demonstrated change from one form to another over long periods of time. Homologous Structures Four-limbed vertebrate animals all have the same bones in the forelimbs, but the bones are shaped differently. In modern terms, this suggests a shared set of genes (for the bones) arising from common ancestry. Vestigial Structures Why do big snakes, like boas, have tiny little hind limbs? Why do whales have hip bones? Vestigial Structures Can you wiggle your ears? (Without using your hands!) Vestigial Structures Ear-wiggling Hips in whales, snakes. muscles in humans. Vestigial organs are greatly reduced in size and function compared with other members of a group and compared with the common ancestor. Analogous Structures The influence of the environment is seen in analogous structures, where less-related species living in similar environments have similar body forms, though often involving different body parts. From variations in a population, the environment selects those that best help an organisms survive in that environment. Analogous Structures Echidna (Australia) Porcupine (North America) Spiny Euphorbia (South Africa) Hedgehog (Europe) Golden Barrel Cactus (North America) Embryological Similarities Are these organisms very different looking? How about now? Lemur Pig Human Similar structures and similar developmental processes in embryos demonstrate a shared set of genes between groups of organisms, suggesting common ancestry. Note: Humans do not become fish or grow gills during development. We do share several embryological features with other vertebrate animals. Cave-dwelling organisms often have small, nonfunctional eyes. Their eyes are examples of: 33% 33% 33% 1. Homologous structures. 2. Analogous structures. 3. Vestigial structures. 1 2 3 A cat’s paw and a human hand have the same bones, but different shape and function. They are: 33% 1. Homologous structures 2. Analogous structures. 3. Vestigial structures. 1 33% 2 33% 3 Biochemical and Genetic Analysis All cells use: • DNA as heritable genetic information • RNA and ribosomes to make proteins • roughly same 20 amino acids to build proteins • ATP to carry energy DNA to Expressed genes Of course, molecular evidence wasn’t available to Darwin. Publication of The Origin inspired scientists to figure out how heredity worked! Biochemical and Genetic Analysis Darwin inferred shared ancestry from shared traits. Modern analysis of DNA demonstrates shared genes for many traits between related species, suggesting descent from a common ancestor. Mice and humans both have the gene for the protein Cytochrome c. Small differences show changes over time as the two groups diverged. There are 3 differences in the Cytochrome C gene between penguins and chickens. There are 13 differences between penguins and horses. What does this tell you? 1. Chickens evolved before horses. 2. Penguins are related more closely to chickens than horses. 3. Penguins and horses have nothing in common. 33% 1 33% 2 33% 3 • Common ancestry is sometimes misunderstood. Check your understanding by looking at these two diagrams. Choose which one best illustrates the statement: Mammoths and modern elephants are separate groups that descend from a single common ancestor. A Mammoths B D A I L Y Elephants Elephants Mammoths Ancestor Ancestor W O R K Natural Selection • Evidence from the natural world showed that change happens to organisms over time. • The important question was how? Darwin and Wallace’s Theory of Natural Selection provided a natural mechanism for change. Natural Selection Variation Within any population, individuals differ from one another in many ways. Natural Selection Inheritance Some differences between individuals are inheritable, and can be passed from parent to offspring. X Natural Selection Differences in survival and reproduction In most populations, more young are born than can survive. Many of the young will die. A few will live long enough to reproduce. Natural Selection Success is not random While random accidents do happen, an individual’s survival depends mainly on the individual’s traits. Those that live and reproduce may pass their traits to the next generation. Natural Selection Selection favors organisms that are best adapted to the environment. Selection does NOT produce new traits, nor does it make organisms “perfect.” Selection is all about survival, and those organisms with traits that help them survive are more likely to reproduce. Natural selection states that pesticide resistance in insects happens when: 1. 2. 3. 4. Nature helps insects by giving them resistance. Those insects with a trait that helps them metabolize the pesticide are more likely to survive. Pesticides cause mutations that make the insects resistant. Over time, insects exposed to pesticides become immune to it. 25% 1 25% 2 25% 3 25% 4 • Let’s return to the wisdom tooth question. In light of what you have learned about natural selection, which of these statements do you think an evolutionary biologist would agree with? • Wisdom teeth are getting smaller because we don’t need them. • Young people can die from impacted, infected wisdom teeth, which takes them out of the gene pool. • We are using wisdom teeth less and less, and that causes them to get smaller and smaller. D A I L Y W O R K Evidence for Selection • In order to construct his theory, Darwin had to gather and examine evidence to discover whether selection happens in nature. • Research continues to refine our ideas of how selection happens and what effects it has on species. Artificial Selection Darwin knew from his studies of pigeon breeding that selection can change organisms, sometimes rapidly. Human selection created many breeds of dogs from wild dogs or wolves as humans chose to breed those animals with traits that the humans wanted. Selection acts on variations Studies have shown that guppies living in ponds with few predators are more brightly colored than those living with many predators. In both populations there are guppies with genes for bright colors. Why are dull colors favored in ponds with many predators? Why are bright colors favored in ponds with few predators? Selection does not cause new variations. Studies on Anole lizards showed that long, fast-moving legs were favored where trees had thicker branches, while shorter, more agile legs were favored where trees were flimsier. If lizards from places where trees had thin branches were moved to a place where trees had thick branches, those with the longest legs survived better. Each generation had more individuals with longer legs because the genes for longer legs were already in the population. • Suppose a flock of ducks were moved to the middle of the Sahara desert. According to Natural Selection which of these is most likely to happen to them? Explain your answer • The ducks would learn to eat new foods and would slowly adapt to the new environment. • With no traits for desert living in their population, the ducks would most likely die and leave no offspring. • Nature would provide the ducks with the traits they need to survive in the desert. D A I L Y W O R K Recap • Evolution is the change in inherited traits in a population over generations. • Fossils, molecular data, and other evidence shows that the genetics of a population do change over time. • Natural Selection, one mechanism for change, is well-supported by evidence.