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May 5, 2000

How does Darwin's theory of natural selection explain the evolution of the Galapagos finches?

Darwin proposed that over a long period of time, species gradually evolve to suit their changing environment, becoming better equipped for their habitat and lifestyle. The Galapagos archipelago is ideal for geographic isolation and speciation. Thirteen species of finches have evolved in the islands from one original species. Perhaps originally, a small flock of birds from the mainland established themselves on an island, eventually becoming different from the original population. Some of these 'new' birds may then have flown off, establishing colonies on neighboring islands and becoming different again.

Due to natural mutation, and as long as the mutation offers a survival advantage in terms of where the finches feed, how they feed and what they feed on, a new species may take advantage of an unoccupied 'niche' in the ecology of an island. For the Darwin finches, the morphological changes can be seen in the size and shape of the beak, which indicate diet.

If a mutation is advantageous, an individual is more likely to breed successfully, thus the same advantage may be inherited by the offspring. Gradually, the advantage is transferred to the entire population over generations, until it is considered a new species of finch. This is evolution through natural selection.

In the Galapagos, this process has occurred repeatedly, forming the thirteen species of Darwin's finches classified today. These include the Ground and Tree finches, the Cactus, Carpenter, Woodpecker, Warbler, Mangrove and Vegetarian finches. Some species live in perfect harmony on the same island, each with its specific function and habitat, ensuring no competition for food.

Listen to Roger Payne's Voice from the Sea piece entitled:
How Finches and Tortoises Contributed to the Theory of Evolution

Read about scientist Dave Anderson's bird studies in the Galapagos in the Scientific American Frontiers: Ask A Scientist feature on PBS Online.

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