The term Neo-Darwinism was given by Georges Romanes. Neo-Darwinism is an improvement of Darwin’s original theory of natural selection in order to remove its defects and incorporate features in tune with present day knowledge. The scientists who have modified Darwin’s theory are called as Neo-Darwinists and their theory is known as Neo-Darwinism. They are Weisman, Mendel, Devries, Fisher and J.B.S. Haldane. The theory of Neo-Darwinism is based on the following facts-
- Variations caused by rearrangement of genes during sexual reproduction.
- Variations arise by mutation.
- Natural Selection.
- Genetic Drift.
The struggle for existence was an important factor for natural selection. The most important objection of this view is variation which Darwin look as heritable. These variations may be due to the environment or due to change in heritable factors. Mendel showed that characters depend upon gene action present on chromosomes. Genes are heritable units and always remain pure. But animals in which sexual reproduction takes place show different kinds of variation due to random assortment and recombination of genes. Mendel showed that mutations take place in genes that produce new characters. Therefore, characters for natural selection leading of organic evolution are produced by mutation and recombination of genes.
Gregor Johann Mendel formulated the science and basic law of genetics in 1865. He obtained many varieties of peas and grew them in his garden. He scientifically obtained results of his cross-breeding. He published his conclusion in 1866.
Mendel’s work remains unknown for 34 years. The significance of his work was recognized only in 1930 when the Law of Segregation was discovered by three different biologists. Devries in Holland, Correns in Germany and Tschermark in Australia. Since then great advances have been made in our knowledge of heredity based on the original discoveries of Mendel. Breeding of plants and animals has become purely a scientific method these days.
Fisher was of opinion that due to natural selection beneficial genes have a tendency to dominate over less beneficial allelomorphs. Mutation has a definite rate which is called mutation pressure. Mutation pressure, selection pressure and genetic drift all along with other factors bring about evolutionary changes.
According to R.A. Fisher and J.B.S. Haldane, the mechanism of selection is based on particular inheritance.
Isolated population has no genetic contact with the parent population. Its gene pool slowly changes due to mutations and differential reproduction. Ultimately this population accumulates sufficient differences that its members fail to interbreed with individuals of the parent population. This creates a new species.