Revision notes of Evolution for NEET

 THEORIES OF EVOLUTION 

        The term ‘Evolution’ was first used by Herbert Spencer in 1852.

        Lamarck (1809) proposed a theory (Lamarckism) that states living things change by inheriting acquired characteristics, e.g. giraffes stretched their necks to reach food and their offspring inherited stretched necks.

        Lamarck’s theory was the first to admit that species changed due to use and disuse of organ and tried to explain it.

        Some evidences in support of Lamarckism are that radish is a biennial crop in cold countries, but it completes its growth in a year in tropical areas. Also European peach, which is deciduous, but it becomes evergreen in India.

        The theory of continuity of germplasms proposed by August Weismann states that germplasm (protoplasm of germ cells) and not the somatoplasm (protoplasm of somatic cells) is inherited.

        Neo-Lamarckism is the modification of Lamarckism, supported by a group of scientists. It explains that only those acquired characters are inherited by offsprings, which influence the germplasms or germ cells.

        Charles Darwin (1859) published the book origin of species by means of natural selection or the preservation of favoured races in the struggle for life, which has been recognised as one of the most important books ever written. A very similar theory was also proposed by Alfred Wallace.

        Essentially, Darwin suggested that random variations take place in living things and that some external agent in the environment selects those individuals which are better able to survive (survival of the fittest). The method of selecting individuals is known as natural selection.

        Speciation Darwin explained the phenomenon of speciation and said that the beneficial adaptations are accumulated and passed on to next generation.

        Some evidences in favour of the theory of natural selection came from modifications seen in the beaks of finches on Galapagos island. Also pedigree of some animals like horse, camel, etc. supports the theory of natural selection.

        Darwin’s was criticised by some scientists because his theory fails to explain arrival of the fittest over specialisation of organs, vestigial organs, somatic and germinal variations, etc.

        Neo-Darwinism is modification of Darwinism proposed by a group of scientists to remove the drawback in the theory of natural selection. It states that the adaptations result from the multiple forces along with natural selection. It combines the idea of natural selection with heredity. It also distinguished between germplasm and somatoplasm.

        Mutation theory was proposed by Hugo de Vries in order to explain the mechanism of evolution. It was published in the book Die mutation theorie (1901) by de Vries. According to this theory, sudden inheritable changes takes place in genomes of an organism due to certain factors called mutation. These mutations are discontinuous variations or saltatory variations.

        Mutation theory was criticised by BM Davis. This theory also failed to explain the role of nature in causing mutations.

        Dobzhansky (1937) in his book Genetics and origin of species provided the initial basis of synthetic theory. Modern synthetic theory of evolution was designated by Huxely in 1942. According to synthetic theory there are five basic factors involved in the process of organic evolution. These are gene mutation, changes in chromosome structure and number, genetic recombinations, natural selection and reproductive isolation. 

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