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Natural Selection The Darwin-Wallace theory of organic change over time. Thinking Question • Some evidence exists that over several centuries, the number of people born with small wisdom teeth or no wisdom teeth has increased. Using your best understanding of Natural Selection, explain how selection could cause this. Darwin’s Evidence • Darwin spent nearly 20 years analyzing evidence as he developed his ideas. He drew on: • His own field work during his voyage on board the Beagle. • His work in classifying barnacles, beetles, and other organisms. Darwin’s Evidence • In addition, Darwin drew on evidence from: • Fossils • Comparative Anatomy • Comparative Embryology Fossils • The fossil record shows that: • Things existed in the past that no longer exist today and • Things exist today that did not exist in the past. • The fossil record is a record of change in the composition of the biosphere over time. What is a fossil? Fossils are rare • Most organisms are eaten or decay after death. • Fossils only form in places where there is little erosion: lake bottoms, deep oceans, etc. • We have little fossil record of beach organisms and alpine communities. Some fossils show transition • Because fossils are rare, and change can be rapid, there are few “missing links” in the fossil record. • However, some fossil sequences of marine organisms do show good evidence of transition from one form to another. Comparative Anatomy • Darwin looked at and described: • Homologous structures • Analogous structures • Vestigial structures Homologous structures • Homologous structures are those that are shared between related organisms, but are slightly altered. • In modern terms, this indicates shared genes (similar structures), but also shows genetic differences led to divergent evolution (modifications of those structures. Homology Analogous Structures • Analogous structures are similar solutions to the same environmental challenges seen in unrelated species living in similar environments. • In modern terms, analogous structures represent convergent evolution. Rather than shared genes, the organisms are under similar selection pressures. Analogy Insects and birds evolved flight independently. Analogy Ancestral seals and penguins that had a streamlined body form were better able to survive. Vestigial Structures • Vestigial structures are those that were well-developed in an ancestor but are much reduced in a descendant. • Vestigial structures show that different selection pressures shape organisms differently. Vestigial Structures All higher vertebrates descend from ancestors with four limbs. What happened in the snake and the whale? Comparative Embryology • Like homology, embryology reveals shared traits between species, demonstrating shared genes. • As embryos develop, newer genes may shape the same structures into different parts: gill arches in fish become gills, but in mammals become parts of the face, jaw, and inner ear. Comparative Embryology Can you tell these three mammals apart? Which one is human? Molecular Comparisons • Shared traits give us some insights into shared genes. Today’s technolgy allows us to analyze genes directly. • Because small mutations accumulate in populations over time, more genetic differences between two different groups of organisms indicates more time since they separated from one another. How Selection Works 1. Variation Exists • All populations vary as the result of the accumulation of small, random mutations over many generations. 2. Inheritance of traits • Inheritable traits (those coded for by genes) are passed directly to the offspring from the parents through genetic information. 3. Differential Survival • More offspring are born than can survive. Many offspring die young. Those with traits best suited to the environment are more likely, though not guaranteed, to survive. 4. Differential Reproduction • Some survivors fail to reproduce. Some have traits that better insure reproduction than others. 5. Differential Inheritance • Survivors that reproduce pass some of their traits on to their offspring. Those with favorable traits may pass those favorable traits on — or not. Natural Selection in Action • Peppered Moth simulation: • http://www6.district125.k12.il.us/~nfi scher/Moth/default.htm Wisdom Teeth, Revisited • Now that we’ve gone through the process of selection, return to the paper on which you responded to the wisdom tooth question. • Draw a line under your original response, and write a second one, using what you just learned about the process of selection: Variation, Inheritance, Differential Survival, Differential Reproduction, Differential Inheritance.
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