Recent knowledge in genetics, molecular biology, cell trilogy, etc. has influenced the triologists to include all the phenomena that guide the process of evolution into one theory called Neo-Darwinism or modern synthetic theory. This theory takes into the consideration of Germoplasm theory of Weismann, Mutation theory of development varies, Laws of inheritance of Mendel, Darwin’s theory of natural selection.Wagner’s factor of isolation and population dynamics.

The basis of this theory was provided by Dobzhansky in his book ‘Genetics and Origin of Species’ while the designation ‘Synthesis Theory’ was proposed by Huxley. Though many other scientists like Muller, Fisher, Wright, Mayr, etc. have contributed a lot other the final shape of the ‘modern Synthetic Theory of Evolution’ was given by Stebbins.

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There are five basic factors involved in the process of organic evolution. These are (i) Gene mutation, (ii) Chromosomal mutation, (iii) Genetic recombination, (iv) Natural selection and (v) Reproductive isolation while the first three factors provide genetic variability; the last two are responsible for giving a direction to the evolutionary process.

1. Gene Mutation:

The heritable characters of an individual depend on the genes those control them. These genes are portions of DNA molecules which have a complex polymer of nucleotides. Any structural change occurring in the chemistry of DNA molecule is called gene mutation. Changes in the genes alter the phenotypic characters of the individuals.

2. Chromosomal Mutation (Chromosomal Aberration):

Specific types of genes present in specific amount of DNA at specific location is the characteristics of a particular species. A change in the number of chromosomes (aneuploidy, polyploidy, etc) or in the structure due to deletion, duplication, addition, inversion, or translocation brings about variations in the organisms phenotypes.

3. Recombination:

Recombination recombination of genes due to crossing over during meiosis is also responsible for bringing about genetic variability among the individuals of the same species, thus, contributing to the heritable variation.

4. Natural Selection:

Natural selection is a guiding of driving force, which utilizes the above raw materials (mutations) and gives a direction to the evolutions. Harmful mutations are eliminated or suppressed while mutants with better survival value are preferred and encouraged. Hence, natural selection is the dynamic force for speciation.

5. Reproductive Isolation:

Isolation plays a great role in speciation and particularly in preserving the identity of a particular species. Various isolating mechanisms operate in nature to isolate small groups of a population reproductively. So they can not breed freely. Moreover, related species are also isolated from each other which prevents hybrid formation.Isolations may be geographical, ecological, psychic, reproductive, etc. But the last one is the most important from evolutionary view point.

Accessory Processes:

Besides the five factors described, there are two accessory processes as follows:

1. Migration and Hybridization:

Migration of individuals from one population to another result in the introduction of new gene into the gene pool.

Hybridization may also occur between two closely related species giving rise to a progeny with altered characteristics than those of the parental population.

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2. Genetic Drift:

In small populations’ chance plays a greater role than the natural selection.

In such populations individuals are rapidly become homozygosity. This may lead to the extinction of the species. But as competition is less in such populations the homozygous individuals may withstand the condition and survive well finally giving rise to this new species.

Stebbins has given an analogy where gene mutations were compared with petrol in auto vehicle genetic recombination with transmission and accelerator and the reproductive isolation with the speed limits and directive signs on the highway.