THEORIES OF EVOLUTION
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The term ‘Evolution’ was first used by Herbert Spencer in 1852.
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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.
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Lamarck’s theory
was the first to admit that species changed due to use and disuse of organ and
tried to explain it.
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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.
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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.
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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.
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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.
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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.
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Speciation Darwin
explained the phenomenon of speciation and said that the beneficial adaptations are accumulated and passed on to
next generation.
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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.
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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.
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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.
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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.
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Mutation theory
was criticised by BM Davis. This theory also failed to explain the role of
nature in causing mutations.
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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.