Bring Science Alive! 8th Grade Integrated L24 Reference Text

The Evolution of Life

ANCHORING PHENOMENON Whales live in water and look like big fish, but they have traits of land-dwelling mammals.

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Phenomenon-Based Storyline As evolutionary biologists, collect data about the survival, anatomy, and evolutionary history of whales. Analyze the data to explain why whales have internal organs similar to those of mammals that live on land. Use your findings to determine which organism is the closest living relative to the whale.

Next Generation Science Standards Performance Expectations MS-LS3-1.

MS-LS4-2.

MS-LS4-3.

MS-LS4-4.

MS-LS4-6.

MS-ETS1-3.

MS-ETS1-4.

Develop and use a model to describe why structural changes to genes (mutations) located on chromosomes may affect proteins and may result in harmful, beneficial, or neutral effects to the structure and function of the organism.

Apply scientific ideas to construct an explanation for the anatomical similarities and differences among modern organisms and between modern and fossil organisms to infer evolutionary relationships.

Analyze displays of pictorial data to compare patterns of similarities in the embryological development across multiple species to identify relationships not evident in the fully formed anatomy.

Construct an explanation based on evidence that describes how genetic variations of traits in a population increase some individuals’ probability of surviving and reproducing in a specific environment.

Use mathematical representations to support explanations of how natural selection may lead to increases and decreases of specific traits in populations over time.

Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.

Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.

Science and Engineering Practices

Crosscutting Concepts

Developing and Using Models • Develop and use a model to describe phenomena.

• Develop a model to generate data to test ideas about designed systems, including those representing inputs and outputs.

Analyzing and Interpreting Data • Analyze and interpret data to determine similarities and differences in findings.

• Analyze displays of data to identify linear and nonlinear relationships.

• Analyze and interpret data to provide evidence for phenomena.

Cause and Effect Phenomena may have more than one cause, and some cause and effect relationships in systems can only be described using probability.

Using Mathematics and Computational Thinking Use mathematical representations to support scientific conclusions and design solutions.

Constructing Explanations and Designing Solutions • Construct an explanation that includes qualitative or quantitative relationships between variables that describe phenomena.

• Apply scientific ideas to construct an explanation for real-world phenomena, examples, or events.

Structure and Function Complex and microscopic structures and systems can be visualized, modeled, and used to describe how their function depends on the shapes, composition, and relationships among its parts, therefore complex natural structures/systems can be analyzed to determine how they function.

Patterns • Patterns can be used to identify cause and effect relationships.

• Graphs, charts, and images can be used to identify patterns in data.

Connections to Nature of Science: Scientific Knowledge Assumes an Order and Consistency in Natural Systems Science assumes that objects and events in natural systems occur in consistent patterns that are understandable through measurement and observation.

Disciplinary Core Ideas LS3.A: Inheritance of Traits Genes are located in the chromosomes of cells, with each chromosome pair containing two variants of each of many distinct genes. Each distinct gene chiefly controls the production of specific proteins, which in turn affects the traits of the individual. Changes (mutations) to genes can result in changes to proteins, which can affect the structures and functions of the organism and thereby change traits.

LS3.B: Variation of Traits In addition to variations that arise from sexual reproduction, genetic information can be altered because of mutations. Though rare, mutations may result in changes to the structure and function of proteins. Some changes are beneficial, others harmful, and some neutral to the organism.

LS4.A: Evidence of Common Ancestry and Diversity • Anatomical similarities and differences between various organisms living today and between them and organisms in the fossil record, enable the reconstruction of evolutionary history and the inference of lines of evolutionary descent. • Comparison of the embryological development of different species also reveals similarities that show relationships not evident in the fully-formed anatomy.

LS4.B: Natural Selection Natural selection leads to the predominance of certain traits in a population, and the suppression of others.

LS4.C: Adaptation Adaptation by natural selection acting over generations is one important process by which species change over time in response to changes in environmental conditions. Traits that support successful survival and reproduction in the new environment become more common; those that do not become less common. Thus, the distribution of traits in a population changes.

ETS1.B: Developing Possible Solutions • A solution needs to be tested, and then modified on the basis of the test results, in order to improve it. • There are systematic processes for evaluating solutions with respect to how well they meet the criteria and constraints of a problem. • Sometimes parts of different solutions can be combined to create a solution that is better than any of its predecessors. • Models of all kinds are important for testing solutions.

ETS1.C: Optimizing the Design Solution • Although one design may not perform the best across all tests, identifying the characteristics of the design that performed the best in each test can provide useful information for the redesign process—that is, some of those characteristics may be incorporated into the new design. • The iterative process of testing the most promising solutions and modifying what is proposed on the basis of the test results leads to greater refinement and ultimately to an optimal solution.

The Evolution of Life

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LESSON 24

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Darwin’s Theory of Evolution Through Natural Selection How can the traits of a population of living things change over time?

Introduction

Vocabulary

In the 1830s, the young Charles Darwin sailed around the world—a trip that would change the history of science. On the voyage, Darwin discovered amazing new plants, animals, and fossils, including this enormous glyptodon, which is the size of a small car!

And although it looks like a turtle, it was actually a mammal, a kind of armadillo. This and other discoveries led Darwin to develop a theory explaining how Earth’s living things evolve, or change over time.

Before Darwin, many scientists thought that species did not change over time. The fossil record told a different story. The extinct glyptodon, for example, was a kind of armadillo. Could it be an ancestor, or a distant relative, of modern armadillos? If so, why was it so big? And why did it have a bony “shell”? Could a fossil species change over time to become a modern species? Darwin’s work helped scientists answer questions like these. Of course, more than one cause may be behind changes in groups of organisms, and scientists may only be able to say that something was a possible or likely cause.

Starting with Darwin’s early life and his voyage around the world, you will learn how Darwin developed his theory of evolution through natural selection. You will see that Darwin’s observations of living things influenced his work and that he was also inspired by ideas from many other people. Finally, you will learn about Darwin’s idea of natural selection and how natural selection produces changes in populations of living things.

trait a characteristic of an organism population a group of individuals of the same species that live and reproduce in the same place genetic variation differences in the inherited traits of different individuals natural selection the process through which individuals in a population with certain inherited traits are more likely to survive and reproduce than individuals with less favorable traits in a specific environment evolution the inherited changes that occur in a population over time adaptation an inherited trait evolved through natural selection that makes a population well suited to living and reproducing in its environment

Next Generation Science Standards Performance Expectations MS-LS4-4. Construct an explanation based on evidence that describes how genetic variations of traits in a population increase some individuals’ probability of surviving and reproducing in a specific environment.

Science and Engineering Practices Constructing Explanations and Designing Solutions Construct an explanation that includes qualitative or quantitative relationships between variables that describe phenomena.

Crosscutting Concepts Cause and Effect Phenomena may have more than one cause, and some cause and effect relationships in systems can only be described using probability.

Disciplinary Core Ideas LS4.B. Natural selection leads to the predominance of certain traits in a population, and the suppression of others.

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1. Darwin’s Voyage on the Beagle

Figure 1 The young Charles Darwin sailed around the world on the Beagle. This map appeared in Darwin’s published account of the voyage and shows the Beagle’s route (in red). The Beagle sailed from England to South America, stopped at the Galápagos Islands, continued across the Pacific, and then went around Australia and Africa back to England.

The Voyage of the Beagle

Galápagos Islands

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Lesson 24

There was once a young man who was fascinated by nature. In order to follow his family’s wishes, though, he tried other career paths, first medicine and then the clergy. Neither worked out. Then, he went on the adventure of a lifetime—a voyage across the world. On this voyage, he began the work that would end up filling his entire life. How did Darwin’s early life shape the revolutionary scientific ideas he came up with later?

Charles Darwin was born in Shrewsbury, England in 1809. As a boy, he developed a great interest in nature. However, his father was a doctor, and Darwin was sent to medical school. Darwin’s studies there went badly, mainly because he found medicine boring. Darwin was also horrified by the surgeries he saw, which were performed without modern painkillers. Soon, he dropped out of school. Worried about his future, Darwin’s family arranged for him to study to become a clergyman. While at school, though, Darwin followed his interests and spent a lot of time working with a professor who studied plants. This professor soon recommended him to Captain Robert FitzRoy, who was about to set sail on his ship, the Beagle.

The primary goal of the Beagle’s voyage was to map the coast of South America. Figure 1 shows the route it took. Captain FitzRoy, who was about the same age as Darwin, invited Darwin along as a companion and the ship’s unpaid naturalist. As the ship’s naturalist, Darwin’s job would be to study nature—particularly the animals and plants they encountered on the voyage.

The Beagle departed from England in 1831, when Darwin was just 22 years old, and returned five long years later. It was not an easy trip—Darwin was extremely seasick at first. However, Darwin did find time to read. One book that he read was by the geologist Charles Lyell.

Lyell’s ideas turned out to be very important to Darwin later in the trip.

Meanwhile, Darwin adjusted to customs at sea. For example, he participated in a traditional initiation ceremony for sailors crossing the equator for the first time. As one of these sailors, Darwin was blindfolded, covered with pitch and paint, and dunked in water. Darwin wrote in his diary that he was “glad enough” when he was finally able to escape.

Once the Beagle reached South America, Darwin began his duties as the ship’s naturalist. He spent a lot of time on land, collecting fossils, plants, and animals and writing long notes describing them. Some of the fossil animals Darwin discovered are shown in the image in this section. Darwin was also struck by some unusual observations he made in South America. For example, he was surprised to find fossils of sea animals high in the mountains. How had they gotten there? An idea came to him when he experienced an earthquake in Chile that caused the land to rise up several feet. This reminded Darwin of something he had read in Lyell’s book while seasick. Lyell argued that some of Earth’s features resulted from gradual processes acting over long time periods. Could repeated small land movements, such as the one caused by the earthquake, raise former sea beds to the height of mountains over very long periods of time? Darwin would come back to this idea.

Meanwhile, the Beagle was about to arrive at the Galápagos Islands.

Darwin collected many unusual fossils in South America. This print from 1862 shows an artist’s reconstruction of three extinct mammals Darwin discovered—giant ground sloths (middle), glyptodons, a kind of giant armadillo (left), and an extinct species of hoofed mammal (top right).

This illustration appeared in Darwin’s original published account of the voyage of the Beagle. It shows the Beagle off the coast of South America.

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2. Darwin Visits the Galápagos Islands In 1835, the Beagle arrived at the Galápagos Islands, located in the Pacific Ocean about 1,000 kilometers from the coast of South America.

Darwin wrote to a friend that the Galápagos would give him a chance to have “a good look at an active volcano,” but it turned out he would find something even more interesting. What did Darwin discover in the Galápagos Islands?

Darwin was fascinated by the life he found in the Galápagos. As he explored the islands, he noticed that many living things there were unique to the islands—they were found nowhere else on Earth. What struck Darwin especially, though, was that many Galápagos species resembled mainland South American species. It almost seemed as if living things had somehow traveled from South America to the Galápagos and then changed to produce new Galápagos species.

Darwin found the many species of Galápagos finches particularly impressive and, long after the Beagle’s voyage was over, he spent time thinking about their traits. A trait is a characteristic of an organism.

Darwin saw that the Galápagos finches varied in the size and shape of their beaks, with each species of finch having a different beak, as shown in Figure 2. Darwin also realized that each beak was well suited to that finch’s diet. For example, finches that ate seeds had large, thick beaks for cracking seeds open. Those that ate insects had narrow beaks for grasping insects. In his account of the Beagle’s voyage, Darwin wrote that it seemed as if one original finch had been modified in different ways for different purposes. But how had this happened?

Darwin worked on answering this question for many years.

Figure 2 Different species of Galápagos finches have beaks of different sizes and shapes. These illustrations, originally published in Darwin’s account of the voyage of the Beagle, show four species of Galápagos finches. The top two species mainly eat seeds, and the bottom two species mainly eat insects.

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Galápagos Finches

3. Darwin Develops a Theory After the voyage of the Beagle, Darwin became a full-time scientist and naturalist. There were two big questions on his mind. Did living things change over time? And how did living things inherit the traits that made them so well suited to their environments?

The French naturalist Jean-Baptiste Lamarck had already argued that living things change over time. Lamarck’s idea was that organisms changed during their lifetimes to become better suited to living in their environments. They then had offspring that inherited these new traits. For example, if giraffes stretched their necks to eat the highest leaves on a tree, their necks would become longer. Then, their offspring would inherit these longer necks. However, there was not much evidence to support this method of change.

After the Beagle’s voyage, Darwin also began to consider the evidence that living things change over time. Darwin had observed that Galápagos species looked like modified versions of mainland South American species. He had also collected sloth fossils and glyptodon fossils that resembled present-day mammals. Had the fossil species changed over time and produced the modern species? But how could that happen? Changes like that had never been observed directly.

Darwin realized that changes in species occurred over very long time periods. Big changes, such as those between fossil sloths and modern sloths, could result from many small changes built up over long periods of time. Darwin owed this idea partly to Lyell. Just as Earth’s features could result from gradual processes acting over long time periods, changes in living things could result from the accumulation of many small changes over a long period of time.

But how do even small changes in living things happen? Darwin’s big breakthrough came when he read the work of Thomas Malthus, an economist. Malthus argued that there was never enough food to keep up with human population growth. As a result, humans would always suffer from famine and misery. Darwin realized that Malthus’s argument applied to all life. All living things were engaged in a constant “struggle for existence” because there was not enough food or resources for all of them to survive and reproduce.

But which individuals survive and reproduce? Do these individuals have specific traits that make them more successful? This led Darwin to his idea of natural selection.

Charles Darwin combined evidence from his own observations with ideas from other thinkers to work out his theory of evolution. This photo shows his study at Down House in Downe, England, where Darwin worked for over 40 years. Darwin’s microscope can be seen on the bench in the window, and his many books line the shelves.

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The individuals in a population show genetic variation in some of their inherited traits. Cactus finches, such as the one shown here, vary in traits such as body size and beak length. Could a shorter or longer beak affect the cactus finch’s ability to get food from a cactus flower?

4. Darwin’s Idea of Natural Selection What was Darwin’s idea of natural selection? What if your class went on a trip to the grocery store, and everyone selects an apple for lunch? You avoid the brown, mushy apples and select a crunchy, red apple. So do most of your classmates. When you compare your class’s apples to the original apples at the store, you find that your class’s apples are redder and crunchier on average. Darwin realized that a process of selection also causes changes in populations of living things over time. How does it work?

Consider a population of living things. A population is a group of individuals of the same species that live and reproduce in the same place. An example of a population is all the cactus finches living in the Galápagos Islands. These cactus finches live in the same area and mate with each other. But remember Darwin’s idea that there are not enough resources for every living thing to survive and reproduce. This is because individuals produce far more offspring than can possibly survive. Because of this, living things are in a constant “struggle for existence.” Which individuals will be able to survive and reproduce?

The individuals in a population are not all identical. They vary the same way the apples at the grocery store varied in color (red or brown) and texture (crunchy or mushy). Genetic variation describes differences in the inherited traits of different individuals. In the Galápagos cactus finch population, there is genetic variation in inherited traits such as body size and beak length. There is variation in body size because some finches are bigger while others are smaller. There is variation in beak length because some finches have longer beaks while other finches have shorter beaks.

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Lesson 24

Darwin proposed that natural selection acts on genetic variation in a population to produce changes in the population. Natural selection occurs when individuals in a population with certain inherited traits are more likely to survive and reproduce than individuals with less favorable traits in a specific environment. In the cactus finch population, finches with longer beaks might be better at getting pollen and nectar from cactus flowers than finches with shorter beaks. Because of this advantage, long-beaked finches might have a higher probability of surviving and reproducing. In other words, a greater proportion of long-beaked finches would survive and reproduce, while a smaller proportion of short-beaked finches would be able to do so. Just as your class favored crunchy red apples at the store, natural selection could favor cactus finches with long beaks.

Darwin reasoned that, over time, natural selection causes the inherited traits that make living things more likely to survive and reproduce to become more common in the population. If long-beaked cactus finches have a greater chance of surviving and reproducing, they will produce more offspring than short-beaked finches. Since the offspring of long-beaked finches will also inherit long beaks, long beaks will become more common in the population over time. Short beaks will become less common. The distribution of traits in the population—that is, how common each trait is—changes.

Natural selection is the key to Darwin’s theory of evolution. Evolution describes the inherited changes that occur in a population over time. Over many generations, natural selection causes populations of living things to evolve adaptations to their environment. An adaptation is an inherited trait evolved through natural selection that makes a population well suited to living and reproducing in its environment.

In the cactus finch population, a long beak is an example of an adaptation. This trait makes cactus finches better able to survive and reproduce in their environment. Of course, more than one cause may be responsible for the evolution of a trait. A long beak, for example, could be useful to cactus finches for many different reasons. Scientists may only be able to conclude that getting food is one likely cause for the origin of the trait. They may then conduct experiments in order to better understand the possible causes.

This cactus finch is peeking out from its nest. If a longer beak makes this finch better able to obtain food, it might be more likely to survive and reproduce than finches with shorter beaks. If so, then longer beaks would become more common in the population through natural selection.

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Key Science Concept

Evolution Through Natural Selection Three factors cause populations to change over time: 1. Genetic variation (different individuals have different inherited traits), 2. Natural selection (individuals with certain inherited traits are more likely to survive and reproduce than those with less favorable traits in a specific environment), 3. Evolution (inherited changes occur in a population over time). Now trace this process through two generations of ground finches.

1st Generation 1 Genetic Variation In a ground finch population on an island in the GalĂĄpagos, there is genetic variation in beak thickness, an inherited trait. Some ground finches have thinner beaks and some have thicker beaks.

2 Natural Selection The most plentiful food in this population’s environment is seeds with hard shells. Ground finches with thicker beaks are better able to crack the seeds. As a result, they obtain more food and are more likely to survive and reproduce. Here, the ground finch with the very thin beak dies of starvation. The surviving ground finches reproduce.

Survivors reproduce

3 Evolution Natural selection causes beak thickness in the population to evolve, that is, change over time. The average beak thickness in the population is thicker now than in the previous generation. The population is better adapted to eating the hard-shelled seeds in its environment.

2nd Generation

1 Genetic Variation In the next generation, the ground finch population still shows genetic variation in beak thickness, an inherited trait.

2 Natural Selection Ground finches with thinner beaks die. The surviving finches reproduce.

Survivors reproduce

3rd Generation 3 Evolution Average beak thickness in the population is even thicker, and the population is even better adapted to eating the hard-shelled seeds in its environment.

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5. Darwin Publishes His Work Once Darwin worked out his theory of evolution through natural selection, you might think that he would be eager to publish his work.

But he did not publish his full scientific findings for decades. Why? Darwin wanted to make sure that he had enough evidence to support his theory. He spent decades working out his ideas very carefully and collecting evidence to support them. A second reason, though, was that Darwin knew that his ideas would cause a furor.

He knew that they conflicted with what many people believed about whether or not organisms, including humans, changed over time.

What finally drove Darwin to publish his work? One day, Darwin received a letter from a younger naturalist named Alfred Russell Wallace. Wallace had independently come up with the idea of natural selection and wanted to know what Darwin thought of it. Devastated at the thought of losing credit for his ideas, Darwin was finally ready to present his work. Darwin and Wallace presented the idea of natural selection together in 1858. A year later, in 1859, Darwin published his theory of evolution in a book called On the Origin of Species by Means of Natural Selection. Today, Darwin’s theory is widely accepted by the scientific community.

Darwin’s On the Origin of Species

by Means of Natural Selection was published in 1859. This photo shows the cover page of the first edition.

LESSON SUMMARY

Darwin’s Theory of Evolution Through Natural Selection Darwin’s Voyage on the Beagle A key event in Charles Darwin’s life was his trip aboard the Beagle.

As the ship’s naturalist, Darwin studied the animals and plants they encountered on the voyage.

Darwin Visits the Galápagos Islands Darwin was struck by the living things he saw on the Galápagos Islands. He was especially interested in the Galápagos finches and their different beaks.

Darwin Develops a Theory As Darwin worked on his theory of evolution, he was inspired by the ideas of many other thinkers.

Darwin’s Idea of Natural Selection Natural selection says that individuals in a population with certain inherited traits are more likely to survive and reproduce than individuals with less favorable traits.

Natural selection causes populations to evolve and become better adapted to living and reproducing in their environments.

Darwin Publishes His Work Darwin spent decades working on his theory of evolution before publishing it. Darwin’s ideas are widely accepted today.

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