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GETTING READY TO LEARN

CHAPTER

10 Principles of Evolution

Preview Key Concepts 10.1 Early Ideas About Evolution

There were theories of biological and geologic change before Darwin.

10.2 Darwin’s ObservationsDarwin’s voyage provided insights into evolution.

10.3 Theory of Natural SelectionDarwin proposed natural selection as a mechanism for evolution.

10.4 Evidence of EvolutionEvidence of common ancestry among species comes from many sources.

10.5 Evolutionary Biology TodayNew technology is furthering our understanding of evolution.

Review Academic VocabularyWrite the correct word for each definition.

competition hybridization phenotype

1. : two or more organisms try to get the same resource

2. : physical characteristics of an organism

3. : the crossing of two different species

Preview Biology VocabularySee how many key terms from this chapter you already know. Rewrite each phrase, using a different word or words for the words in bold.

PHRASE REWRITTEN WITH DIFFERENT WORDS1. In birds, wings and hollow bones

are each an adaptation.In birds, wings and hollow bones are each an

.

2. You may have traits from each of your parents due to heritability.

You may have traits from each of your parents .

3. The hand bones in humans and the flipper bones in seals are ex-amples of homologous structures.

The hand bones in humans and the flipper bones in seals are examples of

.

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Early scientists proposed ideas about evolution. Evolution is the process of biological change by which descendants* come to differ from their ancestors. Much of today’s understanding of evolution is based on Charles Darwin’s work in the 1800s. But Darwin did not come up with the idea of evolution himself. Many other scien-tists contributed important ideas to the study of evolution. Several historical ideas—about species, geology, and the mech-anisms of evolution—are described below. Like all science, the modern understanding of biological evolu-tion builds on hundreds of years of study and research.

Species In the 1700s, a botanist named Carolus Linnaeus came up with a system to organize and name all of the different known types of organisms, or species. A species is a group of organisms that can reproduce and have fertile* offspring. Linneaus’ classification system grouped organisms according to physical similarities. His system also shows evolutionary relationships, and is still in use today.

In Linnaeus’ time, there was a common belief that the organisms that lived on earth were fixed, or that species did not change. Linneaus proposed that species could change. For example, he observed in experiments that two different plant species could cross, and make a new type of plant.

Early Ideas About EvolutionKEY CONCEPT There were theories of biological and geologic change before Darwin.

SECTION

10.1

* ACADEMIC VOCABULARY

descendant offspring, or an organism that is related to another organism from the past

fertile able to breed and have offspring

Student text pages 298–301

There is great diversity in different species of birds.


McDougal Littell Biology164

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In the 1700s, it was also a common belief that Earth was only 6000 years old. But several geologists began to challenge this idea. The geologists believed they had evidence that Earth was much older. The evidence of the old age of Earth was important to Darwin’s development of his ideas.

Mechanisms of Evolution Many scientists in the 1700s did not believe that species could go extinct*. But many scientists thought species could change, or evolve. There were many different ideas, however, about the mechanism of evolution, or how evolution happens. For example, different scientists had different ideas about how environmental changes affect evolution, how changes get passed on to offspring, and what causes biological variation.

How does history affect the development of scientific ideas?

Theories of geologic change set the stage for Darwin’s theory.

The study of fossils led some scientists to conclude that species do go extinct. Fossils are traces of organisms that existed in the past. The locations of fossils in different rock layers provide clues about Earth’s past. But how did those rock layers form? Geologists held different ideas about geologic change.

• Catastrophism (kuh-TAS-truh-FIHZ-uhm) is the idea that past natural disasters—like floods and volcanic eruptions—shaped landforms, and caused species to become extinct in the process.

• Gradualism (GRAJ-oo-uh-LIHZ-uhm) is the idea that landforms were shaped by very slow changes over a long period of time, and not by natural disasters.

The formation of mountains, shown here, results from slow changes over long periods of time.


geology the study of rocks, minerals, and landforms

extinct no longer existing

Interactive Reader 165

How does history affect the development of scientific ideas?

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• Uniformitarianism (YOO-nuh-FAWR-mih-TAIR-ee-uh-NIHZ-uhm) is the idea that the same processes that shaped landforms in the past also shape landforms today. In other words, the geologic processes that shape Earth are uniform, or the same, through time. Uniformitarianism became the favored theory of geologic change and played an important role in the development of Darwin’s theory.

Which idea about geologic change became the accepted theory?

evolution catastrophism

species gradualism

fossil uniformitarianism

1. Name three ideas about geologic change.

2. What is the term for a group of organisms that can reproduce and have fertile offspring?

3. What is the term for the process of biological change by which organisms come to differ from their ancestors?

4. How are catastrophism and gradualism different?

5. How did the ideas of scientists before Darwin infl uence Darwin’s ideas?

10.1 Vocabulary Check

Go back and highlight each sentence that has a vocabulary word in bold.

Mark It Up

10.1 The Big Picture

Uniformitarianism proposes that present geologic processes are the key to the past.

VISUAL VOCAB

Every layer of rock was formed by the uniform laying down of sediment that still occurs today.


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Darwin observed differences among island species.

Darwin spent over 20 years researching biological evolu-tion. He made important observations during his travels on a ship that sailed the coast of South America and the Pacific islands. Darwin was amazed by the variation of traits among similar species that he saw in his travels. The word variation has many common uses, but in biology, variation means the differences in the physical traits among individuals in a group of organisms.

Darwin noticed that there was variation between spe-cies on different islands. This was especially noticeable in the Galápagos Islands, a chain of islands off of the coast of Ecuador in South America. For example, he noticed that birds called finches had different kinds of beaks in areas with different food sources. He found finches with thick beaks in areas with large hard-shelled nuts, and finches with smaller beaks in areas where fruits were available.

These differences seemed to match the environment and the diet of the finches. Darwin realized that species may somehow be able to adapt to their surroundings. An adaptation is a feature that allows an organism to better survive in its environment.

What adaptations did Darwin notice in finches?

Darwin observed fossil and geologic evidence supporting an ancient Earth.

During his travels, Darwin found that some fossils looked similar to living species. This suggested that the living species might have some relationship to the fossil forms. In order for such changes to occur, Darwin figured that Earth must be much older that 6000 years.

Darwin’s ObservationsKEY CONCEPT Darwin’s voyage provided insights into evolution.

SECTION

10.2Student text pages 302–303

Darwin also studied different spe-cies of Galapagos tortoises that are adapted to different environments.

Albemarle Island

Abingdon Island


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What adaptations did Darwin notice in finches?

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Darwin found much evidence supporting the ideas of uniformitari-anism. For example, he found fossil shells of marine* organisms high up in the Andes mountains. Later he experienced an earthquake and saw land that had been underwater get shifted up above the sea level. He put together observations like these, and concluded that daily geologic processes can add up to much bigger changes over a long period of time.

What is one example of evidence from Darwin’s travels that supports an ancient Earth?


marine related to the ocean

variation adaptation

For each term, write brief definitions of its common and biological meanings.

1. variation common meaning:

biological meaning:

2. adaptation common meaning:

biological meaning:

3. What is one example of biological variation that Darwin observed in his travels?

4. What is one example of geologic evidence supporting uniformitarian-ism that Darwin observed in his travels?



Mark It Up



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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 indi-viduals 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 genera-tion 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 off-spring than do other individuals.

Theory of Natural SelectionKEY CONCEPT Darwin proposed natural selection as a mechanism for evolution.

SECTION



insight a clear realization about a topic

beneficial resulting in good; helpful

Humans have changed animal species, such as these different dog breeds, through artifi cial selection.

or



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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 genera-tions in a process he called “descent with modification.”

What is the difference between artificial and natural selection?

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 repro-duction in a particular environment. More individuals will have the trait in every following generation, as long as the environmental conditions stay the same.

The term descent is used in evolution to mean the passing of genetic infor-mation from generation to generation.

VOCABULARY

The white fur of this rabbit allows it to blend in with its environment.


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OVERPRODUCTION

ADAPTATION

VARIATION

jaguar skull 1

jaguar skull 2

jaguar 1jaguar 2

A jaguar may produce many offspring, but not all of young will survive due to competition for resources.

DESCENT WITH MODIFICATION

Because large teeth and jaws are heritable traits, they become more common characteristics in the population.

Jaguars with larger jaws and teeth are able to eat shelled reptiles. These jaguars are likely to survive longer and leave more offspring than jaguars that can eat only mammals.

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.

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 chang-ing climate. There were fewer mammals to eat, and jaguars had to eat reptiles to survive. Variations in jaw and tooth size allowed some indi-viduals to more easily eat shelled reptiles.

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.

What is the difference between the biological term fitness and the common meaning of the word?

Natural selection acts on existing variation.Natural selection cannot make new alleles. It can only work with varia-tion that already exists. In other words, natural selection acts on pheno-types, 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.


What is the difference between the biological term fitness and the common meaning of the word?

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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 hold -ing and eating their food. Over time, this adaptation became typical of the species.

Why might a trait that is an advantage today become a disad-vantage in the future?

artificial selection population

heritability fitness

natural selection

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?



Mark It Up



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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 com-mon ancestor. Today, the concept of evolution ties together all fields of biology.

FossilsBefore 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.

GeographyDuring 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, and predators. The different environmental conditions favored different traits in these migrant organisms. For example, the finches on the different Galápagos 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.

EmbryologyEmbryology 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.

Evidence of EvolutionKEY CONCEPT Evidence of common ancestry among species comes from many sources.

SECTION


Trilobites are extinct organisms that lived in ancient oceans.

See photographs in student text, pg. 311

Visual Connection



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AnatomySome 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.

• Analogous structures (uh-NAL-uh-guhs) are structures that perform a similar func-tion, but are not similar in origin. For exam-ple, 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.

Just because two organisms share similar struc-tures does not mean they are closely related. Homologous structures show common ancestry, while analogous structures do not.

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 four-legged 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.

The wings of birds and butterfl ies are analogous structures.

A whale’s pelvic bone is a vestigial structure—a leftover from an early ancestor.


What is the difference between a homologous structure and an analogous structure?

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Vestigial structures do not become smaller in one individual organ-ism. These structures became smaller over many generations. Today, biologists consider vestigial structures among the most important examples demonstrating how evolution works.

What are vestigial structures?

biogeography analogous structure

homologous structure vestigial structure

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 butterfl y wings are examples of

3. Pelvic bones in whales are an example of

4. What were four lines of evidence Darwin used to support his argument for evolution?

5. How do vestigial structures demonstrate common ancestry?



Mark It Up



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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.

Dorudon 40 million years ago

Ambulocetus natans 50 million years ago

Pakicetus 52 million years ago

Modern -day whale

Fossil evidence supports the idea that whales de-scended from hoofed mammals. Pakicetus had a whale-shaped skull and teeth adapted for hunting fi sh. Ambulocetus natans lived on both land and water. Dorudon had tiny hind legs that were useless on land, similar to modern-day whales.

WHALE EVOLUTION

What is a transitional fossil?

Evolutionary Biology TodayKEY CONCEPT New technology is furthering our understanding of evolution.

SECTION


Paleontology is the study of prehistoric life forms. Paleo- means “ancient,” and -ology means “the study of.”

VOCABULARY



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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 relation-ships 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 informa-tion on evolutionary history.

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 million years ago. You will read more about homeobox genes in Chapter 23.

How does genetic evidence give information about evolution-ary relationships?

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 compari-sons of milk protein genes support the fossil evidence for the relation-ship 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.

VOCABULARY

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.

MOLECULAR EVIDENCE


How does genetic evidence give information about evolutionary relationships?

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As much as we know about life on Earth, there is so much more waiting to be discovered. As the great geneticist Theodosius Dobzhansky (1900–1975) once noted, “Nothing in biology makes sense except in the light of evolution.”

What is the modern synthesis of evolutionary biology?

Paleontology

1. Write a defi nition 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.

4. Name three fi elds of science in which the principles of evolution are used.



Mark It Up


ORGANISM GENE SEQUENCE

Organism A TCAGGAACTA

Organism B ACAGGAAGTA

Organism C TGTAAGTATA


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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 “fi nger” 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 defi nition 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?


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