Hippocampus Biology: Darwin's Theory of Evolution

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Charles Darwin was born in England in 1809. While attending Cambridge University, where he was studying to become a clergyman, he became the student of the botanist Reverend John Henslow. Henslow knew Darwin was interested in nature, and suggested that Darwin go on the worldwide British science expedition aboard the steamship HMS Beagle. From 1831 to 1836, Darwin traveled on the Beagle and studied the plants and animals he found. Darwin's most important observations were made while traveling along the coast of South America and during his time in the Galapagos Islands.

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The Galapagos Islands are located about 500 miles from the west coast of South America. Darwin noticed that while many of the Galapagos organisms were unique to the islands, they resembled the organisms of South America. Many organisms were even specific to a single island but still similar to mainland organisms.

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Darwin collected a large amount of information on the 13 species of finches on the islands. He found that a finch's beak was adapted for the food supply on its home island. Some islands had plants with large, hard-shelled seeds that fell to the ground. The finches on these islands had large, powerful beaks. Other islands had small seeds that had to be picked out of cacti. The finches on these islands had small, thin beaks that enabled them to get at the seeds. Some finches fed mainly on insects or insect larvae. Their beaks were adapted to help them catch their prey. Darwin catalogued the different beaks and how they related to the food source of the finch species.

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While on the Beagle, Darwin read Lyell's paper suggesting that the Earth is very old and constantly changing. Darwin thought that constant change, driven by adaptation to the environment, could also be occurring in organisms, like the finches. He thought the process of adaptation was related to the formation of new species.

When two populations of the same species were isolated from each other, they would adapt to their new environments and become increasingly dissimilar. Eventually the populations would diverge into different species. This would explain why the finches of the Galapagos Islands were similar, but not identical, to the finches on the mainland.

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In 1844, Darwin wrote an essay about his theory of evolution, which he called descent with modification. He believed that all organisms are descended from a single unknown prototype. Over time, organisms have accumulated modifications that make each species unique.

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Darwin proposed that evolution occurs through a process called natural selection. Let's use the length of a giraffe's neck as an example. The basic idea of natural selection is that there is variation within a population, like the variation we see here between neck lengths. In any species, individuals with advantageous traits produce more offspring. The giraffes with longer necks can find more food, so they will survive and produce more long-necked giraffes. The unequal ability of organisms to survive and reproduce leads to gradual changes in a population.

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There are constraints to the process of natural selection. Natural selection only works on variations present in a population of organisms. If all giraffes had the same neck length, natural selection couldn't select for longer necks. Furthermore, natural selection affects only heritable traits—traits that can be passed on to offspring.

If, as Lamarck proposed, a giraffe's neck stretched over its lifetime from reaching for food, the longer neck wouldn't be passed on to the next generation. The longer neck was acquired during the giraffe's lifetime and is not heritable. And natural selection causes changes in a population of organisms. But it doesn’t cause the characteristics of a particular individual to change during its lifetime. A particular giraffe can't evolve to have a longer neck. But over time, a population can evolve to have a larger number of long-necked giraffes.