Theory of evolution

Theory of evolution

Published by: Nuru

Published date: 17 Jan 2022

Theory of evolution in Grade 9 Science, Reference Notes

Theory of evolution:

The major theories of evolution are:

  • Lamarck’s theory or Lamarckism
  • Darwin’s theory or Darwinism

A. Lamarck’s theory or Lamarckism

Jean Baptise de Lamarck of France is biologist and philosopher. He proposed Lamarckism in his book philosophic zoologique in 1809 AD. This theory can be explained in the following points:

  1. Direct environmental effect
  2. Use and disuse of organs
  3. Inheritance of acquired characters

1. Direct environmental effect:

 According to Lamarck, the environment directly influences the modification of the organs in organisms. He believed that due to the changed environment, there is a new need for the organism which modifies the organs of the organism.

2. Use and disuse of organs:

 A constant use and disuse of the organs lead to the gradual weakening of the organs. A constant use and disuse of organs lead to its modification in the form and less used structures reduced and ultimately gets lost.

3. Inheritance of acquired characters:

 The characters obtained by an organism by an organism by direct environmental effect and use and disuse of the organs are called acquired characters. The acquired characters are inherited from one generation to another and new species are originated after a series of generations.

Criticism of Lamarckism are as follows:

  1. Lamarck has not proved experimentally that use and disuse of organs can modify them.
  2. It is absolutely incorrect that new organs can be developed according to the need and wish of the organism.
  3. All the require characters are not inherited into the new generation.

B. Darwin’s Theory or Darwinism

Charles Darwin, an English naturalist wrote a famous book named Origin of Species which was published in 1859 AD. Darwinism is also known as Theory of Natural Selection which can be explained in the following points:

  1. Struggle for existence
  2. Variation and heredity
  3. Natural selection
  4. Origin of natural species 
  5. Rapid rate of multiplication or enormous fertility: 

1. Struggle for existence:

Struggle helps to make the size of its own population constant. Organisms of different species compete with eachother for food and shelter. Here, only one species of organisms lives which struggles more. Struggle may be interspecific or intraspecific. It may be the struggle of individual of different species or of different species.

2. Variation and heredity:

Even though the offspring are very similar to their parents, they are not identical. They differ from each other to some or more extent in size, shape, behavior etc. that is called variation.

3. Natural selection: 

According to the Darwin, out of the vast number of individuals which compete for a place in the world, only those having advantageous variations survive and reproduce. The nature choose only those individuals, which are fittest, only they survive and reproduce successfully.

4. Origin of natural species:

Due to the changed useful characters, the individuals survive and may acquire more useful characters to survive in the struggle.

About the evolution of giraffe, Darwinism suggested that the ancestral of giraffe were believed to have short neck or of different size. When there was shortage of grass, long neckedgiraffe were able to eat leaves or foliage but short necked were not able. Like this, because of competition and natural selection, only long- necked giraffe survived and short necked giraffe disappeared.

5. Rapid rate of multiplication or enormous fertility:

Every species tends to multiply in geometric ratio. It means that population doubling its number in one year increases four times next year.

Criticism of Darwinism are as follows:

  • Darwinism does not explain the origin of variation.
  • It is not always the case that useful variations are selected.
  • Natural selection is not the sole cause of specification but it is not only a guiding factor.
  • Darwinism does not always explain the presence of vestigial organs found in organism.

Mutation:

Mutation theory was given by a Dutch Biologist named Hugo De Vries, in 1901 AD. Mutation is defined as a sudden chromosomal change by which variation occurs in organism. Mutation occurs by the effects of radiation and chemicals called mutagens. An organism affected by mutation is called mutant.

The majority of mutations are harmful, but a very small proportion of them may be useful for an organisms. Mutation is a main factor for evolution. Some of the examples of mutation are black patch on body, disability by birth, cut at lip, six- fingered limbs etc.