| Darwin's finches | |
|---|---|
 | |
| Large ground finch,Medium ground finch Small tree finch,Green warbler-finch | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Passeriformes |
| Superfamily: | Emberizoidea |
| Family: | Thraupidae |
| Groups included | |
Darwin's finches (also known as theGalápagos finches) are a group of about 18species ofpasserine birds.[1][2][3][4] They are well known for being a classic example ofadaptive radiation and for their remarkable diversity in beak form and function.[5] They are often classified as thesubfamilyGeospizinae ortribeGeospizini. They belong to thetanagerfamily and are not closely related to thetrue finches. The closest known relative of the Galápagos finches is the South Americandull-coloured grassquit (Asemospiza obscura).[6] They were first collected whenthe second voyage of theBeagle visited theGalápagos Islands, withCharles Darwin on board as a gentleman naturalist. Apart from theCocos finch, which is fromCocos Island, the others are found only on the Galápagos Islands.
The term "Darwin's finches" was first applied byPercy Lowe in 1936, and popularised in 1947 byDavid Lack in his bookDarwin's Finches.[7][8] Lack based his analysis on the large collection of museum specimens collected by the 1905–06 Galápagos expedition of the California Academy of Sciences, to whom Lack dedicated his 1947 book. The birds vary in size from 10 to 20 cm (4 to 8 in) and weigh between 8 and 38 grams (0.3 and 1.3 oz). The smallest are thewarbler-finches and the largest is thevegetarian finch. The most important differences between species are in the size and shape of their beaks, which are highlyadapted to different food sources. Food availability was different among the islands of the Galapagos and could also change dramatically due to natural events such as droughts. The birds are all dull-coloured. They are thought to have evolved from a single finch species that came to the islands more than a million years ago.[9]
Duringthe survey voyage ofHMSBeagle, Darwin was unaware of the significance of the birds of the Galápagos. He had learned how to preserve bird specimens fromJohn Edmonstone while at theUniversity of Edinburgh and had been keen on shooting, but he had no expertise inornithology and by this stage of the voyage concentrated mainly on geology.[10] In Galápagos he mostly left bird shooting to his servantSyms Covington.[11] Nonetheless, these birds were to play an important part in theinception of Darwin's theory ofevolution bynatural selection.
On theGalápagos Islands and afterward, Darwin thought in terms of "centres of creation" and rejected ideas concerning thetransmutation of species.[12] FromHenslow's teaching, he was interested in the geographical distribution of species, particularly links between species on oceanic islands and on nearby continents. OnChatham Island, he recorded that amockingbird was similar to those he had seen inChile, and after finding a different one onCharles Island he carefully noted where mockingbirds had been caught.[10] In contrast, he paid little attention to the finches. When examining his specimens on the way toTahiti, Darwin noted that all of the mockingbirds on Charles Island were of one species, those fromAlbemarle of another, and those fromJames and Chatham Islands of a third. As they sailed home about nine months later, this, together with other facts, including what he had heard aboutGalápagos tortoises, made him wonder about the stability of species.[13][14]
Following his return from the voyage Darwin presented the finches to theZoological Society of London on 4 January 1837, along with other mammal and bird specimens that he had collected. The bird specimens, including the finches, were given toJohn Gould, the famous Englishornithologist, for identification. Gould set aside his paying work and at the next meeting, on 10 January, reported that the birds from theGalápagos Islands that Darwin had thought wereblackbirds, "gross-beaks" andfinches were actually "a series of ground Finches which are so peculiar [as to form] an entirely new group, containing 12 species." This story made the newspapers.[15][16]
Darwin had been inCambridge at that time. In early March, he met Gould again and for the first time to get a full report on the findings, including the point that his Galápagos "wren" was another closely allied species of finch. Themockingbirds that Darwin had labelled by island were separate species rather than just varieties. Gould found more species than Darwin had expected,[17] and concluded that 25 of the 26 land birds were new and distinct forms, found nowhere else in the world but closely allied to those found on theSouth American continent.[16] Darwin now saw that, if the finch species were confined to individual islands, like the mockingbirds, this would help to account for the number of species on the islands, and he sought information from others on the expedition. Specimens had also been collected by CaptainRobert FitzRoy, FitzRoy's steward Harry Fuller, and Darwin's servantCovington, who had labelled them by island.[18] From these, Darwin tried to reconstruct the locations from where he had collected his own specimens. The conclusions supported his idea of the transmutation of species.[16]
At the time that he rewrote his diary for publication asJournal and Remarks (laterThe Voyage of the Beagle), he described Gould's findings on the number of birds, noting that "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".[19] In the first edition ofThe Voyage of the Beagle, Darwin said that
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.[20]
By the time the first edition was published, thedevelopment of Darwin's theory ofnatural selection was in progress. For the 1845 second edition ofThe Voyage (now titledJournal of Researches), Darwin added more detail about the beaks of the birds, and two closing sentences which reflected his changed ideas:
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.[21][22]
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-groupCactornis, lately brought from Bow Island, in the Low Archipelago. OfCactornis, the two species may be often seen climbing about the flowers of the great cactus-trees; but all the other species of this group of finches, mingled together in flocks, feed on the dry and sterile ground of the lower districts. The males of all, or certainly of the greater number, are jet black; and the females (with perhaps one or two exceptions) are brown. The most curious fact is the perfect gradation in the size of the beaks in the different species ofGeospiza, 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 genusGeospiza 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-groupCerthidea, is shown in Fig. 4. The beak ofCactornis 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. In a like manner it might be fancied thata bird originally a buzzard, had been induced here to undertake the office of the carrion-feedingPolybori of the American continent.[23]
Darwin discussed the divergence of various species of birds in the Galápagos more explicitly in his chapter on geographical distribution inOn the Origin of Species; however, he does not single out the finches:
The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. [In] the Galapagos Archipelago ... almost every product of the land and water bears the unmistakable stamp of the American continent. 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.[24]
Whereas Darwin spent just five weeks in the Galápagos, andDavid Lack spent three months,Peter and Rosemary Grant and their colleagues have made research trips to the Galápagos for about 30 years, particularly studying Darwin's finches.
Females are dimorphic in song type: songs A and B are quite distinct. Also, males with song A have shorter bills than B males, another clear difference. With these beaks, males are able to feed differently on their favourite cactus, the prickly pearOpuntia. Those with long beaks are able to punch holes in the cactus fruit and eat the fleshyaril pulp, which surrounds the seeds, whereas those with shorter beaks tear apart the cactus base and eat the pulp and any insect larvae and pupae (both groups eat flowers and buds). This dimorphism clearly maximises their feeding opportunities during the non-breeding season when food is scarce.
If the population ispanmictic,[25][26] thenGeospiza conirostris exhibits a balanced genetic polymorphism and not, as originally supposed, a case of nascentsympatric speciation. The selection maintaining the polymorphism maximises the species'niche by expanding its feeding opportunity. The genetics of this situation cannot be clarified in the absence of a detailed breeding program, but two loci withlinkage disequilibrium[27] is a possibility.
Another interesting dimorphism is for the bills of young finches, which are either 'pink' or 'yellow'. All species of Darwin's finches exhibit this morphism, which lasts for two months. No interpretation of this phenomenon is known.[28]
For some decades, taxonomists have placed these birds in the familyEmberizidae along with the New World sparrows and Old World buntings.[29] However, theSibley–Ahlquist taxonomy puts Darwin's finches with thetanagers (Monroe and Sibley 1993), and at least one recent work follows that example (Burns and Skutch 2003). TheAmerican Ornithologists' Union, in itsNorth American checklist, places the Cocos finch in the Emberizidae, but with an asterisk indicating that the placement is probably wrong (AOU 1998–2006); in its tentativeSouth American check-list, the Galápagos species areincertae sedis, of uncertain place (Remsen et al. 2007).
A long-term study carried out for more than 40 years by the Princeton University researchersPeter and Rosemary Grant has documented evolutionary changes in beak size affected byEl Niño/La Niña cycles in the Pacific.[37]
Developmental research in 2004 found thatbone morphogenetic protein 4 (BMP4), and its differential expression during development, resulted in variation of beak size and shape among finches. BMP4 acts in the developing embryo to lay down skeletal features, including making the beak stronger.[38] The same group showed that the development of the different beak shapes in Darwin's finches are also influenced by slightlydifferent timing andspatial expressions of a gene called calmodulin (CaM).Calmodulin acts in a similar way to BMP4, affecting some of the features of beak growth like making them long and pointy. The authors suggest that changes in the temporal and spatial expression of these two factors are possible developmental controls of beak morphology.[39] Moreover, these changes in the beak size have also altered vocalizations in Darwin's finches.[5]
In 2015–2016 studies, genome sequencing revealed a 240 kilobase haplotype encompassing theALX1 gene that encodes a transcription factor affecting craniofacial development is strongly associated with beak shape diversity. The blunt beak is a derived characteristic. The haplotype that codes for it contains, among other things, two amino-acid substitutions L110P and I209V.[40][41][42]
Further research from 2016, in which genomes from each of the Darwin's finch species were sequenced, established that asingle nucleotide polymorphism in the high mobility AT-hook 2 gene (HMGA2) locus is significantly associated with variation in beak size (Lamichhaney et al. 2016).HMGA2 codes for a transcription factor which in humans has been associated with variation in height, craniofacial distances, and primary tooth eruption.[43]
In an analysis of the genomes of individuals from threeGeospiza ground finch species found in sympatry (G. fortis,G. fuliginosa,G. magnirostris), 11 out of 32,569 SNPs were identified as representing four independent groups of statistically linked SNPs that together explained 83.6% of the variance in beak size (Chaves 2016). What this means is that only a small fraction of the genome in Darwin's finches is responsible for variation in beak morphology which is consistent with the rapid changes in beak form in response to the varying environments on the Galapagos Islands.
A number of phylogenies, both molecular and morphological, have been produced for Darwin's finches. Many of them have a tendency to divide "species" in a way different from what is currently accepted, which is suggestive of possible issues in the current taxonomy. On one extreme, some (such as Sato's mtDNA study)[44] find the "species" so similar that the entireGeospiza should be one polymorphic species. The same has been said ofCamarhynchus on Floreana Island.[45]
The following is a whole-genome molecular phylogeny from Lamichhaney et al. (2018),[34] with some locality marks from Zink et al. (2019). Whole-genome data has much better resolving power than mtDNA data and is able to clearly differentiate populations from each another.[45]
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outgroup (T. bicolor,L. noctis) | ||||||||||||||||||||||||||||||||||||||||||||||||||
A phylogenetic tree, however, is unable to capture the intraspecific mixture that have happened in the evolution ofGeospiza. For measures of admixture, consult the discussion of gene flow in Lamichhaney et al. (2015),[40] the whole-genome phylogenetic network of Almen et al. (2016),[41] and the SNP admixture estimation in Lamichhaney et al. (2018).[34] One commonly-exchanged region is ALX1, which controls whether the beak is sharp or blunt. The two species now distinguished fromG. difficilis, although not very close to trueG. difficilis on a whole-genome basis, have an unusual amount of shared polymorphisms, suggesting that they may have acquired the morphological similarity from ancient gene flow.[41]
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