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Title Chapter 15 Darwin and Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. History of Evolutionary Thought • Darwin used a variety of data to come to the conclusion that the Earth is very old, not young, and that biological evolution is the method by which species arise and change. • Evolution is now considered the unifying principle of all the biological sciences. It explains both the unity and diversity of life on Earth. • Evolution illustrates that living things share like characteristics because they have a common ancestry. Mid-Eighteenth-Century Contributions • Taxonomy, the science of classifying organisms, was an important endeavor during the mid-eighteenth century. • Chief among the taxonomists was Carolus Linnaeus, who developed the binomial system of nomenclature and a system of classification of living things. • Besides taxonomy, comparative anatomy, the evaluation of similar structures across a variety of species, was of interest to biologists prior to Darwin. • Linnaeus, like other taxonomists of his time, believed in the fixity of species. – Each species had an “ideal” structure and function and also a place in the scala naturae, a sequential ladder of life. – The simplest and most material being was on the lowest rung of the ladder, and the most complex and spiritual being was on the highest rung. – Linnaeus and other taxonomists wanted to determine the ideal characteristics of each species and also wanted to discover the proper rank for each species in the scala naturae. Therefore, for most of his working life, Linnaeus did not even consider the possibility of evolutionary change. Late-Eighteenth-/Early-Nineteenth-Century Contributions • Linnaeus’ hierarchical method of classifying organisms is consistent with modern evolutionary thinking. This is the reason that Linnaeus’ basic method of classification has been modified even until today. Cuvier and Catastrophism • Baron Georges Cuvier used comparative anatomy to develop a system of classifying animals. He also founded the science of paleontology, the study of fossils, and was quite skilled at using fossil bones to deduce the structure of an animal. Lamarck and Acquired Characteristics • Lamarck concluded, after studying the succession of life-forms in strata, that more complex organisms are descended from less complex organisms. He mistakenly said, however, that increasing complexity is the result of a natural force—a desire for perfection— that is inherent in all living things. • To explain the process of adaptation to the environment, Lamarck supported the idea of inheritance of acquired characteristics—that the environment can bring about inherited change. • One example that he gave is that the long neck of a giraffe developed over time because animals stretched their necks to reach food in tall trees and then passed on a long neck to their offspring. • His hypothesis of inheritance of acquired characteristics has never been substantiated by experimentation. • Phenotypic changes acquired during an organism’s lifetime do not result in genetic changes that can be passed to subsequent generations. Darwin’s Theory of Evolution • Geology and Fossils – Darwin’s study of geology and fossils caused him to concur with Lyell that the observed massive geological changes were caused by slow, continuous processes. – Fossils of huge sloths and armadillo-like animals suggested modern forms were descended from extinct forms with change over time; therefore species were not fixed. Fig. 15.4 • Biogeography – Biogeography is the study of the range and geographic distribution of life-forms on Earth. – Darwin reasoned that related species could be modified according to environmental differences. – The Galápagos Islands •Each island had a variation of tortoise; long and short necked tortoises correlated with different vegetation. •The finches of the Galápagos Islands seemed to Darwin like mainland finches, but they exhibited significant variety with regard to their beaks. Natural Selection and Adaptation • Natural selection was proposed by both Alfred Russel Wallace and Darwin as a driving mechanism of evolution caused by environmental selection of organisms most fit to reproduce, resulting in adaptation. • Natural selection is a process consisting of these conditions: – The members of a population have random but heritable variations. – In a population, many more individuals are produced each generation than the environment can support. – Some individuals have adaptive characteristics that enable them to survive and reproduce better. Darwin called the ability to have more offspring, differential reproductive success. – The result of natural selection is a population adapted to its local environment. • Extinction occurs when previous adaptations are no longer suitable to a changed environment. Organisms Have Inheritable Variations • Darwin emphasized that variations were essential to the natural selection process. • New variations are as likely to be harmful as helpful. • The variations that make adaptation to the environment possible are those that are passed on from generation to generation. • Darwin could not state the cause of variations because genetics was not yet established. • Natural selection only operates on variations that are already available in a population’s gene pool. Organisms Compete for Resources • Darwin and Wallace both read an essay by Thomas Malthus, a socioeconomist. • Malthus proposed that human populations outgrow food supply and death and famine were inevitable. • Darwin applied this to all organisms; resources were not sufficient for all members to survive. Organisms Differ in Reproductive Success • In nature, interactions with the environment determine which members of a population reproduce to a greater degree than other members. • Natural selection occurs because certain members of a population happen to have a variation that allows them to survive and reproduce to a greater extent than other members. • Artificial selection is only possible because the original population exhibits a range of characteristics, allowing humans to select which traits they prefer to perpetuate. Therefore, several varieties of vegetables can be traced to a single ancestor. • Darwin used artificial selection as a model when he developed the concept of natural selection • Chinese cabbage, brussel sprouts, and kohlrabi are all derived from a single species, Brassica oleracea. Organisms Become Adapted • Adaptations are especially recognizable when unrelated organisms, living in particular environment, display similar characteristics. For example, manatees, penguins, and sea turtles all have flippers, which help them move through the water. • Differential reproduction generation after generation can cause adaptive traits to be increasingly represented in each succeeding generation. Alfred Russel Wallace (Science Focus Box) • Alfred Russel Wallace (1823-1913) was an English naturalist who independently and simultaneously proposed natural selection as a mechanism for evolution. • In 1858 Wallace concluded changes in species are due to changes in the environment through natural selection. • Wallace wrote a manuscript of these findings and sent it to Charles Darwin for review. Darwin was shocked that Wallace had the same theories that he had. Darwin told Wallace to publish his manuscript at once. • Darwin published Origin of the Species one year later while Wallace was in the field. Fig. 15B Natural Selection Can Be Witnessed • Darwin had formed his natural selection hypothesis by observing the distribution of tortoises and finches on the Galápagos Islands. • Tortoises with domed shells and short necks live on well-watered islands, where grass is available. Those with shells that flare up in front have long necks and are able to feed on cacti. • Similarly, the islands are home to many different types of finches. – The heavy beak of the large, ground-dwelling finch is suited to a diet of seeds. – The thin, sharp beak of a warbler-finch is able to probe vegetation for and spear insects. – The decurved beak of a cactus-finch can find and feed on cactus seeds. • Scientists are currently witnessing natural selection on the Galapagos Islands. Peter and Rosemary Grant have been observing finch beak size change with rainfall. During wet seasons, the offspring have smaller beaks to eat the small seeds, however, in drier seasons, offspring have larger beaks capable of breaking harder seeds that survive the dry weather. Fig. 15.11 • A common example of natural selection is industrial melanism. Prior to the industrial revolution in Great Britain, light-colored peppered moths were more common (90%) than the dark-colored peppered moths (10%). – Following the industrial revolution and increase in pollution, dark-colored peppered moths reached 80% of the peppered moth population. – Once legislation regulated pollution reduction, dark-colored peppered moths reduced to 19% in one of the collecting sites. Evidence of Evolution • Fossil Evidence – Fossils are the remains and traces of past life or any other direct evidence of past life. – Fossils include skeletons, shells, seeds, insects trapped in amber, and imprints of leaves. – Transitional fossils reveal links between groups. • Archeopteryx is an intermediate between reptiles and birds. • Ambulocetus natans is a whale with legs. Fig. 15.13 • Biogeographical Evidence – Both cacti and spurges (Euphorbia) are plants adapted to a hot, dry environment—both are succulent, spiny, flowering plants. Why do cacti grow in the American deserts and most Euphorbia grow in African deserts when each would do well on the other continent? It seems obvious that they just happened to evolve on their respective continents. – Why do so many species of finches live on the Galápagos Island when these same species are not on the mainland? The reasonable explanation is that an ancestral finch originally inhabited the different islands. Geographic isolation allowed the ancestral finch to adapt and evolve into a different species on each island. • Anatomical Evidence – Organisms have anatomical similarities when they are closely related because of common descent. •Homologous structures in different organisms are inherited from a common ancestor. •Analogous structures are inherited from unique ancestors and have come to resemble each other because they serve a similar function. – Vestigial structures are remains of a structure that was functional in some ancestors but is no longer functional in the organism in question. •Most birds have well-developed wings; some bird species have reduced wings and do not fly. •Humans have a tailbone but no tail. •Presence of vestigial structures is explained by the common descent hypothesis; these are traces of an organism’s evolutionary history. Fig. 15.15 – The homology shared by vertebrates extends to their embryological development. – At some time during development, all vertebrates have a postanal tail and exhibit paired pharyngeal pouches supported by cartilaginous arches. Fig. 15.16 • Biochemical Evidence – All living organisms use the same basic biochemical molecules, e.g., DNA, ATP, and many identical or nearly identical enzymes. – Organisms utilize the same DNA triplet code and the same 20 amino acids in their proteins. – This is substantiated by the analysis of the degree of similarity in amino acids for cytochrome c among organisms. – These similarities can be explained by descent from a common ancestor. Fig. 15.17 • Because it is supported by so many lines of evidence, evolution is no longer considered a hypothesis. – Evolution is one of the great unifying theories (GUTs) of biology, similar in status to the germ theory of disease in medicine. The modern synthesis combines Darwin’s theory with genetics • Beginning in the 1930s and 1940s, biologists expressed a conceptual breakthrough when they combine the principles of Mendelian inheritance with Darwin’s theory of natural selection. The result was a unified explanation of evolution known as the modern synthesis, or the synthesis theory of evolution. In this context, synthesis refers to combining parts of several theories to form a unified whole. • Today, the modern synthesis incorporates our expanding knowledge in genetics, systematics, paleontology, developmental biology, behavior, and ecology. • The modern synthesis, which emphasizes the genetics of populations as the central focus of evolution, has held up well since it was developed.