Darwin’s Finches

Common Cactus Finch

In late December 1831, at the age of just 22, Charles Darwin set out on a voyage aboard HMS Beagle as the ship’s naturalist to collect specimens of flora and fauna from around the continent of South America.

After spending over 3 years in South America, HMS Beagle sailed to the Galapagos Islands arriving on the 15th September 1835, where the research Darwin carried out helped him confirm his theory of evolution that was published in On the Origin of Species in 1859.

The crew spent 5 weeks on the islands and in that time Darwin studied the Galapagos’ geology, which he was especially interested in, as well as some of its unique inhabitants including the giant tortoises the islands are famed for.

He was also interested in the mockingbirds of the islands and was surprised to note that they differed from island to island but didn’t take much notice of what he thought were blackbirds, having little knowledge of ornithology.

When he returned to England, Darwin presented the finches, along with other mammal and bird specimens, to the Geological Society of London. The birds were given to John Gould, a renowned ornithologist, for identification, who reported that the birds weren’t blackbirds, but 12 distinct species of finches. A further report by Gould confirmed that the different mockingbirds were also separate species.

Large Cactus Finch
Large cactus finch

The Galapagos finches were nearly identical in many ways to each other as well as to the South American mainland birds. However, they showed wide variations in their size, claws and, in particular, their beaks.

In his book The Voyage of the Beagle, published in 1839, Darwin described in great detail the differences in the finches’ beaks.

“The remaining land-birds form a most singular group of finches, related to each other in the structure of their beaks, short tails, form of body and plumage: there are thirteen species, which Mr. Gould has divided into four subgroups. All these species are peculiar to this archipelago; and so is the whole group, with the exception of one species of the sub-group Cactornis, lately brought from Bow Island, in the Low Archipelago …

“The most curious fact is the perfect gradation in the size of the beaks in the different species of Geospiza, from one as large as that of a hawfinch to that of a chaffinch, and (if Mr. Gould is right in including his sub-group, Certhidea, in the main group) even to that of a warbler …

“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. In a like manner it might be fancied that a bird originally a buzzard, had been induced here to undertake the office of the carrion-feeding Polybori of the American continent.”

Darwin concluded that the shape and size of the finches’ beaks had adapted depending on the local food source and that because the islands were so far from the mainland the finches that had arrived there in the past had evolved over time.

Darwin's Finches

So warbler finches have thin sharp beaks to spear insects, whereas ground finches crack open seeds with much bigger, stronger and blunter beaks.

Darwin also discussed the difference between the birds of the Galapagos in On the Origin of Species:

“There are twenty-six land birds, and twenty-five of these are ranked by Mr. Gould as distinct species, supposed to have been created here; yet the close affinity of most of these birds to American species in every character, in their habits, gestures, and tones of voice, was manifest….

“The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, yet feels that he is standing on American land. Why should this be so? Why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plain a stamp of affinity to those created in America?

“There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which resembles closely the conditions of the South American coast: in fact there is a considerable dissimilarity in all these respects. On the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in climate, height, and size of the islands, between the Galapagos and Cape de Verde Archipelagos: but what an entire and absolute difference in their inhabitants!

“The inhabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America. I believe this grand fact can receive no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the Galapagos Islands would be likely to receive colonists, whether by occasional means of transport or by formerly continuous land, from America; and the Cape de Verde Islands from Africa; and that such colonists would be liable to modification; —the principle of inheritance still betraying their original birthplace.”

Since Darwin’s initial findings many studies have been carried out on the Galapagos finches.

In the 1970s Rosemary and Peter Grant, a British husband and wife team of evolutionary biologists at Princeton University, began spending 6 months each year taking meticulous measurements of Darwin’s finches to observe evolution in action.

The Grants found tiny changes in the shapes and sizes of the finches that correlated with variations in food supply and climate, driven by natural selection and in 2009 they spotted what could be the beginning of a new species of ground finch. After a severe drought, the new type of finch isolated itself from all other species of finches and began breeding only with itself. Whereas Darwin though that it would take a considerable length of time for a new species to appear these findings appear to show that evolutional changes could take place over a much shorter period.

In 2015 researchers completed sequencing the genomes of all 15 species of Darwin’s finches and revealed the key gene responsible for the differences in the birds’ beaks.


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