The Grand Canyon, shown in Figure \(\PageIndex{1}\), is an American icon and one of the wonders of the natural world. It is also a record of the past. Look at the rock layers in the picture. If you were to walk down a trail to the bottom of the canyon,
with each step-down, you would be taking a step back in time. That’s because lower layers of rock represent the more distant past. The rock layers and the fossils they contain show the prehistory of the region and its organisms over a 2-billion-year time span. Although Charles Darwin never visited the Grand Canyon, he saw rock layers and fossils in other parts of the world. They were one inspiration for his theory of evolution. Darwin’s theory rocked the scientific world. In this concept, you
will read why.
What is the Theory of Evolution by Natural Selection?Eighteenth-century Englishman Charles Darwin is one of the most famous scientists who ever lived. His place in the history of science is well deserved. Darwin’s theory of evolution by natural selection represents a giant leap in human understanding. It explains and unifies all of biology. Darwin’s theory actually contains two major ideas:
In Darwin’s day, most people believed that all species were created at the same time and remained unchanged thereafter. They also believed that Earth was only 6,000 years old. Therefore, Darwin’s ideas revolutionized biology. How did Darwin come up with these important ideas? It all started when he went on a voyage. Voyage of the BeagleIn 1831, when Darwin was just 22 years old, he set sail on a scientific expedition on a ship called the HMS Beagle. Darwin was the naturalist on the voyage. As a naturalist, it was his job to observe and collect specimens of plants, animals, rocks, and fossils wherever the expedition went ashore. The route the ship took and the stops they made are shown on the map below. Darwin was fascinated by nature, so he loved his job on the Beagle. He spent more than three years of the five-year trip exploring nature on distant continents and islands. While he was away, a former teacher published Darwin’s accounts of his observations. By the time Darwin finally returned to England, he had become famous as a naturalist. Figure \(\PageIndex{2}\): Voyage of the Beagle. This map shows the route of Darwin’s 5-year voyage on the HMS Beagle. Each stop along the way is labeled. The route starts with Sydney. Along the way they stopped at Hobart, King George's Sound, Cocos island, Mauritius, Cape town, Bahia, Azores, Plymouth, Tenerife, Bhaia again, Rio de Janeiro, Montevideo Islands, Valparaiso, Callao Lima, Galapagos, and then back to Sydney. Darwin and the others on board eventually circled the globe.Darwin’s ObservationsDuring the long voyage, Darwin made many observations that helped him form his theory of evolution. For example:
The Galápagos IslandsDarwin’s most important observations were made on the Galápagos Islands (shown on the map above of the Beagle voyage). The Galápagos Islands are a group of 16 small volcanic islands that are 966 kilometers (600 miles) off the west coast of South America. Individual Galápagos islands differ from one another in important ways. Some are rocky and dry; others have better soil and more rainfall. Darwin noticed that the plants and animals on the different islands also differed. For example, the giant tortoises on one island had saddle-shaped shells, whereas those on another island had dome-shaped shells, as you can see in the photos below. People who lived on the islands could even tell which island a tortoise came from by its shell. This started Darwin thinking about the origin of species. He wondered how each island came to have its own type of tortoise. Other Influences on DarwinScience, like evolution, always builds on the past. Darwin didn’t develop his theory completely on his own. He was influenced by the ideas of earlier thinkers. Writings of Earlier ScientistsThree scientists whose writings influenced Darwin were Lamarck, Lyell, and Malthus.
Knowledge of Artificial SelectionThese weren’t the only influences on Darwin. He was also aware that humans could breed plants and animals to have useful traits. By selecting which plants or animals were allowed to reproduce, they could change an organism’s traits over time. The pigeons in the figure below are good examples. Darwin called this type of change in organisms artificial selection. He used the word artificial to distinguish it from natural selection. Darwin Develops His TheoryDarwin spent many years thinking about the work of Lamarck, Lyell, and Malthus; what he had seen on his voyage; and what he knew about artificial selection. What did all this mean? How did it all fit together? Eventually, it all came together in his theory of evolution by natural selection. It’s easy to see how these influences helped shape Darwin’s ideas, although it actually took Darwin years to formulate his theory. His reasoning went like this:
It's Wallace’s Theory TooDid you ever hear the saying that “great minds think alike?” It certainly applies to Charles Darwin and another English naturalist named Alfred Russel Wallace. Wallace lived at about the same time as Darwin and also traveled to distant places to study nature. Wallace wasn’t as famous as Darwin, but he developed basically the same theory of evolution. While working in what is now Malaysia, Wallace sent Darwin a paper he had written explaining his evolutionary theory. Wallace's ideas served to confirm what Darwin already thought. It also pushed Darwin to finish and publish his book, On the Origin of Species. Published in 1859, the book changed science forever. It clearly spelled out Darwin’s theory of evolution by natural selection and provided convincing arguments and evidence to support it. Applying Darwin's and Wallace’s TheoryThe following example applies Darwin’s and Wallace's theory of evolution by natural selection. It explains how giraffes came to have such long necks, like those shown in the photo below.
Feature: Reliable SourcesIn the first chapter of his book On the Origin of Species, Charles Darwin discussed how artificial selection, also called selective breeding, had been successful in changing the traits of animals, including pigeons, cats, cattle, and dogs. He used this discussion as a springboard to introduce his idea of natural selection as well as to provide support for it. The use of selective breeding to change the traits of other species has a very long history. In fact, archaeological evidence indicates that selective breeding of both plants and animals began as early as 10,000 years ago in the Middle East when previous hunter-gatherers began to domesticate animals and cultivate cereal plants. Around this time, changes in climate led to increasing drought, which forced people to concentrate around permanent water sources. These population concentrations could not be supported by wild animals and plants in the vicinity, providing a stimulus for the invention of agriculture and the use of selective breeding to increase the amount of available food. For thousands of years, species of plants such as wheat and rice and of animals such as goats and sheep were selectively bred and changed from their wild ancestors. In the New World, the wild grain called teosinte, pictured on the left in Figure \(\PageIndex{7}\), was selectively bred by Native Americans to produce larger and more numerous edible kernels. The result was modern maize (commonly called corn), shown on the right in the same picture. Teosinte was very small with fewer grains on it. The modern corn is bulky and with a lot more grain on it. After maize was created, it spread across the Americas and was introduced to Europe by European explorers and traders. Today, maize is still a dietary staple and the most widely grown grain crop in the Americas. The wild ancestors of domesticated wheat and rice were easy to identify because the modern species resemble their wild counterparts. However, that wasn't the case with maize, which looks very different from teosinte. Maize also appeared quite suddenly in the archaeological record, so its origin has been of special interest. Go online to learn more about the selective breeding of teosinte to maize. Use only reliable sources such as university websites to find answers to the following questions:
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What are the evidences that support Darwin's theory of evolution?Darwin used multiple lines of evidence to support his theory of evolution by natural selection -- fossil evidence, biogeographical evidence, and anatomical evidence.
What is the best evidence of evolution similarities?Comparing DNA
Darwin could compare only the anatomy and embryos of living things. Today, scientists can compare their DNA. Similar DNA sequences are the strongest evidence for evolution from a common ancestor.
What does Darwin's theory of evolution explain about the natural world?Darwin proposed that species can change over time, that new species come from pre-existing species, and that all species share a common ancestor. In this model, each species has its own unique set of heritable (genetic) differences from the common ancestor, which have accumulated gradually over very long time periods.
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