Jonathan Wells and Darwin's Finches
Copyright © 2002
[posted: July 21, 2002]
n Chapter 8 of Icons of Evolution, Jonathan Wells examines the case of "Darwin's Finches", and claims that textbooks exaggerate not only the importance of the finches to Darwin's thinking, but also the evidence that they are an excellent example of evolution in action. He also accuses biologists Rosemary and Peter Grant, who spent 30 years studying these birds, of exaggerating the evidence as well. As we shall see, Wells's case is weak. Darwin's Finches remain one of the best examples of adaptive radiation in the literature of evolutionary biology.
Wells begins by giving us a history lesson: "In fact, Darwin did not become an evolutionist until many months after his return to England. Only years later did he look back at the finches and reinterpret them in the light of his new theory. In 1845 he wrote in the second edition of his Journal of Researches: 'The most curious fact is the perfect gradation in the size of the
beaks of the different species of [finches]. 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 has been taken and modified for different ends'. But this was a speculative afterthought, not an inference from evidence he had collected" (p. 162).
Speculative afterthought? Wells seems to be the one doing the speculating. Soon after Darwin arrived back in England in 1837, he received the information from John Gould about the actual taxonomy of the birds he had collected on the Galapagos. Janet Browne writes, in her biography Darwin Voyaging:
"When they met in March , Gould reiterated this opinion [that the Galapagos finches were a new group strictly confined to the Galapagos, and that each bird represented a different species] and told him how the various species also seemed mutually exclusive of each other from island to island... Surprised, Darwin mulled this information over. If each island had its own birds, as Gould suggested, and the archipelago as a whole had its own roster of genera, his shipboard speculations about the instability of species were more accurate than he had thought" (p. 359).
We can see that as early as 1837 the finches played a part in Darwin more fully constructing his ideas on the transmutation of species -- a crucial aspect of his theory. Browne goes on:
"Quickly the finches joined the fossils as extraordinary and intriguingly complex problems he yearned to solve. The more he puzzled, the more tortuous they became. Nor were the finches the end of it. A week later, on 14 March 1837, Darwin went to hear Gould talk at the Zoological Society about his [Darwin's] South American 'ostriches'.The 'Avestrus Petise', he learned, was
not simply a geographical variety of the ordinary rhea as he thought... Gould found sufficient differences to consider it a separate species.... This moment more than any other in Darwin's life deserves to be called a turning point. Poised to insert the relevant information into his Journal, Darwin was tantalized by the week's results. Why should two closely similar rheas agree to split the country between them? Why should different finches inhabit identical inlets?.... Suddenly he caught at a parallel between what the rheas and finches expressed about the modern world and what his fossils were telling him. Where the birds were linked by being spread over a cluster of neighbouring islands, the extinct South American mammals seemed to be connected to modern species in a chronological sense. The geographical relationships mirrored other relationships through time" (pp. 360-361).
So Wells's contention that Darwin only looked back at the finches years later is false. As we have seen, the finches of the Galapagos did play a part in guiding his thinking on a crucial aspect of his theory at a critical early point, and certainly long before 1845. However, even if Wells was correct in his timing, what difference does it make? Darwin published On the Origin of Species in 1859, and any student of Darwin knows the theory developed slowly and only after examining and considering a bewildering number of facts from many different sources. It did not come to him in some kind of shipboard satori on the HMS Beagle. So even if Darwin didn't fully appreciate the finches until 1845, that means they influenced his thinking for 14 years before publication. One wonders what the fuss is all about.
Wells then takes aim at subsequent finch research. He writes, "Birds on different islands probably encountered differences in food supply, leading to natural selection on their eating apparatus -- their beaks. Theoretically, this process could have led over time to the beak differences that now characterize different species. This is a plausible scenario, but the evidence that Lack [in 1947] cited for it was indirect. Difference in finch beaks are correlated with different food sources and the birds are scattered among the various islands(though it is not the case that each island has its own species). The pattern seems to fit Darwin's theory, yet the case would be much stronger if there were some direct evidence for the process" (p. 164).
Wells says one kind of direct evidence would be genetic, and admits that beak shape is "highly heritable". He then goes on to say another type of evidence would be direct observation of natural selection in the wild. He further admits, "That evidence has been supplied by the husband-and-wife team of Peter and Rosemary Grant" (p. 165). He describes the evidence they gathered demonstrating that the correlation of beak size with food supply was established by natural selection, and follows that with a good summary of the observations the Grants made of beak size on the island of Daphne Major after a drought. As the available supply of edible seed dwindled, only tough, hard-to-open seeds were left, and only birds with larger, deeper beaks could eat them. Subsequent generations showed a dramatic increase in overall beak size in the
What exactly is Wells's beef, then? So far he has convincingly shown that Darwin's theory is correct in explaining the adaptive radiation of the finches in the Galapagos. But, it seems, he has a couple of problems:
- He doesn't like the extrapolation the Grants made about directional selection, where they speculated that if the droughts had continued, say one every ten years, then a trend of increased beak size due to selection would be predicted. Wells does not like this extrapolation, because he notes that in the case of the Daphne Major population, the rains returned, and a reversal of beak size over time was observed.In fact, the climate seems to oscillate between drought and plenty of rain, creating a kind of equilibrium. Wells says that no "net" evolution occurred. He also notes: "Some sort of long-term trend would have to be superimposed on the back-and-forth oscillations to produce long-term change. And that is not what the Grants and their colleagues witnessed. Indeed, it would probably take much longer than a decade or two to measure it, even if it were present. Of course, the climate of the Galapagos might change in the future and alter the pattern. But both of these -- an unseen trend and a future change -- are speculation" (p. 170).
If the climate of the Galapagos remained stable like this all of the time, Wells might have a point. But Wells fails to discuss the evidence of the climate in the past. As Peter Grant writes in Ecology and Evolution of Darwin's Finches (1999 ed), Princeton (a book cited by Wells in Icons):
"The climate of the Galapagos has not remained stable over the last 50,000 years. This is known from an analysis of particles and plant products in cores taken from the sediment of El Junco lake on the summit of San Cristobal (Colinvaux 1972, 1984). Inferences can be made about changes in water level, cloud cover, and heat budget from the composition of the cores at different levels.
The present climate has persisted for the last 3,000 years, and it also prevailed about 6,200 and 8,000 years ago. In the intervening period of 3,200 years it was drier, and possibly hotter, than now. Going back further, it was drier before 8,000 years ago. The most different climate regime from the present one occurred from about 10,000 to 34,000 years ago; this was a time of little precipitation or evaporation" (pp. 29-30).
We can see from this, coupled with what we know about how fast selection can influence beak size, that there was more than enough time for significant directional change to occur. So, in reality, far from being pure speculation, the Grants' extrapolation was actually more informed than Wells is willing to reveal to his readers.
- Wells's second point is that there is evidence that some of the species seem to be merging, not diverging. He makes this statement on page 170: "If Darwinian evolution requires that one population merge into two, the opposite would be for two previously separate populations to merge into one". He goes on to say that some hybridization on Daphne Major has resulted in heterosis, that is, superior fitness of the hybrids over the parental stock. He then points out that this, coupled with oscillating selection for beak size and fluctuating climate caused the Grants to conclude "over the long term there should be a selection-hybridization balance". Then he says, "So Darwin's finches may not be merging or diverging, but oscillating back and forth" (p. 171). It is interesting that Wells has accused the Grants of speculative extrapolations, then does exactly the same thing here. Wells has taken the one
example on Daphne Major, and applied it to all of the finches in the archipelago. Is he justified in doing so?
As the Grants themselves point out, hybridization between species in the Galapagos is extremely rare. In addition, where is the evidence of heterosis in hybrids observed outside of Daphne Major? In fact, the vast majority of hybrids that are produced, while fertile, do not mate with each other to produce an F2 generation; rather, they introgressively hybridize with the parental stocks. Wells knows this, because he cites the paper where this is discussed in his research notes (Grant PR & Grant BR, "Genetics and the origin of bird species", 1997, PNAS 94, 7768-7795). Yet Wells makes this blanket statement about Darwin's finches anyway. Not everybody is fooled.
It should also be pointed out that Darwin's theory does not require divergence. On the contrary, the theory requires the ecological context to be taken into consideration. The environmental conditions can prescribe divergence, panmixia -- even extinction. That is the essence of adaptation, and is what drives adaptive radiation. As the ecology goes, so goes the speciation pattern. Peter Grant writes in Ecology and Evolution of Darwin's Finches, "Thus ecological forces were of primary importance in effecting the split, with reproductive factors such as propensity to interbreed and the fertility of the offspring being secondary" (p. 398). Wells does not dwell on the Grants' argument concerning the factors which contribute to the origin and reinforcement of these prezygotic mechanisms, but this argument is the crux of their work. Of course, none of this is apparent to the general reader unfamiliar with ecology. Without that background, and without an understanding of the intricacies and exhaustiveness of the Grant's work in that area, Wells's objections sound impressive.
- Wells also has a problem with the number of species of Darwin's finches. He writes, "Most of the fourteen species of Darwin's finches -- or at least most of the thirteen living on the Galapagos Islands -- remain distinct primarily because of mating behavior. Evidence suggests that the birds choose their mates on the basis of beak morphology and song pattern. The former is inherited, while the latter is learned by young birds from their parents. But one might expect that true species would be separated by more than beak morphology and song pattern". He then says, "Peter Grant acknowledged that if species were strictly defined by inability to interbreed then 'we would recognize only two species of Darwin's finch on Daphne', instead of the usual four" (p. 170). But wait a minute. Are species defined "strictly by inability to interbreed"? Not by any species concept commonly used today. The Biological Species concept as described by Mayr emphasizes reproductive isolation rather than lack of interfertility. The key to the concept is barriers to gene exchange, which can include infertility, but are not restricted to it by any means. Discussions of prezygotic reproductive barriers can be found in any decent textbook. Wells knows this. He also knows that reproductive isolation in the finches on the Galapagos is primarily of the prezygotic type, because his sources by the Grants emphasize it over and over. For example, in "Genetics and the origin of bird species" they say, "First, speciation in birds proceeds with the evolution of behavioral barriers to interbreeding; postmating [postzygotic] isolation usually evolves much later, perhaps after gene exchange has all but ceased". So, Wells has no real point here. However, the general public not familiar with species concepts as used by biologists might consider his arguments authoritative.
The general reader is done a great disservice by this chapter in Icons of Evolution. Jonathan Wells does not sufficiently address the biographical or scientific literature on Darwin's Finches to enable the reader to make an informed decision regarding his argument. He writes, with exquisite irony, "It makes one wonder how much evidence there really is for Darwin's theory". Since, as we have seen, Wells avoids most of it regarding Darwin's Finches, one wonders how much evidence there is to support his book.
Browne, Janet. Charles Darwin Voyaging. 1995, Princeton University Press.
Grant, Peter. Ecology and Evolution of Darwin's Finches. 1999 edition, Princeton University Press.
Grant, PR and Grant, BR. "Genetics and the origin of bird species." (1997) PNAS 94, 7768-7795.
Wells, Jonathan. Icons of Evolution. 2000, Regnery Publishing.
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