On the Galapagos Islands in the Pacific Ocean, close to the equator, there are a variety of different finches, which vary in the shape and size of their beaks. It appears that the finches colonised the Islands from mainland South America, and then diverged in form. The distance between the islands meant that the finches on different islands could not interbreed, so the populations on the different island tended to become distinct. Different populations also became specialised for different food sources, birds with thin, sharp beaks eating insects and birds with large, sturdy beaks eating nuts.
Darwin collected some of these finches when he visited the Galapagos Islands, and it is often stated that the finches were key to the development of his theory of evolution. They are used as evidence for his theory in many textbooks.
Were the Finches important to Darwin in formulating his theory?
School children are often taught that the finches of the Galapagos Islands were very important in helping Darwin to come up with his theory of evolution. For example, the BBC GCSE Bitesize Revision Biology: Old and new species states:
"Darwin studied the wildlife on the Galápagos Islands (a group of islands almost 1,000 km off the coast of Ecuador). He noticed that the finches on the different islands were similar to each other. However, the finches showed wide variations in their size, beaks and claws from island to island. For example, their beaks were different depending on the local food source. Darwin concluded that because the islands are so far from the mainland, the finches that had arrived there had changed over time."Darwin concluded that in each locality one or more individual finch happened to acquire, by random mutation, a beak shape more suitable for the food sources in that locality. These individuals then had a competitive advantage over their fellow finches, enabling them to grow and reproduce more successfully, and pass on their more specialised beaks to successive generations - until eventually the characteristic had spread throughout the finch population in that locality.
The BBC is mistaken in much of what it says here. When he was on the Galapagos Islands, Darwin did not notice that different islands had different finches. Neither did he realise that the finches were closely related despite their differences in beak shape. He did not match different beak shapes to different diets. Even after his return to London, Darwin's biographers note that he "remained confused by the Galapagos finches...unaware of the importance of their different beaks...He had no sense of a single, closely related group becoming specialized and adapted to different environmental niches." (p. 209, Darwin - A. Desmond and J. Moore).
Darwin did not mention the finches in his book The Origin of Species. They only appear in his Journal, being mentioned only in passing in the first edition (1839), and then having a few paragraphs and a picture six years later in the revised edition (1845). The most that Darwin would ever say about finch evolution is found here:
Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends... Unfortunately most of the specimens of the finch tribe were mingled together; but I have strong reasons to suspect that some of the species of the sub-group Geospiza are confined to separate islands. If the different islands have their representatives of Geospiza, it may help to explain the singularly large number of the species of this sub-group in this one small archipelago, and as a probable consequence of their numbers, the perfectly graduated series in the size of their beaks. (pp403-420)
Darwin, Journal of Researches into the Natural History and Geology of the countries visited during the voyage round the world of H.M.S. Beagle, revised edition, Henry Colburn 1845.
So all Darwin did was speculate that the different finches had descended from a common ancestor and had changed to be able to do different things. He was never sure that the different species were from different islands. He certainly never came up with the detailed theory for how the finches diversified which the BBC suggests.
The BBC makes this mistake because a myth has arisen around these Galapagos Finches. They were never known as "Darwin's Finches" until 1936, and the name was popularised by ornithologist David Lack in his book Darwin's Finches (1947). Lack described the detailed account of Finch evolution, recounted by the BBC, and also promoted the myth that the finches had given Darwin important insights into evolution.
"Darwin's Finches" are found repeatedly in school biology textbooks, and the WJEC A-Level Biology syllabus and the Intermediate 2 Biology syllabus mandate their teaching.
What do the Finches demonstrate about evolution?
Though the finches were not important in the work of Charles Darwin, they do tell us something about evolution. In particular, over the past few decades, two scientists have done an excellent long term study on the finches on one of the Galapagos Islands. This is accurately described by the textbook Advanced Biology.(Jones, M., and G. Jones. 1997. Cambridge University Press) The authors recount how from 1977 to 1982 there was a drought on one of the Galapagos Islands, and due to natural selection the average finch beak size became larger…
However, this proved not to be the end of the story. If it continued in this way, the average beak size of G. fortis would continue to get larger and larger. But this has not happened (p. 153)
This cumulative change does not occur for two reasons. (1) There are disadvantages to having a large beak, especially when a bird is young. This can outweigh the advantages. (2) The selection pressure on the island fluctuates. In 1982 the drought stopped and there was selection for birds with small beaks.
It can therefore be argued that the study shows natural limits to evolutionary change. Variation in a species is a good thing, as it gives them the ability to cope with environmental change, but variation does have limits.
Many textbooks do not go into such detail, and simply describe the finches as a good example of a range of species evolving from a common ancestor.
The Galapagos finches afford an excellent example of adaptive radiation. It is assumed by evolutionists that a stock of ancestral finches reached the islands from the mainland and then, in the absence of much competition, evolved to fill many of the empty ecological niches occupied on the mainland by species absent from the islands.” (p. 725) Advanced Biology. Roberts, M., M. Reiss, and G. Monger. 2000. Nelson
Conclusion
The Galapagos finches were not as important to Darwin as is often claimed, but they are a good example of micro-evolution. They show us that finches can vary in their morphology, and that natural selection has a role in this.
This study does not give evidence for macro-evolution, and does not prove that natural selection and random mutation could produce the living world as we know it from simple single-celled ancestors.