Lamarckism:

It is the first theory of evolution proposed by Jean Baptiste de Lamarck (1744-1829) in 1801 and 1809 (in book Philosophic Zoologique) which proposes that organisms undergo changes for adapting themselves to environment and the characters thus acquired are passed on to the next generation. Lamarckism is popularly called ‘theory of inheritance of acquired characters’.

Neo-Lamarckism:

It is an attempted revival of Lamarckism by modifying it and finding evidences for direct effect of environment on germ cells and the effect of somatic cells on germ cells.

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Theory of Germplasm:

The theory of germplasm (Weismann, 1892) states that inheritance of characters from parents to offspring is mediated through germplasm which continues generation after generation-hence “THEORY OF CONTINUITY OF GERMPLASM”. The theory is not completely true because somatoplasm can give rise to germplasm as during regeneration and culture experiments because all cells contain the same sets of chromosomes/genes whether they are somatic or germinal.

Darwinism/Theory of Natural Selection:

Darwinism or theory of natural selection is a theory of organic evolution which states that new species evolve over a long period of time through accumulation of small variations which provide the organisms with structural and functional sperioritv over other in their survival and differential reproduction.

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Neo-Darwinism:

It is a refinement of original theory of natural selection of remove objections. Mutation Theory

It was given by Hugo de Vries in 1902 in his book Species and Varieties their origin by mutation. He studied the inheritance for seven generations, in Evening Primose (Oenothera lamarckiana). He found that most of the offspring resembled parents but a few variants were not connected with the normal characters by any intermediate form.

The modifica­tions were, therefore, called DISCONTINUOUS variations or sports or mutations. They were inherited. The variants, therefore, produced variants and not normal plant’s. Rather a few variants also developed more variations.

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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. A number of mutations have produced new verities, subspecies and even species.

e.g.,

i. Ancon Sheep (1791)

ii. Hornless or Polled Cattle (1889)

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iii. Delicious Apple

iv. Navel Orange

v. Red Sunflower

vi. Hairless Cat

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vii. Double Toed Cat

viii. Cicer gigas etc.

Mutations are mostly retrogressive or negative and rate is also very low as compared to requirement of evolutions. It cannot explain origin of certain phenomenon like symbiosis and mimicry.

Genetic Drift:

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It is the change in number and frequency of genes in small isolated population due to intensive inbreeding causing permanent fixation of some alleles, disappearance of a number of alleles and change in frequency of others.

Isolations:

It is prevention of mating between otherwise interbreeding groups due to physical and biotic barriers. The barriers allow isolated groups to evolve into distinct populations/varieties/subspecies/ species.