Were the Finches important to Darwin in formulating his theory?
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.
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: Variation and inheritance states:
Whilst studying wildlife on the Galapagos Islands [Darwin] noticed that the Galapagos finches showed wide variations - eg in beak shape and size - from island to island. Darwin deduced that these differences made the finches better adapted to take advantage of the food in their particular local environment - thin, sharp beaks prevailing where the birds' main food was insects and grubs, and large claw-shaped beaks where their diet was buds, fruit and nuts. In each locality the finch population had somehow developed beaks which were suitable for that particular environment.
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:
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
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.
Evolution by natural selection...has lately come to function more as an antitheory, called upon to cover up embarrassing experimental shortcomings and legitimize findings that are at best questionable and at worst not even wrong.
Robert B. Laughlin, A Different Universe (New York: Basic Books, 2005)