CHAPTER 22 DARWINISM

1. CHAPTER 22DARWINISM

2. On the Origin of Species by Means of Natural Selection Published November 24, 1859 Two points in The Origin of Species: Today’s organisms descended from ancestral species. Natural selection provided a mechanism for evolutionary change in populations. The basic idea of natural selection is that a population of organisms can change over the generations if individuals having certain heritable traits leave more offspring than other individuals. The result of natural selection is evolutionary adaptation, a prevalence of inherited characteristics that enhance organisms’ survival and reproduction in specific environments. In modern terms, we would say that the genetic composition of the population had changed over time, and that is one way of defining evolution.

3. The Origin of Species was truly radical for its time; not only did it challenge prevailing scientific views, but it also shook the deepest roots of Western culture. Darwin’s view of life contrasted sharply with the conventional paradigm of an Earth only a few thousand years old, populated by unchanging forms of life that had been individually made during the single week in which the Creator formed the entire universe. Darwin’s book challenged a worldview that had been taught for centuries.

4. Plato (427-347 B.C.) and his student Aristotle (384-322 B.C.), held opinions that opposed any concept of evolution. Plato believed in two worlds: a real world that is ideal and eternal and an illusory world of imperfection that we perceive through our senses. Evolution would be counterproductive in a world where ideal organisms were already perfectly adapted to their environments. Plato Aristotle The School of Athens - Raphael

5. Aristotle believed that all living forms could be arranged on a scale, or ladder, of increasing complexity, later called the scala naturae ("scale of nature"). Each form of life had its allotted rung on this ladder, and every rung was taken In this view of life, which prevailed for over 2,000 years, species are permanent, are perfect, and do not evolve.

6. In Judeo-Christian culture, the Old Testament account of creation fortified the idea that species were individually designed and nonevolving. In the 1700s, biology in Europe and America was dominated by natural theology, a philosophy dedicated to discovering the Creator’s plan by studying nature. Natural theologians saw the adaptations of organisms as evidence that the Creator had designed each and every species for a particular purpose. A major objective of natural theology was to classify species in order to reveal the steps of the scale of life that God had created.

7. Carolus Linnaeus (1707-1778), a Swedish physician and botanist, sought to discover order in the diversity of life "for the greater glory of God." Linnaeus specialized in taxonomy, the branch of biology concerned with naming and classifying the diverse forms of life. He developed the two-part, or binomial, system of naming organisms according to genus and species that is still used today.

8. In addition, Linnaeus adopted a system for grouping similar species into a hierarchy of increasingly general categories (phylogeny). Major taxonomic categories; kingdom > phylum > class > order > family > genus > species For example, similar species are grouped in the same genus, similar genera (plural of genus) are grouped in the same family, and so on. To Linnaeus, clustering similar species together implied no evolutionary kinship, but a century later his taxonomic system would become a focal point in Darwin’s arguments for evolution.

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10. The study of fossils also helped lay the groundwork for Darwin’s ideas. Fossils are relics or impressions of organisms from the past, preserved in rock

11. Most fossils are found in sedimentary rocks formed from the sand and mud that settle to the bottom of seas, lakes, and marshes. New layers of sediment cover older ones and compress them into superimposed layers of rock called strata. Later, erosion may scrape or carve through upper (younger) strata and reveal more ancient strata that had been buried. Fossils within the layers show that a succession of organisms has populated Earth throughout time.

12. Paleontology, the study of fossils, was largely developed by French anatomist Georges Cuvier (1769-1832). Realizing that the history of life is recorded in strata containing fossils, he documented the succession of fossil species in the Paris Basin. He noted that each stratum is characterized by a unique group of fossil species and that the deeper (older) the stratum, the more dissimilar the fossils are from modern life. New Old

13. Cuvier even recognized that extinction had been a common occurrence in the history of life. From stratum to stratum, new species appear and others disappear. Yet Cuvier was a staunch opponent of the evolutionists of his day.

14. Instead, he advocated catastrophism, speculating that each boundary between strata corresponded in time to a catastrophe, such as a flood or drought, that had destroyed many of the species living there at that time. He proposed that these periodic catastrophes were usually confined to local geographic regions and that the ravaged region was repopulated by species immigrating from other areas. Landslides Floods Volcanism

15. Competing with Cuvier’s theory of catastrophism was a very different idea of how geologic processes had shaped Earth’s crust. In 1795, Scottish geologist James Hutton (1726-1797) proposed that it was possible to explain the various landforms by looking at mechanisms currently operating in the world. For example, he suggested that canyons were formed by rivers cutting down through rocks and that sedimentary rocks with marine fossils were built of particles that had eroded from the land and been carried by rivers to the sea Hutton explained Earth’s geologic features by the theory of gradualism, which holds that profound change is the cumulative product of slow but continuous processes.

16. The leading geologist of Darwin’s era, a Scot named Charles Lyell (1797-1875), incorporated Hutton’s gradualism into a theory known as uniformitarianism. The term refers to Lyell’s idea that geologic processes have not changed throughout Earth’s history. Thus, for example, the forces that build mountains and erode mountains and the rates at which these forces operate are the same today as in the past.

17. Darwin was strongly influenced by two conclusions that followed directly from the observations of Hutton and Lyell. Lyell Hutton First, if geologic change results from slow, continuous actions rather than sudden events, then Earth must be very old, certainly much older than the 6,000 years assigned by many theologians on the basis of biblical inference. Second, very slow and subtle processes persisting over a long period of time can add up to substantial change.

18. Darwin was not the first to apply the principle of gradualism to biological evolution, however. Toward the end of the 18th century, several naturalists, including Erasmus Darwin, Charles Darwin’s grandfather, suggested that life had evolved as environments changed.

19. One of Charles Darwin’s predecessors developed a comprehensive model that attempted to explain how life evolves: Jean Baptiste Lamarck Lamarck published his theory of evolution in 1809, the year Darwin was born. Lamarck was in charge of the invertebrate collection at the Natural History Museum in Paris. By comparing current species with fossil forms, Lamarck could see what appeared to be several lines of descent, each a chronological series of older to younger fossils leading to a modern species.

20. Lamarck is remembered most for the mechanism he proposed to explain how specific adaptations evolve. It incorporates two ideas that were popular during Lamarck’s era. The first was use and disuse, the idea that those parts of the body used extensively to cope with the environment become larger and stronger while those that are not used deteriorate. Among the examples Lamarck cited were a blacksmith developing a bigger bicep in the arm that wields the hammer and a giraffe stretching its neck to reach leaves on high branches.

21. The second idea Lamarck adopted was called the inheritance of acquired characteristics. In this concept of heredity, the modifications an organism acquires during its lifetime can be passed along to its offspring. The long neck of the giraffe, Lamarck reasoned, evolved gradually as the cumulative product of a great many generations of ancestors stretching ever higher.

22. There is, however, no evidence that acquired characteristics can be inherited. Blacksmiths may increase strength and stamina by a lifetime of pounding with a heavy hammer, but these acquired traits do not change genes transmitted by gametes to offspring. Even though the Lamarckian theory of evolution is ridiculed often today because of its erroneous assumption that acquired characteristics are inherited, in Lamarck’s time that concept of inheritance was generally accepted (and, indeed, Darwin could offer no acceptable alternative). Lance Armstrong Michael Jordan Nolan Ryan

23. To most of Lamarck’s contemporaries, however, the mechanism of evolution was an irrelevant issue because they firmly believed that species were fixed and that no theory of evolution could be taken seriously. Lamarck was vilified, especially by Cuvier, who denied that species ever evolve. In retrospect, Lamarck deserves much credit for his theory, which was visionary in many respects: in its claim that evolution is the best explanation for both the fossil record and the current diversity of life; in its recognition of the great age of Earth; and especially in its emphasis on adaptation to the environment as a primary product of evolution. X

24. The Darwinian Revolution Natural theology still dominated the intellectual climate as the 19th century dawned. Charles Darwin (1809-1882) was born in Shrewsbury in western England. Even as a boy he had a consuming interest in nature. When he was not reading nature books, he was fishing, hunting, and collecting insects.

25. Darwin’s father, an eminent physician, could see no future for a naturalist and sent Charles to the University of Edinburgh to study medicine. Only 16 years old at the time, Charles found medical school boring and distasteful.

26. He left Edinburgh without a degree and shortly thereafter enrolled at Christ College at Cambridge University, with the intent of becoming a clergyman. At that time in Great Britain, most naturalists and other scientists belonged to the clergy, and nearly all saw the world in the context of natural theology.

27. Darwin became the protégé of the Reverend John Henslow, professor of botany at Cambridge. Soon after Darwin received his B.A. degree in 1831, Professor Henslow recommended the young graduate to Captain Robert FitzRoy, who was preparing the survey ship Beagle for a voyage around the world.

28. Darwin would pay his own way and serve as a conversation companion to the young captain. FitzRoy chose Darwin because of his education and because he was of the same social class and about the same age as the captain. (1831-1836) FitzRoy Darwin Sunday Service at Sea by Augustus Earle (ship’s artist)

29. Darwin was 22 years old when he sailed from Great Britain aboard HMS Beagle in December 1831 The primary mission of the voyage was to chart poorly known stretches of the South American coastline. While the ship’s crew surveyed the coast, Darwin spent most of his time on shore, observing and collecting thousands of specimens of South American plants and animals.

30. As the ship worked its way around the continent, Darwin observed the various adaptations of plants and animals that inhabited such diverse environments as the Brazilian jungles, the expansive grasslands of the Argentine pampas, the desolate lands of Tierra del Fuego near Antarctica, and the towering heights of the Andes Mountains. Brazilian Jungle

31. Darwin noted that plants and animals he studied had definite South American characteristics, very distinct from those of Europe. That in itself may not have been surprising. But Darwin also noted that the plants and animals in temperate regions of South America were more closely related to species living in tropical regions of that continent than to species in temperate regions of Europe. x =

32. Furthermore, the South American fossils that Darwin found, though clearly different from modern species, were distinctly South American in their resemblance to the living plants and animals of that continent. South American Ostrich (Rhea) and young Toxodon platensis skull London Natural History Museum Drawing of Toxodon platensis skull that Darwin collected on the voyage Three-toed Sloth (South America)

33. The geographic distribution of species interested Darwin. For example, he was curious about the fauna of the Galápagos, islands of relatively recent volcanic origin that lie on the equator about 900 km west of the South American coast.

34. He learned that most of the animal species on the Galápagos live nowhere else in the world, although they resemble species living on the South American mainland. “indigenous” “Flightless Cormorant” Penguin Giant Tortoise Blue Footed Booby Finch Sally Light Foot Crab

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36. It was as though the islands had been colonized by plants and animals that strayed from the South American mainland and then diversified on the different islands.     

37. Among the birds Darwin collected on the Galápagos were several types of finches that, although quite similar, seemed to be different species. Some were unique to individual islands, while other species were distributed on two or more islands that were close together.

38. Darwin read Lyell’s Principles of Geology while on board the Beagle . Lyell’s ideas, together with his own experiences on the Galápagos, had Darwin doubting the church’s position that Earth was static and had been created only a few thousand years ago.

39. By acknowledging that Earth was very old and constantly changing, Darwin took an important step toward recognizing that life on Earth had also evolved.

40. Soon after returning to Great Britain in 1836, Darwin started reassessing all that he had observed during the voyage of the Beagle . He began to perceive the origin of new species and adaptation to the environment as closely related processes. Could a new species arise from an ancestral form by the gradual accumulation of adaptations to a different environment? Darwin’s Pigeons

41. From studies made years after Darwin’s voyage, biologists have concluded that this is what happened to the Galápagos finches. Among the differences between the finches are their beaks, which are adapted to the specific foods available on their home islands. Darwin anticipated that explaining how such adaptations arise was essential to understanding evolution.

42. By the early 1840s, Darwin had worked out the major features of his theory of natural selectionas themechanism of evolution. The basic idea of natural selection is that a population of organisms can change over the generations if individuals having certain heritable traits leave more offspring than other individuals.

43. However, he had not yet published his ideas. He was in poor health, and he rarely left home. Despite his reclusiveness, Darwin was not isolated from the scientific community. Lyell Already famous as a naturalist because of the letters and specimens he sent to Great Britain during the voyage of the Beagle, Darwin had frequent correspondence and visits from Lyell, Henslow, and other scientists. Joseph Hooker Naturalist

44. In 1844, Darwin wrote a long essay on the origin of species and natural selection. However, Darwin was reluctant to introduce his theory publicly, apparently because he anticipated the uproar it would cause. While he procrastinated, he continued to compile evidence in support of his theory.

45. Lyell, not himself yet convinced of evolution, nevertheless advised 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 letter from Alfred Wallace (1823-1913), a young British naturalist working in the East Indies. The letter was accompanied by a manuscript in which Wallace developed a theory of natural selection essentially identical to Darwin’s.

46. Wallace asked Darwin to evaluate the 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 originality, whatever it may amount to, will be smashed." Lyell and a colleague presented Wallace’s paper, along with extracts from Darwin’s unpublished 1844 essay, to the Linnaean Society of London on July 1, 1858.

47. Darwin quickly finished The Origin of Species and published it the next year. Although Wallace wrote up his ideas for publication first, Darwin developed and supported the theory of natural selection so much more extensively that he is known as its main architect. And Darwin’s notebooks prove that he formulated his theory of natural selection 15 years before reading Wallace’s manuscript.

48. Within a decade, Darwin’s book and its proponents had convinced the majority of biologists that biological diversity was the product of evolution. Darwin succeeded where previous evolutionists had failed, partly because science was beginning to shift away from natural theology, but mainly because he convinced his readers with immaculate logic and an avalanche of evidence in support of evolution. 1831- 1858 = 27 years of study

49. Died April 19, 1882 Buried in Westminster Abbey

50. Descent with Modification The Origin of Species developed two main points: the occurrence of evolution and natural selection as its mechanism “Darwinism” has a dual meaning. It refers to evolution as the explanation for life’s unity and diversity, and it also refers to the Darwinian concept of natural selection as the cause of adaptive evolution. In the first edition of The Origin of Species , Darwin did not use the word evolution until the last paragraph, referring instead to descent with modification, a phrase that condensed his view of life. Caricatures of Darwin would appear in many cartoons