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Chapter 14 Darwin and Evolution 14.1 Darwin’s Theory of Evolution • • • • • Charles Darwin was 22 in 1831. Naturalist aboard the HMS Beagle 5 year voyage through Southern Hemisphere Mission to expand navy’s knowledge of natural resources (water and food) in foreign lands Observation led Darwin to conclude that biological evolution occurs • Contrary to current beliefs of his time Figure 14.1 Voyage of the HMS Beagle. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Charles Darwin HMS Beagle marine iguana Galápagos Islands rhea (ship): © Mary Evans Picture Library/The Image Works; (iguana): © FAN Travelstock/Alamy RF; (rhea): © Nicole Duplaix/National Geographic/Getty Images; (Darwin): © American Museum of Natural History, photo courtesy Geoffrey West. Neg. #326697 • Before Darwin People had an entirely different way of viewing the world. • Earth only a few thousand years old • Species remained the same since creation. Explorers brought back fossils (remains of once-living organisms) found in strata (layers of rock or sedimentary material). Georges Cuvier founded paleontology. • Study of fossils • He believed in fixity of species yet strata showed a succession of different forms over time. • Catastrophism—explains appearance of new forms as replacing old forms due to local catastrophe Figure 14.2 Strata in rock. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Visible strata © Doug Sherman Jean-Baptiste de Lamarck • Evolution occurred and that adaptation was the cause of diversity • Concluded more complex forms descended from less complex forms • Inheritance of acquired characteristics Use and disuse of a structure can bring about inherited change Long neck in giraffes developed from stretching to reach food Not supported—people who were blinded in an accident would have blind children Modern genetics—phenotypic changes acquired during the lifetime do not result in genetic changes that can be passed to offspring Darwin’s ideas close to Lamarck BUT • Species are suited to the environment through no will of their own • Natural selection is the means for speciation • Darwin’s conclusions based on Study of geology, fossils, and biogeography • Charles Lyell’s book Principles of Geology Evidence that the Earth was subject to slow but continuous cycles of erosion and uplift Uniformitarianism—these slow changes occurred at a uniform rate • Darwin concluded the Earth must be very old Figure 14.3a Formation of strata. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. rain weathering and erosion runoff sedimentation ocean geologic uplift settling • Darwin’s conclusions based on Study of geology and fossils • Darwin collected fossils during his voyage that differed from modern forms. • Once he accepted that the Earth must be very old, he began to think there was time for descent with modification (evolution) to occur. • Species are not fixed but change over time. • Darwin’s conclusions based on Study of biogeography • Study of the distribution of life-forms on Earth • Compared animals of South America to England • Did the Patagonian hare resemble a rabbit because the 2 types of animals were adapted to the same environment? Convergent evolution Figure 14.5 The Patagonian hare and European rabbit. Study of biogeography (cont.) • Galápagos Islands—off the coast of South America Too far from mainland for most terrestrial plants and animals to colonize Species there slightly different from mainland forms Where did they come from? Why are they different? • Finches Some were like mainland finches, others quite different Ground-dwelling finch beak size dependent on seed they eat Tree-dwelling finch beak size and shape dependent on their insect prey Could have descended from a mainland finch Speciation—formation of a new species—occurred because the isolated population evolved independently of the mainland population. Figure 14.6 Galápagos finches. Study of biogeography (cont.) • Galápagos Islands tortoises Each island also had its own type of tortoise. Long-necked tortoises in dry areas where food scarce Short-necked tortoises in moist regions with abundant food Adaptation—any characteristic that makes an organism more suited to its environment • Natural selection and adaptation Natural selection • Mechanism for evolution • Process that results in the evolution of organisms well adapted to their environment • Requires: Members of a population to have heritable variation More offspring produced than can be supported Individuals with favorable traits survive and reproduce more than those lacking the traits Across generations, a larger proportion of the population possesses the favorable traits and become adapted to the environment • Uses only variations resulting from genetic changes No direction or anticipation of future needs • Ongoing process—as environment changes, suitability of adaptations changes Figure 14.7 Variation in shells of a marine snail, Liguus fascitus. • Organisms vary in their traits Prior to Darwin, variations were considered imperfections and ignored. Variations are essential to Darwin’s natural selection process. Genes, together with the environment, determine phenotype. Mutations, chromosomal rearrangements, and assortment of chromosomes during meiosis and fertilization can cause new variations to arise. • Organisms struggle to exist Thomas Malthus said death and famine are inevitable because the human population increases faster than the food supply. Darwin applied this to all organisms. • Organisms differ in fitness Fitness—reproductive success of an individual relative to other members of the population • Determined by comparing the number of surviving fertile offspring produced by each member of a population Most-fit individuals capture the most resources and convert into a larger number of viable offspring What determines fitness varies for different populations. Artificial selection—humans choose particular traits In nature, interaction with the environment determines fitness. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 14.9 Mechanism of evolution. Lamarck’s proposal Originally, giraffes had short necks. Giraffes stretched their necks in order to reach food. Darwin’s proposal Originally, giraffe neck length varied. Struggle to exist causes long-necked giraffes to have the most offspring. •Inheritance of acquired characteristics NOT supported by modern genetics •Natural selection With continual stretching, Due to natural selection, most giraffes now have most giraffes now have long necks. long necks. • Organisms become adapted An adaptation may take many generations to evolve. Manatees, penguins and sea turtles all have flippers to move through the water. • Convergent evolution Natural selection causes adaptive traits to be increasingly represented in succeeding generations. Other process for evolution (chapter 15) Only natural selection results in adaptation to the environment. Figure 14.10 Adaptations of the vampire bat. • Darwin and Wallace After the HMS Beagle retuned to England in 1836, Darwin waited 20 years to publish On the Origin of Species. • Gathered evidence to support his hypothesis • Prompted to publish after reading similar hypothesis by Wallace Alfred Russel Wallace • Also an English naturalist and collector at home and abroad • Conceived idea of “survival of the fittest” • Sent essay to Darwin for comment Joint paper presented to Linnean Society • Darwin presented an abstract of On the Origin of Species • Wallace presented “On the Tendency of Varieties to Depart Indefinitely from the Original Type” • Announced to the world that species evolve through natural selection with common descent 14.2 Evidence of Evolutionary Change • Theory of evolution states that all living things have a common ancestor but each is adapted to a particular way of life • Hypotheses become a scientific theory only when a variety of evidence from independent investigators supports the hypothesis. • Theory of evolution is a unifying theme in biology because it can explain so many different observations in various fields of biology • Fossil evidence Fossils trapped in rock strata are the fossil record that tell us about the history of life. Succession of life-forms from the simple to the more complex Find transitional links between groups Archaeopteryx fossils intermediate between reptiles and birds Evolution of whales Figure 14.12 Re-creation Archaeopteryx. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. wing feathers teeth tail with vertebrae claws © Joe Tucciarone/Interstellar Illustrations Figure 14.14 Evolution of whales. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Present 10 MYA Modern toothed whales 20 MYA 30 MYA The reduced hind limbs of Rodhocetus kasrani could not have aided it in walking or swimming. Rodhocetus swam with an up-and-down motion, as do modern whales. 40 MYA Ambulocetus natans Probably walked on land (as do modern sea lions) and swam by flexing its backbone and paddling with its hind limbs (as do modern otters). 50 MYA 60 MYA Hypothetical mesonychid skeleton • Biogeographical evidence Study of distribution of organisms throughout the world Distribution consistent with hypothesis that, when forms are related, they evolved in one locale and then spread to accessible regions Darwin noted that South America lacks rabbits even though environment is suitable • Rabbits evolved elsewhere and had no means of reaching South America. South America, Antarctica, and Australia connected long ago • Marsupials and placental mammals arose at that time. • Marsupials only plentiful in Australia because it drifted away separating them from competition with placental mammals • Opossum only marsupial in the Americas • In some cases marsupial and placental mammals resemble each other. Supports hypothesis that evolution is influenced by environment Figure 14.15 Marsupials of Australia. • Anatomical evidence Vestigial structures • Anatomical features that are fully developed in one group of organisms but reduced and nonfunctional in other similar groups • Most birds have well-developed wings; ostriches do not • Whales and snakes have remnants of hip bones and legs. • Humans have a tailbone. • Presence explained by common descent hypothesis Homologous structures • Anatomically similar structures are explainable by inheritance from a common ancestor. • All vertebrate forelimbs contain the same set of bones organized in similar ways despite dissimilar functions. • Analogous structures—structures serve the same function but are not constructed similarly—bird and insect wings Figure 14.16 Significance of structural similarities. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. bird bat Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. whale cat Figure 14.16 Significance of structural similarities, continued. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. horse human Figure 14.15 Significance of structural similarities, continued. • Homology shared by vertebrates extends to their embryological development At some time during development, all vertebrates have a postanal tail and paired pharyngeal pouches. • In fishes and amphibian larvae, pouches develop into functioning gills. • In humans, first pair of pouches becomes cavity of middle ear and auditory tube, second pair becomes tonsils, and third and fourth pair become thymus and parathyroid glands New structures originate by modifying preexisting structures of one’s ancestors. Figure 14.17 Significance of developmental similarities. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chick embryo Pig embryo eye pharyngeal pouches postanal tail (both): © CBS/Phototake • Molecular evidence All living organisms use the same biochemical molecules, including DNA, ATP, and many nearly identical enzymes. Many developmental genes (Hox) are shared in animals ranging from worms to humans. Life’s vast diversity has come about from only slight differences in the same genes. Cytochrome c is a molecule used in the electron transport chain of all organisms. • Data on the amino acid sequence show that the sequence varies consistent with other data regarding anatomical similarities, and therefore relatedness. Figure 14.18 Significance of biochemical differences. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Types of Organisms monkey pig duck turtle fish moth yeast (Candida) 0 10 20 30 40 50 Number of Amino Acid Differences Compared to Human Cytochrome c human QUIZ in one minute