British Isles North America Asia North Pacific North atlantic Ocean Africa Phillippine Indian Ocean South slands Galapagos Madagascar Society Ascension Mauritius Valparaiso Bourbon island Australia Montevideo King Georges Buenos aires Good Hope Port Desire Hobart South atlantic Zealand Straits of Magellan Islands Cape Horn Tierra del Fuego FIGURE 1.6 The five-year voyage of H. M.S. Beagle. Most of the time was spent exploring the coasts and coastal islands of South America, such as the Galapagos Islands. Darwin's studies of the animals of the galapagos Islands played a key role in his eventual development of the theory of evolution by means of natural selection FIGURE 1.7 Cross section of the Beagle. A 10-gun brig of 242 tons, only 90 feet in length. the beagle had a crew of 74 people! After he first saw the ship, Darwin wrote to his college sea fessor henslow:“Th absolute want of room is an evil that nothing can as naturalist on a five-year navigational mapping expedition different subjects, including the formation of oceanic islands around the coasts of South America(figure 1.6), aboard from coral reefs and the geology of South America. He also H M.S. Beagle(figure 1.7). During this long voyage, Darwin devoted eight years of study to barnacles, a group of small had the chance to study a wide variety of plants and animals marine animals with shells that inhabit rocks and pilings on continents and islands and in distant seas. He was able to eventually writing a four-volume work on their classification explore the biological richness of the tropical forests, exam- and natural history. In 1842, Darwin and his family moved ine the extraordinary fossils of huge extinct mammals in out of London to a country home at Down, in the county of Patagonia at the southern tip of South America, and observe Kent. In these pleasant surroundings, Darwin lived, studied the remarkable series of related but distinct forms of life on and wrote for the next 40 years the galapagos Islands, off the west coast of South America. Such an opportunity clearly played an important role in the development of his thoughts about the nature of life on Darwin was the first to propose natural selection as an earth When Darwin returned from the voyage at the age of 27, e diversity of life on earth. His hypothesis began a long period of study and contemplation. During grew from his observations on a five-year voyage around the next 10 years, he published important books on several the world Chapter 1 The Science of Biology 11
as naturalist on a five-year navigational mapping expedition around the coasts of South America (figure 1.6), aboard H.M.S. Beagle (figure 1.7). During this long voyage, Darwin had the chance to study a wide variety of plants and animals on continents and islands and in distant seas. He was able to explore the biological richness of the tropical forests, examine the extraordinary fossils of huge extinct mammals in Patagonia at the southern tip of South America, and observe the remarkable series of related but distinct forms of life on the Galápagos Islands, off the west coast of South America. Such an opportunity clearly played an important role in the development of his thoughts about the nature of life on earth. When Darwin returned from the voyage at the age of 27, he began a long period of study and contemplation. During the next 10 years, he published important books on several different subjects, including the formation of oceanic islands from coral reefs and the geology of South America. He also devoted eight years of study to barnacles, a group of small marine animals with shells that inhabit rocks and pilings, eventually writing a four-volume work on their classification and natural history. In 1842, Darwin and his family moved out of London to a country home at Down, in the county of Kent. In these pleasant surroundings, Darwin lived, studied, and wrote for the next 40 years. Darwin was the first to propose natural selection as an explanation for the mechanism of evolution that produced the diversity of life on earth. His hypothesis grew from his observations on a five-year voyage around the world. Chapter 1 The Science of Biology 11 British Isles Western Isles Europe Africa Indian Ocean Madagascar Mauritius Bourbon Island Cape of Good Hope King George’s Sound Hobart Sydney Australia New Zealand Friendly Islands Phillippine Islands Equator North Pacific Ocean Asia North Atlantic Ocean Cape Verde Marquesas Galápagos Islands Valparaiso Society Islands Straits of Magellan Cape Horn Tierra del Fuego Falkland Islands Port Desire South Atlantic Ocean Montevideo Buenos Aires Rio de Janeiro St. Helena Ascension North America Canary Islands Keeling Islands South America Bahia FIGURE 1.6 The five-year voyage of H.M.S. Beagle. Most of the time was spent exploring the coasts and coastal islands of South America, such as the Galápagos Islands. Darwin’s studies of the animals of the Galápagos Islands played a key role in his eventual development of the theory of evolution by means of natural selection. FIGURE 1.7 Cross section of the Beagle. A 10-gun brig of 242 tons, only 90 feet in length, the Beagle had a crew of 74 people! After he first saw the ship, Darwin wrote to his college professor Henslow: “The absolute want of room is an evil that nothing can surmount
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 Things
12 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 others 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 seriously 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 similar species varied somewhat from place to place. These geographical patterns suggested to him that organismal lineages change gradually as species migrate from one area to another. On the Galápagos Islands, off the coast of Ecuador, 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 volcanic 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 resemble 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
Inventing the Theory of Natural selection It is one thing to observe the results of evolution, but Geometric quite another to understand how it happens. Darwin's progres great achievement lies in his formulation of the hypothe sis that evolution occurs because of natural selection Darwin and malthus Of key importance to the development of Darwins in sight was his study of Thomas Malthus's Essay on the Principle of Population(1798). In his book, Malth Arithmetic ointed out that populations of plants and animals(in- progression cluding human beings) tend to increase geometrically, while the ability of humans to increase their food supply increases only arithmetically. a geometric progression is one in which the elements increase by a constant factor; for example, in the progression 2, 6, 18, 54, number is three times the preceding one. An arithmetic progression,in contrast, is one in which the elements in FIGURE 1.9 crease by a constant difference; in the progression 2, 6, 10, Geometric and arithmetic progressions. A geometric progression each number is four greater than the preced- increases by a constant factor(e. g, X 2 or X 3 or X 4), while an arithmetic progression increases by a constant difference(e. g, units of 1 or 2 or 3). Malthus contended that the human growt Because populations increase geometrically, virtually curve was geometric, but the human food production curve any kind of animal or plant, if it could reproduce un- only arithmetic. Can you see the problems this difference would checked woule the entire surface of the world within a surprisingly short time. Instead, populations of species remain fairly constant vear after vear, because death limits population numbers. Malthus's conclusion provided the key ingredient that was necessary for Dar win to develop the hypothesis that evolution occurs by natural selection Sparked by Malthus' s ideas, Darwin saw that although every organism has the potential to produce more of Can we doult.. that individuals having spring than can survive, only a limited number actually do survive and produce further offspring. Combining an Owns【5 OcT this observation with what he had seen on the voyage of the Beagle, as well as with his own experiences in breed- oLe. wo ld have the best chance of ing domestic animals, Darwin made an important associ- ation(figure 1. 10): Those individuals that possess supe- suzoLUIng and procreating theit hind? On rior physical, behavioral, or other attributes are more ikely to survive than those that are not so well endowed the othet hand, we may feel sute that By surviving, they gain the opportunity to pass on their favorable characteristics to their offspring. As the fre- variation in cn/ullou quency of these characteristics increases in the popula- tion,the nature of the population as a whole will gradu- ally change. Darwin called this process selection. The driving force he identified has often been referred to as survival of the fittest callwatuzal selection An excerpt from Charles Darwins On tbe Origin of species. Chapter 1 The Science of Biology 13
Inventing the Theory of Natural Selection It is one thing to observe the results of evolution, but quite another to understand how it happens. Darwin’s great achievement lies in his formulation of the hypothesis that evolution occurs because of natural selection. Darwin and Malthus Of key importance to the development of Darwin’s insight was his study of Thomas Malthus’s Essay on the Principle of Population (1798). In his book, Malthus pointed out that populations of plants and animals (including human beings) tend to increase geometrically, while the ability of humans to increase their food supply increases only arithmetically. A geometric progression is one in which the elements increase by a constant factor; for example, in the progression 2, 6, 18, 54, . . . , each number is three times the preceding one. An arithmetic progression, in contrast, is one in which the elements increase by a constant difference; in the progression 2, 6, 10, 14, . . . , each number is four greater than the preceding one (figure 1.9). Because populations increase geometrically, virtually any kind of animal or plant, if it could reproduce unchecked, would cover the entire surface of the world within a surprisingly short time. Instead, populations of species remain fairly constant year after year, because death limits population numbers. Malthus’s conclusion provided the key ingredient that was necessary for Darwin to develop the hypothesis that evolution occurs by natural selection. Sparked by Malthus’s ideas, Darwin saw that although every organism has the potential to produce more offspring than can survive, only a limited number actually do survive and produce further offspring. Combining this observation with what he had seen on the voyage of the Beagle, as well as with his own experiences in breeding domestic animals, Darwin made an important association (figure 1.10): Those individuals that possess superior physical, behavioral, or other attributes are more likely to survive than those that are not so well endowed. By surviving, they gain the opportunity to pass on their favorable characteristics to their offspring. As the frequency of these characteristics increases in the population, the nature of the population as a whole will gradually change. Darwin called this process selection. The driving force he identified has often been referred to as survival of the fittest. Chapter 1 The Science of Biology 13 Geometric progression Arithmetic progression 2 6 18 54 4 6 8 FIGURE 1.9 Geometric and arithmetic progressions. A geometric progression increases by a constant factor (e.g., 2 or 3 or 4), while an arithmetic progression increases by a constant difference (e.g., units of 1 or 2 or 3) . Malthus contended that the human growth curve was geometric, but the human food production curve was only arithmetic. Can you see the problems this difference would cause? FIGURE 1.10 An excerpt from Charles Darwin’s On the Origin of Species.
Natural selection d that Darwin was simply refining Darwin was thoroughly familiar with is theory all those years, although variation in domesticated animals and here is little evidence he altered his began On the Origin of Species with a initial manuscript in all that time detailed discussion of pigeon breeding He knew that breeders selected certain Wallace has the same idea varieties of pigeons and other animals, such as dogs, to produce certain char- The stimulus that finally brought Dar acteristics, a process Darwin called ar- wins theory into print was an essay he tificial selection. Once this had been received in 1858. A young English nat done. the animals would breed true for uralist named Alfred russel walla the characteristics that had been select (1823-1913)sent the essay to Darwin d darwin had also observed that the from Malaysia; it concisely set forth differences purposely developed be he theory of evolution by means of tween domesticated races or breeds natural selection, a theory Wallace had were often greater than those that sep developed independently of Darwin arated wild species. Domestic pigeon Like darwin. wallace had been breeds, for example, show much reatly influenced by Malthus's 1798 greater variety than all of the hundreds essay. Colleagues of Wallace, knowin of wild species of ns four of Darwins work, encouraged him to throughout the world. Such relation- communicate with Darwin. After re ships suggested to Darwin that evolu ceiving Wallace's essay, Darwin ar- ionary change could occur in nature ranged for a joint presentation of their ideas at a seminar in London darwin ing the breeders role in selecting the unsympathetically, as in this drawing from written so long ago, and submitted it next generation-a process Darwin an 1874 publication. for publication called natural selection Darwins theory thus incorporates the hypothesis of evolution, the pro- cess of natural selection. and the mass of new evidence Publication of darwin's the for both evolution and natural selection that Darwin Darwins book appeared in November 1859 and caused an ompiled. Thus, Darwins theory provides a simple and immediate sensation. Many people were deeply disturbed by direct explanation of biological diversity, or why animals the suggestion that human beings were descended from the are different in different places: because habitats differ in same ancestor as apes(figure 1.11). Darwin did not actually their requirements and opportunities, the organisms with discuss this idea in his book, but it followed directly from the characteristics favored locally by natural selection will principles he outlined. In a subsequent book, The Descent of tend to vary in different places Man, Darwin presented the argument directly, building a powerful case that humans and living apes have common an- cestors. Although people had long accepted that humans Darwin Drafts His Argument closely resembled apes in many characteristics, the possibility that there might be a direct evolutionary relationship was un- Darwin drafted the overall argument for evolution by natu- acceptable to many. Darwins arguments for the theory of ral selection in a preliminary manuscript in 1842. After evolution by natural selection were so compelling, howe friends, however, Darwin put it in a drawer, and for tellectual community of Great Britain after the 1860s 16 years turned to other research. No one knows for sure why Darwin did not publish his initial manuscript-it is The fact that populations do not really expand very thorough and outlines his ideas in detail. Some histo- geometrically implies that nature acts to limit rians have suggested that Darwin was shy of igniting public population numbers. The traits of organisms tha criticism of his evolutionary ideas-there could have been survive to produce more offspring will be more little doubt in his mind that his theory of evolution by nat- common in future generations-a process Darwin ural selection would spark controversy. Others have pro called natural selection 14 Part I The Origin of living things
Natural Selection Darwin was thoroughly familiar with variation in domesticated animals and began On the Origin of Species with a detailed discussion of pigeon breeding. He knew that breeders selected certain varieties of pigeons and other animals, such as dogs, to produce certain characteristics, a process Darwin called artificial selection. Once this had been done, the animals would breed true for the characteristics that had been selected. Darwin had also observed that the differences purposely developed between domesticated races or breeds were often greater than those that separated wild species. Domestic pigeon breeds, for example, show much greater variety than all of the hundreds of wild species of pigeons found throughout the world. Such relationships suggested to Darwin that evolutionary change could occur in nature too. Surely if pigeon breeders could foster such variation by “artificial selection,” nature could do the same, playing the breeder’s role in selecting the next generation—a process Darwin called natural selection. Darwin’s theory thus incorporates the hypothesis of evolution, the process of natural selection, and the mass of new evidence for both evolution and natural selection that Darwin compiled. Thus, Darwin’s theory provides a simple and direct explanation of biological diversity, or why animals are different in different places: because habitats differ in their requirements and opportunities, the organisms with characteristics favored locally by natural selection will tend to vary in different places. Darwin Drafts His Argument Darwin drafted the overall argument for evolution by natural selection in a preliminary manuscript in 1842. After showing the manuscript to a few of his closest scientific friends, however, Darwin put it in a drawer, and for 16 years turned to other research. No one knows for sure why Darwin did not publish his initial manuscript—it is very thorough and outlines his ideas in detail. Some historians have suggested that Darwin was shy of igniting public criticism of his evolutionary ideas—there could have been little doubt in his mind that his theory of evolution by natural selection would spark controversy. Others have proposed that Darwin was simply refining his theory all those years, although there is little evidence he altered his initial manuscript in all that time. Wallace Has the Same Idea The stimulus that finally brought Darwin’s theory into print was an essay he received in 1858. A young English naturalist named Alfred Russel Wallace (1823–1913) sent the essay to Darwin from Malaysia; it concisely set forth the theory of evolution by means of natural selection, a theory Wallace had developed independently of Darwin. Like Darwin, Wallace had been greatly influenced by Malthus’s 1798 essay. Colleagues of Wallace, knowing of Darwin’s work, encouraged him to communicate with Darwin. After receiving Wallace’s essay, Darwin arranged for a joint presentation of their ideas at a seminar in London. Darwin then completed his own book, expanding the 1842 manuscript which he had written so long ago, and submitted it for publication. Publication of Darwin’s Theory Darwin’s book appeared in November 1859 and caused an immediate sensation. Many people were deeply disturbed by the suggestion that human beings were descended from the same ancestor as apes (figure 1.11). Darwin did not actually discuss this idea in his book, but it followed directly from the principles he outlined. In a subsequent book, The Descent of Man, Darwin presented the argument directly, building a powerful case that humans and living apes have common ancestors. Although people had long accepted that humans closely resembled apes in many characteristics, the possibility that there might be a direct evolutionary relationship was unacceptable to many. Darwin’s arguments for the theory of evolution by natural selection were so compelling, however, that his views were almost completely accepted within the intellectual community of Great Britain after the 1860s. The fact that populations do not really expand geometrically implies that nature acts to limit population numbers. The traits of organisms that survive to produce more offspring will be more common in future generations—a process Darwin called natural selection. 14 Part I The Origin of Living Things FIGURE 1.11 Darwin greets his monkey ancestor. In his time, Darwin was often portrayed unsympathetically, as in this drawing from an 1874 publication
Evolution after darwin original ancestor would have required a great deal more More evidence time. Using evidence obtained by studying the rates of ra dioactive decay, we now know that the physicists of Dar More than a century has elapsed since Darwins death in wins time were wrong, very wrong: the earth was formed 1882. During this period, the evidence supporting his the about 4.5 billion years ago ory has grown progressively stronger. There have also been many significant advances in our understanding of The Mechanism of Heredity how evolution works. Although these advances have not altered the basic structure of Darwins theory, they have Darwin received some of his sharpest criticism in the area of taught us a great deal more about the mechanisms by heredity. At that time, no one had any concept of genes or which evolution occurs. We will briefly explore some of of how heredity works, so it was not possible for Darwin to this evidence here; in chapter 21 we will return to the the- explain completely how evolution occurs. Theories of he- ory of evolution and examine the evidence in more detail redity in Darwins day seemed to rule out the possibility of genetic variation in nature, a critical requirement of Dar- wins theory. Genetics was established as a science only at The Fossil record the start of the twentieth century, 40 years after the publica- tion of Darwins On the Origin of Species. When scientists Darwin predicted that the fossil record would yield inter- began to understand the laws of inheritance(discussed in mediate links between the great groups of organisms, for chapter 13), the heredity problem with Darwin's theory anished. Genetics accounts in a neat and orderly way for have arisen from them, and between reptiles and birds. We the production of new variations in organisms able in the nineteenth century. Recent discoveries of mi- %copic fossils have extended the known history of life on Comparative Anatomy earth back to about 3. 5 billion years ago. The discovery of Comparative studies of animals have provided strong evi shed light on how organisms have, over this enormous time brates, for example, the same bones are presen, n verte- dence for Darwins theory. In many different types of span, evolved from the simple to the complex. For verte- heir evolutionary past. Thus, the forelimbs shown in brate animals especially, the fossil record is rich and exhib- 1.12 are all constructed from the same basic array of b its a graded series of changes in form, with the evolutionary modified in one way in the wing of a bat, in another way in parade visible for all to see(see Box: Why Study Fossils the fin of a porpoise, and in yet another way in the leg of a horse. The bones are said to be homologous in the differ The Age of the Earth ent vertebrates; that is, they have the same evolutionary ori- gin, but they now differ in structure and function. This con- In Darwins day, some physicists argued that the earth was trasts with analogous structures, such as the wings of birds only a few thousand years old. This bothered Darwin, be- and butterflies, which have similar structure and function cause the evolution of all living things from some single but different evolutionary origi FIGURE 1.12 Homology among vertebrate limbs. The forelimbs of these five vertebrates show the ways in which the relative roportions of the forelimb bones have changed in relation to the particular way of life of Porpoise each organism Chapter 1 The Science of Biology 15
Evolution After Darwin: More Evidence More than a century has elapsed since Darwin’s death in 1882. During this period, the evidence supporting his theory has grown progressively stronger. There have also been many significant advances in our understanding of how evolution works. Although these advances have not altered the basic structure of Darwin’s theory, they have taught us a great deal more about the mechanisms by which evolution occurs. We will briefly explore some of this evidence here; in chapter 21 we will return to the theory of evolution and examine the evidence in more detail. The Fossil Record Darwin predicted that the fossil record would yield intermediate links between the great groups of organisms, for example, between fishes and the amphibians thought to have arisen from them, and between reptiles and birds. We now know the fossil record to a degree that was unthinkable in the nineteenth century. Recent discoveries of microscopic fossils have extended the known history of life on earth back to about 3.5 billion years ago. The discovery of other fossils has supported Darwin’s predictions and has shed light on how organisms have, over this enormous time span, evolved from the simple to the complex. For vertebrate animals especially, the fossil record is rich and exhibits a graded series of changes in form, with the evolutionary parade visible for all to see (see Box: Why Study Fossils?). The Age of the Earth In Darwin’s day, some physicists argued that the earth was only a few thousand years old. This bothered Darwin, because the evolution of all living things from some single original ancestor would have required a great deal more time. Using evidence obtained by studying the rates of radioactive decay, we now know that the physicists of Darwin’s time were wrong, very wrong: the earth was formed about 4.5 billion years ago. The Mechanism of Heredity Darwin received some of his sharpest criticism in the area of heredity. At that time, no one had any concept of genes or of how heredity works, so it was not possible for Darwin to explain completely how evolution occurs. Theories of heredity in Darwin’s day seemed to rule out the possibility of genetic variation in nature, a critical requirement of Darwin’s theory. Genetics was established as a science only at the start of the twentieth century, 40 years after the publication of Darwin’s On the Origin of Species. When scientists began to understand the laws of inheritance (discussed in chapter 13), the heredity problem with Darwin’s theory vanished. Genetics accounts in a neat and orderly way for the production of new variations in organisms. Comparative Anatomy Comparative studies of animals have provided strong evidence for Darwin’s theory. In many different types of vertebrates, for example, the same bones are present, indicating their evolutionary past. Thus, the forelimbs shown in figure 1.12 are all constructed from the same basic array of bones, modified in one way in the wing of a bat, in another way in the fin of a porpoise, and in yet another way in the leg of a horse. The bones are said to be homologous in the different vertebrates; that is, they have the same evolutionary origin, but they now differ in structure and function. This contrasts with analogous structures, such as the wings of birds and butterflies, which have similar structure and function but different evolutionary origins. Chapter 1 The Science of Biology 15 Human Cat Bat Porpoise Horse FIGURE 1.12 Homology among vertebrate limbs. The forelimbs of these five vertebrates show the ways in which the relative proportions of the forelimb bones have changed in relation to the particular way of life of each organism