Theories of organic evolution
The process of organic evolution has been explained by different scientists by formulating different theories.
- Lamarckism (Theory of inheritance of acquired character)
- Darwinism (Theory of Natural Selection)
- Modern Synthetic Theory (Neo-Darwinism)
Lamarckism
One of the first theories of evolution that was published by Jean Baptiste Lamarck in 1809 in the book called Zoological Philosophy. His idea of evolution was based on the observation that the changes in the environment lead to the changed patterns in the species. His conclusion about the evolution was as follows:
- Tendency to grow — The internal forces of life tend to increase the size of an organism i.e the whole body and also the different parts up to a limit which it brings about itself.
- Formation of new organs — The production of new organ in an animal body is the result of the need or want of which has arisen and continuous to be felt by organisms.
- Use and disuse of organs and structures — The continuous use of an organ makes them functional and strong and thus leads to its better development, while continuous disuse makes the organ undeveloped, weakened and finally disappeared. Eg: Giraffe
- Inheritance of acquired characters — The characteristics that are acquired by an organism during its life time due to direct or indirect environmental effect is preserved and transmitted to its offsprings
Objections to Lamarckism
- Mutations are not inherited
- Eyes that are being continuously and constantly develop defects instead of being improved as is suggested by the theory of use & disuse of organs.
- The deadliest blow to Lamarck's theory came from Weismann, the greatest opponent of Lamarckism. Lamarckism explains the improvement of a character or change but not its initiation.
Darwinism
The theory of natural selection that was published by Charles Darwin in 1839 is one of the most important facts of evolution. The elements of this theory are as follows:
- Over production — All organism produce much more offsprings than can actually survive. A fern plant may produce 50 million spores and an insects like a housefly, may lay thousands of eggs.
- Struggle for existence — Due to the over production, there is struggle for existence. The individuals face intraspecific as well as interspecies competition for food, water, air and space.
- Natural Selection (Survival of the fittest) — In nature, organism face everlasting competition or struggle among them. Similarly, organism show some variations, which are proved to be beneficial in facing the environmental calamities.
- Variation and Heredity — The offsprings of the same parents also differ from one another after numerous generations and show variations.
- Origin of species — After several generations over a long period of time, the organism became so different from the individuals of the original species that they constitute a new species.
Objections to Darwinism
- It does not explain about the causes of variations
- Darwin's in fact considered all variations as heritable but today we know that only those variations are inherited which are germinal not somatic
- Darwinism explained the survival of the fittest but not the arrival of the fittest.
Neo-Darwinism
It is a modified form of Darwinism. The Neo-Darwinians like Wallace, Weismann etc believed that natural selection has accounted everything that is involved in evolution. In their dogmatic belief they went further to secure enough proof towards natural selection.
It includes following concepts:
- Mutation — It can be defined as the sudden, random, and heritable changes in the organisms. It was first proposed by Hugo de Vries. According to him, new species are formed due to discontinuous variation caused due to mutation.
- Genetic recombination — The genes or alleles of parental linkage groups separate and new combinations of alleles are formed.
- Natural selection — In nature, organism face everlasting competition or struggle among them. In that competition the stronger win and survive, while the weaker are rooted out.
- Isolation — It is the process of separation of organisms from their original population under physiological geographical factors.
- Genetic drift — The random changes in gene frequencies in a population occurring by chance alone rather than natural selection are called genetic drift.