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Biology: Chapter 15 – Evolution 15-1: The puzzle of life’s diversity Evolution, or change over time, is the process by which modern organisms have descended from ancient organisms. The theory of evolution can explain Earth’s biodiversity. A scientific theory is an explanation of natural events that is supported by evidence and can be tested with new evidence. Charles Darwin added the most to our understanding of evolution. In the 1830’s, Charles Darwin sailed around the world. Darwin made many observations and collected evidence. Darwin observed many organisms. He saw that many plants and animals were very well suited to their environment. Darwin collected fossils, or the preserved remains of ancient organisms. Some fossils were unlike any creatures he had ever seen. He wondered why the species in fossils had disappeared. Darwin’s observations on the Galapagos Islands influenced him most. The islands are near one another but have different climates. Darwin saw that the characteristics of many animals and plants varied noticeably among the different islands. He wondered whether animals on different islands had once belonged to the same species. According to this hypothesis, these separate species would have evolved from an original ancestor species after becoming isolated from one another. 15-2 : Ideas that shaped Darwin’s thinking In Darwin’s day, most European’s thought that the Earth and all of its life-forms had existed for only a few thousand years. They also thought that species did not change. Some scientists of Darwin’s time began challenging these ideas. These scientists influenced the development of Darwin’s theory of evolution. Hutton and Lyell helped scientists recognize that Earth is many millions of years old, and that it had changed over time. These ideas helped Darwin realize that life might change as well. Knowing that Earth was very old convinced Darwin that there had been enough time for life to evolve. Jean-Baptiste Lamarck was one of the first scientists to see that evolution occurred. He also recognized that organisms adapt to their environments. Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime. These traits could then be passed on to their offspring. Over time, this process led to change in a species. Scientists now know that some of Lamarck’s ideas were wrong. However, his general ideas about evolution and adaptation are correct. These ideas influenced Darwin. Economist Thomas Malthus also influenced Darwin. Malthus thought that if human population kept growing, sooner or later there would be insufficient living space and food for everyone. Darwin thought this was true for all organisms. 15-3: Darwin presents his case Darwin was hesitant to publish his ideas because they were so extreme. When he learned that scientist Alfred Russel Wallace had the same ideas, Darwin published On Origin of Species in 1859. In the book, Darwin supplied evidence that evolution has occurred. He also explained his ideas about how evolution occurs. Darwin’s theory was based on artificial selection. In artificial selection, nature provided the variation, and humans selected those variations that they found useful. For example, animal breeders used only the largest hogs, fastest horses, or cows that produced the most milk for breeding. Darwin thought that a similar process occurs in nature. He called this natural selection. This process can be summed up as follows: Individuals differ, and some of the differences can be passed on to their offspring More offspring are produced that can survive and reproduce There is competition for limited resources, or a struggle for existence Individuals best suited to their environment survive and reproduce successfully. In other words, there is survival of the fittest. Fitness is the ability to survive and reproduce in a given environment. It results from adaptations. Adaptations are inherited traits that increase an organism’s chance for survival. Only the fittest organisms pass on their traits. Because of this, a species changes over time. Darwin argued that species alive today descended with modification from species of the past. Darwin also introduced the principle of common descent. This principle holds that all species come from common ancestors. The principle of common descent links all organisms on Earth into a single tree of life. Darwin argued that living things have been evolving on Earth for millions of years. He presented four types of evidence on support of evolution. The fossil record: comparing fossils from older and younger rock layers provides evidence that evolution has taken place. Geographic distribution of living species: the presence of similar but unrelated organisms in similar environments suggests the action of natural selection. Homologous structures of living organisms: homologous structures have different mature forms but develop from the same embryonic tissues. They provide strong evidence that organisms have descended, with modifications, from common ancestors. o Some homologous structures no longer serve major roles in descendants. If the structures are greatly reduced in size, they are called vestigial organs. For example, the appendix in humans is a vestigial organ. It carries out no function in digestion. Similarities in early development: The early stages, or embryos, of many animals are very similar. These similarities are the evidence that the animals share common ancestors. Scientific advances have upheld most of Darwin’s hypotheses. However, evolutionary theory continues to change as new data are gathered and new ways of thinking arise.