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| Theories on Organic Evolution |
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| The occurrence of evolution is undeniably established by the various types of evidences that are available today. However, how it has come about is explained by the several ideas that have been put forth from time to time. These ideas are known as 'theories on organic evolution'. Following are the main theories on the mode of evolution. |
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| It is the name given to the theory proposed by Jean Baptiste Lamarck (1744-1829), the French Zoologist. It is also known as the 'theory of inheritance of acquired characters'. |
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| fig. 5.3 - Jean Baptiste Lamarck |
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| Lamarck has explained his ideas on organic evolution in the book Philosophica Zoologique, published in 1809. He recognised the fundamental continuity underlying the diversity in animals. His theory is the result of his systematic studies and is based on the following three main ideas. |
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| New Needs |
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| Lamarck suggested that new needs arise in the organisms as the ever-changing environment influences them. Changes in the environment create a new need in the organism for adapting to that change, failing which the organism cannot survive. |
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| Use and Disuse of Organs |
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| The appearance of new needs forces the organism to put in additional efforts to fulfil the need. It may become necessary for the organism to put a particular part of its body into more and more frequent use or less and less frequent use. This idea came to be known as use and disuse of organs. Lamarck believed that any particular part of the body that is put into more frequent use, has a tendency to develop stronger and better while any part of the body, which is put into less or no use, has a tendency to gradually disappear. Thus, by a differential use and disuse of various body parts during its life span, an organism would change to some extent and acquire some new characters. |
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| Inheritance of Acquired Characters |
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| According to Lamarck the characters acquired by an organism are transmitted by heredity to the next generation. In every generation, fresh characters are acquired. With the result, after many generations, the changes accumulate to the extent that the species becomes modified into a new one. |
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| fig. 5.4 - Basic Idea of Lamarckism |
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| Lamarck's ideas on evolution may be explained by citing a few examples from his own writings. The most familiar among them are those of Giraffe and Snakes. |
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| fig. 5.5 - Acquired Characters of Giraffe |
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| The Giraffe differs from other mammals in having a long neck and longer fore limbs. The long neck and longer fore limbs, according Lamarck, are the characters that Giraffe acquired in its lifetime. Lamarck suggested that Giraffes started stretching their neck and fore limbs in order to reach leaves of taller plants. They were forced to do so due to the competition that they were facing from other herbivores. The practice of stretching the neck and fore limbs continued for generation after generation resulting in a gradual increase in the length of the neck and fore limbs. This example is cited by Lamarck in support of his view that any organ put into continuous use has a tendency to develop better. |
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| Similarly, Lamarck interprets the absence of limbs in the snakes as an evolutionary change. He suggested that the ancestors of snakes, which had well-developed limbs, faced the problem of increased predation. In order to protect themselves, snakes started crawling on the ground into crevices and holes. Continuous disuse of limbs resulted in limbs becoming shorter and shorter and finally they disappeared in one generation. |
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| August Weismann a German biologist tested the Lamarck's theory by cutting off the tails of the mice generation after generation. If Lamarck's theory was correct, the subsequent generations should have developed shorter and shorter tails. No such shortening of tails was observed. However, even after a very large number of generations, mice continued to develop tails of the same length. |
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| It is the name given to the theory proposed by Charles Darwin (1809-1882). It is also known as the theory of natural selection. He formulated this theory along with another English biologist, Alfred Russel Wallace (1823-1913) in the year 1858. |
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| fig. 5.6 - Charles Darwin |
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| This theory is the result of enormous amount of natural history collected by Charles Darwin on different species of plants and animals, during his voyage on the ship - H.M.S Beagle which lasted for about 5 years. |
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| fig. 5.7 - H.M.S. Beagle |
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| The theory was greatly influenced by the essay on population by Thomas Malthus, which underlined the competition between species for obtaining food. Darwin has given a vivid explanation of his ideas on evolution in the book 'On the Origin of Species by means of Natural Selection' published in 1859. The theory of natural selection is based on the following principles. |
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| Over Production |
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| Darwin found that all organisms have a natural capacity to produce more number of offspring than that survive. He quoted several examples to support this idea, the most classical being that of the elephant. Elephants are the slowest breeders in the animal kingdom. In a life span of about 90 years, a parental pair may produce a maximum of 6 offspring. If all the young of a pair survive, grow and reproduce at the same rate, there would be 19 million elephants in just 750 years! |
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| However, in spite of such an enormous reproductive potential, the number of individuals in each species remains nearly constant over long periods of time. |
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| Struggle for Existence |
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| As a consequence of this prodigal rate of reproduction, Darwin reasoned that there must be a competition between individuals belonging to the same or other species, for sharing food, water and living space. Every individual puts efforts for fulfilling the basic needs such as space for living, food for living, mate for reproduction and also protection from enemies. This competition is known as 'struggle for existence'. |
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| Variations |
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| The differences that an organism shows from its parent or from its related species are called variations. Variations are the rule of nature. Variations help an animal to adjust better to its environment. An animal, which has developed a favourable variation, has a better chance of survival and its offsprings are likely to inherit these variations. |
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| Survival of the fittest |
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| Only those organisms, which have favourable variations, are at a specific advantage over others. Such individuals survive the struggle for existence while those, which are less adapted or not adapted, get eliminated. This idea came to be known as 'survival of the fittest'. |
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| Natural selection |
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| Darwin believed that nature selects only those individuals, which have favourable variations and thereby have competitive advantage over others. This process is known as natural selection. The members of any particular species can develop and survive only when they are able to adapt themselves to the changed environmental conditions, by virtue of their favourable variations. Such variations are inherited and after a number of generations the variations become so prominent that a new species has been formed. Thus, new species develop from the existing ones in a slow and gradual way. This process of formation of a new species is called speciation. Darwin believed that while variations are the raw materials for evolution, natural selection is the force responsible. |
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| fig. 5.8 - Central Theme of Drawinism |
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| Darwin firmly believed that the environment would gradually weed out organisms, which have unfavourable variations, or with no variations, but preserve only those with favourable variations. However, Darwin failed to recognize the source of variations in the organisms. Hence goes the common saying - 'Darwin explained the survival of fittest but not their arrival'. |
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| Since Darwin's theory failed to explain certain facts, particularly the source of variations, several scientists started looking for bridging the lacuna in the theory. One such effort is the mutation theory proposed by Victor Hugo DeVries in 1902. According to this theory, new species arise from the existing one, in a single generation by a sudden change described as mutation. In plant, Oenothera lamarckiana, he found certain variations in one of the generations, from the normal plant. Such plants were found to breed truly giving rise to some more variations. DeVries suggested that large-scale variations occurring in a population are the result of mutations that occur frequently, resulting in the formation of a new species. |
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| Drawbacks and criticism of Darwinism |
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 Darwinism held that small variations form the raw material for evolution, this means that the organs appear as small structures and gradually become perfect after several generations. |
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In certain animals, some organs have developed beyond the stage of usefulness. Eg., Antlers of deer, tusks of elephants etc. These organs instead of providing usefulness to the possessors, offer hindrance in their daily life. |
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According to the theory of natural selection, only the useful organs are selected in the struggle for existence. The vestigial organs serve no function, yet they are preserved generation after generation. |
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| Neo-darwinism |
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| It is the modern theory on organic evolution. It is a synthesis of the ideas from the theory of natural selection and the mutation theory. The theory is the outcome of the thinking of recent biologists such as Huxley, Haldane and Dobzhansky. Neo-darwinism is based on three principles namely; genotypic variations, natural selection and isolation. |
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| Evolution of any organism requires many generations. To be effective in producing long-term evolutionary change, the forces of natural selection must act on populations and not on individual organisms. It is the population, which possesses the variability necessary to undergo evolutionary changes, through space and time. |
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| A more or less genetically isolated unit of population is known as a deme. The sum total of all the different genes in the population is known as gene pool. The evolutionary future of any population depends upon its gene pool. |
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| The frequency of any given gene in a deme, relative to all its alleles in the same locus, is known as gene frequency. In spite of a continuous movement of genes, called gene flow caused by in-breeding, the gene frequencies tend to remain the same. This state of stability is known as genetic equilibrium. Under these circumstances the deme remains genetically unaltered and no evolution occurs in that deme. Evolution takes place only if and when the genetic equilibrium is altered as a result of mutations, environmental changes and natural selection. Two more factors may be responsible for altering the genetic equilibrium. Sometimes variations may occur in the gene frequency, particularly in small population. Such a variation is known as genetic drift. It can produce significant changes in small populations. Another factor, which may alter the genetic equilibrium, is called isolation. It involves the separation of the sub-groups of a population having different genotypes, from the population having different genotypes, from one another. Speciation can occur only if and when the population splits into two or more separate demes, each with its own gene pool. If the populations are isolated, mutation and natural selection can occur independently on them and each of them can develop into a new species. |
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| Natural selection is responsible for maintaining constancy of a species, as well as in changing it. It maintains constancy as long as the environment remains stable, but promotes the formation of new species, if and when the environment changes. |
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