Darwin’s Voyage of Discovery Read the lesson title aloud to students.

Learning Objectives State Charles Darwin’s contribution to science. Describe the three patterns of biodiversity noted by Darwin. Click to reveal each of the learning objectives. Play a free-association word game to activate students’ prior knowledge of Charles Darwin. Call on students at random to say the first word that comes to mind when you say each of the following terms: Charles Darwin, Beagle, Galápagos Islands. Explain that students will learn about Darwin’s voyages and contributions to science.

Darwin’s Epic Journey Ask: What kind of student do you think Charles Darwin was? Answer: He wasn’t a star student. Explain that he preferred nature watching and hunting over studying. His father complained, “You care for nothing but shooting, dogs, and rat-catching, and you will be a disgrace to yourself and all your family.” Yet Darwin would one day come up with what has been called “the single best idea that anyone has ever had”. How did this happen? Tell students that Charles Darwin was born in England on February 12, 1809—the same day as Abraham Lincoln. Eager to see the world, he got the chance in 1831 when he was invited to join the HMS Beagle on the five-year voyage. The captain’s job was to map coastlines and harbors. Darwin was added to keep the captain company. No one knew it, but this would become one of the most important scientific voyages in history. Explain that the Beagle set sail at a time when scientists were revolutionizing views of the natural world. Geologists were suggesting that Earth was ancient and had changed over time. Biologists were suggesting that life had also changed—a process called evolution. Tell students that Darwin developed a theory of biological evolution that offered a scientific explanation for the unity and diversity of life, by proposing how modern organisms evolved through descent from common ancestors. Darwin’s work is so important because it revealed that the living world changes continuously. Evolutionary theory helps us understand and respond to important events such as the emergence of drug-resistant bacteria and new strains of influenza. It helps us to predict the dangers we face if human actions were to drive too many species to extinction. Ask students to define “scientific theory”. Answer: a scientific theory is a well-established, testable scientific explanation of events in the natural world.

Observations Aboard the Beagle Explain that Charles Darwin was fascinated by the diversity of life he saw during his trip. During a single day in a Brazilian forest he collected 68 species of beetles—and he wasn’t looking for beetles! He was intrigued by how well suited to their local environments plants and animals seemed to be. He was impressed by the many ways different organisms obtained food, protected themselves, and produced offspring. He was also puzzled by where different species lived—and did not live. He filled his notebooks with observations. Tell students that Darwin wanted to explain this diversity in a scientific way. He kept observing, asking questions, and formulating hypotheses, seeking larger patterns into which his observations might fit. He came to focus on three patterns of diversity: (1) species vary globally, (2) species vary locally, and (3) species vary over time. Ask: How did the geography of Darwin’s voyage give him far greater exposure to species variability than his fellow scientists back home had? Answer: Darwin crossed many lines of latitude in his voyage (from London, 51º 25’ N, to Cape Horn, 55º 90’ S). This gave him a chance to observe how species had adapted to different biomes and also how species had adapted to the same biomes in different parts of the world. As such, Darwin was exposed to much more variability than were his fellow scientists. Ask: Why is it significant that many of the stops the Beagle made were in tropical regions? Answer: The relatively warm and wet climate of the tropics tends to correlate with high biodiversity. Being in tropical regions exposed Darwin to countless interesting and unfamiliar species. Explain that Darwin noticed that different, yet ecologically similar, species inhabited separate, but ecologically similar, habitats around the globe. In the grasslands of South America, he found flightless ground-dwelling birds called rheas. Rheas look and act a lot like ostriches. Then, when Darwin visited Australia’s grasslands, he found another large flightless bird, the emu. Darwin also noticed that rabbits and other European grassland species didn’t live in similar grasslands in South America and Australia. Australian grasslands, on the other hand, were home to kangaroos and other animals found nowhere else. Discuss in greater detail the example of rheas, ostriches, and emus. Show the class visuals of the three types of birds and their habitats, and call on students to point out how the birds are adapted to their environments. Ask students to infer why large flightless birds are found in grassland habitats around the globe.

Species Vary Locally Tell students that Charles Darwin noticed that different, yet related, species often occupied different habitats within a local area. For example, Darwin found two species of rheas in South America. One thrived in Argentina’s grasslands, while a smaller species was adapted to the colder, harsher grass- and scrublands to the south. Other examples of local variation came from the Galápagos Islands, about 1000 kilometers off the Pacific coast of South America. Explain that these islands are close to one another yet have different ecological conditions. Several islands were home to distinct forms of giant land tortoises. Darwin saw differences among the tortoises but didn’t think much about them. In fact, like other travelers, Darwin ate several tortoises and tossed their remains overboard without studying them closely! Then Darwin learned from the islands’ governor that the tortoises’ shells varied in predictable ways from one island to another. Someone who knew the animals well could identify which island an individual tortoise came from, just by looking at its shell. Direct student attention to the tortoise images. Call on students to describe differences between the two tortoises. Discuss how the differences between the tortoises are related to the differences in their environments. Challenge students to infer, based on the discussion, why groups of islands like the Galápagos are good places to study evolution. Ask: What other groups of islands might be good places to study evolution? Sample answers: the Hawaiian Islands, the islands of the Caribbean

Species Vary Over Time Armadillo 0.5 m 3 m Glyptodonts Tell students that Charles Darwin also collected fossils. Scientists already knew that these remains formed a fossil record that told a story of organisms no longer living, although researchers didn’t yet know how to read and interpret that record. Darwin noted that the fossil record included many extinct animals that were similar to, yet different from, living species. The most striking examples were fossils of extinct giant armored animals called glyptodonts. Glyptodonts look like giant versions of modern armadillos, which live in the same area. Darwin wrote: “This wonderful relationship in the same continent between the dead and the living, will, I do not doubt, hereafter throw more light on the appearance of organic beings on our earth, and their disappearance from it, than any other class of facts.” Ask: What similarities and differences do you see between these two animals? Sample answer: The armadillo is much smaller than the glyptodont, but the two organisms have similar shells, tails, and body shapes. 3 m Glyptodonts

Putting the Pieces of the Puzzle Together Tell students that on the voyage home, Charles Darwin thought about the patterns he’d seen. When he returned to London, he sent his specimens to experts for identification. They set the scientific community abuzz. His Galápagos mockingbirds belonged to three separate species found nowhere else! The little brown birds Darwin thought were wrens, warblers, and blackbirds were actually all species of finches! They, too, were found nowhere else, although they all resembled a single common finch species from South America. Yet the island species were all different from the mainland species and from each other. The same was true of tortoises, marine iguanas, and many plants that Darwin collected. Darwin was stunned by these discoveries. Explain that the evidence caused him to wonder if species might not be fixed and unchanging, as people thought back then. Could organisms change over time, through some natural process? Could Galápagos species have evolved from common South American ancestors? He spent years actively researching and filling notebooks with ideas about species and evolution.