Evolution: Evolution is defined as the history and development of race.
THEORIES OF EVOLUTION
Lamaism (Theory of inheritance of acquired characters)
• Jean Baptiste de Lamarck (1744-1829) was the greatest of French naturalists, who for the first time suggested a complete theory of evolution (organic evolution).
• Lamarck’s theory was published in 1809 (year of Darwin’s birth) in his book ‘philosophies zoologist’.
• Lamarck’s theory was most severely criticized by Cuvier, who greeted it by calling it ‘novella folic’ (foolish)
• Lamarck’s theory is also known as ‘theory of inheritance of acquired characters’ or ‘theory of use and disuse of organs’.
• Lamarck arranged his theory in the form of four postulates:
- Internal forces tend to increase the size of the body.
- Formation of new organs is the result of the need or want continuously felt by organisms. It is also known as doctrine of desires.
- Development and power of action of an organ is directly proportional to its use.
- All changes acquired by the organism during its life are transmitted to offsprings by the process of inheritance.
• The above - mentioned fourth postulate is the most controversial and is known as ‘Lamarck Doctrine’.
• Lamarck believed in direct influence of environment of the individual.
• The greatest blow to Lamarck’s came from the work of the German scientist August Weizmann.
• Weizmann cut the tails of mice for 80 generations, but no single mouse was born without a tail.
• In 1886, Weizmann formulated his famous theory ‘theory of continuity of germplasm’
• Neo – Lamarck's believe that only those characters, whatever be their origin and nature, that could affect germplasm are heritable and others are not.
DARWINISM
• Darwin was a Britain naturalist; in 1831 at the age of 22 he was appointed upon a world survey ship of the British government, HMS Beagle.
• Alfred Russel Wallace (1823-1913) also travelled widely. Evolutionary ideas similar to those of Darwin developed independently in Wallace’s mind.
• Both, Darwin and Wallace jointly propounded the ‘theory of natural selection’.
• The thinking of both Darwin and Wallace was greatly influenced by essays on ‘principles of population’ written by T. R Malthus.
• Darwin’s theory of evolution by natural selection can be summarized as follows:
- Over production
- struggle for existence (intraspecific, interspecific and with environment)
- Heredity and evolution
- Natural selection (Darwin) or survival of the fittest (Spencer).
- Changing environment needs structural modifications.
- Origin of species • Struggle for existence may be intraspecific (between an individual of the same species), interspecific (between different species) and extra specific (between an individual and its environment).
• There is more competition for survival between same animals of same niche.
• Continuity of existence of the fittest was called ‘survival of the fittest’ by Herbert Spencer, while Darwin called it natural selection.
• Darwin and Wallace published a joint paper titled ‘origin of species’ in 1858.
• Later in 1859, Darwin published his detailed theory in his book titled ‘on the origin of species by means of natural selection’.
• Some drawbacks of Darwin’s theory are:
- He considered minute fluctuating variations as principal factors.
- He did not distinguish between somatic and germinal variations.
• To explain the transmission of characters from generation to generation, Darwin proposed ‘pan genesis theory’ in 1868 which stands completely discarded.
• One major criticism against Darwin’s theory was his failure to give a satisfactory explanation for variations.
• Darwinism does not take into account of variations with heredity.
• According to the Neo - Darwinism, both mutations and natural selection are responsible for evolution.
• The chief Neo - Darwinian’s are Weizmann, Mendel and de Vries.
Mutation Theory
• Hugo De Vries (1901), a Dutch Botanist, pioneered the theory of mutations to explain the mechanism of evolution.
• The plant on which de Vries had experienced was evening primrose.
• The mutation observed by de Vries in evening primrose essentially was a chromosomal number variant.
• Mutations are discontinuous variations, called ‘sports’ by Darwin and ‘salutatory variations’ by Bateson.
• Mutations are generally harmful and recessive.
• Role of mutations in evolution is genetic variations.
• Mutations are the changes which may or may not be inherited.
• Mutation theory successfully explains progressive and retrogressive evolution, continuation of vestigial and overspecialized organs, struggle for existence and survival of the fittest along - with inheritance of useful mutations and formation of new species.
• Germinal mutation is a change that is inherited.
• Hereditary variations in plants have been produced by the use of x-rays.
• Frequency of a mutated gene in a population expected to increase if that gene selected by nature.
• The possibilities of hereditary and evolutionary changes are greater in species that reproduces by sensual means.
• Organic evolution would not have taken place if individuals in a population did not show gene variations.
You must be logged in to post a comment.