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Darwin and Natural Selection Charles Darwin 1809-1882 Charles attended boarding school as a boy, where he developed an interest in Natural History. Father sent him to study medicine in Edinburgh. Turned off by barbaric surgery practices and dull lectures. Joined Natural History society, read Buffon, Cuvier, Lamarck, others. Darwin left Edinburgh determined not to be a physician. Attended Cambridge, thinking he would be a clergyman. At Cambridge, he read Herschel (philosophy of science), Humboldt (geological distribution of organisms), Lyell. After graduation in 1831, Darwin was encouraged by friends to apply to be a captain’s companion and naturalist on board the Beagle. December 27, 1831: Joins Captain Fitzroy on board the Beagle for a fiveyear journey. Voyage of the H.M.S. Beagle Official purpose of the voyage was to survey the South American coast. During the voyage, Darwin observed a volcanic eruption, an earthquake, and noted geologic features suggesting an ancient earth. In South America, found fossils of ancient sloths, observed living rheas (ostrich-like birds) and llamas, many other animals not described by naturalists. Collected huge numbers of specimens. The Galapagos In September of 1835, the Beagle reached the Galapagos Islands. Fascinated by the giant tortoises. Brought three back alive. Made detailed observations of finches, mockingbirds. Realized that they resembled birds on the mainland, yet were different. Where did they come from? Besides the Galapagos, Darwin observed organisms on Tahiti and other Pacific islands. Later developed theories of coral reef building. Darwin’s questions Why the replacement of species geographically; that is, why are the species on islands not exactly the same as those on the nearby mainland? Why the replacement of species in time? Where do species come from in the first place? Change in species appears to be the answer — but how do species change? Natural Selection Darwin rejected the idea that all members of a population are shaped equally by the environment and that all change together. Selection does NOT look like this: Generation 1 Generation 100 Generation 200 Natural Selection Rather, selection begins with variation. Some variants in a population may be at an advantage, some at a disadvantage. Those with a strong advantage are more likely to have offspring. “Fitness” is the ability to survive. “Success” is passing one’s traits to the next generation. Natural Selection Natural Selection looks more like this. Suppose in a population of horses, black horses get hotter in the sun and are more likely to die of heat exhaustion. Generation 1 Generation 2 Natural Selection But suppose global climate changes alter the local environment. It becomes cooler and cloudier. Now the black horses have an advantage because their black coats help them stay warmer. Natural Selection • Notice the difference: • Buffon and Lamarck would say, “The horses need to be darker, so they will get blacker and blacker each generation.” • Darwin says, “Black horses are better able survive and reproduce each generation, so there will be more and more black horses each generation.” Darwin’s New Questions • Darwin noted two difficulties in his original theory of Natural Selection: • Where did variation come from in the first place? • How did favorable traits get passed on to the offspring, if traits of each parent are blended in the offspring (as was commonly believed at the time)? In the 20th Century... • Darwin’s questions led other scientists to explore heredity, which led to the science of genetics. • Genetic research in the 20th century supported natural selection as a plausible mechanism, and raised new questions and new possibilities. Darwin, Buffon, or Lamarck? 1. “Members of the dog family that migrated to North America were shaped during fetal development by the American environment. They became wolves, which were best adapted to the environment.” 2. “The original wolf ancestors that arrived in North America had a wide variety of traits. Some traits were favorable, some were not. Those with most favorable traits, such as heavy fur, sharp fangs, and pack hunting behaviors, tended to survive better, and these survivors gave rise to modern wolves.” 3. “Small dog-like ancestors had to run after prey and survive in the cold North American winters. Running exercised their legs, which grew longer each generation. Exposure to cold caused them to grow thicker coats generation by generation. Slowly, the dog-like creatures became modern wolves. Thinking question • In groups, discuss and answer this question: Buffon, Lamarck, and Darwin all thought that species changed over time, and each developed a theory that explained how that change might occur. Why is Darwin’s mechanism, Natural Selection, still accepted (though modified) by today’s biologists, but Buffon’s and Lamarck’s theories are not?