1. Darwin Presents His Case
Read the lesson title aloud to students.

2. Learning Objectives
Describe the conditions under which natural selection occurs.
Explain the principle of common descent.
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Print a blank family tree, readily available online, and share it with the class. Explain how the tree can be used to show an individual’s descent from his or her ancestors.
Discuss how a family tree is similar to an evolutionary tree. Tell students Charles Darwin created the first known evolutionary tree, which they will learn about in this lesson.

3. Darwin Publishes
Tell students that Charles Darwin worked out the main elements of his theory about natural selection soon after reading Thomas Malthus and thinking about artificial selection. His scientific friends urged him to publish his brilliant arguments. Darwin did write up a complete draft of his ideas, but he put the work aside and didn’t publish it for another 20 years because he knew that many scientists, including some of his own teachers, ridiculed Jean-Baptiste Lamarck’s ideas. Darwin’s theory was just as radical, so he wanted to gather as much evidence as he could to support his ideas before he made them public.
Explain that, in 1858, Darwin reviewed an essay by Alfred Russel Wallace, a British naturalist working in Malaysia. Wallace’s thoughts about evolution were almost identical to Darwin’s! Not wanting to get “scooped,” Darwin moved forward with his own work.
Tell students that Wallace’s essay was presented together with some of Darwin’s observations at a scientific meeting in The next year, Darwin published his work, On the Origin of Species, which described his ideas in detail. Wallace had the right idea, but Darwin had data and observations to support his hypotheses. Darwin’s contribution was to describe a natural process, a scientific mechanism, that could operate like artificial selection. In On the Origin of Species, he supported his ideas with arguments from Malthus and Lamarck.

4. The Struggle for Existence
Tell students that Thomas Malthus’s work had convinced Charles Darwin that if all populations have the potential to produce more offspring than can survive, members of that population will compete for a finite supply of environmental resources. Darwin described this competition as the struggle for existence. But which individuals would succeed in surviving and reproducing?
Explain that here is where natural inheritable variation took center stage in Darwin’s thinking. He hypothesized that individuals with certain types of inherited variation are better suited, or adapted, to life in their environment than others. Members of a predatory species that are faster or have longer claws or sharper teeth can catch more prey. And members of a prey species that are faster or better camouflaged can avoid being caught.
Draw students’ attention to the image.
Ask students to explain how this image demonstrates these concepts.
Answer: A scorpion fish’s coloring is an example of camouflage—an adaptation that allows an organism to blend into its background and avoid predation.

5. Variation and Adaptation
Scarlet king snake
Tell students that any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment is called an adaptation. Adaptations can involve body parts or structures, like a tiger’s claws; colors, like those that make camouflage or mimicry possible; or physiological functions, like the way a plant carries out photosynthesis. Many adaptations also involve behaviors, such as the complex avoidance strategies prey species use.
Click to reveal the first label.
Ask: Are these two snakes the same species or two different species?
Answer: two different species.
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Explain that the scarlet king snake is exhibiting mimicry—an adaptation in which an organism copies, or mimics, a more dangerous organism.
Ask: How might mimicry be adaptive for the scarlet king snake?
Answer: Although the scarlet king snake is harmless, it looks like the poisonous eastern coral snake, so predators avoid it, too.
Eastern coral snake

6. Survival of the Fittest
Ask students to describe how this image shows possible adaptations.
Answer: The fox is adapted for hunting. One protective adaptation of the crane is this defensive behavior meant to scare off predators.
Tell students that Charles Darwin, like Jean-Baptiste Lamarck, recognized that there must be a connection between the way an organism “makes a living” and the environment in which it lives. According to Darwin, differences in adaptations affect an individual’s fitness. Fitness describes how well an organism can survive and reproduce in its environment.
Explain that individuals with adaptations that are well suited to their environment can survive and reproduce and are said to have high fitness. Individuals with characteristics that are not well suited to their environment either die without reproducing or leave few offspring and are said to have low fitness. This results in differential reproductive success or, as some have termed it, survival of the fittest. Note that survival here means more than just staying alive. In evolutionary terms, survival means reproducing and passing adaptations on to the next generation.
Ask: If an organism produces many offspring, but none of them reaches maturity, do you think the organism has high or low fitness? Explain your answer.
Answer: If an organism produces many offspring, but none of them reaches maturity, then it has low fitness, because its offspring will not pass on their traits to the next generation.
Make sure students understand the terms adaptation and fitness, because these terms are basic to their comprehension of the process of natural selection. Have students apply these concepts to specific cases. Ask them to describe examples of adaptations and explain how they increase the fitness of the organisms in which they occur.

7. Natural Selection
Tell students that Charles Darwin named his mechanism for evolution natural selection because of its similarities to artificial selection. Natural selection is the process by which organisms in nature with variations most suited to their local environment survive and leave more offspring. In both artificial and natural selection, only certain individuals produce offspring. But in natural selection, the environment—not a farmer or animal breeder—influences fitness.
Explain that natural selection occurs in any situation in which more individuals are born than can survive (the struggle for existence), natural heritable variation affects the ability to survive and reproduce (variation and adaptation), and fitness varies among individuals (differential reproductive success). Well-adapted individuals survive and reproduce. From generation to generation, populations continue to change either as they become better adapted or as their environment changes. Note that natural selection acts only on inherited traits because those are the only characteristics that parents can pass on to their offspring.
Direct students’ attention to the image, which shows a hypothetical example of natural selection.
Ask students to guess how many eggs grasshoppers can lay at one time.
Answer: Grasshoppers can lay more than 200 eggs at a time.
Ask: Will all of these offspring survive?
Answer: Only a small fraction of these offspring survive to reproduce.
Remind students that this is one piece of evidence for evolutionary theory: Organisms produce more offspring than can survive.
Click to reveal the next image.
Ask students to explain the component of evolutionary theory shown here.
Answer: There is variation in nature, and certain heritable variations—called adaptations— increase an individual’s chance of surviving and reproducing. In this population of grasshoppers, heritable variation includes yellow and green body color. Green coloration is an adaptation: green grasshoppers blend into their environment and so are less visible to predators.
Ask students to describe the image.
Answer: Because their green color serves to camouflage them from predators, green grasshoppers have a higher fitness than yellow grasshoppers. This means that green grasshoppers survive and reproduce more often than do yellow grasshoppers in this environment.
Ask students to describe how this set of images demonstrates natural selection and fitness.
Answer: Green grasshoppers become more common than yellow grasshoppers in this population over time because: (1) more grasshoppers are born than can survive, (2) individuals vary in color, and color is a heritable trait, and (3) green individuals have a higher fitness in their current environment.
Extend the example by having students apply the same concepts to somewhat different conditions. For each step in the figure, describe an alternative situation, and ask students to explain how natural selection would be affected. For example, ask them to explain how selection might be different if there were brown grasshoppers in addition to yellow and green grasshoppers.

8. No Such Thing as Perfect
Explain that natural selection does not make organisms “better.” Adaptations don’t have to be perfect—just good enough to enable an organism to pass its genes to the next generation. Natural selection also doesn’t move in a fixed direction. There is no one perfect way of doing something, as demonstrated by the diverse styles of pollination.
Tell students that if local environmental conditions change, some traits that were once adaptive may no longer be useful, and different traits may become adaptive. This can lead to a great diversity of adaptations in species living in different environments. And if environmental conditions change faster than a species can adapt to those changes, the species may become extinct. We now know that natural selection is not the only mechanism driving evolution.
Explain that many different styles of pollination have evolved among flowering plants. Oak tree flowers are pollinated by wind. Apple tree flowers are pollinated by insects. Neither method is “better” than the other. Both kinds of pollination work well enough for these plants to survive and reproduce in their environments.
Students commonly have the misconception that challenges to Charles Darwin’s view of evolution as a slow, steady process mean that natural selection is no longer accepted by scientists. Make sure students understand that evolution by natural selection is not in dispute. Rather, it has just been extended to include observations of evolution occurring at a relatively rapid rate under certain conditions. In other words, Darwin’s theory has been modified and improved, but his basic principles remain unchallenged within the scientific community.
Ask students to give at least two reasons why the following statement is not true: “The goal of natural selection is to produce perfect organisms.”
Sample answer: Because natural selection is a natural process, it cannot have goals. Further, natural selection could never produce “perfect” organisms because environments change, and no organism is “better” or “more perfect” than any other in adjusting to future changes.

9. Common Ancestry
Tell students that natural selection depends on individuals’ ability to reproduce and leave descendants. Just as well-adapted individuals in a species survive and reproduce, well-adapted species survive over time. Charles Darwin also proposed that living species are descended, with changes over time, from common ancestors—an idea called descent with modification.
Explain that, over many generations, diverse adaptations to changing resources that are limited could cause a single species to split into two or more new species. For evidence of descent with modification over long periods of time, Darwin pointed to the fossil record. Note that this process requires enough time for all this descent with modification to occur!
This reminds us how James Hutton and Charles Lyell’s work supported Darwin’s theory: Deep time gave enough time for natural selection to act. Darwin’s idea that natural selection and adaptation can produce new species offered an explanation for the diversity of life shown by the very first evolutionary tree. This “tree-thinking” implies that all organisms are related. Look back in time and you will find common ancestors shared by tigers, lions, and cheetahs. Look further back and you will find ancestors that these felines share with dogs, then horses, and then bats. Further back still is the common ancestor that all mammals share with birds, reptiles, and fish. Far enough back are the common ancestors of all living things.
Tell students that, according to the principle of common descent, all species—living and extinct—are united by descent from ancient common ancestors, and exhibit diversity due to natural selection and adaptation. A single “tree of life” links all living things.
Explain that this page from one of Darwin’s notebooks shows the first evolutionary tree ever drawn. This sketch shows Darwin’s explanation for how descent with modification could produce the diversity of life. Note that, just above the tree, Darwin wrote, “I think.”

10. Comparing Terminology
Natural selection vs. evolution
Natural selection vs. artificial selection
Adaptation vs. fitness
Natural selection is a mechanism of evolution.
In artificial selection humans select those variants they find useful. In natural selection organisms that are fitter will produce more offspring.
Tell students that natural selection and evolution are often used interchangeably—but they are not the same thing.
Have students work in pairs to write a sentence that describes the difference between these concepts.
Ask a volunteer to share their answer. Discuss this and other answers.
Answer: Natural selection is a mechanism of evolution.
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Make sure students understand that organisms can evolve in other ways, too: through random mutation, lateral gene transfer, genetic drift, and gene shuffling in sexual reproduction.
Have students work in pairs to write a sentence that describes the difference between the next two sets of terms.
Ask volunteers to write their answers on the board. Discuss this and other answers.
Answer: In artificial selection humans select those variants they find useful. In natural selection organisms that are fitter will produce more offspring.
Answer: Adaptations can increase an organism’s fitness.
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Adaptations can increase an organism’s fitness.