Darwin’s finches are a classic and historically important example of a species radiation (sometimes called an “adaptive” radiation, but that implies a specific assertion about the cause of the radiation which may not be appropriate in all cases). During the five weeks that Darwin spent on the Galapagos in September, 1835, he made a number of observations of these birds, but they did not occupy his time or attention more than any other aspect of this remarkable archipelago of islands. It seems that Darwin did not recognize all of the finches as finches, thinking some were of an entirely different group of birds, and in some cases, the variety seen across some of 13 or so species was initially interpreted by Darwin to represent a notably large range of variation in a single species. Please remember, Darwin was a rock man more than he was a bird man, at the time.
repost from gregladen.com
John Gould was a bird man, the most famous of his time at least in England, and he is the one who re-identified specimens mistaken by Darwin to be three or four different kinds (blackbirds, grosbeaks, and finches) as representing about a dozen different finches that had diversified into distinctly different forms.
|There is even a Vampire Finch. And, even more amazingly, there is a Vampire Finch Movie!(click here to find out more…)|
There are two separate dimensions to variation in these finches. The overarching pattern is that different species are found on different islands, owing to a combination of genetic drift and local adaptation, the other is that different finches living in overlapping geographical ranges had different adaptations for differing diets. One gets the impression that Darwin was struck more by this first aspect of variation than the second. This pattern of variation … island by island … also applied to other animals such as the tortoises.
The phrase “Darwin’s Finches” was first advanced, or at least popularized, by David Lack, the famous ornithologist who also advanced a version of group selection theory, in the 1940s.
Here is a list of the species cribbed from Wikipedia:
- Genus Geospiza
- Large Cactus-finch, Geospiza conirostris
- Sharp-beaked Ground-finch, Geospiza difficilis
- (Vampire Finch, Geospiza difficilis septentrionalis)
- Medium Ground-finch, Geospiza fortis
- Small Ground-finch, Geospiza fuliginosa
- Large Ground-finch, Geospiza magnirostris
- (Darwin’s Large Ground-finch, Geospiza magnirostris magnirostris – possibly extinct [1957?])
- Common Cactus-finch, Geospiza scandens
- Genus Camarhynchus
- Vegetarian Finch, Camarhynchus crassirostris – sometimes separated in Platyspiza
- Large Tree-finch, Camarhynchus psittacula
- Medium Tree-finch, Camarhynchus pauper
- Small Tree-finch, Camarhynchus parvulus
- Woodpecker Finch, Camarhynchus pallidus
- Mangrove Finch, Camarhynchus heliobates
- Genus Certhidea
- Warbler Finch, Certhidea olivacea
- Genus Pinaroloxias
- Cocos Island Finch, Pinaroloxias inornata
The adaptive story here is about the relationship between ecological niches and the species that evolve “into” them. For example, wherever there is wood, there is rotten wood. Wherever there is rotten wood, there will be grub-like things and other insects and creepy crawlies living in the wood. Wherever there are creepy crawlies living in wood, there will be woodpeckers.
But the woodpeckers are not always woodpeckers per se. In the Galapagos, there is the so-called woodpecker finch. This little bird gets at the grubs using the needle of a cactus. (A finch that uses tools!) Other finches take the role elsewhere filled by ground feeders or nut cracking birds. Indeed, there appears to be no native Vampires in the Galapagos, thank goodness for the Galapagonians, but there is a Vampire Finch: A subspecies of the sharp-beaked ground finch the feeds occasionally on the blood of dead seagulls and such. I wonder if there is an as yet undiscovered Wolfman Finch or maybe a Bigfoot Finch? … but I digress..
Here are a few selected passages from “Narrative of the Surveying Voyages….” published by Darwin in 1839. First we detect hints of Natural Selection in the form of recent and rapid evolution in a rat:
Of mammalia a large kind of mouse forms a well-marked species. From its large thin ears, and other characters, it approaches in form a section of the genus, which is confined to the sterile regions of South America. There is also a rat which Mr. Waterhouse believes is probably distinct from the English kind; but I cannot help suspecting that it is only the same altered by the peculiar conditions of its new country.
And the first mention of the finches in this text, embedded within a broader discussion of the bird fauna:
In my collections from these islands, Mr. Gould considers that there are twenty-six different species of land birds. With the exception of one, all probably are undescribed kinds, which inhabit this archipelago, and no other part of the world. …
Although the species are thus peculiar to the archipelago, yet nearly all in their general structure, habits, colour of feathers, and even tone of voice, are strictly American. The following brief list will give an idea of their kinds. 1st. A buzzard, …; 2d. Two owls; 3d. Three species of tyrant-flycatchers …; 4th. A sylvicola, …; 5th. Three species of mocking-birds, …; 6th. A finch, with a stiff tail and a long claw to its hinder toe, closely allied to a North American genus; 7th. A swallow ….; 8th. A dove, …
and finally ….
…9th. A group of finches, of which Mr. Gould considers there are thirteen species; and these he has distributed into four new sub-genera. These birds are the most singular of any in the archipelago. They all agree in many points; namely, in a peculiar structure of their bill, short tails, general form, and in their plumage. The females are gray or brown, but the old cocks jet-black. All the species, excepting two, feed in flocks on the ground, and have very similar habits. It is very remarkable that a nearly perfect gradation of structure in this one group can be traced in the form of the beak, from one exceeding in dimensions that of the largest gros-beak, to another differing but little from that of a warbler. Of the aquatic birds I have already remarked that some are peculiar to these islands, and some common to North and South America.
Talk of these finches without mention of the Grants would be wrong. Peter and Rosemary Grant have been working on the Galapagos Island Daphne Major for over thirty years. Their approach has been to label, identify, get to know and follow around every single finch on the little island from birth to death. Of course they capture and measure them and they collect copious and detailed data on environmental conditions.
The Grants have documented the way in which the finch species endemic to this island both live and die at the mercy, ultimately, of rainfall, which affects the availability of different kinds of food. This in turn, as they have carefully documented, affects the range and mode of beak shape of the birds across each population over generational time. In other words, they have documented Natural Selection in Darwin’s finches. A book written by Jonathan Weiner describing their work for the general public is a nice read and should be of interest to anyone who has gotten this far in this post! (So go read it!)
This is part of the following series of posts:
Charles Darwin Bicentennial
Charles Darwin Bicentennial – A Tangled Bank
Charles Darwin Bicentennial- Beagle and The Voyage
Charles Darwin Bicentennial – Coral Reefs
Charles Darwin Bicentennial – Finches
Charles Darwin Bicentennial – Gauchos
Charles Darwin Bicentennial – Iguanas, a “most disgusting, clumsy lizard…
Charles Darwin Bicentennial – Notebooks
Darwin, C. R. 1839. Narrative of the surveying voyages of His Majesty’s Ships Adventure and Beagle between the years 1826 and 1836, describing their examination of the southern shores of South America, and the Beagle’s circumnavigation of the globe. Journal and remarks. 1832-1836. London: Henry Colburn.
Sulloway, Frank. 1982. The Beagle collections of Darwin’s finches (Geospizinae). Bulletin of the British Museum (Natural History) Historical Series 43:2(49-94).
Wikipedia on Darwin’s finches [click me]