Darwin's Evidence Table 1.1 Darwin,s Evidence that Evolution occurs One of the obstacles that had blocked the acceptance of any theory of evolution in Darwins day was the incorrect FOSSILS notion, widely believed at that time, that the earth was 1. Extinct species, such as the fossil armadillo in figure 1. 8, ly a few thousand years old. Evidence discovered during most closely resemble living ones in the same area, Darwins time made this assertion seem less and less likely suggesting that one had given rise to the other The great geologist Charles Lyell(1797-1875), whose 2. In rock strata(layers), progressive changes in characteristics can be seen in fossils from earlier and earlier layers Principles of Geology(1830)Darwin read eagerly as he sailed on the Beagle, outlined for the first time the story of GEOGRAPHICAL DISTRIBUTION an ancient world of plants and animals in flux. In this 3. Lands with similar climates, such as Australia. South Africa world, species were constantly be ning extinct while California, and Chile, have unrelated plants and animals, ers were emerging. It was this world that Darwin sought to indicating that diversity is not entirely influenced by climate and environment 4. The plants and animals of each continent are distinctive all South American rodents belong to a single group, What darwin saw structurally similar to the guinea pigs, for example, while most of the rodents found elsewhere belong to other When the Beagle set sail, Darwin was fully convinced that species were immutable. Indeed, it was not until two or OCEANIC ISLANDS three years after his return that he began to consider seri- 5. Although oceanic islands have few species, those they do ously the possibility that they could change. Nevertheless, have are often unique(endemic)and show relatedness to during his five years on the ship, Darwin observed a number one another, such as the galapagos tortoises. This suggests of phenomena that were of central importance to him in hat the tortoises and other groups of endemic species the rong his ultimate conclusion(table 1. 1). For example, in developed after their mainland ancestors reached the islands ch fossil beds of southern South america. he observed and are, therefore, more closely related to one anothe fossils of extinct armadillos similar to the armadillos that 6 Species on oceanic islands show strong affinities to those on still lived in the same area(figure 1. 8). Why would similar the nearest mainland. Thus, the finches of the galapagos iving and fossil organisms be in the same area unless the Islands closely resemble a finch seen on the western coast of South america. The Galapagos finches do not resemble the earlier form had given rise to the other? birds on the Cape Verde Islands, islands in the Atlantic Repeatedly, Darwin saw that the characteristics of Ocean off the coast of Africa that are similar to the lar species varied somewhat from place to place. Galapagos. Darwin visited the Cape Verde Islands and geographical patterns suggested to him that organismal lin- many other island groups personally and was able to make eages change gradually as species migrate from one area to such comparisons on the basis of his own observations. another. On the Galapagos Islands, off the coast of Ecua dor, Darwin encountered giant land tortoises. Surprisingly, these tortoises were not all identical. In fact, local residents and the sailors who captured the tortoises for food could tell which island a particular tortoise had come from just by looking at its shell. This distribution of physical variation uggested that all of the tortoises were related, but that they had changed slightly in appearance after becoming isolated on different islands In a more general sense, Darwin was struck by the fact that the plants and animals on these relatively young vol canic islands resembled those on the nearby coast of South America. If each one of these plants and animals had been created independently and simply placed on the Galapagos Islands, why didn't they resemble the plants and animals of islands with similar climates such as those the coast of Africa, for example? Why did they resem b)Arm ble those of the adjacent South American coast instead? FIGURE 1.8 The fossils and patterns of life that Darwin observed or Fossil evidence of evolution. The now-extinct glyptodont(a) the voyage of the Beagle eventually convinced him that was a 2000-kilogram South American armadillo, much larger than volution had taken place. the modern armadillo(b), which weighs an average of about 4.5 kilograms. (Drawings are not to scale 12 Part I The Origin of Living Things12 Part I The Origin of Living Things Darwin’s Evidence One of the obstacles that had blocked the acceptance of any theory of evolution in Darwin’s day was the incorrect notion, widely believed at that time, that the earth was only a few thousand years old. Evidence discovered during Darwin’s time made this assertion seem less and less likely. The great geologist Charles Lyell (1797–1875), whose Principles of Geology (1830) Darwin read eagerly as he sailed on the Beagle, outlined for the first time the story of an ancient world of plants and animals in flux. In this world, species were constantly becoming extinct while oth￾ers were emerging. It was this world that Darwin sought to explain. What Darwin Saw When the Beagle set sail, Darwin was fully convinced that species were immutable. Indeed, it was not until two or three years after his return that he began to consider seri￾ously the possibility that they could change. Nevertheless, during his five years on the ship, Darwin observed a number of phenomena that were of central importance to him in reaching his ultimate conclusion (table 1.1). For example, in the rich fossil beds of southern South America, he observed fossils of extinct armadillos similar to the armadillos that still lived in the same area (figure 1.8). Why would similar living and fossil organisms be in the same area unless the earlier form had given rise to the other? Repeatedly, Darwin saw that the characteristics of simi￾lar species varied somewhat from place to place. These geographical patterns suggested to him that organismal lin￾eages change gradually as species migrate from one area to another. On the Galápagos Islands, off the coast of Ecua￾dor, Darwin encountered giant land tortoises. Surprisingly, these tortoises were not all identical. In fact, local residents and the sailors who captured the tortoises for food could tell which island a particular tortoise had come from just by looking at its shell. This distribution of physical variation suggested that all of the tortoises were related, but that they had changed slightly in appearance after becoming isolated on different islands. In a more general sense, Darwin was struck by the fact that the plants and animals on these relatively young vol￾canic islands resembled those on the nearby coast of South America. If each one of these plants and animals had been created independently and simply placed on the Galápagos Islands, why didn’t they resemble the plants and animals of islands with similar climates, such as those off the coast of Africa, for example? Why did they resem￾ble those of the adjacent South American coast instead? The fossils and patterns of life that Darwin observed on the voyage of the Beagle eventually convinced him that evolution had taken place. Table 1.1 Darwin’s Evidence that Evolution Occurs FOSSILS 1. Extinct species, such as the fossil armadillo in figure 1.8, most closely resemble living ones in the same area, suggesting that one had given rise to the other. 2. In rock strata (layers), progressive changes in characteristics can be seen in fossils from earlier and earlier layers. GEOGRAPHICAL DISTRIBUTION 3. Lands with similar climates, such as Australia, South Africa, California, and Chile, have unrelated plants and animals, indicating that diversity is not entirely influenced by climate and environment. 4. The plants and animals of each continent are distinctive; all South American rodents belong to a single group, structurally similar to the guinea pigs, for example, while most of the rodents found elsewhere belong to other groups. OCEANIC ISLANDS 5. Although oceanic islands have few species, those they do have are often unique (endemic) and show relatedness to one another, such as the Galápagos tortoises. This suggests that the tortoises and other groups of endemic species developed after their mainland ancestors reached the islands and are, therefore, more closely related to one another. 6. Species on oceanic islands show strong affinities to those on the nearest mainland. Thus, the finches of the Galápagos Islands closely resemble a finch seen on the western coast of South America. The Galápagos finches do not resemble the birds on the Cape Verde Islands, islands in the Atlantic Ocean off the coast of Africa that are similar to the Galápagos. Darwin visited the Cape Verde Islands and many other island groups personally and was able to make such comparisons on the basis of his own observations. FIGURE 1.8 Fossil evidence of evolution. The now-extinct glyptodont (a) was a 2000-kilogram South American armadillo, much larger than the modern armadillo (b), which weighs an average of about 4.5 kilograms. (Drawings are not to scale.) (a) Glyptodont (b) Armadillo