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BS1013 ESSAY: Adaptive radiation of Darwin’s Finches «Both in space and time, we seem to be brought somewhat near to that great fact, that mystery of mysteries, the first appearance of new beings on this earth». (Campbell, Reece, Biology 8th ed. p.487) The changes of evolution take place in all parts of biology of animals and any change must result in a better adaptation of the living animal. Darwin’s finches consist one of the stronger example of that successful adaptation to the environment. Picture 1: Galapagos Islands http://www.iciclesoftware.com/galapagos/gifs/mapgalap2.gif They were first collected by Charles Darwin during his voyage on the Beagle ship, in a group of volcanic islands, Galapagos Islands. While these islands are located west of south America, they had been colonized by organisms which they gave rise to new species. Some of the finches are unique to individual islands whereas some others live on more neighboring islands. Thirteen of these species reside in Galapagos Islands and just a single one on Cocos Island. However, finches are highly adapted to the specific environment. As numerous of their habits and characters have evolved by the descent with modification in order to achieve a better matching with their home islands. MYROFORA PANAGI Page 1 Like all birds, finches are vertebrate animals and were evolved by parallel evolution i.e. they have evolved independently, having no common ancestor later than an early reptilian stage. Furthermore, the brownish or black body with size between 10 – 20 cm, the short tail and short rounded wings are some general features that relate the species with each other. But a finch’s most obvious adaptation is its beak which ranges from delicate and thin to stout and large. «Only the variety of their beaks and the number of their species excite attention-small finch-like beaks, huge finch-like beaks, parrot- like beaks, straight wood-boring beaks, decurved flower-probing beaks, slender warbler-like beaks, are species which look very different and species which look closely similar»(D.Lack, 1936,Darwin’s finches,p.11). This is due to the different food sources and more usually characterize a different genera; Geospiza, Camarhynchus, Certhidea, Pinaroloxias. Picture 2: Darwin’s Finches are divided according their food source. http://www.math.ucla.edu/~tao/finch.jpg The heavy finch-like beaks characterize large, medium and small ground- finches of Geospiza magnirostis, G.fortis and G.fuliginosa (species of subgenus Geospiza).These species habit in coastal zone and feed mainly by seeds MYROFORA PANAGI Page 2 especially during the breeding season, but also eat flowers, buds, fruits, young leaves and large caterpillars. Sometimes spiders and small insects may be part of their diet. On the other hand, the ground-finch G.difficilis lives in hunid zone and has a sharp beak. Geospiza scandens and Geospiza conirostris species have long decurved beak and chiefly feed on opuntia (cactus feeding). The three species of Camarhynchus genera; C.psittacula, C.pauper, C.parvulus have thick, short and somewhat decurved beak for a diet consist of insects. While the birds are searching for their meals in trees, in agile movements, «they examine the twigs, bark and leaf clusters, and also excavate shallow holes in soft wood» (D.Lack, 1936, Darwin’s finches, p.58). Finches may feed on ground and include nectar, buds, young leaves and large caterpillars (and grain) in their food source. C.pallidus and C.heliobates prefer to dig insects out of wood, by the using of twigs, thus they have stout and straight beak. The vegetarian one (finch), C.crassirostris feed primarily on leaves, buds and fruits. As for Certhidea olivacea species, their diet consists of small insects which can be found in leaves, twigs, as well as, in ground or air. This type of finch has a slender beak and generally has similar habits and appearance with a warbler. In contrast with other subgenus, the genus Pinaroloxias refers to the Cocos finche. This finch is feed mainly with insects both from ground and trees and its beak looks like Certhidea , except it is a little longer and somewhat decurved. The main characteristic of the finch is the unusually long, grooved and bifid tongue. It must be mentioned that according to Dr.Cliff Tabin biologist «the bone morphogenetic protein 4 (BMP4) and its differential expression during development is responsible for the variation in beaks' size and shape among finches» (http://en.wikipedia.org/wiki/Darwin%27s_finches). In addition, the modification of the beak affects the singing of finches. Males use singing as an attractive agent of the opposite sex especially during the breeding season. The speciation of Darwin’s finches was contribute by main three factors; the ecological opportunity, the geological isolation and evolutionary change. The ecological opportunity refers to the absence of predators which accelerated the adaptive radiation of finches, and permit evolution in certain directions which would otherwise be impossible. Apart from the complete freedom from enemies, for a long time these areas were without food competitors. But «event at the present time there are few other passerine birds in the MYROFORA PANAGI Page 3 Galapagos, and these are predominantly insectivorous species» (D.Lack, 1936, Darwin’s finches, p.118), without any access on other finch diets. Although the speciation has become more successful by the variety of islands, which has permitted the migration of finches between them, the great distances have been a disadvantage for interbreeding. However, geological isolation is illuminated by the single species of Darwin’s niches in Cocos Island. Furthermore, the evolutionary change can be occurred through the combination of natural selection, genetic drift (chance events that change allele frequencies) and founder effect i.e. «loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population» (Campbell, Reece, Biology 8th ed. p.468). To conclude, the archipelago is a little world within itself. Charles Darwin pointed out that by «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» (D.Lack, 1936, Darwin’s finches, p.10). Finches are undoubtedly adaptive by the action of natural selection in order to be favorable spread through the population. SCIENTIFIC CLASSIFICATION OF THE FINCHES: Kingdom: Animalia Phylum: Chordata Class: Aves Order: Passeriformes Family: EmberizidaeGeospiza GENERA Camarhynchus Certhidea Pinaroloxias (http://en.wikipedia.org/wiki/Darwin%27s_finches) MYROFORA PANAGI Page 4 REFERENCES: Essay material: D.Lack, 1936, Darwin’s finches Campbell, Reece, Biology 8th ed. http://en.wikipedia.org/wiki/Darwin%27s_finches http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciat ion.html Pictures: Pic1: http://www.iciclesoftware.com/galapagos/gifs/mapgalap2.gif Pic2: http://www.math.ucla.edu/~tao/finch.jpg MYROFORA PANAGI Page 5
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