Explain the history of Darwin's Finches?

The most well known evidence upon which Darwin based his theory was the distribution pattern of the Galapagos finches. The Beagle's voyage included stops in the Galapagos Islands, about 900 kilometers (560 miles) off the west coast of Ecuador in South America. The primary purpose of the voyage was to chart the coastal waters, but Darwin, as ship's naturalist, combed the Brazilian coast and the islands, collecting specimens at every chance.

Darwin noticed that the flora and fauna of Brazil differed markedly from those in other parts of the world. Furthermore, he observed that although animals living in the Galapagos were to be found nowhere else in the world, they did appear to be most closely related to those on the South American continent. This was true of the finches Darwin collected. Darwin collected 13 different species of finches that, while bearing close resemblance to each other, were distinctly different.

Darwin theorized that the following process must have occurred. The original ancestral finches were probably blown from South America to the Galapagos Islands by tropical storms. Once there, some birds "island-hopped" to the other islands, but were for the most part isolated because finches are generally not sea-going creatures. Over many years, the isolated populations gradually adapted to the different environmental conditions prevailing on each island.

For instance, the food sources available to the finches on the islands varied. Some islands had an abundance of plants that produced seeds with thick shells; other islands had plants with predominantly small seeds; other islands had large insect populations, or fruits; or a rich cactus community. Darwin proposed that the finches evolved into separate and distinct species, reflecting these environmental differences. These differences are best observed in the finches' beaks. Finches with large crushing beaks are found on those islands that have large seeds. Other finches have bills that easily grasp insects, some have parrot-like beaks, which are efficient in eating fruits, and one species even uses cactus spines as a tool to pry out insects from bark!

Eventually, these and other differences became more and more pronounced, to the extent that the species were no longer able to interbreed and share the same gene pool. This process of forming a separate species is known as speciation. Each of the species survived and evolved because they were able to utilize different portions of the available resources. This is known as resource partitioning. Today, there are 14 species of finches that inhabit the Galapagos Islands that scientists believe evolved from one common ancestral species.