Biology (B)

Darwin’s Observations

Most people in the world did not become aware of the theory of evolution until 1859, when Charles Darwin published his book On the Origin of Species by Means of Natural Selection. This book described the observations and evidence that he collected over 20 years of intensive research, beginning with a five-year voyage around the world on a British research ship, the HMS Beagle. During this five-year voyage, Darwin was able to make observations about plants and animals spread around the world, and to collect specimens to study when he returned to England. Each time the Beagle stopped at a port to do some trading, Darwin went on land to explore and look for the local plants, animals, and fossils. One of the most important things Darwin did was to keep a diary. He took extremely detailed notes and drawings about everything he saw as well as his thoughts.

The Galápagos Islands

The around the world voyage of the HMS Beagle was mostly to map the coastline of South America. Darwin’s best known discoveries were made on the Galápagos Islands, a group of 16 volcanic islands near the equator about 600 miles from the west coast of South America. Darwin was able to spend months on foot exploring the islands. Darwin’s Theory of Evolution by Natural Selection was a result of his observations and over 20 years of examining the specimens he had collected and sent back to England, many of which came from these islands.

Darwin was amazed by the array of life he saw on the Galápagos Islands. He saw animals unlike anything he had ever seen before. Darwin was struck by how the same kind of animal differed from one island to another. For example, the iguanas (large lizards) differed between islands. The members of one iguana species spent much of their time swimming and diving underwater for seaweed, while those of another iguana species lived on land and ate cactus. In England, he was accustomed to watching cormorants fly, so he was surprised to find flightless cormorants on the islands alongside flying cormorants.

Giant Tortoises

Giant tortoises, large enough for two men to ride on, plodded across the islands and foraged on super tough leaves. Some of the tortoise species were found on only one island. Darwin was fascinated by the number of ways that organisms were well-suited to their environments. Even the tortoise shells were specially adapted to the conditions. Tortoises that ate plants near the ground had rounded shells, while the tortoises that stretched their necks to reach plants higher in shrubs had shells that bent upwards, allowing them to stretch their necks upward.

Darwin's Finches

The most extensively studied animals on the Galápagos are the finch species (birds). When Darwin first observed the finches on the islands, he did not even realize they were all finches. But when he studied them further, he realized they were all the same type of bird, and that each island had its own distinct species of finch. The birds on different islands had many similarities, but their beaks differed in size and shape.

In his diary, Darwin pointed out how each animal is well-suited for its particular environment. The shapes of the finch’s beaks on each island were well-matched with the seeds available on their particular island, but not the seeds on other islands. A larger and stronger beak was needed to break open large seeds and a small beak was needed to feed on some of the smallest seeds.

Darwin also noticed how different species were distributed around the world. The finch, tortoise and other species found on the Galápagos Islands were similar to species on South America, the nearest continent. Yet they also differed. Likewise, species he saw on islands near Africa were similar to, but different from species on Africa.

Return to England

When Darwin returned to England five years later, he did not rush to announce his discoveries. Unlike other naturalists before him, Darwin did not want to present any ideas unless he had strong evidence supporting them. Instead, once Darwin returned to England, he spent over twenty years examining specimens, talking with other scientists and collecting more information before he presented his theories. Darwin’s observations eventually resulted in the Theory of Evolution by Natural Selection. His now famous book, The Origin of Species is a diary of his explorations and discussion on how he interpreted his observations.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/