5. Mutation theory can explain the origin of new varieties or species by a single gene mutation e.g. Darwin and a scientific contemporary of his, Alfred Russel Wallace, proposed that evolution occurs because of a phenomenon called natural selection . 2. Finally, there are a number of examples in which there is reduction in the size of organs e.g. The plants of next generation were again subjected to self pollination to obtain second generation. Hugo de Vries cultured O. lamarckiana in botanical gardens at Amsterdam. �@�P( This theory is based on the comparison between the contemporary species of his time to fossil records. Even slow breeding organisms reproduce at a rate which is much higher than required e.g., an elephant becomes sexually mature at 30 years of age and during its life span of 90 years, produces only six offsprings. Beagle” (Fig. So the number of a species will be much more than can be supported on the earth. ]��e�{+;�t, ����]h4�g5x�.�(��slD��7@�0�� ��! 3. Copyright 10.

Any factor which reduces the chances of interbreeding between the related groups of living organisms is called an isolating mechanism. So the useful variations go on accumulating and after a number of generations, the variations become so prominent that the individual turns into a new species. Various sources of genetic variability in a gene pool are: These are sudden, large and inheritable changes in the genetic material. Evolutionary theory explains the existence of homologous structures adapted to different purposes as the result of descent with modification from a common ancestor. Natural selection is a natural process through which life evolves. ���)|�h�o�D��u Development of flightless birds like ostrich from flying ancestors due to continued disuse of wings as these were found in well protected areas with plenty of food. In the absence of reproductive isolation, these variants freely interbreed which lead to intermixing of their genotypes, dilution of their peculiarities and disappearance of differences between them. 7.40. 3 0 obj He suggested that germplasm is with special particles called “ids” which control the development of parental characters in offsprings. In this theory, Charles Darwin proposed the concept of natural selection as the mechanism of evolution. 2. It stressed on adaptation to the environment as a primary product of evolution. The Theory of Evolution based on the process of Natural Selection was first propounded by Charles Darwin in his book ‘On the Origin of Species’ published in 1859. Differential reproduction states that those members, which are best adapted to the environment, reproduce at a higher rate and produce more offsprings than those which are less adapted. So it is also called Darwin-Wallace theory. Darwin and Wallace’s views about evolution were presented in the meeting of Linnean Society of London by Lyell and Hooker on July 1, 1858. This competition between living organisms for the basic needs of life like food, space, mate etc., is called struggle for existence which is of three types: Between the members of same species e.g. Effect of environment on germ cells through the somatic cells e.g.

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