10.3. Theory of Natural Selection

Transcription

1 SECTION 10.3 Theory of Natural Selection KEY CONCEPT Darwin proposed natural selection as a mechanism for evolution. Student text pages Several key insights* led to Darwin's idea for natural selection. The variation of similar species among islands, fossil evidence, and geologic events convinced Darwin that evolution occurs. But he still wondered how evolution occurs. Here, you will read about some of Darwin's reasoning that led him to his idea for natural selection. Artificial Selection Darwin noticed that plants and animals that are raised by humans had variations in traits that were not seen in their wild relatives. Think of all the different breeds of dogs that you have seen. In a process called artificial selection, humans select individuals with the traits they desire, and then breed them to produce more individuals with those traits. Heritability In order for artificial selection to occur, traits must be heritable. Heritability (HER-ih-tuh-BIHL-uh-tee) is the ability of a trait to be passed down from one generation to the next. Things that are acquired in an organism's life, like a broken bone, are not heritable. Natural Section Darwin reasoned that a process similar to artificial selection could happen in nature. In artificial selection, humans are the source of selection. In natural selection, the environment is the source of selection. Natural selection is a process in which individuals that have inherited beneficial* adaptations produce more offspring than do other individuals. Humans have changed animal species, such as these different dog breeds, through artificial selection. * ACADEMIC VOCABULARY insight a clear realization about a topic beneficial resulting in good; helpful Interactive Reader 169

2 Struggle for Survival Darwin was influenced by the work of an economist named Thomas Malthus. Malthus proposed that resources like food, water, and shelter were limits to human population growth. Darwin reasoned that a similar struggle happened in nature. Darwin saw great variation within populations of organisms. A population is all the individuals of a species that live in an area. He saw individuals with adaptations that matched their environment. Darwin proposed that these adaptations arose over many generations in a process he called "descent with modification." What is the difference between artificial and ter2.... "/ natural selection? r Natural selection explains how evolution can occur. Darwin was not the only scientist studying evolution during this time. Another scientist named Alfred Wallace independently developed an explanation of how evolution occurs. Wallace's explanation was very similar to Darwin's. In the late 1850s, the ideas of Darwin and Wallace were presented to the scientific community. There are four main principles to the theory of natural selection: variation, overproduction, adaptation, and descent with modification. Variation Individuals of a species differ due to genetic variation. Heritable differences are the basis for natural selection. Overproduction Organisms have more offspring than can survive. This results in competition among offspring for resources. Adaptation Some individuals have certain variations that allow them to survive better than other individuals in their environment. These individuals are "naturally selected" to live longer and produce more offspring that also have those adaptations. Descent with modification Over time, natural selection will result in species with adaptations that are beneficial for survival and reproduction in a particular environment. More individuals will have the trait in every following generation, as long as the environmental conditions stay the same. The white fur of this rabbit allows it to blend in with its environment. The term descent is used in evolution to mean the passing of genetic information from generation to generation. 170 McDougal Littell Biology

3 Let's apply these four principles to an example of natural selection. About 11,000 years ago, jaguars faced a shortage of food due to a changing climate. There were fewer mammals to eat, and jaguars had to eat reptiles to survive. Variations in jaw and tooth size allowed some individuals to more easily eat shelled reptiles. A jaguar may produce many offspring, but not all of young will survive due to competition for resources. ADAPTATION Jaguars with larger jaws and teeth are able to eat shelled reptiles. These jaguars are likely to survive longer and leave more offspring thin jaguars that can eat only mamm Some jaguars, such as jaguar 1 shown here, may be born with slightly larger jaws and teeth due to natural variation in the population. Some variations are heritable. jaguar skull 1 jaguar skull 2 Because large teeth and jaws are heritable traits. they become more commo characteristics in the population. In biology, the term fitness is a measure of the ability to survive and reproduce. In the example above, jaguars with larger teeth and jaws had higher fitness after the change in climate. maw Am; What is the difference between the biological term fitness ana the common meaning of the word? Natural selection acts on existing variation. Natural selection cannot make new alleles. It can only work with variation that already exists. In other words, natural selection acts on phenotypes, or physical traits, and not on genetic material itself. Changing environments As an environment changes, different traits become beneficial. Think about the jaguars. When mammals were their main food source, small teeth and jaws were beneficial. But when the environment changed, larger teeth and jaws became beneficial to better eat reptiles. Because the environment constantly changes, a trait that is an advantage today may be a disadvantage in the future. Interactive Reader 171

4 Adaptations as compromises Adaptations can sometimes be thought of as compromises. One example of an adaptive compromise is the panda bear's "thumb." Modern pandas have five digits that are like your fingers and a sixth digit that functions like a thumb. This digit is actually a wrist bone. Ancestral pandas with bigger wrist bones had an advantage in holding and eating their food. Over time, this adaptation became typical of the species. /7-raiN n4s a Why might a trait that is an advantage today become a disadi. 4.1 V12.,/ P3 vantage in the future? artificial selection heritability natural selection population fitness Go back and highlight each sentence that has a vocabulary word in bold. 1. In the list above, draw an arrow pointing to the term that describes the process by which humans breed animals or plants for certain traits. 2. Circle the term that describes a mechanism for evolution in nature. 3. Underline the term that means the ability of a trait to be passed from one generation to the next. 4. Box the term that is a measure of the ability to survive and reproduce. 5. What is the difference between the meanings of the terms evolution and natural selection? 6. What are four main principles to the theory of natural selection? 172 McDougal Littell Biology

5 10.4 SECTION Evidence of Evolution KEY CONCEPT Evidence of common ancestry among species comes from many sources. Student text pages Evidence for evolution in Darwin's time came from several sources. Genetic inheritance was not known in Darwin's time. But Darwin supported his ideas with evidence from many other sources fossils, geography, embryology, and anatomy. His evidence was very strong, and left no doubt in the minds of scientists that all organisms share a common ancestor. Today, the concept of evolution ties together all fields of biology. Fossils Before Darwin, scientists studying fossils knew that organisms changed over time. Fossilized organisms were different in different layers of rocks. The bottom layers of rock are the oldest, and contain fossils of more ancient organisms. The upper layers of rock are the youngest, and contain fossils of more recent organisms. Findings in the fossil record support Darwin's idea of descent with modification. Geography During Darwin's travels, he noticed that plants and animals on islands looked similar to species on the mainland, but not exactly the same. He hypothesized that organisms from the mainland had migrated to the islands. Different islands had different food sources, climates, predators. The different environmental conditions favored different traits in these migrant organisms. For example, the finches on the different Galapagos islands have different shapes of beaks that evolved in response to the different island habitats. This is an important part of biogeography, the study of the distribution of organisms around the world. Trilobites are extinct organisms that lived in ancient oceans. Embryology Embryology is the study of embryos and their development. Darwin noticed that the embryos of different species may look very similar, although the adult species look very different. For example, fish, birds, reptiles, and mammals all have gill slits as embryos. These slits develop into gills in fish, but develop into ears and throats in mammals. The similar features of embryos in very different organisms suggest evolution from a distant common ancestor. Visual Connection See photographs in student text, pg. 311 Interactive Reader 173

6 Anatomy Some of Darwin's best evidence came from comparing the body parts of different species. He found that some organisms have body parts that are similar in structure, but might be used differently. Homologous structures (huh-mahl-uh-guhs) are features that are similar in structure, but appear in different organisms and may have different functions. For example, think of the front limbs of humans, bats, and moles. The front limbs of these organisms share similar bone structure, but each organism uses these limbs differently. Homologous structures appear across many different species, and offer strong evidence for common descent. Analogcnis structures (uh-nal-uh-guhs) are structures that perform a similar function, but are not similar in origin. For example, both birds and butterflies have wings. They both use their wings to fly, but their wings do not share a common origin. Bird wings have bones, and butterfly wings do not. J US L MU:MSC MU 01 gainsins siiare S11111E11- SU LIC.- tures does not mean they are closely related. Homologous structures show common ancestry, while analogous structures do not. g4,22, -F ' The wings of birds and butterflies are analogous structures. What is the difference between a homologous structure and an analogous structure? Structural patterns are clues to the history of a species. Vestigial structures (veh-stihj-ee-uhl) are small leftover organs or structures that had a function in an early ancestor. For example, snakes and whales have tiny pelvic bones. Even though neither organism walks, their ancestors were fourlegged animals. These pelvic bones are vestigial structures. The appendix is an example of a vestigial structure in humans. The appendix is a remnant of an organ that helped to digest certain plant material eaten by human ancestors. The human appendix has lost the ability to digest this material, and actually has no known function. A whale's pelvic bone is a vestigial structure a leftover from an early ancestor. 174 McDougal Littell Biology

7 Vestigial structures do not become smaller in one individual organism. These structures became smaller over many generations. Today, biologists consider vestigial structures among the most important examples demonstrating how evolution works. 4iirlAZ What are vestigial structures? vistapl.."- A Vocabulary Check Mar t Up N: " Go back and highlight. ' biogeography analogous structure each sentence that Age of homologous structure vestigial structure has a vocabulary. word in bold. Complete each sentence with the correct term from the list above. 1. The forelimbs of humans, bats, and moles are examples of 2. Bird wings and butterfly wings are examples of 3. Pelvic bones in whales are an example of 5. What were four lines of evidence Darwin used to support his argument for evolution? 6. How do vestigial structures demonstrate common ancestry? Interactive Reader 175

8 10.5 SECTION Evolutionary Biology Today KEY CONCEPT New technology is furthering our understanding of evolution. Student text pages Fossils provide a record of evolution. Paleontology (PAY-lee-ahn-TAHL-uh-jee) is the study of fossils or extinct organisms. The fossil record is not complete. One reason for this is because most organisms do not form fossils after they die. Fossils form only in particular environmental conditions. However, fossil evidence that does not support evolution has never been found. Darwin wondered why he did not find fossils that showed transitions between different groups of organisms. Since Darwin's time, many of these transitional fossils have been found. For example, fossils have been found of a transitional species in the evolution of whales. This organism had a whalelike body, but still had the limbs of land animals. Fossils continue to provide new information and evidence for current ideas about evolution. Paleontology is the study of prehistoric life forms. Paleo- means "ancient," and -ology means "the study of.:' Dorudon 40 million years ago " 44 T*, trorit.dril'r,, Ambulocetus natans 50 million years ago ,, Ikterer,,,-, - e-,i.,t',--1,..di - ri,,,,,,... -,' --,...k...,. 4,,..., Pakicetus 52 million years ago Fossil evidence supports the idea that whales descended from hoofed mammals. Pakicetus had a whale-shaped skull and teeth adapted for hunting fish. Ambulocetus natans lived on both land and water. Dorudon had tiny hind legs that were useless on land, similar to modern-day whales. What is a transitional fossil? 176 McDougal Littell Biology

9 Molecular and genetic evidence support fossil and anatomical evidence. The fields of genetics and molecular biology have added strong support to Darwin's theory of natural selection. All living things have DNA, share the same genetic code, and make most of the same proteins from the same 20 amino acids. Comparisons of DNA and protein sequences can be used to show evolutionary relationships between different organisms. The more related two organisms are, the more similar the sequences will be. Because there are thousands of genes in even simple organisms, DNA contains a huge amount of information on evolutionary history. MOLECULAR EVIDENCE Some particular genes are found in many organisms from fruit flies to humans and therefore give evidence of a very distant common ancestor. For example, homeobox genes, which control development, are found in a wide range of organisms. These genes are even found in organisms that lived 600 minion years ago. You will read more about homeobox genes in Chapter 23. polistr.4.,, How does genetic evidence give information about evolutionary relationships? Hippopotamus TCC TGGCA GTCCA GTGGT Humpback whale CCC TGGCA GTGCA GTGCT Molecular evidence also supports the idea that whales descended from hoofed mammals. As shown above, the DNA sequences of a whale and a hippopotamus are very similar. Evolution unites all fields of biology. Scientists continue to actively study evolution through natural selection. The theory of natural selection combined with genetics is sometimes called the modern synthesis of evolutionary theory. New tools are providing more data than ever before. Modern tools from different fields of study add to what has been discovered through fossil evidence. For example, as shown in the figure above, the comparisons of milk protein genes support the fossil evidence for the relationship between whales and hippopotamuses. New discoveries are limited only by the time and resources of scientists. The basic principles of evolution are used in all fields of science, including medicine, geology, geography, chemistry, and ecology. For example, the idea of common descent helps biologists understand where new diseases come from and how they might be best treated. Synthesis means the combination of different ideas to form a new thing. Interactive Reader 177

10 As much as we know about life on Earth, there is so much more waiting to be discovered. As the great geneticist Theodosius Dobzhansky ( ) once noted, "Nothing in biology makes sense except in the light of evolution." AN What is the modern synthesis of evolutionary biology? Paleontology Go back and highlight each sentence that - has a vocabulary word in botd. 1. Write a definition for the term paleontology. 2. Why are transitional fossils important information for tracing the course of evolution? 3. This chart shows the sequence of a particular gene in three different organisms. Which two organisms do you think are most closely related? Explain your answer. ORGANISM GENE SEQUENCE Organism A TCAGGAACTA Organism B ACAGGAAGTA ' Organism C TGTAAGTATA 4. Name three fields of science in which the principles of evolution are used. 178 McDougal Littell Biology

11 Chapter 10 Review. 1. Fill in the chart below with examples of different lines of evidence for evolution. Field of Study Fossils Geography Embryology Anatomy Evidence 2. The. panda's "thumb" is actually a sixth "finger" made from a wrist bone, not from the same bone parts as the human thumb. These bones are an example of which type of structures? a. analogous structures b. homologous structures c. vestigial structures d. evolutionary structures 3. Which idea best describes our modern understanding of geologic change? a. gradualism b. catastrophism c. uniformitarianism d. biogeography 4. Explain why the following sentence is true: "An evolutionary advantage today may be an evolutionary disadvantage in the future." 5. Write a brief definition of the term natural selection. 6. What is a heritable trait? 7. Based on what we know today about genetics, what accounts for variation in a population? Interactive Reader 179