Adaptive Radiation of Chordates:
Adaptive Radiation of Chordates means emergence of diverse group of organisms from the ancestral form in numerous different forms which adapt to various environments. Adaptive radiation explains rapid speciation of the single or few species to fill several ecological niches. It frequently happens when species is introduced into the new ecosystem, or when the species can survive in environment which was formerly unreachable or which has rapidly changed.
The dynamics of adaptive radiation is observed within context that within the relatively short time, several species evolve from the single or few ancestor species. From this great number of genetic combinations, only few can survive long term. After rapid development of several new species, numerous or most of them die out as quickly as they emerged. Surviving species are approximately perfectly adapted to new environment. Rise and fall of new species is now progressing very slowly, compared to initial outburst of species. Adaptive radiation is aided by geographic isolation whereby a population of animals reaching the new area slowly becomes isolated from parent stock and finally becomes distinct over time. Even in the given area, it is likely to have diversity of habitats that may make varied populations of same animals. For instance, in area with deciduous forest, grassland and marsh etc., you may find different species of the organism inhabiting every niche. It is also likely to contain two or more species inhabiting one place/area but specialised to take different foods or to forage in various ways. The group/species may enter the new adaptive zone and by natural selection start slow changes that may lead to them doing things entirely different from usual pattern of parent stock. Such specialization eventually results in number of genetically different but similar-looking species over time. This usually happens when the species colonizes new habitat in which it has little or no competition. For example, a flock of one species of bird may arrive on some sparsely populated islands. Finding little or no competition, birds may evolve rapidly into the number of species, every adapted to one of the available niches. Most celebrated example of adaptive radiation is that reported by Charles Darwin he reasoned that one species of finch (a bird) colonized islands thousands of years ago and by slow process of adaptation gave rise to 14 species of finch-like birds which exist there now. Darwin seen that birds look much related but vary greatly in appearance of shape of the bills, that arose from adaptation to the mode of eating. Few species have large beaks for cracking seeds. Others contain smaller beaks for eating vegetation, and still others featured long, thin beaks for eating insects.
It is thought that birds developed from the single species, of which over time, radiated in several species which adapted to different niches available on islands. Darwin seen that mainland finches were all of one kind, having short straight beaks for crushing seeds. Though, he recorded 13 species which fell in six main kinds, each possessing beak particularly adapted for dealing with particular kind of food. The Galapagos finches exploit the wide range of ecological niches that on mainland are already occupied by other groups of birds. It was seen that large ground finches that were closest to mainland finches in food and habit, had typical finch-like beak for crushing seeds. In contrast to large ground finches, cactus ground finches possess long straight beak and split tongue for getting nectar out of the flowers of prickly pear cactus available there. Vegetarian tree finch, on other hand, has curved parrot-like beak with which it feeds on buds and fruits. Those which feed on insects (insectivorous tree finches) contain the similar beak that they utilize for feeding on beetles and other small insects. Then there is warbler finch that is so like a true warbler that at first it was believed to be one. It utilizes its slender beak for feeding on small insects that it catches as it flies like true warbler. But most extraordinary of all Galapagos birds is woodpecker finch. This looks likes true woodpeckers in its skill to climb up vertical tree trunks and bore holes in wood in search of insects. True woodpecker utilizes its long tongue to look for insect, while woodpecker finch, not having long tongue, chooses small stick or cactus spine in beak and pokes this in hole. When insect emerges bird drops stick and devours insect. Fairly apart from evolutionary implications, this stays the interesting case of tool-using by the animal other than humans.
There are 3 basic kinds of adaptive radiation:
1. General adaptation: In this condition, species develops the radically new skill that allows it reach new parts of its environment. For e.g. birds able to fly.
2. Environmental change: Here a species, on the contrary to other species in ecosystem, productively survives the radical changed environment and then branches in new species which cover new ecological niches created by environmental change. The example of result of the environmental change is rapid spread and growth of mammalian species after extinction of dinosaurs. Radical environment change can also be caused by hurricanes and tsunamis which can destroy life in area to near extinction.
3. Archipelagoes: This refers to isolated ecosystems, like islands and mountain areas like Galapagos Islands visited by Charles Darwin. New species which upon founding itself experiences rapid divergent evolution as was case with Darwin's finches.
Birds differ in size from 10 to 20 cm and weigh between 8 and 38 grams. Smallest is Warbler Finch and largest is Vegetarian Finch. Most significant differences between species are in size and shape of the beaks; beaks are extremely adapted to different food sources.
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