Title (Campbell Biology Series) Lisa A. Urry, Micheal L. Cain, Steven A. Wasserman, Peter V. Minorsky, Rebecca B. Orr, Neil A. Campbell - Campbell Biology, 12th Edition-Pearson (2020) (1)-4-9.pdf ✅

CHAPTER 22 Descent with Modification: A Darwinian View of Life 471 show that traits acquired by use during an individual’s life are not inherited in the way proposed by Lamarck (Figure 22.4). Lamarck was vilified in his own time, especially by Cuvier, who denied that species ever evolve. In retrospect, however, Lamarck did recognize that the fact that organisms are well- suited for life in their environments can be explained by gradual evolutionary change, and he did propose a testable explanation for how this change occurs. event, such as a flood, that had destroyed many of the species living in that area. Such regions, he reasoned, were later repop- ulated by different species immigrating from other areas. In contrast to Cuvier’s emphasis on sudden events, other scientists suggested that profound change could take place through the cumulative effect of slow but continuous pro- cesses. In 1795, Scottish geologist James Hutton (1726–1797) proposed that Earth’s geologic features could be explained by gradual mechanisms, such as valleys being formed by riv- ers. The leading geologist of Darwin’s time, Charles Lyell (1797–1875), incorporated Hutton’s thinking into his pro- posal that the same geologic processes are operating today as in the past, and at the same rate. Hutton’s and Lyell’s ideas strongly influenced Darwin’s thinking. Darwin agreed that if geologic change results from slow, continuous actions rather than from sudden events, then Earth must be much older than the widely accepted age of a few thousand years. It would, for example, take a very long time for a river to carve a canyon by erosion. He later reasoned that perhaps similarly slow and subtle processes could produce sub- stantial biological change. However, Darwin was not the first to apply the idea of gradual change to biological evolution. Lamarck’s Hypothesis of Evolution Although some 18th-century naturalists suggested that life evolves as environments change, only one proposed a mechanism for how life changes over time: French biologist Jean-Baptiste de Lamarck (1744–1829). Alas, Lamarck is pri- marily remembered today not for his visionary recognition that evolutionary change explains patterns in fossils and how organisms are well suited for their environments, but for the incorrect mechanism he proposed. Lamarck published his hypothesis in 1809, the year Darwin was born. By comparing living species with fossil forms, Lamarck had found what appeared to be several lines of descent, each a chronological series of older to younger fos- sils leading to a living species. He explained his findings using two principles that were widely accepted at the time. The first was use and disuse, the idea that parts of the body that are used extensively become larger and stronger, while those that are not used deteriorate. Among many examples, he cited a giraffe stretching its neck to reach leaves on high branches. The second principle, inheritance of acquired characteristics, stated that an organism could pass these modifications to its offspring. Lamarck reasoned that the long, muscular neck of the living giraffe had evolved over many generations as giraffes stretched their necks ever higher. Lamarck also thought that evolution happens because organisms have an innate drive to become more complex. Darwin rejected this idea, but he, too, thought that variation was introduced into the evolutionary process in part by inher- itance of acquired characteristics. Today, however, our under- standing of genetics refutes this mechanism: Experiments CONCEPT CHECK 22.1 1. How did Hutton’s and Lyell’s ideas influence Darwin’s think- ing about evolution? 2. MAKE CONNECTIONS Scientific hypotheses must be test- able (see Concept 1.3). Applying this criterion, are Cuvier’s explanation of the fossil record and Lamarck’s hypothesis of evolution scientific? Explain your answer in each case. For suggested answers, see Appendix A. c Figure 22.4 Acquired traits cannot be inherited. This bonsai tree was “trained” to grow as a dwarf by pruning and shaping. However, seeds from this tree would produce offspring of normal size. CONCEPT 22.2 Descent with modification by natural selection explains the adaptations of organisms and the unity and diversity of life As the 19th century dawned, it was generally thought that species had remained unchanged since their creation. A few clouds of doubt about the permanence of species were begin- ning to gather, but no one could have forecast the thunder- ing storm just beyond the horizon. How did Charles Darwin become the lightning rod for a revolutionary view of life? Darwin’s Research Charles Darwin (1809–1882) was born in Shrewsbury, in western England. Even as a boy, he had a consuming inter- est in nature. When he was not reading nature books, he was fishing, hunting, riding, and collecting insects. However, URRY8743_12_SE_C22_PRF.indd 471 11/6/19 10:48 AM
472 UNIT FOUR Mechanisms of Evolution of Geology during the voyage. He experienced geologic change firsthand when a violent earthquake shook the coast of Chile, and he observed afterward that rocks along the coast had been thrust upward by several meters. Finding fossils of ocean organisms high in the Andes, Darwin inferred that the rocks containing the fossils must have been raised there by many similar earthquakes. These observations reinforced what he had learned from Lyell: Physical evidence did not support the traditional view that Earth was only a few thousand years old. Darwin’s interest in the species (or fossils) found in an area was further stimulated by the Beagle’s stop at the Galápagos, a group of volcanic islands located near the equator about 900 km west of South America (Figure 22.5). Darwin was fasci- nated by the unusual organisms there. The birds he collected included several kinds of mockingbirds. These mockingbirds, though similar to each other, seemed to be different species. Some were unique to individual islands, while others lived on two or more adjacent islands. Furthermore, although the ani- mals on the Galápagos resembled species living on the South American mainland, most of the Galápagos species were not known from anywhere else in the world. Darwin hypoth- esized that the Galápagos had been colonized by organisms that had strayed from South America and then diversified, giving rise to new species on the various islands. Darwin’s Focus on Adaptation During the voyage of the Beagle, Darwin observed many exam- ples of adaptations, inherited characteristics of organisms that enhance their survival and reproduction in specific envi- ronments. Later, as he reassessed his observations, he began to perceive adaptation to the environment and the origin of new species as closely related processes. Could a new species arise Darwin’s father, a physician, could see no future for his son as a naturalist and sent him to medical school in Edinburgh. But Charles found medicine boring and surgery before the days of anesthesia horrifying. He quit medical school and enrolled at Cambridge University, intending to become a clergyman. (At that time, many scholars of science belonged to the clergy.) At Cambridge, Darwin became the protégé of John Henslow, a botany professor. Soon after Darwin graduated, Henslow recommended him to Captain Robert FitzRoy, who was readying the survey ship HMS Beagle for a voyage around the world. Darwin would pay his own way and serve as a conversation partner to the young captain. FitzRoy, who was himself an adept scientist, accepted Darwin because he was a skilled naturalist and they were of similar age and social class. The Voyage of the Beagle Darwin embarked from England on the Beagle in December 1831. The primary mission of the voyage was to chart stretches of the South American coast that were poorly known to Europeans. Darwin, however, spent most of his time on shore, observing and collecting thousands of plants and animals. He described features of organisms that made them well suited to such diverse environments as the humid jungles of Brazil, the expansive grasslands of Argentina, and the towering peaks of the Andes. He also noted that the plants and animals in temperate regions of South America more closely resembled species living in the South American tropics than species liv- ing in temperate regions of Europe. Furthermore, the fossils he found, though clearly different from living species, distinctly resembled the living organisms of South America. Darwin also spent much time thinking about geology. Despite repeated bouts of seasickness, he read Lyell’s Principles PACIFIC Pinta OCEAN Genovesa The Galápagos Islands Equator Marchena Fernandina Santa Fe San Cristobal Floreana Isabela Santa Cruz Santiago Española 0 4020 Kilometers ATLANTIC OCEAN PACIFIC OCEAN NORTH AMERICA Darwin in 1840, after his return from the voyage SOUTH AMERICA Great Britain Andes Mtns. Cape Horn Cape of Good Hope Brazil Argentina Chile Equator Malay Archipelago AFRICA EUROPE HMS Beagle in port Tasmania AUSTRALIA New Zealand PACIFIC OCEAN . Figure 22.5 The voyage of HMS Beagle (December 1831–October 1836). URRY8743_12_SE_C22_PRF.indd 472 11/6/19 10:48 AM
CHAPTER 22 Descent with Modification: A Darwinian View of Life 473 (a) Cactus-eater. The long, sharp beak of the common cactus finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp. (b) Insect-eater. The green warbler finch (Certhidea olivacea) uses its narrow, pointed beak to grasp insects. (c) Seed-eater. The large ground finch (Geospiza magnirostris) has a large beak adapted for cracking seeds found on the ground. . Figure 22.6 Three examples of beak variation in Galápagos finches. The Galápagos Islands are home to more than a dozen species of closely related finches, some found only on a single island. A striking difference among them is their beaks, which are adapted for specific diets. MAKE CONNECTIONS Review Figure 1.20. Circle the most recent common ancestor shared by the three species that eat insects. Are all of the descendants of that ancestor insect-eaters? book, titled On the Origin of Species by Means of Natural Selection (commonly referred to as The Origin of Species), and published it the next year. Although Wallace had submitted his ideas for publication first, he admired Darwin and thought that Darwin had developed and tested the idea of natural selection so extensively that he should be known as its main architect. Within a decade, Darwin’s book and its proponents had convinced most scientists that life’s diversity is the product of evolution. Darwin succeeded where previous evolutionists had failed by presenting a plausible scientific mechanism with immaculate logic and an avalanche of supporting evidence. from an ancestral form by the gradual accumulation of adap- tations to a different environment? From studies made years after Darwin’s voyage, biologists have concluded that this is indeed what happened to a diverse group of finches found on the Galápagos Islands (see Figure 1.20). The finches’ various beaks and behaviors are adapted to the specific foods avail- able on their home islands (Figure 22.6). Darwin realized that explaining such adaptations was essential to understanding evolution. His explanation of how adaptations arise centered on natural selection, a process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than do other individuals because of those traits. By the early 1840s, Darwin had worked out the major features of his hypothesis. He set these ideas on paper in 1844, when he wrote a long essay on descent with modification and its underlying mechanism, natural selection. Yet he was still reluctant to publish his ideas, in part because he anticipated the uproar they would cause. During this time, Darwin contin- ued to compile evidence in support of his hypothesis. By the mid-1850s, he had described his ideas to Lyell and a few oth- ers. Lyell, who was not yet convinced of evolution, neverthe- less urged Darwin to publish on the subject before someone else came to the same conclusions and published first. In June 1858, Lyell’s prediction came true. Darwin received a manuscript from Alfred Russel Wallace (1823–1913), a British naturalist working in the South Pacific islands of the Malay Archipelago (see Figure 22.2). Wallace had developed a hypothesis of natural selection nearly identical to Darwin’s. He asked Darwin to evaluate his paper and forward it to Lyell if it merited publication. Darwin complied, writing to Lyell: “Your words have come true with a vengeance . . . . I never saw a more striking coincidence . . . so all my original- ity, whatever it may amount to, will be smashed.” On July 1, 1858, Lyell and a colleague presented Wallace’s paper, along with extracts from Darwin’s unpublished 1844 essay, to the Linnean Society of London. Darwin quickly finished his Ideas from The Origin of Species In his book, Darwin amassed evidence that descent with modification by natural selection explains three broad obser- vations about nature—the unity of life, the diversity of life, and the striking ways in which organisms are suited for life in their environments. Descent with Modification In the first edition of The Origin of Species, Darwin never used the word evolution (although the final word of the book is “evolved”). Rather, he discussed descent with modification, a phrase that summarized his view of life. Organisms share many characteristics, leading Darwin to perceive unity in life. He attributed the unity of life to the descent of all organisms from an ancestor that lived in the remote past. He also thought that as the descendants of that ancestral organism lived in various habitats, they gradually accumulated diverse modifications, or adaptations, that fit them to specific ways of life. Thus, Darwin thought of evolution as a process in which both descent (shared ancestry, resulting in shared characteristics) and modification (the accumulation of differences) can be observed. Mastering Biology HHMI Video: The Origin of Species: The Making of a Theory URRY8743_12_SE_C22_PRF.indd 473 11/6/19 10:48 AM
474 UNIT FOUR Mechanisms of Evolution Darwin reasoned that over a long period of time, descent with modification eventually led to the rich diversity of life we see today. He viewed the history of life as a tree, with mul- tiple branchings from a common trunk out to the tips of the youngest twigs (Figure 22.7). In his diagram, the tips of the twigs that are labeled A–D represent several groups of organ- isms living in the present day, while the unlabeled branches represent groups that are extinct. Each fork of the tree repre- sents the most recent common ancestor of all the lines of evo- lution that subsequently branch from that point. Darwin thought that such a branching process, along with past extinction events, could explain the large morphological gaps that sometimes exist between related groups of organisms. As an example, let’s consider the three living species of elephants: the Asian elephant (Elephas maximus) and two species of African elephants (Loxodonta africana and L. cyclotis). These closely related species are very similar because they shared the same line of descent until a relatively recent split from their common ancestor, as shown in the tree diagram in Figure 22.8. Note that seven lineages related to elephants have become extinct over the past 32 million years. As a result, there are no living species that fill the morphological gap between the elephants and their near- est relatives today, the hyraxes and manatees. Extinctions like those in Figure 22.8 are common. In fact, many evolutionary branches, even some major ones, are dead ends: Scientists estimate that over 99% of all species that have ever lived are now extinct. As in Figure 22.8, fossils of extinct species can document the divergence of present-day groups by “filling in” gaps between them. Artificial Selection, Natural Selection, and Adaptation Darwin proposed the mechanism of natural selection to explain the observable patterns of evolution. He crafted his argument carefully, hoping to persuade even the most skepti- cal readers. First he discussed familiar examples of selective breeding of domesticated plants and animals. Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits, a process c Figure 22.7 “I think . . . .” In this 1837 sketch, Darwin envisioned the branching pattern of evolution. Branches that end in twigs labeled A–D represent particular groups of living organisms; all other branches represent extinct groups. 60 34 24 Millions of years ago Years ago 5.5 2 104 0 †Barytherium †Mammut †Stegodon †Platybelodon †Mammuthus †Deinotherium †Moeritherium Elephas maximus (Asia) Loxodonta africana (Africa) Loxodonta cyclotis (Africa) Hyracoidea (Hyraxes) Sirenia (Manatees and relatives) Common ancestor of living elephant species . Figure 22.8 Descent with modification. This evolutionary tree of elephants and their relatives is based mainly on fossils—their anatomy, order of appearance in strata, and geographic distribution. Note that most branches of descent ended in extinction (denoted by the dagger symbol, †). (Time line not to scale.) VISUAL SKILLS Based on this tree, approximately when did the most recent ancestor shared by Mammuthus (woolly mammoths), Asian elephants, and African elephants live? URRY8743_12_SE_C22_PRF.indd 474 11/6/19 10:48 AM