Developing a Theory of Evolution

Transcription

1 CHAPTER 8 Developing a Theory of Evolution Specific Expectations In this chapter, you will learn how to... C2.1 use appropriate terminology related to evolution (8.1, 8.2) C2.3 analyze, on the basis of research, and report on the contributions of various scientists to modern theories of evolution (8.1) C3.1 explain the fundamental theory of evolution, using the evolutionary mechanism of natural selection to illustrate the process of biological change over time (8.1, 8.2) C3.2 explain the process of adaptation of individual organisms to their environment (8.2) The word evolution is often used to describe a process of change over time. Cars, fashion, language, and many other things are said to evolve. Evolution in the biological sense is the process by which hereditary changes occur in one or more characteristics within a species over long periods of time, usually over many generations. Evolution by natural selection is a scientific theory that explains how Earth s vast biodiversity developed in the past, continues to develop in the present, and will continue to develop in the future. Many theories have been developed through collaborative efforts, and the theory of evolution is no different. As you will learn in this chapter, the theory of evolution is built on the work of many contributors and is based on different kinds of evidence, such as fossils. The fossils shown here are trilobites. These fossils were found in rock that is about 515 million years old. There were more than species of trilobites living in Earth s oceans, but they disappeared about 260 million years ago. 324 MHR Unit 3 Evolution

2 Launch Activity Forming a Hypothesis Many theories are developed and refined as a result of collaborating with team members and with other teams. In this activity, you will collaborate with members in your group and with another group to try to identify an animal from its paper fossil bones. You can assume that all the fossil bones in your envelope are from the same animal. Materials paper fossils printout of fossil remains of various animals Procedure 1. Work in groups of four. Open your envelope of fossils, and remove three fossil bones. Do not look at the other fossil bones in your envelope. Try arranging the bones in different ways to help you form a hypothesis about the identity of the animal. Record the group s hypothesis or hypotheses in a table like the one below. Fossil Data Step Number of Bones Total Number of Bones Hypothesis Notes Remove three more bones from your envelope. If necessary, revise your hypothesis and record your new hypothesis in your table. 3. Remove four more bones from your envelope. If necessary, revise your hypothesis and record your new hypothesis in your table. 4. Compare your findings with those of another group. If necessary, revise your hypothesis and record your final hypothesis in your table. 5. Compare your fossil assembly with the printout of fossil remains of other animals. Note any similarities and differences. 6. Return your fossil bones to the envelope, and return the envelope to your teacher. Questions 1. Did your group reach an agreement on the identity of the animal? Explain why or why not. 2. From looking at the fossil and the printout of fossil remains, what could you say about how and where this animal lived? 3. What features of the nature of science do you think this activity demonstrates? Chapter 8 Developing a Theory of Evolution MHR 325

3 SECTION 8.1 Scientific Contributions to a Theory of Evolution Key Terms paleontology catastrophism uniformitarianism inheritance of acquired characteristics theory of evolution by natural selection evolution survival of the fittest descent with modification Scientific knowledge develops as people observe the world around them, ask questions, and seek answers to their questions. A scientific hypothesis is a statement that provides one possible answer to a question, or one possible explanation for an observation. Hypotheses are tested to determine their validity, mainly through experiments, observation, developing models from data, or a combination of these activities. Hypotheses that consistently lead to successful predictions and explanations are sometimes synthesized into a general statement that explains and makes successful predictions about a broad range of observations. Such a statement is called a scientific theory. The question Where did we come from? has been asked and debated for thousands of years. Many of the earliest ideas about the origins of life were strongly influenced by religion and philosophy. These ideas suggested that all forms of life have existed, unchanged, since their creation. In the 1600s, however, scholars in Europe began to use a system of empirical studies to explain the natural world. Empirical studies involve observation and experiment to form ideas and hypotheses about nature. The first scientist to carry out empirical studies of the natural world was John Ray ( ), in England. He developed a classification system for plants and animals based on anatomy and physiology. This system was later extended by the Swedish naturalist Carolus Linnaeus ( ). Both systems helped scientists of the time recognize and think about similarities and differences between organisms. Georges-Louis Leclerc, Comte de Buffon ( ) One of the first people to publicly challenge the idea that life forms are unchanging was French naturalist Georges-Louis Leclerc, Comte de Buffon. His 44-volume Histoire Naturelle compiled his understandings of the natural world. In this work, Buffon noted the similarities between humans and apes, and speculated that they might have a common ancestor, suggesting that species change over time. In other writings, Buffon suggested that Earth was much older than 6000 years, as was commonly believed. Buffon s ideas were revolutionary for his time. By 1830, however, other scholars from many other areas of inquiry paleontology, geology, geography, and biology began to share their ideas to explain how life could change with the passage of time. Figure 8.1 Mary Anning was very skilled at collecting fossils. As a result, she made many important contributions to the field of paleontology. paleontology the study of ancient life through the examination of fossils The Science of Paleontology Fossils are important to the study of evolution. A fossil is the preserved remains of a once-living organism. Fossils include specimens preserved in amber, permafrost, dry caves, and the more common fossils preserved as rock. Mary Anning ( ), shown in Figure 8.1, was a fossil hunter in England. Her most important discovery was the first plesiosaur, an aquatic reptile. French naturalist Georges Cuvier ( ) was doing important work with fossils around Anning s time. Cuvier examined Anning s drawing of the plesiosaur to see if it was genuine. Cuvier s acknowledgment of her work essentially made Anning respectable in the scientific world, a difficult achievement for a young woman in a male-dominated field. Georges Cuvier is largely credited with developing the science of paleontology the study of ancient life through the examination of fossils. Cuvier found that each stratum (layer of rock) is characterized by a unique group of fossil species. He also found that the deeper (older) the stratum, the more dissimilar the species are from modern life (see Figure 8.2). As Cuvier worked from stratum to stratum, he found evidence that new species appeared and others disappeared over the passage of time. This evidence showed that species could become extinct. 326 MHR Unit 3 Evolution

4 A B C younger stratum with more recent fossils thicker layer of sediment A fossil is formed when an organism falls into a body of water and settles in the sediment. The sediments, brought by rivers or streams to larger bodies of water, keep the organism or parts of the organism from decomposing. More sediment is laid down on top of older sediments and on top of remains of the organism. These additional layers of sediment compress lower strata, and then these lower strata turn into rock. Over time, many strata of rocks are formed. Sometimes, each of the strata contains fossils. Movements of the soil and erosion of the rock can result in fossil-laden rocks being exposed above water level. older stratum with older fossils Figure 8.2 Deep rock strata (layers of rock) are older than strata that are closer to the surface. Different species of fossilized organisms can be found in different sedimentary rock strata. This is evidence that not all life forms came into existence at the same time. Catastrophism To explain his observations, Cuvier proposed the idea that Earth experienced many destructive natural events in the past, such as floods and volcanic eruptions. These events, which he called revolutions, were violent enough to have killed numerous species each time they occurred. Cuvier s idea is now called catastrophism. Cuvier suggested that these revolutions, or catastrophes, corresponded to the boundaries between the strata he studied. He also thought they were limited to local geographical regions, and that the area would be repopulated by species from nearby unaffected areas. This is how Cuvier explained the appearance of fossils of species that did not exist anymore. Charles Lyell ( ) Other scientists had ideas that differed from Cuvier s theory. Scottish geologist Charles Lyell rejected catastrophism. He proposed instead, based on the work of geologist James Hutton, that geological processes operated at the same rates in the past as they do today in a process called uniformitarianism. Lyell reasoned that, if geological changes are slow and continuous rather than catastrophic, then Earth might be more than 6000 years old. He also theorized that slow, subtle processes could happen over a long period of time and could result in substantial changes. The forces that build and erode mountains, for example, and the rate at which such geological change happens, are no different today than they were in the past. Floods in the past had no greater power than floods that occur today. This idea inspired naturalist Charles Darwin and others. If Earth is slowly changing, they wondered, could slow, subtle changes also occur in populations? catastrophism the idea that catastrophes such as floods, diseases, and droughts periodically destroyed species living in a particular region, allowing species from neighbouring regions to repopulate the area uniformitarianism Charles Lyell s theory (based on Hutton s theory) that geological processes operated at the same rates in the past as they do today Learning Check 1. What is the empirical system? 2. Why were Buffon s ideas revolutionary for his time? 3. Contrast uniformitarianism and catastrophism. 4. Georges Cuvier explained the appearance of fossils of species that did not exist anymore. Why is this an important contribution to science? 5. Do you think Cuvier s ideas on catastrophism suggest the idea that organisms change over time? Explain your answer. 6. Do you think that the same geological processes operate today as they did in the past, as Lyell suggests? Is anything different today? Explain your answer. Chapter 8 Developing a Theory of Evolution MHR 327

5 inheritance of acquired characteristics the idea that characteristics acquired during an organism s lifetime can be passed on to its offspring theory of evolution by natural selection a theory explaining how life has changed, and continues to change, during Earth s history evolution the process of genetic change in a population over time Jean-Baptiste Lamarck ( ) In his book Philosophie Zoologique, French naturalist Jean-Baptiste Lamarck outlined his ideas about changes in species over time. By comparing current species of animals with fossil forms, Lamarck observed what he interpreted as a line of descent, or progression, in which a series of fossils (from older to more recent) led to a modern species. He thought that species increased in complexity over time, until they achieved a level of perfection. Lamarck hypothesized that the organisms would become progressively better adapted to their environments. At the time, many thought that body parts that were used extensively to cope with conditions in the environment would become larger and stronger. Following this reasoning, giraffes stretched their necks to eat the foliage from tall trees. Over time, they would pass on this stretched neck condition to their offspring, resulting in tall giraffes that can eat from the tops of trees. (It is important to note that giraffes did not develop their long necks in this way.) Lamarck called his idea the inheritance of acquired characteristics. Lamarck also suggested that body parts not used would eventually disappear. This idea is called use and disuse. Lamarck provided a hypothesis for how the inheritance of characteristics from one generation to the next might happen. More importantly, he noted that an organism s adaptations to the environment resulted in characteristics that could be inherited by offspring. At the time, there was little understanding of cell biology and no understanding of genetics. The idea of inheriting acquired characteristics was generally accepted to explain observations that species are not static and could change. Even Charles Darwin, who is credited with developing a comprehensive theory to explain how change in populations can occur, accepted Lamarck s idea of inheritance and acknowledged Lamarck in his writing. But Lamarck s ideas were controversial to the many people who firmly believed that species never changed. Charles Darwin ( ) In 1831, 22-year-old Charles Darwin left England on the HMS Beagle, a British survey ship. The primary purpose of the expedition was to map the coast of South America. The journey also provided Darwin with an opportunity to explore the natural history of various countries and geographical locations. Figure 8.3 outlines the Beagle s journey. At first, Darwin did not always understand the significance of many of his observations. Years later, however, many of these observations (as well as ideas and observations resulting from new work by Darwin and others) became important to his theory of evolution by natural selection. Darwin s main observations, and the questions he asked about these observations, are summarized in Table 8.1. Figure 8.3 The five-year voyage of the HMS Beagle took Darwin around much of the world. He spent most of his time exploring the coast and coastal islands of South America. Tahiti Galapagos Is. Callao Valparaiso Tierra del Fuego Azores Bahia England Rio de Janeiro Canary Is. Cape Verde Is. Montevideo Falkland Is. Ascension Is. St. Helena Is. Mauritius Cocos Is. King George Sound Sydney New Zealand Hobart 328 MHR Unit 3 Evolution

6 Table 8.1 Darwin s Observations and Questions Arising from Them Observations 1. The flora and fauna of the different regions the Beagle visited were distinct from those Darwin had studied in England and Europe. For example, the rodents in South America were structurally similar to one another but were quite different from the rodents Darwin had observed on other continents. 2. Darwin observed fossils of extinct animals, such as the armadillo-like glyptodont, that looked very similar to living animals. Questions If all organisms originated in their present forms during a single event, Darwin wondered, why was there a distinctive clustering of similar organisms in different regions of the world? Why were all types of organisms not randomly distributed? Why would living and fossilized organisms that looked similar be found in the same region? A glyptodont, an ancient 4 m, 2 tonne animal from South America 3. The finches and other animals Darwin saw on the Galapagos [guh-la-pa-gos] Islands closely resembled animals he had observed on the west coast of South America. A modern armadillo from South America (1.5 m) Isabela Santiago Why did the Galapagos species so closely resemble organisms on the adjacent South American coastline? The Galapagos Islands, shown in this satellite image, include more than 20 small volcanic islands located approximately 1000 km off the coast of Ecuador, some of which are shown here. They formed at approximately the same time and have similar abiotic conditions. 4. Galapagos species (such as tortoises and finches) looked identical at first, but they actually varied slightly between islands. Each type of Galapagos finch, for example, was adapted to eating a different type of food based on the size and shape of its beak. Ten finch species that occur on one of the islands, Santa Cruz, are shown here. warbler finch (Certhidea olivacea) Fernandina Santa Cruz cactus ground finch (Geospiza scandens) Why was there such a diversity of species in such a small area? Could these species have been modified from an ancestral form that arrived on the Galapagos Islands shortly after the islands were formed? woodpecker finch (Cactospiza pallida) small insectivorous tree finch (Camarhynchus parvulus) large insectivorous tree finch (Camarhynchus psittacula) sharp-beaked ground finch (Geospiza difficilis) small ground finch (Geospiza fuliginosa) medium ground finch (Geospiza fortis) vegetarian tree finch (Platyspiza crassirostris) large ground finch (Geospiza magnirostris) 5. Through his experience with artificial selection (breeding pigeons and studying breeds of dogs and varieties of flowers), Darwin knew that it was possible for traits to be passed on from parent to offspring, and that sexual reproduction resulted in many variations within a species. Could a process similar to artificial selection also operate in nature? Chapter 8 Developing a Theory of Evolution MHR 329

7 survival of the fittest the idea that the organisms that are the fittest leave the most offspring, so those organisms win the struggle for survival; phrase coined by John Spencer SuggestedInvestigation ThoughtLab Investigation 8-A, Comparing the Ideas of Lamarck and Darwin descent with modification Darwin s theory that natural selection does not demonstrate progress, but merely results from a species ability to survive local conditions at a specific time Darwin, Wallace, and the Theory of Evolution by Natural Selection After his trip on the Beagle, Charles Darwin began to propose answers to his questions, and to organize his and others observations into a comprehensive theory to explain how species change over time. Alfred Russel Wallace ( ), another British naturalist, reached conclusions that were similar to Darwin s. Darwin and Wallace accepted that populations changed as time passed, but they were unclear how populations changed. An essay by economist Thomas Malthus ( ), called Essay on the Principles of Population, provided them with a key idea. Malthus proposed that populations produced far more offspring than their environments (for example, their food supply) could support. He said that the populations were eventually reduced by starvation or disease. According to Darwin and Wallace, individuals with traits that helped them survive in their local environments were more likely to survive to pass on these traits to offspring. They reasoned that competition for limited resources between individuals of the same species would select for individuals with favourable traits traits that increased their chances of surviving to reproduce. Thus, a growing proportion of the population would have these traits in later generations and, as time passed, the population as a whole would have them. This is the idea of survival of the fittest, and Darwin called this process natural selection. He published his ideas in 1859 in a book whose title is often shortened, for convenience, to The Origin of Species. He proposed that all life descended from some unknown organism. As descendants of this organism spread out over different habitats during the millennia, they developed adaptations that helped them better survive in their local environments. Darwin s theory of natural selection showed how populations of individual species became better adapted to their local environments over time. His ideas are summarized as follows: Organisms produce more offspring than can survive. Therefore, organisms compete for limited resources. Individuals of a population vary extensively, and much of this variation is heritable. Individuals that are better suited to local conditions survive to produce more offspring. Processes for change are slow and gradual. Descent with Modification Darwin did not use the word evolution in the original edition of The Origin of Species. Instead, he spoke of descent with modification. Darwin felt the word evolution implied progress, the notion that each generation was somehow improving in some way. Natural selection does not demonstrate progress; it has no set direction. It results from the ability of certain individuals in any population to survive local environmental conditions and to pass on the traits that helped them survive in the first place. Activity 8.1 Building a Theory In this activity, you will research the work of several scholars whose work influenced Darwin. Materials computer with Internet access print resources Procedure 1. Using print or Internet resources, research how the following contributed to Darwin s theory of evolution: Comte de Buffon, Charles Lyell, Thomas Malthus, Jean-Baptiste Lamarck, and Alfred Russel Wallace. 2. Create a presentation that includes the following: one contribution from each that is not mentioned in this textbook, and the evidence that supports it societal influences at the time of each contributor how Darwin was influenced by their work Question In your opinion, which contributor was the most influential to the development of Darwin s theory of evolution by natural selection? Explain your answer. 330 MHR Unit 3 Evolution

8 Section 8.1 REVIEW Section Summary Scientific theories explain facts and connect them in a comprehensive way, enabling scientists to make predictions about new situations and experimental outcomes. The question of whether living things have changed over the course of Earth s history has been considered by many different philosophers and scholars. In Histoire Naturelle, Buffon challenged the idea that life forms are unchanging and that Earth was 6000 years old. Cuvier founded the science of paleontology and proposed catastrophism as an explanation for fossil history. Geologist Charles Lyell noted that Earth s geological features were in a slow, continuous cycle of change, which he called uniformitarianism. Lamarck proposed the idea of inheritance of acquired characteristics, which suggested that parents passed on learned adaptations to the environment, which resulted in evolution. Darwin brought together his own observations from his journey on the HMS Beagle, his observations from his selective breeding, and the work of many other great thinkers to develop his theory of evolution. His theory proposes natural selection as the mechanism for how new species arise from ancestral species in response to the local environment. Review Questions 1. K/U Explain how the diagram shown below is evidence that not all life forms came into existence at the same time. 2. C Imagine yourself to be Cuvier, examining fossils. You find the fossil of a species of fish in one stratum but not in the next highest stratum. Write a brief letter to a peer that proposes an explanation for your observations. 3. C Using a graphic organizer, show the difference between catastrophism and uniformitarianism, and how these ideas relate to the development of the theory of evolution. 4. K/U How did Lyell s observations about changes in Earth s geological features inspire naturalists ideas about changes in life forms on Earth? 5. A How might Lamarck have explained an elephant s long trunk? 6. A An athlete breaks her leg. Years later she has a child who walks with a limp. Is this an example of evolution? Explain your answer. 7. K/U How is the work of Malthus related to the concept of survival of the fittest? 8. K/U Describe the contributions of the following people to the understanding of evolution. a. Cuvier c. Wallace e. Lamarck b. Malthus d. Lyell 9. C Create a concept map showing the individuals from this section whose contributions led to the development of the theory of evolution by natural selection. State their contributions. 10. A Nature writer Wallace Stegner once wrote of a population of trout in a mountain lake that were in a Malthusian dilemma. Explain what Stegner meant. 11. T/I Explain why Darwin used the phrase descent with modification rather than evolution. 12. K/U Describe, using two examples, how Charles Darwin used observations of the world around him to develop his hypothesis about how species might change with the passage of time. 13. C Draw a concept map that summarizes Darwin s four main ideas related to evolution. 14. T/I Much of the theory of evolution has been developed by interpreting certain observations or by making logical inferences about these observations. Outline the inferences that Darwin and other scientists made from each of the following observations. a. Some species found on islands are very similar to species found on neighbouring continents. b. No two individuals are exactly alike. c. Resources, such as food, are limited. 15. A Explain how breeding dogs would provide you with observations that would support Darwin s theory of evolution by natural selection. Chapter 8 Developing a Theory of Evolution MHR 331

9 SECTION 8.2 Sources of Evidence for Evolution Key Terms fossil record transitional fossil vestigial structure biogeography homologous structures analogous structures embryology fossil record the remains and traces of past life that are found in sedimentary rock; it reveals the history of life on Earth and the kinds of organisms that were alive in the past In The Origin of Species, Darwin assembled a group of facts that had previously seemed unrelated. Darwin certainly was not the only person to conclude that life had changed over long periods of time, but he was the first person to publish these ideas in a comprehensive manner. Darwin s ideas were developed, for the most part, by his observations of the distribution of organisms throughout the world (as outlined in Table 8.1 on page 329). Before and after publication of The Origin of Species, biologists, geologists, geographers, and paleontologists provided a wealth of information that supported and strengthened what Darwin called and today s scientists also call the theory of evolution by natural selection. Fossils: Evidence for the History of Life Sedimentary rock with fossils provides a fossil record of the history of life by showing the kinds of species that were alive in the past, such as those shown in Figure 8.4. For instance, when people examined the Burgess Shale fossil beds in British Columbia, they found fossils of animals that lived in an ancient ocean during the Cambrian period, over 500 million years ago. In addition to micro-organisms and soft-bodied animals, the Burgess Shale fossil beds also preserved some of the earliest animals with hard parts to be seen in the fossil record. Some of the fossilized animals found in the Burgess Shale are ancestors of animals that are common today. Others have long been extinct and are unlike anything in our modern oceans. The geological time scale in Figure 8.5 shows approximately when organisms first appear in the fossil record. Opabinia Pikaia trilobites Figure 8.4 The animals unearthed in the Burgess Shale lived over 500 million years ago during a period called the Cambrian explosion, when there was a sudden increase (on a geological scale) in the diversity of animal species. 332 MHR Unit 3 Evolution

10 Eons Cenozoic Eras Phanerozoic Paleozoic Mesozoic 135 million years ago Flowering plants first appear in the fossil record 225 million years ago Dinosaurs and mammals first appear in the fossil record 400 million years ago Seed plants first appear in the fossil record; tetrapods and insects first appear in the fossil record Millions of Years ago million years ago Hominids first appear in the fossil record 160 million years ago Birds first appear in the fossil record 300 million years ago Reptiles first appear in the fossil record 450 million years ago Large terrestrial colonization by plants and animals Periods Quaternary Tertiary Cretaceous Jurassic Triassic Permian Carboniferous Devonian Silurian Ordovician Cambrian Late 520 million years ago First vertebrates; first land plants million years ago Cambrian explosion creates diverse animal life 900 Precambrian Archaean Proterozoic Middle Late Early Middle 543 million years ago Shelled animals first appear in the fossil record 1.5 billion years ago Multicellular eukaryotic organisms first appear in the fossil record million years ago Bilateral invertebrate animals first appear in the fossil record billion years ago Eukaryotic cells first appear in the fossil record Early 3.5 billion years ago Fossils of primitive cyanobacteria first appear in the fossil record billion years ago Prokaryotic cells first appear in the fossil record Hadean 4500 Figure 8.5 This geological time scale illustrates the approximate appearance in the fossil record of various organisms during Earth s 4.6 billion year history. Note that this illustration is not to scale. Chapter 8 Developing a Theory of Evolution MHR 333

11 Evidence from the Fossil Record The fossil record provides the following evidence: Fossils found in young layers of rock (from recent geological periods and usually closer to the surface) are much more similar to species alive today than fossils found in older, deeper layers of rock. For example, paleontologists have used fossils to trace the evolution of the modern camel. As you can see in Figure 8.6, the camel ancestor from the Miocene epoch is more similar to the modern camel than the ancestors from the more distant epochs. Fossils appear in chronological order in the rock layers. So, probable ancestors for a species are found in older rocks, which usually lie beneath the rock in which the later species is found. Not all organisms appear in the fossil record at the same time. For example, the fossil history of vertebrates shows that fish are the oldest vertebrates. In subsequent layers, the fossils of other vertebrates amphibians, reptiles, mammals, and birds appear. This reinforces scientific evidence that amphibians evolved from ancestral fish, reptiles evolved from ancestral amphibians, and both mammals and birds evolved from different groups of reptiles (mammals first, and then birds). It is important to remember that these changes were slow and took millions of years. Figure 8.6 Paleontologists have used fossils to trace the evolution of the modern camel. The Paleocene, Eocene, Oligocene, and Miocene epochs are subdivisions of the Cenozoic era. Paleocene Eocene Oligocene Miocene Present 66 million years ago tooth leg bone skull 54 million years ago 37 million years ago 26 million years ago transitional fossil a fossil that shows intermediary links between groups of organisms and shares characteristics common to two now separate groups vestigial structure a structure that is a reduced version of a structure that was functional in the organism s ancestors Evidence from Transitional Fossils The original fossil record gave only scattered snapshots of ancestral forms. Scientists wondered about the gaps between these snapshots. The ongoing discovery of hundreds of transitional fossils fossils that show intermediary links between groups of organisms has helped scientists better understand the evolutionary process and relationships between groups of organisms. Transitional fossils link the past with the present. For example, scientists have found fossilized whales that lived 36 to 55 million years ago. These fossils link present-day whales to terrestrial ancestors. Basilosaurus and Dorudon were ancient whales that had tiny hind limbs but led an entirely aquatic life. Dorudon was about the size of a large dolphin, about 5 m long. It had a tiny pelvis (located near the end of its tail) and legs about 10 cm long. These characteristics would have been useless to an animal that lived an aquatic life. Structures that are the reduced forms of structures that were functional in the organism s ancestors are called vestigial structures. The pelvic bone in the Dorudon whale and in some modern whales, such as baleen whales is called a vestigial pelvic bone. Ambulocetus, a transitional form that was discovered more recently (announced in 1994), had heavier leg bones. Scientists hypothesize that it lived both on land and in water. In Figure 8.7, compare Ambulocetus with a modern toothed whale, as well as two other ancestors of present-day whales, Pakicetus and Rodhocetus. The discovery of Pakicetus and Rodhocetus has filled gaps in the fossil record of whales. 334 MHR Unit 3 Evolution

12 Pakicetus attocki lived on land, but its skull had already evolved features characteristic of whales. Ambulocetus natans likely walked on land (as modern sea lions do) and swam by flexing its backbone and paddling with its hind limbs (as modern otters do). Rodhocetus kasrani s small hind limbs would not have helped it swim, much less walk. Modern toothed whale Figure 8.7 Fossil evidence suggests that modern toothed whales evolved from a terrestrial ancestor, Pakicetus attocki. Basilosaurus and Dorudon, not shown in this illustration, appear more recently in the fossil record, after the appearance of Rodhocetus. Archaeopteryx: A Transitional Fossil In 1995, the fossil of a previously unknown dinosaur called Atrociraptor (savage robber) was discovered near Drumheller, Alberta. Atrociraptor was a small meat-eating dinosaur, about the size of a 10-year-old child. It is thought to be a close non-birdlike relative of Archaeopteryx. Fossils of Archaeopteryx show a transitional stage in the fossil record because this species had characteristics of both reptiles (dinosaurs) and birds. Archaeopteryx had feathers, but, unlike any modern bird, it also had teeth, claws on its wings, and a bony tail. Evidence from Biogeography Biogeography is the study of the past and present geographical distribution of organisms. Many of the observations that Darwin and Wallace used to develop their theories were based on biogeography. Darwin and Wallace hypothesized that species evolve in one location and then spread out to other regions. Biogeography supports this hypothesis with examples such as the following: Geographically close environments (for example, desert and forest habitats in South America) are more likely to be populated by related species than are locations that are geographically separate but environmentally similar (for example, a desert in Africa and a desert in Australia). So, for instance, cacti are native only to the deserts of North, Central, and South America. They are not found naturally in other deserts in the world, such as those in Australia and Africa. Animals found on islands often closely resemble animals found on the closest continent. This suggests that animals on islands have evolved from mainland migrants, with populations becoming adapted over time as they adjust to the environmental conditions of their new home. For example, the lizards found on the Canary Islands, off the northwest coast of Africa, are very similar to the lizards found in west Africa. Fossils of the same species can be found on the coastline of neighbouring continents. For example, fossils of the reptile Cynognathus have been found in Africa and South America. How can this be explained? The location of continents is not fixed; continents are slowly moving away from one another. About 510 million years ago, the continents of Africa and South America were joined in one supercontinent, called Gondwana, as shown in Figure 8.8. Closely related species are almost never found in exactly the same location or habitat. South America Gondwana Africa biogeography the study of the past and present geographical distribution of species populations India Antarctica Australia Figure 8.8 As the southern supercontinent Gondwana broke apart about 150 million years ago, the land masses that became the current continents of Africa, Australia, South America, and Antarctica were isolated from each other. Chapter 8 Developing a Theory of Evolution MHR 335

13 Learning Check 7. What is the fossil record? 8. Explain two ways in which the fossil record has helped scientists understand that organisms change over time. 9. Why are transitional fossils important? 10. How does the existence of vestigial pelvic bones in whales refute Lamarck s idea of use and disuse? 11. Compare each drawing in Figure 8.6 and describe the changes that you see from one epoch to the next. 12. Explain the following sentence: Islands have many unique species of animals and plants that are found nowhere else in the world. homologous structures structures that have similar structural elements and origin but may have a different function Evidence from Anatomy Vertebrate forelimbs can be used for various functions, such as flying (birds and bats), running (horses and dogs), and swimming (whales and seals). Despite their different functions, however, all vertebrate forelimbs contain the same set of bones, organized in similar ways. How is this possible? The most plausible explanation is that the basic vertebrate forelimb originated with a common ancestor. Homologous structures are those that have similar structural elements and origin but may have a different function. The limbs shown in Figure 8.9 have similar structures, such as number of bones, muscles, ligaments, tendons, and blood vessels. These structural elements are arranged, however, to be best suited for different functions: walking, flying, or swimming. Homologous structures are similar because they were inherited from a common ancestor. As you can see in Figure 8.9, homologous structures differ in their anatomy based on an organism s lifestyle and environment. For example, the bones in a horse s leg are larger and heavier than the bones in a bat s wing. Human Frog Bat Porpoise Horse Figure 8.9 These vertebrates have the same basic arrangement of bones (as indicated by the colours), but the bones have different uses. analogous structures structures of organisms that do not have a common evolutionary origin but perform similar functions Homologous structures can be similar in structure, function, or both. The limbs in Figure 8.9 are structurally similar. The lower limbs of the human, frog, and horse perform the same function: movement on land. Functional similarity in anatomy, however, does not necessarily mean that species are closely related. The wings of insects, birds, bats, and pterosaurs (extinct flying reptiles) are similar in function but not in structure. For example, bones support bird wings, whereas a tough material called chitin [KYE-ten] makes up insect wings. All of these organisms evolved independently of one another, and they do not share a common ancestor with wings. Body parts that perform similar functions, even though the organisms do not have a close common evolutionary origin, are called analogous structures. Analogous structures evolve in species of different origin who live in similar ecological units. 336 MHR Unit 3 Evolution

14 Homologous Hair In mammals, hair is homologous. Compare variations in the functions of mammalian hair by completing Activity 8.2 below. Activity 8.2 Homologies of Hair Mammals are the only animals that have hair. Among mammalian species, hair can vary in length, density, texture, and colour. The basic structure of hair, however, is the same in all mammals. Each hair has a central medulla that is surrounded by a dense cortex, which contains most of the pigment granules that give each strand of hair its colour. A layer called the cuticle covers the cortex. The scales of the cuticle are specific to a particular genus or even species of mammals. Thus, mammalian hair has a common origin, yet may serve different functions. In this activity, you will investigate variations in the functions of mammalian hair. cuticle pigment granules medulla cortex Cross Section of a Hair Materials computer with Internet access print resources Procedure 1. Work in a group of three or four. cuticular scale 2. Each person in your group should choose a different type of mammalian hair from the following list: stout, strong hairs of a porcupine dense underfur, or underhairs, of a sea otter whiskers (vibrissae) of a cat long, thick hair of a woolly mammoth horn of a rhinoceros, which is made of densely packed hair thick mane of a lion scales of a pangolin, which are modified hairs soft, fluffy underfur (qiviut) of a musk-ox 3. Using print and Internet sources, research the structure of the hair you have chosen. Research how the animal s lifestyle and habitat might explain the particular function(s) of its hair. Questions 1. Based on the information you collected and your understanding of natural selection, hypothesize how the structure of the hair is related to abiotic conditions in an animal s environment. Write a hypothesis stating how the variations might have arisen from the basic hair structure of a common mammalian ancestor. 2. Present your findings to the others in your group in a written or oral report, a computer presentation, or another form that is easily shared. 3. In a graphic organizer, describe one similarity and one difference in the adaptation of the hair studied by the members of your group. porcupine sea otter cat woolly mammoth rhinoceros lion pangolin musk-ox Chapter 8 Developing a Theory of Evolution MHR 337

15 embryology the study of early, pre-birth stages of an organism s development Evidence from Embryology Embryology is the study of early, pre-birth stages of an organism s development. Embryology has also been used to determine evolutionary relationships between animals. The embryos of different organisms exhibit similar stages of embryonic development. For example, all vertebrate embryos have paired pouches, or out-pocketings, of the throat. In fish and some amphibians, the pouches develop into gills. In humans, the pouches become parts of the ears and throat. At certain stages in the development of the embryo, the similarities between vertebrates are more apparent than the differences, as you can see in Figure The similarities between embryos in related groups (such as vertebrates) point to a common ancestral origin. It follows that related species share both adult features (such as basic arm-bone arrangements, as discussed earlier) and embryonic features (such as the presence of paired pouches in the throat). Fish Chicken Pig Human Figure 8.10 Similarities in the embryos of fish, birds, and mammals provide evidence of evolution of species from a common ancestor. Describe the differences and similarities between the fish and the chicken embryos. Evidence from DNA As you learned in Section 1.2, the evolutionary relationships between species are reflected in their DNA. Since DNA carries genetic information, scientists can determine how closely related two organisms are by comparing their DNA. If two species have similar patterns in their DNA, this indicates that these DNA sequences must have been inherited from a common ancestor. For example, by studying gene sequences, scientists have determined that dogs are related to bears and that whales and dolphins are related to ungulates [UN-gya-lets] (hoofed animals such as cows and deer). The use of modern technology has led to many discoveries that support Darwin s theory. Scientists now know how species pass on their traits to their offspring, and how the genes for these traits could change by mutation, as you learned in Section 7.1. Current evolutionary theory connects genetics with the theory of natural selection, and how natural selection operates on populations. Thus, genetic evidence and our understanding of heredity and mutations lend support to hypotheses that stem from observations of fossils, anatomy, biogeography, embryology, and DNA relationships. 338 MHR Unit 3 Evolution

16 STSE BIOLOGY Connections T. rex and chickens share a common ancestor? After excavating a T. rex fossil in 2003, scientists found it was too big to transport by helicopter. The scientists carefully broke the thigh bone in half to ship the bone. The results of later tests on the broken bone were surprising the bone held preserved soft tissues! These tissues, shown in the photograph below, included connective tissue, blood vessels, and possibly even blood cells. SOFT TISSUE In 2007, the fossil of the 68-million-year-old T. rex was tested for the first time to see if dinosaurs could be shown to share genetic markers with modern animals. The examination of dinosaur fossils allows scientists to understand how life on Earth has changed over time. The discovery of soft tissue allows new tests to be performed. It is possible that many more dinosaur bones contain soft tissue samples. soft tissue The soft tissue from the T. rex was almost perfectly preserved. Two independent tests on the soft tissue found in the fossil indicate that the T. rex is likely related to the present-day chicken. This new research provides molecular evidence that supports hypotheses that a common ancestor linked birds and dinosaurs. Molecular evidence suggests that the Tyrannosaurus rex and the chicken are related. (These images are not to scale.) In previous studies, physical similarities between early bird fossils and dinosaur fossils supported this link. For example, some fossils showed that the earliest birds had feet very similar to dinosaur feet. Several dinosaur fossils also show evidence of feathers. THE TEST A group of scientists at North Carolina State University introduced a protein to chicken and the T. rex soft tissue. The protein reacted strongly in the presence of the collagen found in chickens. (Collagen is a protein found in the connective tissue of animals.) A similar reaction was observed when the protein was administered to the dinosaur tissue. This indicates a molecular similarity between chicken tissues and dinosaur tissues. In another study performed by a team of researchers from Harvard Medical School, scientists obtained protein sequences from the T. rex soft tissue. The amino acid sequence in the proteins was similar to the amino acid sequence in chickens, showing clear support for an ancestral link between chickens and dinosaurs. (Amino acids are compounds that form proteins.) Connect to Scientific Inquiry Before researchers tested the reaction of protein and collagen in the soft tissue of chickens and dinosaurs, they had to formulate a hypothesis. Write a hypothesis that they might have tested. Chapter 8 Developing a Theory of Evolution MHR 339

17 Section 8.2 REVIEW Section Summary The theory of evolution connects facts to provide a logical framework that explains how life on Earth has changed and is still changing. Charles Darwin and Alfred Russel Wallace both developed hypotheses to explain natural selection. New discoveries of fossils, called transitional fossils, help fill in the gaps in the fossil record. Homologous structures have similar structural elements and origin but may have a different function. Analogous structures perform similar functions, even though the organisms do not share a recent common ancestor. The fossil record, biogeography, anatomy, embryology, and relationships in DNA all provide evidence for evolution. Review Questions 1. K/U Charles Darwin was not the only person to discuss the idea of evolution. Why is his name often synonymous with the idea of evolution? 2. T/I Evolutionary biologist John Haldane once said, I d give up my belief in evolution if someone found a fossil rabbit in the Precambrian. Explain this quote. 3. K/U How does the discovery of transitional fossils in the fossil record help you understand the evolutionary events of the past? 4. C Choose a fossil (either one described in this textbook or another one you have researched) and describe what information it provides that helps you understand evolution. 5. K/U A scientist finds a rare organism a whale with hind legs. Explain how this finding is evidence for evolution. 6. A The island of Madagascar is thought to have split from Africa about 150 million years ago. Discuss the types of organisms you would expect to find on this island. Explain your reasoning. Africa Madagascar The island of Madagascar is just east of Africa. 7. T/I Make a hypothesis about what species changes and environmental changes you would expect to see on Madagascar, after a long period of time, if it were somehow reconnected to mainland Africa today. How might a scientist test your hypothesis? 8. K/U Describe how comparing the anatomy of animals is used to support the theory of evolution by natural selection. 9. K/U What kinds of structures are the arm of a human and the forelimb of a horse? 10. K/U Explain the difference between analogous structures and homologous structures. 11. K/U Are bird wings and bat wings homologous structures or analogous structures? Explain your answer. 12. K/U Define vestigial structure. Provide an example. 13. T/I Scientists discover that the pharyngeal (gill) pouches are similar in the early embryological stages of vertebrates such as snakes, cats, bats, and human embryos. Yet, the later stages of development show many more differences between these organisms. Explain why the early stages are so similar. 14. C Create a concept map that shows how different types of scientific evidence support the theory of evolution. Go to Using Graphic Organizers in Appendix A to learn more about making a concept map. 15. T/I A person tells you that evolution, like the big bang, is just a theory. Explain to the person what a theory means in a scientific sense, and provide four facts that support the theory of evolution. 16. T/I Baleen whales, such as grey and humpback whales, have teeth and body hair while they are embryos, but they lack these features as adults. What does this tell you about the evolutionary history of these animals? 17. C Use a diagram such as a flowchart to illustrate the process of evolution by natural selection in a giraffe population competing for leaves in the high canopy of trees. Go to Using Graphic Organizers in Appendix A to learn more about making a flowchart. 340 MHR Unit 3 Evolution