Theories of Evolution
Various theories have been postulated by different evolutionists. Most important ones are;
1-Lamarcks theory of inheritance of acquired characters(Lamarckism)
2-Darwin theory of natural selection (Darwinism)
LAMARCKISM (Theory of inheritance of Acquired Characters)
This theory was proposed by French biologist Lamarck in 1809.It is popularly known as ’The inheritance of Acquired Characters in Organisms’ and comprises of four assumptions-
(1)Living organisms and their parts tend to increase in size continuously due to internal forces of life(Origin of need).
(2)Formation of new organ in the body of an organisms is the result of a new need and new movement which this need initiates and maintains in the body(Effect of need).
(3)If an organ is used continuously and constantly it will tend to become highly developed, whereas disuse results in its degeneration (Use and Disuse).
(4)Modifications acquired during the life time of an individual are inherited by its offspring. It means that changes are cumulative over a period of time(Inheritance of Acquired characters).
DARWINISM (Theory of Natural Selection)
Darwinism is the term coined for the explanation offered by Charles Darwin for the origin of species by Natural Selection. Darwin explained how evolution might have occurred in nature. This explanation has been has been beautifully elaborated by Darwin in his book entitled’’ The origin of Species by Natural Selection’’. The theory comprises of following postulates as given under:
(1)Over production-All living being have an inherent tendency to produce offspring’s of their own kind in large number for the perpetuation of their race. The number of their offspring is much more than can be supported by a particular environment.
(2)Limited space and food-The space in the universe remains constant. The ultimate source of food for plants and animals also remain constant. Therefore, carrying capacity of the environment does not allow the population to grow indefinitely.
(3)Struggle for existence-According to Darwin individuals multiply in geometric ratio whereas space and food available remains constant, so there is an intense competition and threefold struggle for existence. The struggle is intraspecific, interspecific, and struggle with the environment.
(4)Variations and heredity-The offspring’s are similar to their parents and also exhibit some resemblance to each other. But they are not identical. They differ to some extent in shape, size, colour and behaivour etc..
(5)Natural Selection and Survival of the Fittest-During struggle for existence, only some offsprings are able to survive and reach adulthood. Those who become adult reproduce offsprings similar to them .This is called Reproduction.
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