Darwin's Finches
From SkepticWiki
Contents |
[edit] Introduction
Darwin's finches are a group of closely related birds living in the Galapagos archipelago, which are, for this reason, also known as the Galapagos finches. They are of interest to naturalists because they display an interesting example of adaptive radiation --- that is, despite their underlying genetic and morphological similarities, the different species of finch display different adaptations to the different lifestyles they live. For this reason they have become a textbook example of adaptive radiation.
Many people think that these finches inspired Darwin to think of the theory of evolution. As we shall see, this is not true in the least.
[edit] Darwin, the finches, and the myth
Darwin's connection to the finches is that he collected specimens of nine of the thirteen species when he visited the Galapagos Islands during the second voyage of the H.M.S. Beagle. At the time, Darwin noticed neither their close underlying resemblance, nor, indeed, the fact that they were all finches. This was discovered only when Darwin presented his collection to the Geological Society of London, when they were examined by the noted ornithologist John Gould.
The fact that they are called "Darwin's finches", together with their use in biology textbooks as a standard example of adaptive radiation, has led many people to overestimate their importance in the development of Darwin's ideas.
But reference to Darwin's notes of the voyage and his letters, and his failure even to grasp the fact that they were all finches, shows that Darwin did not, as legend suggests, undergo some sort of evolutionary epiphany on discovering the finches. Nor, even with the benefit of hindsight, having figured out the theory of evolution, could he ever make much out of them, for he was unaware both of their true geographical distribution and of the marked differences in habit, diet, and mating calls that differentiate the species. It was only in the twentieth century that the finches took on their role as a stock example of adaptive radiation.
For more information on the development of the myth of Darwin and the finches, the reader should consult Frank J. Sulloway's paper Darwin and His Finches: The Evolution of a Legend.
The fact is that Darwin never explicitly mentions the Galapagos finches in the Origin of Species, as the modern reader can verify very easily by performing a text search on an electronic version of Darwin's book. However, Darwin does make some remarks about the fauna, and even the birds, of the Galapagos in general terms. It seems likely that his references to the Galapagos fauna, together with the fact that today the finches are the species one naturally thinks of when thinking of the Galapagos, has made many people think that Darwin had the finches particularly in mind in these passages. But this is not the case.
We shall look at a couple of these references to the Galapagos is the following two sections.
[edit] The Galapagos and biogeography
The fauna and flora of the Galapagos present naturalists with a puzzle in biogeography that only evolutionary thinking can solve. These organisms live in an environment very different from mainland South America, and have very different lifestyles. And yet in their underlying morphology (and, as we know today, their underlying genetics) they show strong affinities to the inhabitants of the mainland. We can hardly improve on Darwin's own words on this topic:
- The most striking and important fact for us is the affinity of the species which inhabit islands to those of the nearest mainland, without being actually the same. Numerous instances could be given. The Galapagos Archipelago, situated under the equator, lies at a distance of between 500 and 600 miles from the shores of South America. Here almost every product of the land and of the water bears the unmistakable stamp of the American continent. There are twenty-six land birds. Of these twenty-one, or perhaps twenty-three, are ranked as distinct species, and would commonly be assumed to have been here created; yet the close affinity of most of these birds to American species is manifest in every character in their habits, gestures, and tones of voice. So it is with the other animals, and with a large proportion of the plants, as shown by Dr. Hooker in his admirable Flora of this archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, 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 plainly the 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 closely resembles 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 the climate, height, and size of the islands, between the Galapagos and Cape Verde Archipelagos: but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape Verde Islands are related to those of Africa, like those of the Galapagos to America. Facts, such as these, admit of 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 from America, whether by occasional means of transport or (though I do not believe in this doctrine) by formerly continuous land, and the Cape Verde Islands from Africa; such colonists would be liable to modification--the principle of inheritance still betraying their original birthplace.
But however much Darwin may have been inspired by the species of the Galapagos, the finches in particular played no particular part in this --- because Darwin could have had no idea that the finches were particularly American. As Sulloway explains:
- It is often claimed that Darwin was impressed by the American character of his Galapagos finches [...] But Darwin’s finches played no role in this aspect of his evolutionary insight. Rather it was the mockingbirds, the flycatchers, the dove, and numerous other typically American species that established this generalization about the Galapagos avifauna. The finches, in contrast, were placed with the Fringillidae in the nineteenth century, and this family of birds was then believed to be worldwide. It is only in this century [i.e. the twentieth] that the Fringillidae and Emberlzidae, under which Darwin’s finches are now classified, have been distinguished as families of Old and New World finchlike species, respectively.[1]
If Darwin had only known that his finches were peculiarly American, then doubtless he could have used this fact as further support for his argument; and, with the benefit of hindsight, we can do so. But Darwin himself couldn't and didn't.
[edit] Doubtful species
One possible reference to the finches comes in the section headed "DOUBTFUL SPECIES" in Chapter II of the Origin of Species:
- The forms which possess in some considerable degree the character of species, but which are so closely similar to other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank them as distinct species, are in several respects the most important for us [...] Many years ago, when comparing, and seeing others compare, the birds from the closely neighbouring islands of the Galapagos Archipelago, one with another, and with those from the American mainland, I was much struck how entirely vague and arbitrary is the distinction between species and varieties.
Although Darwin does not explicitly mention the finches, he may have had them in mind as one instance of his argument, for in The Voyages of the Beagle, he writes:
- 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. The largest beak in the genus Geospiza is shown in Fig. 1, and the smallest in Fig. 3; but instead of there being only one intermediate species, with a beak of the size shown in Fig. 2, there are no less than six species with insensibly graduated beaks. The beak of the sub-group Certhidea, is shown in Fig. 4. The beak of Cactornis is somewhat like that of a starling, and that of the fourth subgroup, Camarhynchus, is slightly parrot-shaped. 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.
Tellingly, the last sentence was only added by Darwin in the second and subsequent editions of the Voyage of the Beagle, after Darwin had begun to think in evolutionary terms.
[edit] Creationist nonsense
Darwin's finches are so famous that even creationists have heard of them, despite their notorious lack of interest in natural history. Obviously, they are as wrong about the finches as about every other aspect of the natural world to which scientists have drawn their attention. The gist of their nonsense is that the observation that the finches can interbreed somehow proves Darwin wrong about something. The following is a reasonable (and mercifully short) sample of the blather that they have produced on this subject:
- Darwin collected what he regarded as 9 finch species during his voyage on Beagle (1831-1836). These finches were classified as separate species based on their beak shape, size, color, feeding etc. Darwin's argument sounded so good, no-one bothered to test it by seeing if they were really separate and could not interbreed and produce fertile offspring. Now it has been discovered that Darwin's finches can interbreed and produce fertile offspring if given the opportunity, so they are really one species, and provide no evidence for the evolution of new species, and never have. This historic first and foundational evidence for Darwin's theory turns out to be false.
Now, this is as wrong as can be. As we have seen, it was not Darwin who classified the finches that bear his name; the notion that the finches inspired Darwin is the purest myth; and since he never explicitly mentions them in the Origin of Species, they cannot reasonably be described as the "historic first and foundational evidence for Darwin's theory". Nor does any biologist (still less Darwin) claim that they have undergone complete reproductive isolation.
Moreover, the fact that Darwin's finches can breed between species and even genera[2] proves that he was absolutely right when, writing generally about "the birds of the Galapagos", he described them under the heading of "DOUBTFUL SPECIES", and wrote that they provided a striking example of "how entirely vague and arbitrary is the distinction between species and varieties."
The discovery that the finches can interbreed thus proves Darwin correct. By one set of criteria, i.e. the finches' morphology, habits, songs, diet, and usual reluctance to interbreed, they would be classified as separate species, and indeed were so classified by John Gould, and are still so classified by modern taxonomists. On the ther hand, a strict application of the "Biological Species Concept" would require us to class all those species and even genera that are capable of interbreeding as different varieties of the same species. When we look at Darwin's finches, the question of whether we are looking at varieties, subspecies, species, or genera depends, as Darwin said, on an "arbitrary" choice as to how to define species.
So far from denying that different species of finches could interbreed, Darwin asserts that "the canary-bird has been crossed with nine distinct species of finches" in chapter IX of the Origin of Species. If he had known about the interbreeding of the Galapagos finches, he might well have cited it as a further illustration of his point. The very concept of evolution implies that there should not be a single, unambiguous, universally applicable definition of "species" that fits our intuitive notions of what "species" should mean. For more information on this subject, see our main article on Species.
Finally, we may note that, as usual, creationists are barking up the wrong tree in trying to attack Darwin, since he had the first word about evolution, not the last. If they wish to attack the theory of evolution, they need to attack what biologists maintain now --- such as the fact that the species of Darwin's finches can interbreed, which was, of course discovered and published by evolutionary biologists.
