I can explain the development of the theory of evolution by natural selection
The 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 represents a giant leap in human understanding. It explains and unifies all of biology.
An overview of evolution can be seen at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/0/GcjgWov7mTM (17:39).
Darwin’s theory of evolution 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.
In 1831, when Darwin was just 22 years old, he set sail on a scientific expedition on a ship called the HMS Beagle. He 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 in Figure below. You can learn more about Darwin’s voyage at this link: http://www.aboutdarwin.com/voyage/voyage03.html.
Voyage of the Beagle. This map shows the route of Darwin
Darwin was fascinated by nature, so he loved his job on the Beagle. He spent more than 3 years of the 5-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.
During the long voyage, Darwin made many observations that helped him form his theory of evolution. For example:
On his voyage, Darwin saw giant marine iguanas and blue-footed boobies. He also dug up the fossil skeleton of a giant ground sloth like the one shown here. From left: Giant Marine Iguana, Blue-Footed Boobies, and Fossil Skeleton of a Giant Ground Sloth
Darwin’s most important observations were made on the Galápagos Islands (see map in Figure below). This is a group of 16 small volcanic islands 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, while those on another island had dome-shaped shells (see Figure below). People who lived on the islands could even tell the island a turtle 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.
Science, 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.
These 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 animals were allowed to reproduce, they could change an organism’s traits. The pigeons in 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.
Artificial Selection in Pigeons. Pigeon hobbyists breed pigeons to have certain characteristics. All three of the pigeons in the bottom row were bred from the common rock pigeon.
Did 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. He also traveled to distant places to study nature. Wallace wasn’t as famous as Darwin. However, he developed basically the same theory of evolution. While working in distant lands, Wallace sent Darwin a paper he had written. In the paper, Wallace explained his evolutionary theory. This served to confirm what Darwin already thought.
Darwin spent many years thinking about the work of Lamarck, Lyell, and Malthus, what he had seen on his voyage, and artificial selection. What did all this mean? How did it all fit together? It fits together in Darwin’s theory of evolution by natural selection. It’s easy to see how all of these influences helped shape Darwin’s ideas.
For a discussion of the underlying causes of natural selection and evolution see http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/5/DuArVnT1i-E (19:51).
It took Darwin years to form his theory of evolution by natural selection. His reasoning went like this:
Wallace’s paper not only confirmed Darwin’s ideas. They pushed him to finish his book, On the Origin of Species. Published in 1859, this 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.
Darwin's Theory of Evolution through Natural selection can be summarized in four basic steps:
The following example describes how these steps could have changed a population of giraffes.
African Giraffes. Giraffes feed on leaves high in trees. Their long necks allow them to reach leaves that other ground animals cannot.
As this example shows, chance variations may help a species survive if the environment changes. Variation among species helps ensure that at least one will be able to survive environmental change.A summary of Darwin's ideas are presented in the Natural Selection and the Owl Butterfly video: http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/3/dR_BFmDMRaI (13:29).